The @IPCC_CH report is out, so let's talk about #drought. There are major advances in the report on this. In the last report (8 years ago) scientists weren't sure whether some of the bad droughts we were seeing were caused by humans. Things have changed. 1/
First, what is a #drought? It's complicated! You need low rainfall, but things that "remove" water from land (wind, temperature, evaporation) are also important. In Chapter 8 you'll find a diagram with all these factors. We tried to make it simple, but 🤣 2/
Where has human influence made drought worse so far? Two hot spots are western North America and the Mediterranean. These places are labeled in the SPM with brown hexagons and double dots (more confidence in human influence) 3/
What is interesting about these places is that it is not low amounts of rain that has been causing bad droughts. It's high temperatures (a consequence of rising greenhouse gases) leading to so-called "hot droughts" 4/
"Hot drought" happens because when the atmosphere is warmer, it is also thirster. A warm atmosphere evaporates more water out of soils, making droughts worse. So for the same reduction in rainfall, you get a more severe drought 5/
An example: the 2012-2014 drought in California. An analysis of tree ring data by @thirstygecko & @locallyabsent demonstrated that this drought was not unusual in terms of low rain but once you considered soil moisture, it was amongst the worst in 800 years! 6/
Let's shift to the future. What can we expect under higher emissions? This map from FAQ 8.3 highlights places where projections suggest droughts will get worse. 7/
Unsurprisingly, many semi-arid places (western NA, Med, Chile, SW Australia, S Africa) are expected to get drier but some wet places are on this map as well, like the Caribbean and the Amazon. 8/
Figure 8.19 shows in more detail how the projected changes scale with emissions scenario. In this plot, Vapor Pressure Deficit (VPD) is used as a metric for how "thirsty" the atmosphere is. Soil moisture reflects the balance of precipitation, evaporation, and transpiration. 9/
Bottom line: More emissions = thirstier atmosphere = more severe droughts. If we cut emissions sooner rather than later we can avoid the worse-case scenarios. /fin #ClimateReport #IPCC
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