In March and April of 2022, a heatwave gripped India and Pakistan, harming their economies, agricultural sectors, and human health. After an extreme weather event like this, it is natural to ask how much of the heatwave, flooding, or drought can be attributed to climate change. The answer is not only scientifically interesting but also relevant to the effective communication of the impacts of climate change.
Any attempt to answer the question, though, must begin with a precise definition of what “how much” means. For example, does it mean “how much more,” as in how much hotter did climate change make the heatwave? Or does it mean “how much more likely,” as in how much more often do we expect heatwaves like this?
The climate science community and the media seem to have coalesced around the second; in the case of this year’s heatwave in India and Pakistan, the headline from the Guardian was “Deadly Indian heatwave made 30 times more likely by climate crisis.”
Focusing on the “how much more likely” question fits well with a more general mantra that has emerged in this area of climate science: “A small change in the average means a large change in extremes.” (see also here and here).
This statement is supposed to communicate that average warming in a given place may only be a degree or two, but extremes like heatwaves will change by much more.
Some argue that the emphasis on the change in the likelihood of extremes effectively communicates the contribution of climate change (IPCC handbook). But the word “effectively” does not mean the same thing to everyone. To some, the most effective communication is the kind that raises concern about climate change the most and is thus best at persuading people to act. This can be in tension with the goal of most effectively communicating our best understanding of the situation (e.g., Stephen Schneider famously highlighted the tradeoff between being “effective” and being honest in climate communication).
After all, it is not necessarily true that a small change in the average means a large change in extremes. To see why, ask “how much more?” rather than “how much more likely?” The same analysis that yielded the “30 times more likely” result for the India/Pakistan heatwave also concluded that climate change made the heatwave 1°C hotter than it would have been in a preindustrial climate. Average warming in this region of the world has actually been slightly more than 1°C, meaning that, in this case, a small change in the average has meant a smaller change in extremes. This is true more often than not: temperatures averaged throughout the year are warming faster over land than the hottest temperatures of the year (IPCC AR6 Ch 11).
The “how much more likely” framing and its associated inflation of the influence of climate change on extreme events thus only serves to confuse the issue. The confusing part of the framing becomes apparent when we unpack it a little further.
All heatwaves are the result of certain conditions coming together in the atmosphere—often the movement of already warm air into a region, sunny skies, and sinking air that traps heat near the surface. These types of situations occur with or without climate change, and their frequency is not necessarily changing in the long term (although that is an active area of research).
The “30 times more likely” idea misleads one into thinking that the weather patterns that cause heatwaves are occurring 30 times more frequently. But in fact, their frequency has not changed appreciably, they are just occurring under conditions that are 1°C warmer.
Indeed, the “30 times more likely” figure is the product of some puzzling reasoning. After a heatwave occurs, researchers have to define what constitutes the heatwave before they can assess how much more likely it has become. The way this is done in practice is the heatwave is defined as only counting as a heatwave if it was exactly as hot as observed or hotter. With this definition in place, the researcher can quantify how much more likely climate change has made this type of heatwave where “type of heatwave” refers to crossing that precise temperature threshold. But this definition has the strange effect of defining out-of-existence heatwaves that were slightly less hot. Under this definition, a 1-degree cooler heatwave is no heatwave at all.
By analogy, imagine a basketball player has an average vertical leap of 25 inches. Sometimes they jump higher and sometimes lower. On very rare occasions, say 1-out-of-100 times, they jump as high as 28 inches (the jumps here are similar to heatwaves). Now let’s imagine that they begin wearing new shoes that boost all their jumps by one inch (this is similar to how background climate warming warms all temperatures). The shoes do not cause them to jump more, but wearing them means that when they do jump, they will reach 28 inches or more 1-out-of-10 times rather than 1-out-of-100.
Now let’s say we see the player jump 28 inches in their new shoes, and we want to quantify the impact of the new shoes on this leap. We could, bizarrely, say that a jump only counts as a jump if it is 28 inches or more. That would allow us to ask, “how much more likely” are similar jumps, and we would find that they are ten times more likely (1-out-of-10 compared to 1-out-of-100). But that’s misleading because it makes it seem like the shoes caused the player to jump more. Only a marketer for the shoe company would argue that reporting the 10-fold increase is a clearer way to quantify the impact of the shoes than simply stating that the shoes increase all jumps by 1 inch (but do not increase the frequency of jumps).
The exception would be if 28-inch jumps represented a particularly meaningful threshold, like if the player needed to jump at least that high in order to be able to dunk the basketball. Similarly, if a given temperature value is associated with a major impact threshold, it is relevant to quantify “how much more likely” it is to breach that threshold.
However, if a meaningful threshold is not being discussed, it is almost always more clear to ask “how much more?” instead of “how much more likely?”. In the case of the basketball player, the new shoes caused the jump to be 1 inch higher, and in the case of the Indian/Pakistani heatwave of 2022, climate change caused the heat to be 1°C hotter.
This is the most effective way to communicate the science, and it should be favored over alternatives that do more to motivate political action than they do to convey our best understanding of the situation.