Extreme weather happens with or without climate change, so the extent to which various extreme events are attributable to human activity is of great scientific and public interest.
Of all extreme weather, heatwaves are the most straightforward to connect to climate change, but the way this is quantified and communicated can be confusing. When a heatwave occurs, headlines based on scientific studies will often read something like:
“This heatwave was made 30 times more likely because of human-caused climate change”.
Yet such language frames heatwaves as distinct events that either occur or do not. In reality, heatwaves lie on a spectrum of intensity; they are a matter of degrees. Human-caused climate change generally makes all temperatures hotter, including the temperatures experienced during heatwaves. Thus, a much more intuitive and useful way to think about how human-caused climate change has affected a heatwave is to express it in terms of the intensity of the heatwave:
“This heatwave was 3°f hotter because of human-caused climate change”.
Intensity framing avoids some of the confusing aspects of the explanations centering around likelihood, namely that such likelihood framing relies on the nonsensical assumption that if the temperature were one-degree cooler, there would have been no heatwave at all (this is because the likelihood being expressed is technically the likelihood of it being at this temperature or hotter). Because of this, the likelihood framing gives the false impression that the entirety of the heatwave was due to human influence rather than just the additional degrees that were actually contributed.
Likelihood framing has proliferated not because it is a useful way to think about the issue but because it offers much larger sounding headline numbers than the more intuitive intensity framing. To help the public better understand the relationship between climate change and extremely hot temperatures, we will occasionally offer updates pairing standard, IPCC-generated estimates of the human influence on notable temperatures with intensity framing.
Our first example is the record-breaking heat that occurred on July 19 this year in Europe. Lincolnshire, England, for example, recorded a record temperature of 40.3°C (104.5°F). The UK Met Office is already reporting on the extreme heat through the prism of likelihood framing:
“The chances of seeing 40°C days in the UK could be as much as ten times more likely in the current climate than under a natural climate unaffected by human influence.”
But how much has climate change contributed to the intensity of this extreme heat? How many extra degrees has it piled on? That is left out of the reporting.
Much research has been done on the relationship between global warming and extreme heat locally (see Chapter 11.3 in the most recent IPCC Working Group 1 report). The most common way to look at how human influence affects extremely high temperatures is to investigate modeled and observed long-term changes in the hottest daily maximum temperature experienced in a given year or month (the variable is referred to as TXx). Both global climate models and observations indicate that, over land, these extremes tend to increase about 25% more than the increase in global mean temperature, but this varies significantly by region and season. This enhancement is roughly the same, even when more rare temperatures are investigated than TXx, like 1-in-50-year daily temperature extremes.
Returning to the current heatwave in the UK, below is a map of the model-calculated enhancement of extreme daily high temperatures (TXx) in July relative to global mean warming, where numbers above 1 (green, yellow, red) indicate faster warming of extremes than the global mean and numbers below 1 (blue) indicate slower warming of extremes than the global mean.
Ratio Between Extreme Temperature Warming and Global Warming
For Lincolnshire, England (black dot), we see that July extreme temperatures are enhanced by about 32% relative to the global mean. This implies that in a preindustrial climate, a 40.3°C (104.5°F) temperature would have been 38.7°C (101.7°F). In a future world that experiences 3°C of global warming, this day would be about 42.7°C (108.9°F).
Effect of Climate Change on This Daily High Temperature
We believe that this intensity framing is more useful than the likelihood framing both in terms of conceptualizing human influence on the climate and informing society of what it needs to be prepared for in the future. Knowing that a certain temperature threshold will be crossed many more times in the future than in the present is only meaningful if a specific impact corresponds to that threshold. But most impacts scale with the intensity of the heatwave; they are a matter of degrees. Thus quantifying how humans influence the intensity of heatwaves gives us our best understanding of the events themselves as well as how impacts have and will be affected.