Extreme Weather #10 – Trends in Droughts - Overview

In #1-6 we looked at trends in Tropical Cyclones. In #7-#9 we looked at trends in extreme rainfall and floods.

Now we move onto droughts.

The simplest idea about droughts is they occur when there’s a drop in rainfall over some time period.

Rainfall is relatively easy to measure. However, as a caveat, the world is a big place and different datasets have differences.

Now suppose the rainfall in a given region is the same this year as 50 years ago, but it’s 2°C warmer. More water will evaporate from the surface. In some cases this will lead to more droughts than 50 years ago.

So we can’t just look at the easy measurement of rainfall. We need to measure evaporation. That’s a lot harder as we don’t have measurements.

Why don’t we just use measurements of soil moisture? We don’t have that data either. In some places we do, but nothing like enough for a global assessment.

There’s another related drought type - hydrological. This means, for example, the rivers have dried up, the water table is depleted, and so on. Data is likewise limited.

So we have three categories of droughts:

• rainfall

• soil moisture

• hydrological

If we had perfect data on, say, soil moisture we would still have to decide how to categorize droughts. We could look at:

• 3-month droughts - frequency

• 7-12 month droughts - frequency

• Greater than 12 month drought - frequency

• Area of droughts that cover a given time period

• Severity of any given drought category, i.e. how bad the soil moisture deficit is

We’ll see some of these different metrics in some papers in future articles.

In the next article we’ll look at rainfall deficits.

The current IPCC report, AR6, presents a complex picture of recent trends, so as a wrap up, here’s what the 2012 Special Report on Extremes said about droughts, in plain English:

We don’t know whether droughts are going up or down globally.

The actual text is in the notes below.

---

Notes

From p. 171 of SREX (2012):

Following the assessment of observed changes in the AR4 (Chapter 3), which was largely based on one study (Dai et al., 2004), subsequent work has drawn a more differentiated picture both regionally and temporally.

There is not enough evidence at present to suggest high confidence in observed trends in dryness due to lack of direct observations, some geographical inconsistencies in the trends, and some dependencies of inferred trends on the index choice.

There is medium confidence that since the 1950s some regions of the world have experienced more intense and longer droughts (e.g., southern Europe, west Africa) but also opposite trends exist in other regions (e.g., central North America, northwestern Australia)

---

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

IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp.