Droughts are climatic extremes that are hostile enough for many science fiction and post-apocalyptic stories to use near-permanent droughts for apocalyptic scenarios (Water world notwithstanding). And for good reason – drought is part of the picture based on the reality of modern climate change, as the combined trends of precipitation and evaporation are bringing drier conditions to some regions. But understanding the trends is a challenge: more rain is being delivered to other regions, drought conditions are naturally variable and historical rainfall data is limited.
Researchers typically turn to tree rings for past drought files. When compiling records of many trees, historical maps called "dry atlas" have been constructed for various regions and can span nearly a millennium. These can provide incredible historical information, including events such as the megadroughts of the western United States between 800 and 1300 CE. But each atlas of drought is only part of the global picture.
A new study led by NASA Kate Marvel groups together all these regional dry atlases – along with recent data and simulations of the climate model – to see what they can tell us about human impacts on the drought since 1900.
A global atlas
The researchers' approach was similar to other studies that examined patterns of climate change for a human fingerprint. Many simulations of the climate model were averaged to highlight the expected long-term drought trend driven by man-induced climate change. Real world data were then analyzed to see if a similar trend emerged from the typical range of natural variability. In this case, these tree-based drought atlases provided the natural variation of drought around the world.
This type of analysis reduces things to a signal-to-noise ratio – the stronger the signal caused by humans stands out from the natural and noisy variability, the clearer it is that humans are changing the climate. When the changes are large enough, the trend completely leaves the reach of past events and enters into what is essentially unmapped territory.
The simulations of the model with human emissions of greenhouse gases reflect the general patterns observed in the data, with changes towards the driest in North America and Europe, but increasingly humid conditions around India and western China . But the way this unfolds throughout the 20th century is interesting, prompting researchers to place parentheses around three periods: 1900-1949, 1950-1975, and 1981 until the present.
A Tale of Three Trends
During the first half of the century, the sign of man-made drought trends became increasingly clear, crossing the statistical confidence level of 90%. But in the 1950s, 60s and early 70s, global temperature dropped, partly due to a rapid increase in aerosol pollution reflecting sunlight. This brought the signal back to reach the noise.
Interpreting this as an aerosol, however, is complicated. Aerosol pollution affects the precipitation of complex forms that are different from greenhouse gas-guided heating, and these interactions are still a weak point for the climate model simulations. So while researchers see some hints that global drought trends over this 25-year period resemble the effects of aerosol pollution, they can not say anything with confidence.
The last period begins in 1981 when a couple of new drought datasets become online. Data change again in the direction of a signal caused by greenhouse gases, but they do not escape the range of natural variability. Because it is a short period of time, this range can more easily obscure a trend.
As model simulations continue in the future using a scenario of high greenhouse gas emissions, the researchers also looked at where these trends are headed. This shows that we can expect the human signal to emerge clearly from noise in the near future. The global signal crosses the statistical confidence level of 99% in the mid-2030s, although most individual regions take longer. Europe and parts of Asia will emerge in the 2040s, while North America and Mexico will follow in the 2060s, due to their greater natural variability.
So what can you get out of it all? The notion that there is a potentially detectable influence of aerosol pollution on drought patterns in the 1950s, 1960s and 1970s is quite remarkable. But more broadly, the authors note that "multiple sets of observation and reconstruction data using tree-ring data confirm that human activities were likely to affect the worldwide risk of droughts since the early 20th century."
And that risk will only grow as the world keeps warming up. "The human consequences of this," the researchers write, "particularly drying in large parts of North America and Eurasia, are likely to be severe."
Nature, 2019. DOI: 10.1038 / s41586-019-1149-8 (About DOIs).