Posted on 15 August 2023 by John Mason, BaerbelW

On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added "At a glance" section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a "bump" for our ask. This week features "Is extreme weather caused by global warming?". More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.

At a glance

Have you experienced an extreme weather event?

The answer to that question first requires a definition for 'extreme' weather. What threshold must be passed for 'bad' weather to take on the distinction of being 'extreme'?

The IPCC Sixth Assessment Report (AR6) defines an extreme weather event as, “an event that is rare at a particular place and time of year”. So we're getting somewhere now, although we're still left with 'rare', which is not a precise term. But we have to be pragmatic about such things. It's fair to say that torrential rain, for example, is common enough seasonally in the world's Monsoon belts but infrequent in deserts, so 'rare' in that latter case is a justifiable word to use.

When those Monsoon-affected parts of the world experience torrential rain sufficient to submerge vast areas of a country beneath flood-waters, we can agree that's pretty extreme, too. Basic physics tells us that for every degree Celsius of extra warmth, air can carry 7% more moisture. So the potential for heavier rains in a warming world is obvious. The IPCC use strictly-defined categories of probability. In AR6, the probability of an increase in heavy precipitation events is given as "Likely on a global scale, over a majority of land regions". That probability is described in terms of their, "increased frequency, intensity, and/or amount of heavy precipitation". 'Likely' means they are 66-100% certain this will happen.

National weather agencies are able to compare weather events against a baseline for which they have good data-coverage. In most countries, such coverage has been in place since the mid 20th Century, but in some, such as the UK, the data go back for another 100 years. So if a record in terms of heat or daily rainfall amount does get broken, that's significant.

In July 2022, for example, the UK saw extraordinarily high temperatures with a daily maximum of 40.3°C recorded at Coningsby in Lincolnshire. This was the first time 40°C had ever been recorded in the UK. But more astonishingly, a total of 46 other weather stations exceeded the previous UK record of 38.7°C. In addition, overnight minimum temperatures widely exceeded anything recorded before. That scorching heat wave, coming on top of drought conditions, had tremendous impacts both in terms of lives lost and fire-related damage. Again, that's significant.

The problem is that in a warming world, 40°C days in the UK can be expected to become more frequent as the decades pass by. In a world where global warming continues unabated, yesterday's extreme becomes next century's normal. The trend of, "warmer and/or more frequent hot days and nights over most land areas", is described in AR6 as, "virtually certain". In the strictly-defined categories of probability adopted by the IPCC, 'virtually certain' can only be used where there is 99-100% probability.

So the take-home is that some, but not all weather-types are liable to be amplified in their severity and frequency by global warming. Heat, drought, fire-weather and long-duration heavy rains: surely that's enough to be dealing with.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above

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In case you'd like to explore more of our recently updated rebuttals, here are the links to all of them: