Extreme Weather #12 – Trends in Droughts - Soil Moisture - Rainfall & Evaporation
In #11 we looked at “droughts from rainfall”, i.e., rainfall deficits. There were regional variations but no obvious global trend.
In fact, we expect more rainfall as the earth warms (warm air holds more moisture) so a question to return to in a later article is why there hasn’t been an obvious increase in rainfall, i.e., why there hasn’t been a reduction in “droughts from rainfalls”.
We can measure rainfall. Think - a little bucket that fills up with rain and someone comes around every day, takes a measurement, and writes that number down in a notebook. Of course, the measurement is often more sophisticated in recent times. But we can all appreciate it’s a measurement that can easily be taken.
The other side of soil moisture is evaporation. If it’s hotter, all other things being equal, we expect more evaporation and so on the margins, more places in droughts.
The probem? There is no simple instrument we can stick in the ground next to the rainfall bucket to measure evaporation.
So, when we are attempting to consider recent trends we are looking at calculations.
Let’s start with Spinoni et al 2019 referenced by AR6. We saw the “rainfall only” drought map, for 12-month droughts, in the last article (right hand column below). This version includes the evaporation calculation in the left column, with the bottom row being the all important change over time:
We can see that the bottom left map is worse than the bottom right map. Red = more droughts. Blue = less droughts.
Here’s their calculation by region of area in drought, with blue being the “rainfall only” drought estimate, while the red is the “rainfall + evaporation” drought estimate:
It’s clear that the red line is worse than the blue line. The red line includes the calculation of evaporation.
Here is the commentary from the paper:
According to the SPEI-12 [rainfall plus evaporation] indicator, the increase of drought frequency is approximately 9.7% and the absolute increase of areas in drought is 3.3%. According to SPI-12 [rainfall only], however, an opposite slight decrease of drought frequency and areas in drought can be observed. On average, both indicators point towards longer, more severe, and slightly more intense droughts, but they diverge on drought frequency and areas in drought
So this is bad news.
Here’s some commentary from AR6, p.1573-4:
A lack of sufficient soil moisture, sometimes amplified by increased atmospheric evaporative demand, result in agricultural and ecological drought..
..Overall, evidence from global studies suggests that several land regions have been affected by increased soil moisture drying or water balance drying in past decades..
..For AR6 regions, several studies suggest an increase in the frequency and areal extent of soil moisture deficits, with examples in East Asia, Western and Central Europe, and the Mediterranean. Nonetheless, some analyses also show no long-term trends in soil drying in some AR6 regions – for example, in Eastern North America and Central North America, as well as in North Eastern Africa.
In the notes below I’ve included my summary of the lengthy table you can find at the end of AR6 for this category of drought.
So, this is bad news. Soil moisture droughts have increased. This is primarily because higher temperatures mean more evaporation, while at the same time global rainfall hasn’t significantly changed.
However, many recent papers raise a very important question on droughts which is not discussed in this section of AR6. We’ll look at that in the next article.
Notes
AR6 table on droughts summarised for “agricultural/ecological droughts”.
Red is bad - more drought. Blue is good - less drought. MC, is “medium confidence” - which covers all of the changes. We’ll cover attribution in later articles but I’ve highlighted the changes which are attributed to human activity.
References
Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
A new global database of meteorological drought events from 1951 to 2016, Jonathan Spinoni et al, Journal of Hydrology: Regional Studies (2019)
I'm not sure how anyone can say soil moisture decreases are "bad news" and say they are related to warming, when it's been cooling for the last 8 years.
In #10 (Into to Drought), you wrote that there are three kinds of droughts:
1) [reduced] rainfall,
2) [reduced] soil moisture [due to decreased precipitation and increased transevaporation]
3) hydrological (which may be less water in reservoirs, rivers and ground water)
We can measure any reduction in precipitation, and there has probably been a trivial increase in precipitation because warmer air can hold more water. However, I believe you said we had no systematic way of monitoring changes in soil moisture. Above, Spinoni et al 2019 report the change drought in terms in terms of "drought events" per decade, which is not a very robust way to quantify something. More comprehensive monitoring alone can cause an increase in detected events. With limited ability to systematically measure soil moisture, our ability to count the number of times and places that cross the threshold for being counted as a "drought event" can't be very robust. Yield of crops per acre might be a more meaningful measure of the IMPACT of increasing drought AFTER MITIGATION. So might falling levels of ground water in wells. Does the IPCC discuss other metrics for drought.
We have the same problems with in homogeneity in the record of TCs "events". As our surveillance has improved, our count of the number of TCs qualifying as Category 1 hurricanes and Category 3 or 4 ("major" hurricanes) has improved. In Kossin's paper, homogenizing the hurricane record by downgrading the resolution of images to a common low-resolution at the beginning of the satellite period reduced the count of TCs and major TCs in the past two decades by about 30%. Crossing a threshold to be counted as a TC or major TC is less reliable method of quantifying TCs (IMO) than more continuous monitoring such as Accumulated Cyclone Energy. I suspect the same applies to quantifying droughts by "events".