The Climate in Emergency

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

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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, areĀ  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.