In a warming world, we expect to see increases in some extreme weather events. The science is pretty clear that in some parts of the world, drought and heat waves have and will continue to increase. In other areas, more severe storms along with precipitation and flooding have increased. Drought, heat waves, and floods are examples of changes to weather and climate patterns that will have costs for human society.
It’s tricky to discern not only whether past extreme weather have changed, but also whether human-caused global warming is a factor. Scientists need high-quality records that go back many decades to see if there is any trend towards increasing or decreasing extreme weather. But weather is quite variable. We can see a rise or fall in extreme weather events with no apparent cause, human or natural.
While these trends tend to be shorter (over days, weeks, or perhaps months), some trends can have longer durations. How can we identify trends prior to high-quality instrumentation and how can we discern whether the extreme weather we see now is within natural variability? Those are the questions addressed in a new publication in the Journal of Geophysical Research by lead author Dr. Benjamin Cook and his colleagues.
The title of the article, “Spatiotemporal drought variability in the Mediterranean over the last 900 years” clearly indicates that this study considers almost a millennium of drought records and focuses attention on the Mediterranean region.
The authors use a powerful database called the Old World Drought Atlas to look back in time prior to modern instruments. This atlas is a collection of tree-ring data that measures drought using the Palmer Drought Severity Index. Within the Mediterranean region, there are 106 different tree-ring datasets. These datasets go back various lengths of time but since 1100 CE, the region is accurately sampled.
The Palmer Drought Severity Index accounts for changes in precipitation as well as changes in evaporation and storage of water within soils. By using this measurement, the authors are able to find multiple droughts throughout the Mediterranean region. Often times, the droughts occur simultaneously in different parts of the region. For instance, droughts are likely to occur at the same in in both the Western Mediterranean (Spain, Portugal, Morocco, Algeria, Tunisia) and in the east (e.g. Greece and Turkey).
What did the authors find regarding the recent droughts in the Mediterranean? Well in recent decades, there has been a persistent and long-duration drought in the regions of Greece and the Levant region. The authors found that although the Greek droughts have been severe, they do not deviate from droughts that have been observed in the past.
In the Levant region, the recent drought (1998–2012) exceeds what they have seen in the past 900 years. In fact, the recent drought is likely the driest period in the last 900 years and very likely the driest period in the last 500 years.
Here is what Dr. Cook told me:
To really understand the extent to which climate change is affecting extreme events, like droughts, you need to understand the full range of natural variability. This is why paleoclimate is so important – it gives us a way to extend our understanding way past the relatively short instrumental record of the last 100 to 150 years. In this study, we found that the recent drought in the Levant region of the eastern Mediterranean is likely worse than any comparable period of the last 900 years. This provides some independent support for other studies that have argued that climate change is intensifying drying in the region.
To be clear, this study doesn’t prove that droughts are getting worse, and it doesn’t prove that human emissions of greenhouse gases are making droughts more severe. In fact, proving such statements would be nearly impossible. What this study does is put recent extreme weather into historical context.
We know human carbon emissions will warm the world. We know that a warmer world affects precipitation and evaporation and this should affect droughts. We are observing that recent droughts are likely to worst on record. We also know that droughts and other extreme weather events have tremendous societal and economic consequences.
Was the recent extreme Mediterranean drought caused by us? Probably. Or, if not caused by us, certainly made worse. Can we prove it? Probably not. However, it is clear to people like myself that study this topic that we have enough information to make decisions about the future planet we want to live in. It is clear that if we want to avoid further increases in extreme weather, we need to reduce our emissions of global-warming gases.