In June of 2021, the Pacific Northwest had an unprecedented seven-day heat wave. At one point that week, the thermometer hit 108 degrees F in Seattle. About 100 people died here; many more suffered from the effects of extreme heat in a city where air conditioning has traditionally been an unnecessary luxury.
Events like that have been happening more frequently around the world, and, in some cases, they have occurred repeatedly in specific locations. This weekend's read is a paper by a group of Columbia University researchers looking at these "heatwave hotspots" and pointing out that the current climate change models tend to be right in one way but wrong in another important one.
Their main beef is that climate scientists tend to build their models around predicting the average (or "mean") temperature in a particular location. This is indeed an important metric, and average temperatures have been rising globally and in most regions of the world. That has implications for crops, for wild species, and for nature in general: Even a change of a couple of degrees can push something over a threshold leading to dramatic change. An ice-covered area of the planet where the average temperature changes from 31 degrees F to 33 degrees is going to see more of its ice melt away — and more water flowing. As another example, some amphibians' eggs are temperature sensitive: If incubated above a certain temperature, they produce males, and below that temperature, they produce females. Changing the temperature a couple of degrees can radically change the male-to-female ratio of an entire species.
But while the average temperature is important, the researchers point out that it doesn't tell us anything about the likely range of temperatures in that location. There are places where the temperature doesn't vary much over the course of the year: Honolulu, for example, has an average temperature of 74 F in January and 82 F in August — an only 8-degree difference — while Manchester, New Hampshire, has an average of 25 F in January and 73 F in July, a full 48 degrees apart. And the data shows that since 1950, not only are the range of mean temperatures widening in many places, but the maximum and minimum temperatures are also getting farther apart. The weather in those locations is getting more extreme, what researchers call a "widening tail," as extreme events like the 2021 heat wave here become more common.
To be clear, it isn't happening everywhere; there are even some places where the range of temperatures is shrinking. But there are clearly several new "hotspots" of extreme heat around the globe, some in surprising locations. Here's a map showing what's happened to the "upper tail" of temperatures across the globe since 1958: Redder areas have seen higher highs, and bluer areas have seen the range of temperatures decrease.
There are a couple of large areas: northernmost Canada and Greenland and northwest Europe, most notably. And then there are several small red "hotspots" sprinkled around without any obvious pattern. Some are coastal, while others are inland. Some are near the equator, while others are near the poles. Some are highly populated areas, some are largely uninhabited. Even the large areas are mystifying: Northern Canada is getting more extreme, but northern Russia is getting less extreme. Northwest Europe is getting more extreme, but here in the Pacific Northwest. the extremes have slightly contracted.
Here's another map, showing how often the traditional climate models underestimate extreme high temperatures (the redder, the more wrong the models are):
The researchers believe there are three main factors that contribute to a local hotspot:
Soil moisture deficiencies, or, as they say, "dry gets hotter."
Surface air temperatures.
High-pressure systems.
Any one of these three can lead to warmer weather, but together, they build on each other and create even larger temperature increases. This matches our local experience: The 2021 heat wave here featured a large high-pressure system that parked over us for days, drying out the region and preventing cool air from flowing in from the Pacific Ocean.
Why should we care? Because extreme heat (and cold) cause their own emergencies. In the Seattle area, 100 people died during the 2021 heat wave — and it would have been much worse if the heat wave wasn't predicted about seven days in advance, giving local officials a bit of time to prepare. In the places where extreme events are becoming more common, a different set of investments need to be made at all levels — individuals and families, businesses, policymakers and governments — to prepare. Contrast Phoenix, Arizona, with Seattle: In Phoenix, extreme heat is a daily occurrence for at least six months of the year, but it expects that and has built out the infrastructure to deal with it. Manchester plans around extreme cold every winter.
The researchers argue, convincingly, that the climate models need to be improved so they more accurately predict the locations that are likely to see more extreme weather, so locals can learn and plan accordingly. They make a couple of suggestions for how the models can be improved. One is to make them higher-resolution, so that the small local hotspots can be more clearly identified — though, unfortunately, we can't go back in time to make the last 75 years of data collection more granular, so, at least in the short term, this might be difficult. They also argue that new AI technologies might help generate more accurate models, but they note that machine-learning algorithms don't explain the reasoning behind the model, so we might get better predictions without understanding the "why" behind them.
This report is a good reminder that there is still much about climate change we don't understand. Average temperatures are going up globally and in several regions of the world, but temperatures are also getting more extreme, with greater frequency in many (but not all) locations. At the same time, the impact of climate change is not being felt uniformly around the globe, and there are hotspots, both large and small, that dot every major landmass. We are not yet proficient at predicting shifts in weather extremes, but the clock is ticking away, and there is a need to figure that out quickly so the communities most impacted can prepare.
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