The Climate in Emergency

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


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Nor’easters

Last week, I spent three days huddled inside because of high winds rattling the house and ripping dead branches off of swaying trees–and I live in Maryland, where the storm (“Winter Storm Riley,” officially) was relatively minor. What we saw was nothing, compared to what the people in coastal Massachusetts experienced.

Now we’re preparing for another one (“Quinn”). And some meteorologists expect another storm after that.

What Are Nor’easters?

This week’s storms are nor’easters. They’re not unusual, although the recent one was an extreme example. Like hurricanes, they are very large low pressure systems that bring wind and rain (or snow) and last for several days. Unlike hurricanes, they draw their power, not from warm water (there wasn’t any under Riley) but from the interaction between warm and cold air masses. They generally form in winter. In the case of Riley, a storm system moved east across the US, then drove the rapid development of a very intense low pressure area just off the coast, which then moved north and gradually east. On satellite images, the thing looks like a hurricane, a massive pinwheel of swirling cloud off the coast. While too far out in the Atlantic now to influence my weather directly, Riley still exists. It’s busy causing damaging surf on Puerto Rico from thousands of miles away.

Nor’easters seldom approach hurricane force winds. Typically, these storms are gusty, not windy, a serious inconvenience, but not a danger, unless you have bad luck (such as an unusually weak tree limb right above your car). Rily was the most intense I’ve seen, and around here it was only in the high tropical storm-force range.

The lesser winds do not make these storms mild.

For one thing, nor’easters have much larger peak wind fields than hurricanes do. While a hurricane might have sustained winds of 90 miles an hour near its center, most of the area the storm passes over will get much weaker winds, say 50 or 60 miles per hour. A strong nor’easter will blast the same 50 or 60 miles per hour over the same large area, it just lacks the 90 mph core.

Second, wind is not the most destructive aspect of a hurricane, it’s just the easiest way to compare storms to each other. The size of the wind field, the speed the storm travels (and hence how long it spends in any one place), the size of its storm surge, and how much it rains are all much more important in terms of its destructive power–and above all, there is the question of what it hits. A low-lying, heavily populated area where the people lack both money and political power is where the disaster happens. And nor’easters have large wind-fields, heavy precipitation, sometimes heavy coastal flooding, and can persist for days.

And, as with hurricanes, when we get a bad one (or several) people start asking about climate change.

Nor’easters and Climate Change

Meteorologists can be quick to point out that individual storms can’t be linked to climate change, which both is and is not true. One recently referred to efforts to draw the link as “witch-craft.” That’s at best disingenuous.

We can absolutely prove that climate change is making nor’easters worse, for the same reason that climate change is making hurricanes worse. First, the single most dangerous aspect of either storm is coastal flooding, which is unquestionably worse now that the sea level is several inches higher than it was when most existing infrastructure was built and when the data used to define flood zones for insurance purposes were gathered. The apparent sea-level rise varies from place to place, because geological forces are also in play making the ground rise in some places and fall in others, but climate change can claim about eight inches of it world wide, due to a combination of thermal expansion (things, including oceans, expand when they heat up) and glacier melt. That means every coastal flood event, including all hurricanes and all nor’easters, are  eight inches worse than they would otherwise have been.

Eight inches doesn’t sound like much, until you imagine them inside your living room.

Also, a warmer planet means more humid air, which means wetter storms. In the winter, as long as the air temperature is below freezing (which isn’t really very cold), that means more snow–more closed roads, more fallen trees and snapped power lines, more collapsed roofs, more car accidents, more missed days of school. All of this should sound very familiar to some readers right about now. All that white stuff? Yup, it’s a symptom of climate change, not a negation of it. In warmer weather, wet storms means rain which means flooding. That’s ruined houses, damaged roads, washed-out bridges, soaked earth–leading to toppled trees and snapped power lines–and drownings.

We’ve been through this already with hurricanes; climate change does not have to cause individual storms, or even make a certain type of storm more likely or more intense, in order to directly cause more storm damage.

But can climate change cause nor’easters? Yeah, it kind of looks like they can.

Connecting the Dots

To tell this story, we have to cover a bit of atmospheric anatomy.

Remember the polar vortex? It was all over the news a few years ago, but I haven’t heard of it of late. It still exists, though. Actually, there’s two of them. Or sometimes three.

The polar vortex is not a type of storm, but rather either of two long-term atmospheric features–this sounds a little different than the last time I explained it, because the two features tend to get mixed up in public discussion, and I only recently learned that they are distinct.

Originally, “polar vortex” meant a circular pattern of winds that forms in the stratosphere around the pole in winter. It’s also called the polar night jet, because the sun does not rise in the winter at its latitude. The winds blow from west to east and divide cold polar air from warmer air at lower latitudes–the stratosphere is a layer that begins several miles up, above where weather happens. But in recent years, the term has also been applied to the jet stream, a circular pattern of winds in the troposphere–a much lower layer–also at a boundary between warm and cold air, but much farther south. The jet stream meanders, across the latitudes covered by the United States and southern Canada. The jet stream exists winter or summer, and its shape and location help determine whether any given area gets warm, tropical air or cold, arctic air this particular week.

Ok, so, definitions taken care of, what does either polar vortex have to do with climate change or Winter Storm Riley?

A lot of the strange weather we’ve had in recent years has been caused by extreme waviness in the jet stream. Because the jet marks the boundary between warm air and cold air, an extreme meander means that warm air flows much farther north than normal over here, while cold air flows much farther south than normal over there. At the same time, weather systems tend to persist longer and move slower than normal. Rainy weather becomes catastrophic floods. Dry, hot weather becomes killer heat waves and droughts. The extra waviness is likely caused by global warming, especially the loss of Arctic sea ice. As the planet warms, the polar regions warm faster than the rest of the planet, decreasing the contrast between the warm and cold regions and weakening the jet stream that lies at their boundary. Weak jets are slow and wavy.

So climate change doesn’t cause snowstorms in Florida by some magical method of “global weirding,” but instead through a fairly straight-forward form of atmospheric messiness, a weakened and wobbly boundary between warm and cold caused directly by the warming Arctic.

The next bit is less certain, as in not all scientists agree, but a weak and waving jet stream could be one of the mechanisms able to put pressure on the polar vortex and cause it to temporarily break down and allow warm air in over the pole. Such an event is, sensibly enough, called a Sudden Stratospheric Warming, or SSW. Although the stratosphere itself doesn’t have weather in the normal sense of the word, it can influence the weather of the troposphere, resulting in odd weather several weeks later–such as cold snaps, warm periods, or violent storms. SSWs appear to be natural (we have only been measuring stratospheric temperatures for a few decades, now, so it is hard to be sure), and their frequency has not increased, but some computer models suggest an increase could happen, and the extra-wavy jet stream could make it happen–or could already be making it happen. It takes a while to gather enough data to document a change in events that don’t happen every year.

Riley (and presumably its sibling-storms, to some extent) was triggered by a particularly severe SSW, one which ripped the polar vortex in two and triggered a bizarre winter heat wave in which parts of the Arctic rose above freezing for days on end. There’s no sun up there, remember, yet the ice started melting instead of growing–a bad sign. That triggering is not in doubt. And the SSW could have been triggered by a weak and wavy jet stream, which is itself caused by melting sea ice (notice the ominous cycle implied there?). Melting sea ice is, rather unambiguously, a symptom of global warming.

That “maybe” in the middle of the causal chain remains, but this is very close to a linkage between climate change and a single storm. Anyone who claims differently is going to have to marshal a much better argument than claiming “witchcraft” to convince me otherwise.

 

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Snowtastrophe

So, in case anybody didn’t know, Boston is sitting under about six feet of snow right now. Six feet.

Just to put that in perspective, if you had a ground-level door in Boston, with no porch or overhang above it,  and you opened that door, you wouldn’t see the outside. Mostly, you would see a wall of snow. Unless your indoor lights were on, most of the wall would appear black, because even a few inches of snow completely block light. The very top of the cliff would look blue or white, and you’d be able to see the sky through a six-inch gap between the snow and the top of the door. And all of it has fallen in the past thirty days.

More heavy snow is on the way this weekend.

Predictably, certain people are yammering that so much snow disproves global warming, while others point out, correctly, that climate change actually causes more snow. I agree that global warming = more snow does sound counter-intuitive, but we all know that climate change is complex, bringing floods to some places and droughts to others, etc. Perhaps snowy Boston is similar? But no. Actually, the thing with snow is not complex at all.

Here’s the deal; when the air is below freezing, precipitation falls as snow. That means that until a given area warms so much that it no longer freezes in winter, it will continue to get snow. If a big, wet, storm moves in during freezing weather, it will get a lot of snow. A giant blizzard (or several of them in a row) is what a flood looks like in New England in February.

And indeed, while the Boston area has been colder than average for February this year, it hasn’t been that much colder. It hasn’t dropped to zero (Fahrenheit) this month yet, although one below-zero night is average for February in Boston. The record lows for each day in the first twelve days of February are all below zero–and no cold weather record has been broken in early February in Boston for eighty-one years (in contrast, of the heat records for the first twelve days of this month, six were set over the last twenty-five years).

So, we’re not looking at especially cold weather right now. What we’re looking at is a flood that happens to be frozen.

And for New England, climate change generally takes the form of floods, some of them catastrophic. Temperatures have risen dramatically as well, but most of the change has involved nighttime lows, when most people are asleep. It is the flooding most people notice. The event that we’re seeing now is comparable in scope to Tropical Storm Irene, Superstorm Sandy, and all the other major floods, named and unnamed, that have wet New England in recent years. At least fifteen people have already died (and that figure is six days old), counting those who succumbed to the same storms in other areas. Snow storms typically kill through traffic accidents and heart failure triggered by the effort of shoveling. Very heavy snowfalls, like these, can collapse roofs from the weight. I have not heard of anyone being under a roof when it collapsed, but it must happen. Boston alone has or will spend over twenty million dollars on snow removal and other blizzard-related costs from just the storms of the past month. The snow season still has another month to go.

Where I live, in Maryland, we’ve hardly had any snow all year–just a light dusting a few times and a couple of flurries. We see the New England storms on the news, but the TV coverage usually makes it look like a giant pain in the neck and not much more. And my friends in New England all seem to be fine, if a little tired of the snow, so a mere inconvenience is all it is for many. But, it’s important to realize that it’s more than that for some people, and the regional infrastructure–which was not designed to deal with this much snow–is being severely strained. This is an extreme weather event and it is dangerous.

How does it relate to climate change?

Generally speaking, a warmer atmosphere carries more water and so delivers more floods. When it rains, it pours is the weather-mantra of the new age. But specifically, this series of storms is linked, not so much to warm air, but to warm water.

My friend, the science educator/weather geek explained to me that:

The snowiness is being caused by an upper level and persistent trough of low pressure. There is a strong High pressure ridge over the Western US that is bringing warm weather to the great plains and wet/cold weather to the eastern third of the US. Not sure when it will move away, probably not for another couple weeks.

The reason why this ridge of high pressure causes different kinds of weather in different places is that air rotates around it clockwise. So that rotation is pulling warm air up from Mexico into the Great Plains (and ruining Garrison Keilor’s winter), to the west of the trough, while pulling cold weather down from Canada to the East. Climate change may be making these sorts of things more common or more severe, but it is nonsensical to ask whether a single storm reflects a trend–trends are only visible across time.

In any case, so we’ve got persistently cold, damp weather in the Eastern part of the country periodically bubbling up into storm systems, some of which intensify into nor’easters along the coast. A nor’easter is an extra-tropical cyclonic storm that feeds off of cold air and a warm ocean. They might loosely be considered winter hurricanes (though they can happen in summer, too), because they bring wind and coastal flooding (with snow or rain) in a similar way.

BUT this February, sea temperatures have been abnormally high. As of a few days ago, sea surface temperatures off of Cape Cod (meaning many miles off the coast–in the Gulf Stream and beyond) were twenty-one degrees Fahrenheit  above normal for this time of year. That doubled the amount of moisture in the air, dramatically increasing the amount of snow that a system feeding in the region could dump.

Again, it’s hard to say if one pool of warm water is climate change, because climate change is a trend not an event, but we do know the ocean is getting warmer. And when it gets warmer, Boston gets buried under six feet of snow.