How Global Warming Causes Extreme Weather: New research

I want to tell you about what may be the most important research result in the area of climate change in recent years. First, a little background.

We know from paleoclimate studies that the Earth's climate system changes from time to time enough to leave a mark. For example, it is widely thought that during the "ice ages" (periods of extensive or moderate glaciation) over the last couple of million years, areas that are currently very dry had a lot more water. Some combination of rain and evaporation (more rain and less evaporation) conspired to fill playas (dried up lakes) or salt lakes (like the Great Salt Lake in Utah) with so much fresh water that inland basins filled and started to drain out to the sea. It is hard to imagine how the weather would have been so different to make the arid regions of the American West into very wet places, long term, but it happened.

As we head towards a warmer and warmer planet, one would think that whatever happened during the ice ages would be the opposite of what we would expect in the future. To some extent that is almost certainly true, as certain regions will likely be much dryer in a heated up world than they were during the cooler ice ages. But some patterns of climate change are not simply characterized by temperature. The pattern of movement of air, and the pattern of moisture in the air, can be different from one climate system to another in very complex ways. Perhaps (this is very conjectural) the recent intense rains we see in the American West would be a common phenomenon in a warmed world. Perhaps the phenomenon of ARkStorm, a very rare situation where several "pineapple express" style storms happen over a single winter, large ones, in rapid sequence, filling the dry valleys of the American West with giant lakes and wiping out low lying villages and most of the crop land. That kind of feels like the Pleistocene when the great inland deserts were converted int great inland lakes! Or, perhaps the multe-year california drought that we experienced up until just a few months ago will become the "normal" situation.

Don't get me started, but it is not difficult to imagine a world in which the American west has 4 to 10 year long droughts punctuated with a couple of winters in a row sufficiently wet to fill those lakes, so we get both!

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ADDED: Jet Streams, Extreme Weather and Other Things with Stefan Rahmstorf

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Here is the point of all this: Back in the 1950s through the early 1980s, by my estimation, North America (and probably many regions in the world) experienced stereotypical storm patterns that were like storm patterns seen over many centuries, though with the occasional interruption for something strage for a few years. 1860 - 1861 were strange years out west. The 1930s were strange in a lot of places. But what happened startomg around 1980 or so was different.

Prior to 1980s, storms in North America came from certain directions, were more common during certain times of years, dropped a range of precipitation amounts on the ground, and rarely were severe in the amount of rain that occurred. After 1980, the timing and various aspects of the physical nature of those storms including their apparent directionality and the speed which with they passed through, changed.

For example, in Minnesota, at the Twin Cities airport, there was an average of about 1.64 above 2 inch rainfall events per year for the hundred years of record keeping before 1971. In the time following, to the present, that number went up to about 2.7. Since about 2000 that number has gone to close to 3.5. Meanwhile, out east, the frequency of large blizzards has gone from one every few years to at least one in most years.

Putting this a slightly different way, the chance in a given year of having a major storm around these parts has more than doubled, with that doubling happening well within the lifetime of most of the people who live in the region.

I've written before about a special class of research on climate change that I have always regarded as among the most important. The authors of that earlier work overlap with the most recent work, with a key player being Stefan Rahmstorf. Rahmstorf and his colleagues, a few years ago, tackled an interesting problem that others had also noticed, and for which a number of explanations were floating around. Speaking of floating, this work surfaced and got some real traction when the Rocky Mountains near Boulder, and up near Calgary, were each hit by a really bad and very special kind of storm.

In each case, the storm system was trained along a very curvy and slow moving jet stream. When the jet stream slows it curves, or when it curves, it slows, or, really, kinda both. When that happens, if there is a big wet storm following along in the air system that itself creates/is created by the jet stream, that storm also slows. Fed by a more or less unending supply of moisture, such a storm can drop a lot of rain on a given region. We tend to think of the most severe storms as being fast moving, and they often are. Hurricanes can be pretty darn fast. Rapidly moving fronts coming off a dry line are associated with either tornado outbreaks or derecho storms. But these big and slow jet stream mediated storms are very very wet. Calgary was badly flooded, like no one has seen before. Boulder was very badly flooded more than in anyone's memory.

Storms like this happen now and then, and can be found in historical records, and may have even happened in or near Calgary or Boulder at some time in the past. But since Calgary, we have had many many more such storms. Here in Minnesota, we've had a few. St. Louis had one. Texas had had a bunch of them. They've happened in China, Japan, all across Europe.

These storms, which are associated with a jet stream that is curvy and slow, are now common, and they were once rare.

What is the climate change connection? How do we know that global warming causes this?

There are a couple of different lines of evidence. First, as noted by the earlier work, and exemplified in the graph I put at the top of the post, which I made a couple of years ago, researchers have noticed that these curvy jet stream are more common. Another reason to think this is that curvy jet streams are expected to be associated with an Arctic that is warming more rapidly than the rest of the planet.

How does that work? I can explain it in general terms that will probably make some atmospheric scientists yell at me, but that I think is close to reality and also understandable by the average science-savvy civilian.

The big features of the weather system on a planet with an atmosphere have to do with heat reaching equilibrium across the planet. There is more heat near the equator, less near the poles, so it is all about heat moving from the equators to the poles, but also, hot stuff, water or air, making that journey. This hypothetically sets up an interesting phenomenon in the atmosphere that can be thought of as a giant twisting donut --- a plain round donut shaped donut, not some hipster cream filled donut -- just north of the equator, and another donut just south of it. Air is moving up in altitude at the equator, cools and spreads away from the equator, then drops back down to the surface, and heads back to where it was originally heated to get warmed up again.

This pair of giant twisting donuts of air helps set up another giant twisting donut of air to the north and south, and those donuts can, in turn, set up another, and so on. On a small planet like Mars, with a thin atmosphere, there may be one single donut per hemisphere. On giant heavy atmosphere planets like Jupiter and Saturn, you get many such donuts, and an overall striped appearance from a distance.

On earth, you get one really well defined donut (in each hemisphere) then a poorly defined donut, then another donut that is fairly well defined but that is also rotating in a circle, around the pole, much like a donut that got partly stepped on, rolled out the door of the donut shop, and his heading down the street.

Now here's the thing, the explanation you can understand once you rap your head around these donuts: If the difference in heat at the equator vs. at the poles is great, the donuts are well defined and energetic. If the difference in heat between the equator and the poles is less, the donuts become less well defined, wobbly, and curvy.

The upper atmosphere region between adjoining donuts and the jet stream are more or less the same thing. So, as the arctic warms faster than the rest of the planet, the donuts change their configuration and you get curvy jet streams that can set up to remain in the same location over long periods of time.

Simple.

There was, however, a major missing part of this theory, and Michael Mann, climate scientist, joined the Rahmstorf et al team to fill in that blank. It is very difficult to be sure that a climatic phenomenon is either a) for real or b) characterizable as you've witnessed it, when you are looking at it for just a few years. If there is a change in climate because of the above described effects, there are not too many years of data allowing us to track it, observe its variations, or to figure out exactly how it works. This is complicated by several factors. For example, an alternate but similar explanation for the waves themselves, and the weather that comes with them, is the warming of the North Pacific. Hell, it could be both factors, because both factors may reduce the heat differential between the midriff and heads of the planet.

There are two obvious solutions to this problem. One is to sit back and wait a hundred years or so and collect data then consider the problem with a lot more information at hand. I'm sure climate scientists are busy doing this as we speak, but it may take a while! The other is to use climate modeling to simulate long periods of time, and see if quai-resonant waves and changes in the weather pattern are associated with anthropological global warming.

Michael Mann told me "that there is now a detectable influence of anthropogenic climate change on jet stream dynamics associated with extreme, persistent weather events like the 2010 Russian heat wave/wildfires, 2011 Texas heat wave/drought, 2013 European floods, etc. This is the first article, in my view, to demonstrate a robust such connection."

This research involved combining some 50 climate models that comprise the CMIP5 project, and historical observations of climate over time. They found that under conditions of a warming Arctic, "standing waves" (quasi-resonant waves in other parlance) formed, just as we've seen during recent bad weather events. Above, I focused on rainfall events, but drought, extreme fire conditions, etc. are the other side of the coin, or rather, the other side of the jet streams. A persistent standing wave in a jet stream can cause a few nice and sunny days to transform into several years lack of rain, and a drought.

"Both the models and observations suggest this signal has only recently emerged from the background noise of natural variability. We are now able to connect the dots when it comes to human-caused global warming and an array of extreme recent weather events," said Mann.

Here is the abstract of the paper:

Persistent episodes of extreme weather in the Northern Hemisphere summer have been shown to be associated with the presence of high-amplitude quasi-stationary atmospheric Rossby waves within a particular wavelength range (zonal wavenumber 6–8). The underlying mechanistic relationship involves the phenomenon of quasi-resonant amplification (QRA) of synoptic-scale waves with that wavenumber range becoming trapped within an effective mid-latitude atmospheric waveguide. Recent work suggests an increase in recent decades in the occurrence of QRA-favorable conditions and associated extreme weather, possibly linked to amplified Arctic warming and thus a climate change influence. Here, we isolate a specific fingerprint in the zonal mean surface temperature profile that is associated with QRA-favorable conditions. State-of-the-art (“CMIP5”) historical climate model simulations subject to anthropogenic forcing display an increase in the projection of this fingerprint that is mirrored in multiple observational surface temperature datasets. Both the models and observations suggest this signal has only recently emerged from the background noise of natural variability.

Historical data and cutting edge modeling and analysis strongly indicates that global warming, caused by human release of greenhouse gas, is increasing the frequency of persistent weather extremes such as very wet or very dry conditions. This paper looked at the northern hemisphere summer.

We have long passed the point where you can say with a straight face, "you can't attribute a given weather event to global warming." Climate change is change in climate; weather is climate today, climate is weather long term. Weather generally carries the climate change signal, and some of the weather is very different than it was prior to recent decades because of that change. You can't separate a given weather event from global warming.

Michael E. Mann, Stefan Rahmstorf, Kai Kornhuber, Byron A. Steinman, Sonya K. Miller & Dim Coumou, 2017, Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events.