The Climate in Emergency

A weekly blog on science, news, and ideas related to climate change


Roofs for Heaven

So, our roof sprung a leak.

I realize some might question my sharing this small bit of personal woe, but really, what homeowners don’t have an issue now any then? In fact, I’m told that roofs need to be replaced routinely–it’s an expected hassle, especially for some of the shorter-lived roof types. That means you, too, might be in the market for a new roof, and if you’re not now, (if you’re lucky enough to own a home) you will be someday in the future.

So what kind of a roof do we want?

The thing is that roof-shopping is an opportunity, a chance to make decisions about some small corner of the built environment instead of simply accepting it as a given. What sort of roof do I want? Can I have one that’s different? That’s especially mine? That better suits my tastes and values and generally makes me happy to live beneath?

Can, in other words, a roof be a response to the challenge and crisis of climate change?

Yes, it can.

Roof-Related Issues

First, let’s take a look at how roofs are related to climate change. In my reading, I’ve identified three broad categories of relevance: roofs have carbon footprints, so it’s possible to choose a roofing type with a small one; roofs have an impact on the energy use of the house; and some roofs have additional tricks, such as generating renewable electricity.

All of these together, plus such practical matters as cost, become part of the picture for making a final choice.

The Roof’s Footprint

Roofs, like everything else, have a carbon footprint. One way for a homeowner to respond to the climate crisis is to get a roof with a smaller footprint. I was able to find a study that did compare the carbon footprints of various roofing types, at least in Australia, but unfortunately it did not include asphalt shingle, which is the simplest and least-expensive in our area.

The study compared typical residential roofs of sheet metal, clay tile, and concrete tile, including in each case the wooden frame beneath (though not, apparently, insulation). The analysis looked at both greenhouse emissions and embodied energy for each from cradle to grave and found out that the footprint of metal roofing can differ radically depending on whether it is recycled afterwards. The abstract of the paper (reading the full text would require money I don’t have) did not include all of the relevant numbers, so I had to do some math. “CO2e-” means carbon dioxide equivalent, recognizing that there are other greenhouse gasses and their warming potential varies. CO2e- is a way to quantify and express warming potential as a single figure regardless of which greenhouse gasses in what relative quantities are involved.

It’s also not clear from the abstract how much of each roofing material was involved–obviously, larger roofs would have bigger numbers–but the roof in the analysis was the same size in each case. “T” presumably stands for metric tonne, which is somewhat larger than the American ton.

  • Clay tile has a carbon footprint of 4.4 t of CO2e-.
  • Sheet-metal roofing that is not recycled has a carbon footprint of 9.85 t of CO2e-.
  • The carbon footprint of concrete tile wasn’t given, but is intermediate.
  • Sheet metal roofing that is recycled “can obtain significant carbon and embodied energy saving benefits (i.e. 71–73%) compared to clay tile or concrete roof covers,” a grammatically ambiguous statement that seems to suggest the following:
    • Metal roofs, if recycled, have a carbon footprint of 1.27 t of CO2e-
    • Concrete tile has a carbon footprint of 4.7 t CO2e-


  • Metal, if later recycled = 1.27
  • Clay = 4.4
  • Concrete = 4.7
  • Metal, if not later recycled, = 9.85

Curiously, concrete tile, not never-recycled metal, has the highest embodied energy.

So, what about other materials? Unfortunately, figures from different analyses aren’t directly comparable, since analyzing carbon footprints requires making a lot of arbitrary decisions about what to include and what not to include–and each person who does such an analysis ends up making those decisions differently. So I can’t combine results from multiple studies. Anyway, I wasn’t able to find life-cycle analyses of other roofing materials.

I did find an American analysis of the carbon footprints of disposing of various construction materials, including asphalt shingles, and the article did comment on the footprint of construction. Curiously, shingles made with a core of fiberglass felt have a lower footprint than those made of more natural-seeming paper felt, since paper absorbs water and must be dried, a process that takes energy. The fiberglass variety are already far more common.

Another surprise was that incinerating the shingles lowers the footprint of disposal because they can replace other fuels for energy generation, including fuels that have more greenhouse gas emission for the same amount of energy–shingles can’t be incinerated in most facilities because of “impurities” (I’m guessing this means air-quality concerns?) but are accepted as fuel for cement kilns in Europe. Asphalt shingles can also be melted down and added to the asphalt mixes used in roadways, which reduces the total amount of asphalt used and thus lowers emissions related to sourcing the material. Asphalt shingles can, in theory, be recycled into new asphalt shingles, but it’s technically difficult and nobody is doing it. The shingles can’t be composted. None of that is very useful for a homeowner, though, especially as I don’t have access to a European cement kiln.

And I wasn’t able to compare the footprint of asphalt shingles to that of metal roofing.

The Roof’s Role in the House’s Footprint

What type of roof a house has can alter the energy use (and thus the carbon footprint) of the house as a whole. For example, a white roof reflects heat and keeps the house cooler, reducing the temptation to use the air conditioner. A black roof absorbs heat and keeps the house warmer, reducing the need for heating in winter. Which one is better depends on the local climate where the house sits and whether the occupants tolerate cold or heat better. There may, in the future, be coatings available that will darken or lighten in response to temperature, but as yet we must pick a color.

Different materials also vary in their ability to conduct heat into the house, so asphalt shingles will warm a house more than a metal roof of the same color–both because asphalt absorbs a lot of heat and because asphalt roofs are designed to transfer as much heat as possible from the shingles to the house beneath, since otherwise the shingles get too hot and are damaged by heat.

A roof with excellent insulating capacity will keep the house temperature from varying as much, an advantage in both hot and cold weather. The R-value (insulating ability) of a whole roof typically depends largely on a layer of insulation, because the roofing surface tends to have a low R-value no matter what it’s made of, but they do vary, so a material with a higher R-value is better, all else being equal. Asphalt shingle is .44 and wooden shingle is .97. I wasn’t able to get a figure for metal roofing, but it is similar to that of asphalt.

Roofs with Benefits

There are roofs that do more than simply cover the top of the house.

Green roofs, that is roofs designed to support living plants, reduce local air pollution, reduce storm-water run-off, provide animal habitat (depending on what’s planted up there), and can even grow food (if the roof is accessible enough to harvest). They also sequester carbon, although the chance of that carbon remaining sequestered very long is slim–most have to be replaced after about 40 years.

Solar roofs incorporate solar panels and generate electricity.

And, while it’s a bit off-topic for us here, flat roofs surfaced in gravel provide nesting habitat for certain birds (nighthawks, for example).

Roofing Materials

It’s no good just asking what roofing material is “better for the climate” in a vague way. To make a decision, we have to know what kinds of benefits we’re talking about. Now, we do know, so we can get on with exploring specific materials.

Asphalt Shingle

Asphalt shingles are made out of sheets of felt (either paper-based or, more commonly, fiberglass) that have been soaked in a thick type of asphalt and sprinkled with coarse sand. They are popular because they are cheap and, in the short term, sturdy (though quality can vary a lot). Unfortunately, their R-value is low, they conduct a lot of heat into the house, and they don’t last very long. Most must be replaced about every 20 years, meaning that however large their carbon footprint is, a sixty-year-old house roofed in asphalt has three such footprints, not one.


Metal roofs are moderate in cost (higher than asphalt but lower than some other options) and relatively long-lasting, on the order of 60 years. Their R-value is low, but they conduct very little heat into the house, especially if given a heat-reflective coating. If the metal will be recycled at the end of their service, their total carbon footprint is quite low. They resist most kinds of damage very well, don’t burn (important, as climate change makes wildfires more frequent!), and don’t support the growth of moss or algae, though they can be dented by hail. Many different styles are available–metal roofs can be made to look like several other materials–but do have one aesthetic disadvantage; rain falling on them is very noisy.

Wood Shingles

Wooden shingles are carbon neutral, or close to it (processing and transportation surely involve some emissions), can be made with reclaimed wood from other buildings, and unlike almost all other options, they can be composted upon retirement. The price is only a little higher than asphalt shingle. While at first consideration they seem excellent, I have noticed that even buildings with wood shingle siding almost never have wood shingle roofs. I’m not sure I’ve ever seen a wood shingle roof, come to think of it. Why not?

Wood shingle is actually not recommended from an environmental perspective, at least not by people who don’t sell wood shingle. The problem is two-fold. First, while they can last 30 years under good conditions, wood shingles are vulnerable to rot, and many don’t last even as long as asphalt shingles do–that makes them less economical and increases their environmental impact. Perhaps more importantly, they are relatively dark in color, so they absorb heat. I assume they could be painted, but at the cost of much of their aesthetic value. They are also less fire-resistant than most other roofing types.

Green Roofs

Green roofs have all the advantages of a garden, plus they’re great at keeping the house cool. Unfortunately, the weight of the soil and water means that not all buildings can support these roofs. Installation is also expensive, though not necessarily more so than higher-end forms of traditional roofing.

Green roof designs are categorized by soil depth (and therefore what kinds of plants can grow on them) and by how much maintenance, including irrigation, they need. The categories are labeled “intensive,” “semi-intensive,” and “extensive.” It is possible to have a roof that is only partially covered by a garden and is otherwise more traditional.

Solar Roofs

Solar roofs involve tiles that are each little solar panels. I’m not sure what their virtues as roofing are, and they are more expensive that traditional solar panels; their primary advantage is that they don’t look like solar panels. It’s a moot point for my husband and I anyway, as we live in a forest.

Passive solar energy, in which water is heated in the roof, might also count as a “solar roof,” but is again rendered moot for us by trees.

Other Roofs

There are other roofing options, such as slate, rubber (made to look like slate), concrete tiles, clay tiles, and good-old-fashioned thatch. I love the idea of thatch, and I’ve heard it performs very well. If gathered locally, its carbon footprint could be virtually zero. However, dry thatch likes to catch on fire, and the chance of our finding a qualified thatcher in Maryland is just about nil. The other options are either expensive or hard to find or both, and also easily damaged by hail or other impacts (perhaps not the best thing as climate change makes extreme weather more likely).

That being said, slate has a very low carbon footprint and is reputed to be environmentally excellent, according to multiple sources, most of which sell slate. Clay and concrete have moderate carbon footprints, as already noted. I have not found figures for the other “others.”

The View From (or of) the Roof

Since it doesn’t look like I can have thatch or a green roof, I’ll be pushing for metal. If we end up priced out of that, white-painted asphalt shingle will do, though paint doesn’t stick to shingle very well. Metal is much easier to make white, and I like the fact that it lasts much longer.

Our area has hot summers and cool, but not cold, winters. We are also prone to wind–in the decade or so I’ve been here, we’ve had to cope with hurricanes, nor’easters, a derecho, a tornado, and frequent blustery days (it’s too windy to bike for several days in any typical week). We therefore need a roof that resists wind, rain, and stuff falling on the roof (such as tree branches) and that can keep our house as cool as possible in the summer. We do not need help keeping the house warm in the winter, especially since we heat with sustainably harvested wood, not with fossil fuel. Given our forested lot, solar shingles don’t make sense, especially since we buy renewably-sourced electricity anyway. And cost is a consideration because are not independently wealthy.

Your considerations, and thus your conclusions, may differ.



The Carbon Footprint of College?

Image shows a green, leafy college campus, looking down a concrete walkway--a second walkway branches off to the left. In the angle between the two walkways is a group of bike racks on a mulched surface. Otherwise the ground is grassy. There is only one bike in the rack. To the left of the walkway is a lawn, some small trees, and a very long, ivy, covered three-story building. To the right is lawn and a row of trees. The sky is partly cloudy. There are no people visible.

Photo by Ryan Jacobson on Unsplash

Some weeks ago, I wrote a post on the carbon footprint of medicine, but I could not find all the information I wanted. While I found estimates of the total footprint of medicine in various countries and on certain specific aspects of medicine (emergency transport, surgery, pharmaceuticals), there were some big gaps I could not fill. I speculated that since hospitals and residential colleges have certain things in common, it might be interesting and informative to look up the carbon footprint of college campuses.

Well, guess what?

There are lots of articles out there on how colleges are reducing their footprints, but virtually nothing on what those footprints actually are. A typical piece might boast that such-and-such a school has reduced their emissions by 73% since 2010, but without saying 73% of what, and without giving a breakdown of where those savings had been found or what aspects of the school were responsible for those emissions. I doubt the information is being hidden, it’s just that I’m asking questions few other people are asking, and that always makes information hard to find online.

When I can’t find answers to write about, I write about the questions.

Questioning College Emissions

How one asks a question dictates the sort of answer one gets–and what can be done with that information. For example, consider the difference between the following two questions:

  • Which American colleges have reduced their carbon footprint the most in the past ten years?
  • Which American colleges have the lowest carbon footprints currently?

Either list seems like a reasonable answer to a vague question, like “which colleges are greenest?” and yet the two lists are likely to be completely different. And neither one offers any clue as to how small a college footprint could actually get and still offer an excellent education.

So what questions do I want answers for?

The Breakdown

I want to know where the emissions in a given school come from. A pie chart would be nice. Schools could be compared not only by their totals but also by their scores in several subcategories–one school might have very high emissions associated with heating and cooling, for example, while another has a big transportation score. Now since I’ve only ever been a student, not a staff member or an administrator, there are things I don’t know about how schools operate. The following is my current best guess for how the greenhouse gas emissions of a school might be broken down.

On-campus Housing

This category can be further broken down into electricity, fuel use, coolant (from air conditioning and refrigeration), waste disposal, and possibly some other categories. The figure will be zero for non-residential schools, that doesn’t mean non-residential schools are automatically better; if dorm-living has a lower footprint than off-campus living, residential schools might be better for the planet despite having a bigger institutional footprint.

On-campus Food-service

Again, a category that will vary in ways that really must be put in context. For example, my undergraduate school did not have classes. Instead, it had residencies several times per year. During residencies, we all ate in the dining hall, so the school definitely had food service, but only for about 12 weeks per year, and only for about a third of the student body, plus faculty, at any given time. It’s per-student food-service figure must have been extremely low, relative to traditional residential schools, for reasons that had nothing to do with energy efficiency in the kitchen. And, as with housing, a school’s food-service emissions must be compared not just to those of other schools but also to those of students who don’t eat on campus.


While most emissions categories need to be expressed as per-student figures, with grounds-keeping the important context is probably not the size of the student body but the size of the outdoor portion of the campus. Again, I’m thinking of my undergrad program, which held its residencies on a campus that had been built for a student body much larger than ours. A tricky situation arises for schools that rent space–does maintenance of the grounds go on the school’s tally sheet or not?

Academic Facilities

By “academic facilities” I mean buildings other than dorm rooms and dining halls, the sort of buildings that both residential and non-residential schools have in common and by which they can be directly compared. I am not sure whether sports facilities ought to be included here or not. And do, for example, the pastures of an agricultural college or the forests of a forestry school count as academic facilities or grounds?


Shows a classroom with about ten or twelve adult students sitting in chairs watching a man deliver a PowerPoint lecture. The man, presumably the professor is standing behind a lecturn and is dressed casually. The students are also dressed casually. The professor is on the other side of the room from the camera and is hard to see. The classroom is well-lit and has large windows with curved tops and a ceiling with a large raised area in the middle with what look like noise-dampening panels in it. The room as a whole has a somewhat fancy look.

Photo by NeONBRAND on Unsplash

I have taken college classes that likely had carbon emissions of zero, but I’ve also had classes that involved driving hundreds of miles to field study sites, and the school offered some that required air travel. Courses that involve chemistry experiments or animal care might have significant footprints, too. Some classes use much more electricity than others–should that be counted as part of the footprint of the class, or simply subsumed into “academic facilities”?

Non-academic Travel

“Academic travel” is that undertaken as part of a class, such as driving on a field trip, or when a professor visits a site in order to prepare for an upcoming field trip. Such trips count under “classes.” Non-academic travel is students and professors and other staff going to and from school. It’s a tricky category, because it’s not directly under the control of the school. If a student wants to commute to class from the other side of the country by private jet, there’s not much the school can do about it.

And yet schools can take steps to minimize the necessity of travel, such as by providing on-campus housing. Schools can also make lower-impact forms of travel more practical, such as by installing bike racks and having bikes students may borrow for free, or, for large schools, by creating a local transit system that runs on biodiesel. A school could also have EV charging stations on campus (providing the school has a sustainable source of electricity) or it could produce and sell biodiesel. There are lots of options, and schools should be held accountable for taking those options. Perhaps a school’s non-academic travel figure could be an estimation of the travel emissions of an average student based on survey data and how far from campus students live.

Construction and Renovation

Building stuff has a carbon footprint, particularly if cement is involved. There can be some tricky judgment calls, though, since a college that builds a new LEEDS-certified lecture hall will have more emissions that year than one that doesn’t, even though the hall may be an effective investment in the school’s sustainability long-term. Also, some building materials either release greenhouse gasses over time or absorb them. Finally, building-related emissions have to be seen in context. Some years ago, my grad school put up a bike shelter on campus in order to encourage students to bike rather than drive. The bike shelter sits on a poured concrete pad, and although much of the assembly was manual, heavy machinery was also involved. So that shelter was definitely responsible for some greenhouse gas emissions. However, if it accomplished its mission, it’s also responsible for reducing emissions from the non-academic transportation category, a possibility that must be accounted for when judging its construction.

A fair footprinting method would have to average construction-related emissions over the projected life of the building (demolition-related emissions would also have to be included), and the impact of the building on other aspects of the school’s footprint would also have to be considered somehow.

Legacies and influences

What emissions occur on campus are not the only issue for colleges. A school could score a nice, big zero in all of the above categories, but if it is training students specifically to work in the fossil fuel industry (certain branches of geology, for example, are most applicable to fossil fuel prospecting), then its carbon footprint can’t really be zero. Likewise, a school that has invested its endowment in fossil fuels has a big footprint no matter what else it does. How such indirect emissions might be calculated, I’m not sure, but they would have to be included somehow.

What Are These Questions For?

All of the above categories would, ideally, show where a school might improve. The school itself could use the information in its planning, and outside entities (prospective students, for example) could assess how serious the school really is about climate change.

For example, a school might put up a new bike shelter and put out a press release about how “green” it is. OK, but if the school’s non-academic transport score is actually pretty good already and its academic buildings score is excessive, then the bike shelter begins to look like green-washing.

Of course, to make such an assessment, we’d have to know what all these scores should be. At the very least, we need to be able to compare the scores of each school to some kind of relevant average.

Defining such averages, never mind collecting enough data to calculate them, would be difficult.

A Modest Proposal

It’s not that nobody is comparing colleges based on environmental considerations. A search for “the greenest colleges” actually yields a lot of information, including carefully-compiled rankings. But if listings of carbon footprints broken down by category exist, I haven’t found them, yet.

Considering the matter carefully, I begin to suspect they don’t exist. There are too many places in the above list where a simple question (“What is a given school’s greenhouse gas emissions for academic buildings?”) shatters into dozens of sub-questions about definitions and methods and fairness. And to be truly useful, the assessment I’m envisioning would have to be done for a fair portion of all the schools in the US (or whatever country one was curious about), and then all those data would have to be compiled into several different categories so schools could be compared fairly.

That’s not a blog post. That’s not even a master’s thesis (calculating the footprint of a single school and making recommendations would be a master’s thesis). It’s a PhD dissertation.

A white mug full of coffee sits in the foreground on a plain wooden table lit by defuse sunlight. At least two wooden chairs are next to the table, but they are out of focus. At the far end of the table is a very out-of-focus object that might be a person sitting at the table facing the camera. The rest of the room is too out-of-focus to make out. On the cop, in small, black print, is written the word "begin."

Photo by Danielle MacInnes on Unsplash

And it’s quite likely no one’s gotten around to doing it yet.

Fortunately, there are people out there looking for dissertation topics, so if you’re one of them, or if you know someone who is, may I humbly suggest this one? Send me the results when you’re done.

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The Carbon Footprint of Space Flight

So, what is the carbon footprint of a rocket launch?

The short answer is that nobody knows. Because rockets inject various substances into the upper atmosphere–both on the way up and the way down–they have the potential to chemically alter the atmosphere, and thus the planet’s greenhouse effect, in addition to whatever greenhouse gas emissions are associated with construction and launch. And nobody has studied the issue yet.

Other aspects of the carbon footprint of space travel are possible to calculate, but are still not quite straight-forward.

Footprinting Rockets

Reportedly, a single Space Shuttle launch released 28 tons of carbon dioxide, but I haven’t been able to find out how that was calculated. Is that the burning of rocket fuel only? Does it include all associated ground-based transportation, from moving the shuttle out to the launch pad to the morning commute of everybody who works in Mission Control? The problem with carbon footprinting is that there is seldom an unambiguous distinction between the footprint of one activity and the footprint of a different activity. What do you include? What do you exclude? Why? The best you can do is be clear about what you did include so that a fair comparison can be made with the footprints of other activities.

Some rocket launches can be described in terms that are seriously misleading; hydrogen is a popular rocket fuel, and burning hydrogen yields water as its exhaust, not carbon dioxide. While water vapor is also a greenhouse gas, the amount of water vapor in the atmosphere depends on how much the atmosphere can hold, not how much steam is released. Look at just the launch of a hydrogen-burning rocket, therefore, and we seem to see a carbon footprint of zero–yay! But hydrogen cannot be simply collected, like so much firewood–it must be produced. And the production of hydrogen takes energy, which comes from….

It comes from wherever the production plant gets its electricity, but it’s a good bet much of that energy comes from coal-fired plants. Not only is coal itself an incredibly carbon-intensive fuel, but look at how many times this energy must transform on the way to the rocket. The chemical energy in the coal is converted to heat, which boils water to make steam, which spins a turbine, which makes electricity, which converts water to hydrogen–that’s at least six transformations! As per the Second Law of Thermodynamics, each time energy is transformed some of it is lost, meaning there is a lot less energy in that hydrogen than there was in the coal. The rocket would have a much smaller carbon footprint if it could just burn coal directly (except coal is too heavy to use in rockets).

Ultimately, the carbon footprint of one rocket is less important than the carbon footprint of the entire space industry; are we talking about a few rocket launches per year, or are we talking about thousands, or hundreds of thousands of them? The difference matters.

The reason for the launch might matter, too.

Space flight is not only a source of carbon emissions and other pollutants, it is also a critical part of climatological and meteorological research. Without data from satellites, we would not be able to effectively model the changing climate, nor would our predictions of extreme weather events be as accurate. GPS satellites and communications satellites are also important in research, not to mention facilitating communication that would otherwise require actually going somewhere, with associated carbon emissions. Conceivably, the true footprints of some launches could be less than zero if those satellites lead to meaningful climate action policies or make possible reduced carbon footprints on Earth.

The Future of Spaceflight and Climate Sanity

The reason I began thinking about all of this is I’ve been wondering whether the carbon-sane future can include space flight.

Climate sanity is all about energy; the function of fossil fuel use has been to give us more energy than the biosphere can spare, and it is this over-use of energy that is destabilizing the climate and collapsing biodiversity. The enhanced greenhouse effect is the mechanism by which the climate is being destabilized, but the overdraft of energy is the ultimate cause. If there were a way to access just as much energy by some means other than fossil fuel use, the planet would be pushed to crisis by some other mechanism.

It follows, therefore, that humanity, to become sustainable, must go on an energy diet. The change need not be painful; greater efficiency and new technology could ensure that standards of living remain about the same, or even improve, for most people. But some energy-intensive practices will have to be left behind.

Will space flight be one of them?

I’m specifically thinking of those various research and communications satellites, not manned space flight or military applications, both of which we really can do without, and not brief trips to the edge of space and back down again. How much energy is necessary to launch a satellite into orbit, and will that energy be available under a sustainable energy budget?

Calculating the energy required to put up a satellite is fairly easy, if you happen to know the relevant equations and don’t have a learning disability involving math (meaning I’m out of luck, but maybe you’re not).

Here is an article discussing part of the process.

The short answer is that it takes over 31 million joules of energy to put one kilogram of something into orbit, except that all those joules need to come from somewhere, and the fuel necessary to deliver that much energy is going to weigh more than a kilogram, meaning you’ll need more energy to lift the fuel….It sounds as though any rocket must be infinitely large in order to lift itself and its ever-increasingly large fuel supply, and obviously that’s not true, but rockets do have to be much more powerful than casual thought might suggest–and if the payload is bigger, the rocket must be much bigger, in order to account for the extra fuel, and the extra fuel necessary to carry the extra fuel, etc.

Calculating the total energy need for a launch thus requires an extra layer of math, but the process still sounds fairly straight forward, provided you know what you’re using for fuel (so you know how much weight you’ve got per joule), have the right equations, and, once again, are not me.

Then you’d have to figure out whether that much energy is available under the new energy budget, and that involves….

A Simpler Way to Figure It

So, how does a writer who cares about science but can’t do math figure out whether the climate-sane future has rockets in it?

I don’t know how to work the various equations, but I have seen an object launched into orbit aboard the Antares rocket. The Antares, as configured for launch to the ISS, is almost 43 meters long, or about 140 feet, and about 12 feet across. Total burn time, counting both stages, is about six minutes. The first stage burns 64,740kg, or 142727 pounds, of a highly-refined form of kerosene. The second stage burns 12,815kg, or 28252 pounds, of polyurethane. It can deliver a payload of up to 5,400kg, or 11,905 pounds. That’s the equivalent of about three average-sized cars.

A gallon of kerosene weighs 6.8 pounds, meaning we’re looking at well over 20,000 gallons of kerosene. That’s a lot, but offhand it doesn’t sound like more than could exist in a post-petroleum future–and yes, aviation-grade kerosene can be made from plant-derived oils. I’m not sure how much sense it makes to attempt to calculate gallons of polyurethane, but it’s going to be much smaller than the kerosene figure. And polyurethane, too, can be made from plant-derived oils.

Some time ago I attempted to calculate the per-gallon prices of various fuels in a post-petroleum world. Because I can’t begin to anticipate market forces in such an economy, I did not calculate the prices in money but rather by comparison to food. The two main biofuels, ethanol and biodiesel, can both be made out of edible substances: corn and soybeans or Canola seed, respectively. So, for every gallon of ethanol, how much corn must be removed from the human food-stream? For every gallon of biodiesel, how much soy or Canola seed must be lost? I didn’t mean that fuel must always be made from food, only that the comparison provides an intuitively accessible way to understand both the economic and ecological cost of fuel production in terms that are going to be relevant no matter what type of economy we end up using.

I have not attempted similar calculations for either kerosene or polyurethane–we’re interested in ballpark figures at present, in getting to the right scale, so for our purposes, the biodiesel figures are probably close enough.

If so, then the first stage of an Antares rocket alone burns the economic/ecological equivalent of enough food for at least 164 people–possibly as many as 657, depending on what kind of oil you start with–to eat for a year. And that’s not counting the energy involved in refining and chemically converting the oil into fuel, building the rocket, building the satellites, transporting materials or finished parts, running all necessary computers and communications equipment, and covering the loss of the occasional rocket the blows up during launch (as I saw an Antares do). And don’t forget all the people involved with all of this, who need to eat and so forth.

It’s a lot, but it’s not so much that the United States could not collect the resources for a launch every year or three, which is really all that should be necessary for satellites for science, GPS, and communication.

And this is for launch of the Antares, a launch vehicle certainly capable of delivering a good-sized payload to low-Earth orbit, but it’s certainly not the only way to get the job done. A smaller payload–or a launch vehicle made of lighter materials–would need dramatically less fuel, thanks to the issue we explored earlier of needing fuel to lift the fuel. Launching a rocket from the upper atmosphere (a balloon lifts it up to the launch “site”) reduces drag on the rocket and again reduces fuel dramatically.

Lunar colonies and space tourism are probably still out, but those applications of space flight that yield the biggest benefits to us here on Earth sound doable.

I had initially assumed that the carbon-neutral future would have to do without spaceflight. Now it looks like I was wrong.

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The Carbon Footprint of a Beagle

So, we just got a beagle.

We already had one beagle, but after the death of her co-dog (a Lab/pit mix) last month, she’s been lonely, so we got her a companion. His name is Reilly, and he is sweet and affectionate and already causing trouble in his distinctively charming and beaglish way.

This seems like a good time to cover a topic I’ve been interested in for a while, the relationship between climate change and pets

The Carbon Footprint of Pets

Turns out, there have been serious scientific studies of the carbon footprint of dogs and cats. Results vary, but the general consensus tends to be that pets, collectively, have a large carbon footprint because there are a lot of them and dogs and cats eat mostly meat, which is a carbon-intensive food.

There are a couple of interesting points, here.

First, these studies may be studies of the carbon footprint of pet food, not pets. One research team is quoted as having looked at dog food only, based on the assumption that other aspects of dog care have minimal impact. Their assumption may be correct, but personally I’d like to see a study that examined all aspects of dog (and cat) care so we could check the accuracy of that assumption. I’m also amused by their conclusion, that big dogs have a larger carbon footprint than small dogs, since big dogs eat more. Personally, I’m not sure why anyone would assume the non-food aspects of dog care have minimal impact (a complicated question involving lots of data most of us don’t have) but then perform and publish a formal study on whether big dogs eat more than small dogs do.

Second, sorting out the carbon footprint of food may be trickier than it appears. For example, pet food is often made, in part, from meat by-products, which humans can’t eat. By-products are essentially waste for which a market has been created, stuff that would not exist if the primary product (muscle meat for human consumption) were not being produced. So is it really fair to assign the carbon footprint of the meat by-product to the dog who eats it rather than to the human whose demand for steaks created that steer in the first place?

The carbon footprint of food can vary a lot, as we know from studies of human diets. For example, beef and lamb are much more carbon-intensive than chicken. I’d like to see a detailed break-down of several different kinds of pet food and the different aspects of their production.

To Pet or Not to Pet

What does the question “what is the carbon footprint of a pet?” really mean? We could ask about the carbon footprint of Reilly and what we, his guardians, can do to make him a “greener” dog. Alternatively, we could be asking about our own carbon footprint and whether not having Reilly would make my husband and I “greener” people.

And since Reilly’s personal impact on the climate would presumably be about the same no matter who had him, the latter question really boils down to the draconian “should Reilly be alive?”

In a similar spirit we might debate, or refuse to debate, the lives of human children. Indeed, since humans have huge carbon footprints, especially in the so-called “developed” world, some list “having a child” as the worst thing a person can do to the planet, even worse than airplane travel, car travel, or eating meat.

My husband and I don’t have children, and environmental impact is part of the reason, but phrasing the decision as a measurable reduction of our carbon footprint as a couple seems very wrong.

What if the child in question were the next generation’s Rachel Carson?

The very idea of reducing a child to a carbon footprint is offensive. Reducing Reilly in such a way is less so, but still pretty bad.

But Haven’t There Always Been Dogs?

There is an argument to be made for having fewer dogs and cats in total. Their collective environmental impact is not negligible, and most humans could get along without them quite well (I said most, not all). But if all dogs and cats suddenly vanished, would the carbon footprint of humanity really shrink? Or would some other use be found for meat by-products?

Perhaps more to the point, would climate change really slow?

This whole line of questioning reminds me of cows. There is an argument to be made for having fewer head of cattle, too, after all, since their environmental impact is quite large, and we can eat other things. But when I brought up such an argument a while back, a friend of mine posed an interesting question; haven’t there always been cows?

And yes, cows are not new. I’m fairly sure there are a lot more now than there used to be, but surely before the modern mountain of moo there were other ungulates, bison and caribou, antelopes and takhi and quagga, to take up the slack.

Ok, those last two aren’t exactly ruminants, but you get the point. The only way large herds of cattle could actually change the climate would be if the total number of ruminants, domestic or otherwise, had grown–and how would such increased stock find enough to eat if something else hadn’t changed?

The same question applies to dogs and cats. If these animals have not simply replaced their wild counterparts but actually exist now in excess of the total historical animal mass, where did the excess food come from and why isn’t it accounted for in the historical carbon balance, where the carbon each animal releases came ultimately from plants and returned to plants again for no net change?

Some other source of energy must be fueling the swelling populations, something from outside the old balance–fossil, presumably, in one way or another. In other words, if the total population of dogs (or cattle or humans) has grown too large for the planet, it is a symptom, not a cause, of our problem.

As useful as carbon footprint calculation can be, it’s possible to get lost in the weeds here and miss the larger picture, which is that the climate is changing because the concentration of greenhouse gasses is rising, period.

Reilly can’t introduce additional carbon to the system. He just can’t. If he is alive because of such an introduction, his death at some shelter would not begin to solve the problem.

Take Home Messages

Yes, certainly it makes sense to feed pets the most climate-friendly diet possible. And people who are bound and determined to buy a pet from a breeder might seriously consider a little vegetarian, like a rabbit, instead of a big carnivore, like a retriever–shift the market in a more climate-friendly direction.

But you are not going to fight climate change by not getting that beagle from the shelter.

Let’s keep our collective eye on the ball, the ball being to get off fossil fuel completely as soon as possible. Only then can we fix the problem that causes all the other problems.

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Solar Impulses

This past week, I saw a documentary on the flight of the Solar Impulse 2, the first airplane to circumnavigate the globe without fuel–the plane is solar powered. It’s a great story.

The visionary behind the project, Bertrand Piccard, is the latest in a long line of brilliant dare-devil explorers who have been building and piloting record-breaking balloons and submarines and the like for generations. His great-uncle, Jean Felix Piccard, was the historical inspiration for that Star Trek captain with a very similar name, and the real and fictional Piccards actually bear a bizarre physical resemblance; Bertrand looks like a relative of Jean-Luc. The airplane itself is one of those objects everybody insisted could never be built, could never work–to have enough solar cells to generate enough power, the plane would have to be very big, but big planes need even more power to fly, so the plane would have to be even bigger, which would mean…unless the plane were absurdly light and under-powered (and still big), in which case it would be hard to fly and prone to break if a cloud looked at it funny. Impossible. But Captain Piccard assembled a team, said “make it so,” and they did, and it worked, and there you go.

Just to give everyone due credit, the plane had two pilots who took turns, Mr. Piccard and Andre Borschberg, and a large team of engineers and other mission-support personnel, without whom the project would not have worked.

Obviously, part of the motivation for the whole project was the coolness factor. Mountaineers climb Everest “because it’s there,” and Piccards probably invent and pilot unusual flying machines or submarines for similar reasons. But the specific mission for the Solar Impulse 2, and the thing that brings it under the purview of this blog, was to raise awareness for renewable energy. While the plane itself is far from practical (it can only carry a single person–the pilot–and only under ideal conditions), its existence suggests greater things to come and, as Mr. Piccard is fond of pointing out, everything is more difficult in the sky, so if solar power can work even marginally for an airplane, there’s no excuse for not using it on the ground.

All of this is laudable. There is a long history of impractical-seeming exploration leading to very practical technical innovation, and there is much to be said for crazy stunts as a way to get media attention. If flying around the world in an extremely fragile experimental airplane gets you on TV saying “climate change is real and important and we have to do something!” than I am all for it. These people are doing it right, making a difference.

Also, based on his appearance on the documentary, I find Bertand Piccard impossible not to like. He positively glows with a kind of driven, excitement, the kind of delighted passion usually called “childlike,” except it’s also obvious that you’d better not get in his way. He’s probably hard to live with, but as I don’t have to live with him, I’m free to just think he’s really cool. And he’s good-looking, so that helps.

I point all this out in order to make sure my next question is not misunderstood:

What was the carbon footprint of this project?

I suspect somebody has calculated the answer, but finding the number is not really the point–I’m sure the footprint was huge. Consider just two aspects of the project. First, the plane took off from Abu Dhabi, and eventually returned there, triumphant, but that’s not where it was built. The documentary clearly showed the Solar Impulse 2 arriving at the Abu Dhabi airport inside the belly of a giant cargo plane. That cargo plane was not solar powered. Second, the Solar Impulse 2 can carry only one human at a time, but it had two pilots who alternated. One pilot would land and, I assume, go sleep in a hotel for three days, and the next pilot would board and take off. That means that the relief pilot, not to mention the ground crew and the specialized portable hanger, must have flown (in non-solar aircraft) to the meeting place. Since weeks or months sometimes went by between the legs of the journey, the pilots probably flew home sometimes, too.

It’s not that the project was necessarily carbon-heavy as such things go, but it obviously wasn’t carbon-light, either, and it definitely wasn’t a flight around the world using no fuel. The airplane that doesn’t use fuel requires the support of those that do.

As I said, the value of the project was as an early proof of concept and as a stunt designed to trigger necessary conversations. As such, it was a good and important project. But I’d like to suggest a follow-up:

How about a team of people go around the world ACTUALLY with zero fossil fuel?

Or, better yet, several teams, and have them race? They’ll be walking, biking, sailing, rafting, and in some areas using plug-in hybrid cars and possibly some experimental technology. The race will provide both audience interest and an incentive for teams to innovate, rather than simply walking and sailing for three or four years. Infrastructure and technology will be tested and explored, possibly triggering useful innovations, such as bike lanes and walkable city designs. Local people will appear in interviews on BBC and PBS with translators doing voice-overs. It will be great.

Because we know that climate change isn’t really a technological problem. Better technology will help, but we could do a lot more to combat climate change with the technology we have. The problem is cultural and political, and requires cultural and political solutions.

A big, attention-grabbing demonstration of the zero-carbon transportation tools we already have might help.

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Oh, Christmas Tree!

The other day, my mother asked me whether she ought to switch to artificial an Christmas tree, for environmental reasons. This question has been addressed by other authors (please check those links for my source information), and the short answer is “no.”

(Don’t you like straightforward answers, Mom?)

But why the answer is no is interesting, as are the exceptions–my husband and I use an artificial tree, for example.

Natural Christmas Trees

You’d think this would start with a side-by-side comparison of pros and cons of each option. After all, using a natural tree involves cutting down a tree, and that can’t be good, right? But while I admit that cutting is bad for the individual tree, that’s not how conservation works. The health of the land as a whole doesn’t depend on the longevity of individual trees, but on the functioning of a whole system. While it’s possible to imagine Christmas trees being cut in environmentally destructive circumstances, I’ve never actually heard of the Christmas tree trade being a major driver of deforestation. Instead, Christmas trees are generally grown on farms–and a Christmas tree farm is a much better bet, environmentally speaking, than, say, a housing development. The growing trees do provide some wildlife habitat, protect and develop soil, and sequester carbon.

Most of the carbon sequestered by a growing tree is, of course, released when the tree dies and the wood rots or burns, but the farm as a whole holds carbon as generations of Christmas trees grow there. And while transporting the cut tree does involve carbon emissions, but depending on how far the trees have to travel and what happens to them after Christmas, these emissions can be minimal. Typically, half of a tree’s total carbon footprint comes from the trip the family makes to bring it home. If you drive less than ten miles to get the tree, and especially if the tree is mulched afterwards, rather than landfilled, your Christmas tree can actually be carbon-negative–that it, it fights global warming, rather than adding to it.

Even if you do drive farther for your tree, its carbon footprint is still dramatically smaller than that of an artificial tree.

Artificial Trees

It might be possible to produce sustainable artificial Christmas trees, but that’s not what is available in the stores. Artificial trees are almost always made of a combination of PVC plastic and steel, which are both carbon-intensive materials. They are recyclable, but virtually no recycling centers are prepared to disentangle the two, so artificial trees are typically treated as trash. The trees are also almost all made in China, meaning that they travel much farther (at a much greater carbon cost) than real trees normally do.

It is true that real trees are used only once and artificial trees can be used over and over–but if the live tree you’re comparing it to was carbon-negative, that’s irrelevant. The real tree is always going to be better. As for comparisons with live trees that do have carbon costs, estimates vary from five to 20 years, as to how many years an artificial tree must be used before its annual carbon cost starts to equal that of the real tree.

Most people replace their artificial trees after only six years.

Exceptional Trees

Whether artificial or real trees are better in the abstract is one question. “Which tree should I use?” is a completely different question. For example, our artificial tree is second-hand, and it likely would have been thrown away had we not taken it. Arguably, the environmental cost of the tree belongs at the feet of its original owners, since their decision not only paid for its manufacture, but also made certain it would one day need to be disposed of. We got the tree for free, environmentally speaking, and it saved us from having to buy any tree of any kind for well over ten years, now (my husband doesn’t remember when he got it, but it was here when I arrived).

You could also make your own artificial tree out of sustainably-sourced materials. You could also decorate a houseplant as your Christmas tree–balled and burlapped trees usually die, and spruces grown in pots as Christmas trees are only slightly more likely to make it, but you could decorate a Norfolk pine or another species that does well as a houseplant. You can do a little research to determine whether locally-grown trees are available in your area, whether Christmas trees can be mulched in your area (if you have a yard, you can also set your post-Christmas tree outside to provide cover for wild birds) and, if you want a live tree, you can make sure to pick it up from someplace less than ten miles from home (depending on the gas mileage of your vehicle).

In short, which tree you should use (assuming you want one at all) depends, in part, on your situation.


The Carbon Footprint of a Book

So, I’ve got a book coming out.

Technically, this is a second edition. Last summer, I published my first novel, To Give a Rose, but a few months later my publisher had to drop the project for reasons that had nothing to do with me. After much difficulty and confusion, I have finally found a way to get my book back into print; it’s due out next month.

Of course, this will make me responsible for a huge weight of paper product when (hopefully!) I sell lots of copies. Of course I’m concerned about the environmental impact of all of this, so I set out to do some research, beginning with the search term “carbon footprint of a book.” What I found was interesting and somewhat contradictory and uncertain.

How Carbon Footprinting Works

The problem is that carbon footprinting anything is complex and uncertain. In theory, to find the carbon footprint of an object, you look at how it’s made, how it’s transported, how it functions, and what happens to it when it’s disposed of, add up all the sources of greenhouse gasses in all these processes, and there you go. The figure is usually expressed as pounds (or kilograms, or tons, or tonnes) of carbon dioxide equivalent–different greenhouse gasses have different warming potentials, so for simplicity we use the warming potential of carbon dioxide as a kind of standard.

The problem is that in practice literally adding up all associated greenhouse gas emissions is usually impossible. Our economy is so complex, and manufacturing chains are so long, that a single product–in this case, a book–might involve resources sourced in dozens of countries and handled in multiple factories in a dozen different countries. That’s hundreds or even thousands of steps, each of which could have its own separate greenhouse gas emissions.Totally unworkable.

Carbon footprinting depends on imagining a simplified version of whatever manufacturing process you’re looking at, one that has a carbon footprint approximately the same size as the real one. But this simplification process is always a judgment call, and different analyses of the same product can yield very different results.

There are two other sources of complication.

One is that similar products might be products of very different manufacturing processes. A book printed in the United States using paper made from American trees might have a different footprint than one printed in the UK on paper made from European trees because of differences in the forestry practices and energy grids of each country.

The other complication is that it can be hard to determine what belongs in a given object’s footprint and what does not. For example, the footprint of a book should clearly include emissions associated with felling and milling the tree used to make the paper, but should it also include the lost carbon sequestration potential of that tree? What about the car the logger used to get to the job site to fell the tree? What about the Freon in the air conditioner of that car, if the logger used the air conditioner on the way to work? And so on. Clearly one has to draw a line somewhere, but where? A particularly vexing version of this problem comes up with recycled paper. Obviously, processing the same fibers twice uses more energy than processing them only once, so recycled paper ought to have a higher carbon footprint than non-recycled paper–unless you consider that the carbon footprint of the initial processing belongs to the first, “virgin” generation of paper only, in which case the recycled paper’s footprint might be much lower.

Again, judgment calls abound and can differ.

All things considered, carbon footprinting is only a rough tool useful for estimation. Its best application is probably for comparison of several alternatives all analyzed according to the same set of judgment calls–for example, a comparison among different protein sources or different types of energy generation. The technique does not yield definitive figures. An object cannot have a known carbon footprint the same way it can have a known weight or size or calorie count.

Carbon Footprints of Books

A Canadian paperback

I was able to find several different versions of carbon footprint assessments of books. The most extensive was probably one published in the Journal of Industrial Ecology, which presented the footprint of a paperback book printed in Canada on American paper. Unfortunately, that journal does not make itself available for free and I don’t have money. I was only able to read the Abstract (summary), which is available for free but does not have as much detail as I’d like.

The study came up with the figure of 2.71 kilograms CO equivalent (CO2-eq) per book, based on a production run of 400,000 books mostly distributed in North America. That figure applied only to the book through its production up to sale. The study also looked at three different end-of-life scenarios for these books (how long they last, how they are finally disposed of, etc.), but unfortunately the Abstract didn’t describe those scenarios or list their results.

One curious result of the study was that post-consumer recycled paper had a much higher footprint than virgin fiber. As noted earlier, that could be due, in part, to debatable judgment calls in the analysis method, which the Abstract did not fully describe. However, the non-recycled paper they analyzed came from a mill that used wood residue and other byproducts to generate power, thus substantially reducing reliance on fossil fuel and yielding paper with a lower footprint. Presumably, a recycled paper plant would not have access to such residue and is therefore much more likely to depend entirely on fossil fuel.

A Finnish hardback

This analysis comes from a brochure on the environmental impact of Finnish book production. The brochure describes its methods in detail and is both easy to read and thorough. To read it yourself, click here.

Among many other interesting facts, the brochure asserts that a single book has a carbon dioxide equivalent of 1.2 kilograms. Again, that leaves out the impact of the book’s disposal. Does a Finnish hardback really have less than half the carbon footprint of a Canadian paperback? We can’t really say, because the two studies are not directly comparable, but it is possible–especially if Finland has a less carbon-intensive power grid than Canada does.

The brochure further states that the vast majority of a printed book’s footprint is in the production of its paper and in the printing process–fiber supply and transportation contribute relatively little (at least in Finland).

An American book

I also found a reference to an analysis of the American printing industry that gave roughly 4 kg CO2-eq per book and listed the use of virgin paper as far and away the highest contribution to the footprint–in apparent direct contradiction to the other two analyses. Probably the discrepancy is again due, at least in part, to details of how the analyses were completed.

What About eBooks?


What about books that don’t require paper? eReaders themselves have a carbon footprint associated with manufacture, transport, and disposal. These devices also have other environmental impacts associated with the production of metals, heavy metals, and plastics which are important but are outside the scope of the article. According to at least one study, the carbon footprint of the ereader alone is cancelled out after a few years because of all the paper books it replaces.

Curiously, the number of paper books replaced could be much higher than it might at first appear, since they don’t just replace the printed books that people read but also the printed books that people don’t read. Roughly a third of all the books that arrive at a book store are never sold. These go back to the publisher and are either pulped and recycled or added to the waste stream. Presumably, some percentage of books are actually thrown out soon after purchase as well. Incinerating or landfilling paper releases its carbon, meaning that the carbon footprint of a book in the trash is higher than that of a book in a library. If each printed book in a personal collection has a shadow-footprint of sibling-books that never made it, then switching to ebooks could carry substantial carbon savings.

That’s assuming that more ebooks actually mean fewer printed books and fewer printed books wasted, something that is not necessarily true.

Books online

But there is a big difference between a book on a machine and a book on a shelf–once a book is printed and shipped, it causes no further emissions until its eventual demise. If the book lasts as long as the tree would have, which is quite possible, its eventual yielding of its carbon could be no different than the eventual rotting of an old tree. In any case, the longer the book lasts and the more people read it, the lower the carbon footprint of reading it gets. With an ebook, reading it requires electricity every single time–and for books stored in the cloud, electricity must be constantly in use to keep those books available on servers. I am unclear whether that continued electricity usage has been included in the calculation of the footprint of ebooks.

The internet uses a fantastic amount of energy, though exactly how much seems debatable. That’s the bad news. The good news is that companies with a large online presence can use their economic muscle to build renewable energy capacity and some have done so. eBooks could therefore be a potential driver of conversion away from fossil fuel use, if the industry chooses to put its weight in that direction.

Bringing It All Together

If indeed most of a book’s footprint is due to paper manufacturing and printing, if that figure is not unique to Finland, that suggests that whether a book is printed on recycled paper is actually a lesser consideration. The real bang for the buck, as far as shrinking footprints are concerned, lies in making paper manufacturing and printing more efficient and less carbon-intensive.

In both cases, the mechanical efficiency of the plants themselves could probably be improved, but I really don’t know. The big deal is almost certainly the power grid; the carbon footprint of a book depends largely on the emissions of the power grid its production is plugged into. Since the power grid also determines much of the emissions related to ebooks, shrinking the carbon footprint of a book is really about transitioning the power grid off of fossil fuels–and paper mills, printing companies, and internet servers can all help drive the transition by demanding capacity that other users can then tap into.

So, What Does this Mean for Writers and Readers?

Arguably, the carbon footprint of a book belongs to its reader–personal carbon footprinting assumes personal responsibility for anything we buy. Since the same footprint can’t belong to two people, that would mean authors don’t bear the weight of the carbon emissions of their books–but that way lies paralysis. A reader seldom has an opportunity to choose low-carbon books over high-carbon ones, and in any case, reading materials are seldom a significant part of household footprints. Readers are unlikely to drive any sort of change, here. Writers have a little more power.

Writers can ask their publishers to take certain steps towards “greening” their processes. The publisher may or may not say yes–we live in an era when writers who are not J.K. Rowling or Stephen King have very little power–but we do have options and can explore them. I have already asked the printer I’m using now to use a paper with a higher post-consumer recycled content when possible and they said yes. This was before I found out that recycled paper might have a higher carbon footprint, but I’ll stick with it, since it sounds like virgin paper is only lighter on carbon because its production has access to alternative power generation. That implies that recycled paper could be just as climate-friendly if the mill that produces it buys renewable power. I therefore intend to ask whether we can get paper from a company that does so.

Climate-friendly paper might not be available, but it’s worth asking. If enough people ask, it might become available.