Why flows matter as well as flames

Dynamics! Dynamics!

As Europe cleans up following Storm #Ciarán's unthinkably hurricane-force winds and heavy rains, we're seeing a swarm of "it's worse and it's coming faster than we thought" climate stories. James Hansen says "the 1.5 degree limit is deader than a doornail" and could be exceeded this decade. (Other scientists point out that Hansen's prediction of +4.8◦C by 2100 is just as likely as the prediction that we'll get to something more like +2.8◦by the end of the century.) Antarctic sea ice just had its least-successful month since scientists started keeping records in 1881.

In general, 2023 has been a really hot year.

A different kind of post on X, though, caught my attention last week, and it ties neatly into Ciarán's "bomb cyclone" status.

Prominent climate scientist Dr. Stefan Ramstorf of the Potsdam Institute for Climate Impact Research has a paper out now examining 39 studies of recent weather extremes and running through examples of increasing numbers of heatwaves, droughts, floods, and wildfires. His point is that the rapid increase in the number of these events each year exceeds what scientists might have predicted based only on increasing GHG emissions.

Here's his mildly exasperated post:

Somehow "some commentators" (Rahmstorf doesn't say who) missed the central point: although even laypeople will understand that the number of heat extremes will increase in a heating world, the extremes are *really* extreme, off-the-charts bananas extreme, and so are extreme rainfall and wildfire events, and they're accompanied by regional periods of extreme cold (remember Texas and the Pacific Northwest in 2021).

The paper explains that this is happening because "dynamic changes can further exacerbate thermodynamic driven changes" and points out that scientists are increasingly focusing on dynamics.

So what, you might ask, are dynamics? They are flows and swirls of water and air around our spinning globe, these days expressing themselves in the form of wavier, slower, and sometimes splintered jet streams as well as changing ocean and atmospheric circulation patterns. They are the processes that help lead to the development and evolution of extreme weather events.

Thermodynamics, on the other hand, have to do with heat and heat transfer: warmer air can hold more water vapor (so for every one degree increase in heat, the same volume of air can hold seven percent more water), heat extremes worldwide are increasing because GHG trap more heat, in a hotter world there is greater evaporation and so greater rainfall, and the warmer oceans are leading to more powerful hurricanes.

Rahmstorf's point is that although most of the extreme weather constantly assaulting human existence—wildfires raging with unprecedented speed and intensity, heat waves killing thousands, extraordinary droughts rendering regions increasingly uninhabitable, etc.—can be traced to our warmer atmosphere, which is packing more energy and water (thermodynamics), atmospheric dynamics are playing an important role at regional scales.

For example, waves of air far above the Earth that help transfer heat from the tropics toward the poles (and colder air from the poles to the tropics) can become stuck in ways that create fearsome heatwaves in mid-latitude areas that we might have thought of as temperate, like California or Colorado. Europe is seeing incredible heat waves that are related to the jet stream splitting and then getting locked in place. It's the weakening jet stream that led to the heat wave in the Pacific Northwest in 2021 and the extraordinary floods in Europe. Bloating in air circulation patterns eight miles above the earth (patterns that we used to think of as transferring heat from the equator to the poles) is leading to searing droughts in the southwestern US and Mexico.

Of course, thermodynamic changes are related to, or drive, dynamic changes: the Arctic is warming, which is making jet stream patterns wavier and more unstable; oceans are warmer, which is changing the relatively stable ocean circulation flows on which our civilization has relied for thousands of years.

These days, climatologists are focused on untangling and isolating the contributions of changing dynamics to extreme weather, Rahmstorf tells us, and it is difficult work, "represent[ing] a current frontline of research." Understanding and predicting the accelerating and amplifying patterns shaped by jet streams and ocean currents will be extremely important on a region-by-region basis, because these patterns will make some parts of the world experience even more extreme heatwaves (or polar-style cold spots) or extraordinary deluges than other regions.

Even if GHG emissions are stopped tomorrow, these uncanny, extreme events driven by changes in dynamics will not cease for centuries. "Just think of a former once-in-5000 year event which at 1.5 ◦C warming may have become a once-in-50 year event," Rahmstorf and his co-author warn. "It will take many decades until we have seen all the possible extreme events a 1.5 ◦C warmer world has in store for us."

Dynamics played an important role in Ciarán: an unusually strong jet stream that had shifted further south than usual steered the storm rapidly across the North Atlantic. Thermodynamics drove those dynamics: the surface of the sea was unusually warm in the western Atlantic, and that drove the boundary between warmer air and colder air farther south and made that boundary particularly sharp—which created the jet stream that propelled the storm.

I predict we'll all become expert (or believe we are) on jet streams and ocean circulation patterns. It's a reminder never to be reductive about any subject. Particularly the weather.