The Climate in Emergency

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


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If We Stopped Tomorrow

What would happen* if we stopped causing climate change tomorrow?

It’s a fantasy, obviously, though an appealing one. It’s also food for a lot of interesting thought. What would life be like? What kind of climate would we be left with? Would climate change stop right away, or would there be residual change? Here, I’m going to explore the climate part of the question; if humans stopped producing greenhouse gas emissions right now, how would the climate respond?

For simplicity, our scenario is that all humans everywhere simply vanish and that all our machinery shuts itself down safely at once–I’ll ignore complications caused by unattended machinery blowing itself up and so forth. I want to be clear that I do not actually think my whole species should go extinct, I just don’t want to get pulled off topic by an overly complex scenario.

When do greenhouse gas emissions stop?

Emissions of different greenhouse gases stop at different times in our scenario. These gases are carbon dioxide, methane, nitrous oxide, and two groups of related gases, the chlorofluorocarbons and the hydrofluorocarbons (CFCs/HCFs), plus water vapor. I’m going to ignore water vapor here because the primary way its atmospheric concentration varies is not from emissions but from changes in the hydrologic cycle.

So, in our scenario, fossil fuel use and its carbon dioxide emissions stop immediately–but that’s only 57% of total greenhouse gas emissions worldwide by weight. Another 20% of the total is carbon dioxide from other sources, such as forest fires or aerobic decomposition. 14% is methane, 8% is nitrous oxide, and 1% is CFCs/HCFs. These gases come from different processes and some of these processes would continue a while.

Nitrous oxide comes largely from the production and use of nitrogen fertilizer. Its emissions should therefore drop off pretty quickly in our scenario. CFC/HCFC comes from industry and refrigeration and would therefore drop off much more slowly as abandoned refrigeration units slowly broke down and leaked. But the real issue would be methane and non-fossil-fuel-related carbon dioxide.

If the world were simple, then after our piles of wood and paper and other biomas finished burning or rotting (that might take a few years), atmospheric carbon oxide levels should stabilize. The only remaining emissions would be from natural wildfire or decay and that carbon would be taken up again as other plants grew. But the world is not simple. One of the things climate change is doing is shifting some places from forest to savanna. It’s unclear how much of that shift has happened yet, but it’s quite possible that some of our forests are essentially dead trees walking, so to speak. They won’t get the rain they need to survive and when they die they will be replaced by grass, shrubs, and the occasional tree, not forest. In that case, their carbon won’t be recovered, driving the atmospheric concentration up. One of the nightmare scenarios we’re looking at is if climate change caused by forest dieback becomes enough to cause further dieback–a runaway positive feedback cycle in which the planet starts warming itself.

If that nightmare feedback loop has not started yet, I doubt it would under our scenario, given the substantial emission cuts from the end of fossil fuel use. But elevated CO2 emissions will persist at least as long as it takes those forests doomed by climate change to die and rot or burn.

Methane levels might actually not drop in our scenario. Methane occurs as a fossil fuel and is also produced by anaerobic decomposition at the surface. Agriculture is a major source, mostly from rice cultivation and animal husbandry, and these emissions would probably taper off pretty quickly. Our vast herds of cattle are not going to survive us for very long. But landfills and leaky fossil fuel facilities will keep producing methane for a long time–only we won’t be here to capture and burn off those emissions (burning converts methane to carbon dioxide, which is actually a good thing because methane is a much more powerful greenhouse gas). So those emissions could actually increase without us. I do not have enough information to calculate what the net result would be. And the nightmare scenario is that melting permafrost liberates enough methane to warm the planet enough to melt more permafrost and release more methane….

So, what we’re looking at is that if humans vanished and neither nightmare cycle has begun yet, total greenhouse gas emissions would drop immediately by somewhere around 60% and then probably decrease further over a period of years. When the system would reach equilibrium seems unclear. The relative contributions of each gas would change dramatically as well, with methane becoming co-dominant with CO2 by weight. Since methane is both more powerful and less persistent in the atmosphere, this shift would be very important to anyone running climate models of our scenario.

How long will the climate keep warming after emissions stop?

Even if the atmospheric concentrations of all the greenhouse gases stabilized today (which under our scenario they would not), the global climate would continue to warm for a period of years. This lag between cause and effect is actually a very familiar principle; if physics didn’t work this way, cooks would not have to use timers because food would become fully cooked the instant it went on the stove or into the oven. Earth’s climate has a longer lag than it might otherwise because we have oceans and water can swallow a huge amount of energy before changing temperature, but basically things just take a while to warm. The experts aren’t sure, but Earth’s lag is probably around 40 years–which means we are now experiencing the consequences of the greenhouse gas emissions of the 1970’s.

In our scenario, then, the loss of humans does not start to show on our climate for another couple of decades. Only then will the planet start responding to the dramatic decreases in emissions.

How long will sea level keep rising after the warming stops?

Here is another familiar principle: ice takes time to melt.Glacial dynamics are a bit more complicated, since they receive new snow as well as lose meltwater and they move, but when scientists say a certain amount of melting is “locked in,” that basically means that a certain amount of ice already has the conditions necessary to melt. It’s like an ice cube set out on the table at room temperature; that ice cube is going to melt away to nothing even if the air in the room does not get any warmer. Because glaciers are very big, some of the melting now locked in might take thousands of years–or it might go faster. Scientists aren’t sure, and of course the rate of melt is likely to increase because the temperature will keep rising (for at least 40 years!), but however long the process takes, the melting we have already triggered will cause at least three feet of sea level rise, probably more.

How long will greenhouse gas levels stay elevated?

Under our scenario, and assuming those cycles of viciousness aren’t in operation yet, greenhouse gas levels would level off as soon as emissions stopped and then eventually start falling. How long would it take for the atmosphere to return to something close to what it was before? The answer depends on which gas you’re looking at.

CFCs/HCFs and their kin vary a lot. Some can stay in the atmosphere for thousands of years, some for less than a year. I do not know how many of each kind we have up there and in what proportions, but we’re looking at a process that begins immediately and lasts for a very long time. Nitrous oxide breaks down in the stratosphere and takes just over a century to do it. Methane is quick, lasting only about 12 years (my source does not say what any of these chemicals becomes afterwards–I am suspicious that methane may become carbon dioxide, a complicating factor!).

Carbon dioxide is the tricky part, since it can leave the atmosphere by several different means. Much of it is absorbed into the ocean pretty quickly, where it no longer causes the greenhouse effect but instead causes ocean acidification. Also, this mechanism only works if there is more CO2 in the air than what the water near the surface can absorb. The upper layers of the sea are getting “full” now, meaning that not much more CO2 will go into the water until ocean mixing brings new water up to the surface. Chemical weathering of rocks also absorbs CO2, as does, of course, photosynthesis. And that last is the complicated one.

If the distribution of plants across the globe is roughly stable, then carbon sequestration by photosynthesis will be roughly matched by carbon emissions from fire and decay. But reforestation–and the re-establishment of wetlands–could become a powerful force for carbon sequestration with humans out of the way. Unless environmental damage has in some way precluded regrowth, which is possible, and unless the nightmare cycle has begun.

Without factoring in regrowth, somewhere above 65% of our carbon dioxide will be absorbed by the oceans in the next 20 to 200 years and the rest will drop very gradually, finally reaching equilibrium after a few thousand years. If plant regrowth proves significant, the process could go faster, maybe much faster–there is evidence that reforestation following the conquest of the Americas caused the Little Ice Age. In our scenario, it would be the entire world regrowing.

So what’s the scenario?

Bringing all of this information together, we can fill out the details of this scenario.

Humans either vanish or somehow become ecologically negligible in November of 2015. Right away, that very month, greenhouse gas emissions drop by about 60% and then continue dropping gradually over a period of years. Atmospheric concentrations of these gases also start to drop right away, though more gradually. Within a few years, meaningful reforestation begins in some areas, possibly balancing out climate-related deforestation elsewhere.

But the global average temperature keeps climbing–and it’s climbing faster than ever because the oceans have absorbed enough energy that now they’re warming rapidly, too. Extreme weather gets more so. If there are any humans left, they are having a very rough time of it. Somewhere around 2055, the climate begins to stabilize, although what it looks like by that point is anybody’s guess.

But by that point the atmospheric concentration of methane has fallen and leveled off at whatever its new normal is. Carbon dioxide levels are starting to fall meaningfully. I don’t know whether there is the same lag on cooling as there is on warming, but by sometime around the turn of the century I’m guessing the planet has started cooling again–and the cooling gradually accelerates over the following century as nitrous oxide starts to break down and as more and more carbon dioxide is absorbed by the oceans and by growing plants.

All this time, the sea level is rising. Water creeps gradually across the hurricane-ravaged ruins of many of the world’s major cities and upstream into previously fresh areas of the world’s rivers. Oysters grow on the streets of Manhattan.

I’m guessing that the cooling will take much longer than the warming, because greenhouse gas levels will stay somewhat elevated for thousands of years. The  planet would also see a lot of delayed effects of the warming–along the lines of changing plant growth patterns or changing ocean salinity triggering various feedback loops. I don’t know what those loops would be or when they might occur. At some point the pace of change would slow enough that the biosphere will start to recover–but recovery from a mass extinction takes about ten million years.

Feeling depressed?

I don’t mean this as an exercise in pessimism. I mean it as an illustration of what optimism looks like at this point, what we can look forward to in the best possible scenario we can anticipate. If being limited to this as optimism bothers you, consider how the next generation will feel if we do not get our butts in gear right now.

 

  • Note: After writing this, I’ve thought of a bunch more complications that might change the details of the picture I’ve given. I stand by my factual statements, but my suppositions might be muddy. Creating a detailed, accurate climate projection is not my intention, though–that requires a supercomputer I don’t have. The point is to draw attention to the questions, to the issues of lag and lingering emissions–to provide food for thought.
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Alaska Burning

Alaska is on fire at the moment.

Well, not all of it, but the state’s wildfire Preparedness Level is at 4. The scale only goes up to 5, so PL 4 means the state is starting to have real trouble dealing with its fires and needs help from other states. For a state, or region to go to Level 4 is not all that unusual–the state and regional wildfire response systems are not designed to be self-sufficient–but the fires are not inconsequential, either. In recent weeks, both forests and tundra in Alaska have burned–and some of the fires have been quite large and dangerous.

Fires are not exactly a new thing in Alaska, but there are more of them now, for a variety of reasons including the current successional stage of previously logged forests, the effects of fire-suppression policies, and, yes, climate change. Alaska’s climate is changing much faster than that of more temperate areas, becoming both hotter and drier. And the fires, in turn, might be causing dramatic changes to both the climate and ecology of the region.

In forests

Recent research suggests that larger, more intense and frequent fires might dramatically alter forest compositions that have been stable (despite repeated natural climate changes) for six thousand years–although the forests themselves could then act to slow further changes.

In the interior of Alaska, there are essentially two main types of forest; most areas are dominated by black spruce, berry bushes, and moss, but there are forests of aspen and other deciduous trees as well. Both types of forest burn, perhaps every hundred years or so, but after the fire, the same type of forest eventually grows back. The result is a mosaic of different forest communities that has kept the same pattern since before the pyramids were built. Basically, each forest type produces its own distinctive type of forest floor. Because spruce forest floors are very thick and wet, they don’t burn down to bare soil, whereas the thin deciduous leaf-litter layer does. After a fire, the two different forest floor types guide ecological succession in different directions so that, in time, black spruce and aspen each return to the areas where they grew before.

As Alaska dries out, however, the black spruce forest burns more intensely and more often, destroying its distinctively thick duff. Once the soil is bare, the deciduous trees can move in–and there they stay.

The neat thing about ecology, though, is that nothing is simple–as the number of deciduous groves in interior Alaska increases, it seems likely that the situation will stabilize itself because the deciduous trees do not burn as easily and may act to slow down and break up large fires. These trees are paler in color, too, and they release more water back into the air and so may act to cool the region somewhat. Both effects may act to protect the remaining black spruce forests, at least for a while.

All by itself, changes in the composition of Alaska’s forests is not necessarily a disaster, although we don’t know for sure that it isn’t, either. Both the human cultures in the region and much of its wildlife have developed ways to use both types of forests in different ways, and it is not obvious what changing the proportion and distribution of the two types is going to do. Change is not automatically bad, but the fact that we are changing something this old should certainly give us pause.

Of more obvious, clear-cut concern is the fact that black spruce forests, with their thick, slowly-decaying duff, are a carbon-sink. That is, they take in more carbon than they release and thus are one of the reasons global warming is not worse than it already is. The loss of these duff layers, either because forests convert to deciduous communities or because spruce forests can no longer build up as much duff between more frequent, more intense fires, is already starting to convert Alaska’s forests into a net carbon source.

That’s a problem.

In the tundra

Much of Alaska is still treeless tundra, plant communities dominated by shrubs, mosses, grasses, and lichens. The tundra, too, is a net carbon sink, because huge amounts of organic matter build up in the soil and do not rot. The layers of ligroundving and dead organic matter also insulate the soil, helping to keep the permafrost from melting. The permafrost, in turn, keeps groundwater close to the surface and keeps buried methane trapped. As permafrost melts, some lakes are actually draining away, destroying important habitat for fish and for migratory birds. And, of course, that methane is bubbling up–methane is a much more powerful greenhouse gas than carbon dioxide is.

Alaska’s forests have permafrost as well, but it is discontinuous–rather like big, underground boulders of ice. Beneath the tundra, the permafrost is more like bedrock.

The thing is, when the tundra burns–as it may be doing more often now, in part because northern Alaska is getting more lightening strikes because of its warmer weather–it’s not just the thin living laying but also the soil that goes up in smoke. A tundra fire can release as much carbon dioxide as a forest fire can. Without as much insulation, and given the much darker color of the charred surface, the permafrost beneath can then melt all the faster.

Positively problematic

I have written before about how positive feedback loops are anything but positive in the colloquial sense of good or happy. A positive feedback loop is a self-intensifying cycle, such as where rising temperatures melt permafrost, releasing methane, which makes temperatures rise faster, melting more permafrost….

The really scary thing here is that initiating these loops–pushing systems to the point where they start releasing greenhouse gasses–means that even if we stopped burning fossil fuel tomorrow, climate change might continue to get worse. We are losing the option to save ourselves.

That isn’t an argument to give up, of course–no situation is so bad that it cannot be made worse, and that means no situation is so bad that we  cannot make things better by our restraint. But it does mean that the hour is later than we might think. The Earth is a live thing, and it has been protecting us from ourselves to some extent–but it won’t do so forever. To those of you who are doing the equivalent of calmly reading the paper while your house burns around you; it is time to get up, now.