‘Multi-year’ droughts have become more frequent, drier and hotter over past 40 years
Droughts spanning multiple years have become drier, hotter and more frequent over the past 40 years, according to new research.
The study, published in Science, finds that the global land surface affected by these extreme events has expanded at a rate of nearly 50,000 square kilometres (km2) per year in the past four decades – an area larger than Switzerland each year.
The authors identify multi-year droughts – which can last from years to decades – that occurred around the world between 1980 and 2018.
They find that multi-year droughts can cause significant declines in vegetation in ecosystems such as grasslands. These impacts can also translate into severe impacts for humans, including water scarcity.
Study author Dr Dirk Karger, a senior researcher at the Swiss Federal Research Institute (WSL), tells Carbon Brief:
“Everybody was talking about droughts, [that they] will be more [frequent] with climate change, but there [was] no clear database where we could look. We finally have a good baseline of what is happening…[and] provide a new way of thinking about the impact the [multi-year droughts have].”
§ Long-lasting drought
“Multi-year” droughts – those lasting at least two years and for as long as decades – can have dramatic impacts on nature and societies.
These long-lasting events can deplete soil moisture and leave rivers, lakes and reservoirs parched. This, in turn, can result in “devastating impacts”, such as massive crop failures, tree mortality or reduced water supply, according to the study.
(For more on the different ways that droughts can be defined, see Carbon Brief’s guest post.)
In recent years, multi-year droughts have occurred around the world, including Chile, the western US and Australia. For example, a 2015 study found that the so-called “megadrought” that persisted in Chile from 2010 to 2019 led to a “marked decline in water reservoirs and an extended forest fire season”.
The new study maps the distribution of multi-year drought events between 1980 and 2018. It identifies droughts by looking at a “drought index” based on changes in rainfall and potential evapotranspiration, which measures the amount of water that escapes the soil and plants into the atmosphere.
The researchers also rank the drought events by their severity – based on a combination of the extent and duration, along with the magnitude of the drought index. Then, they use the index to estimate the impact of multi-year droughts on global vegetation.
They identify more than 13,000 multi-year drought events during the four-decade study period, spanning every continent except Antarctica.
The map below shows the location and characteristics of the 10 most severe events, with the colours representing each individual drought and its length.
The longest multi-year drought occurred in the eastern Congo basin. It lasted for almost a decade, from 2010 to 2018, and affected an area of almost 1.5m square kilometres (km2).
The study finds that the most affected ecosystems by these extreme events are temperate grasslands.
However, not all multi-year droughts result in significant damage to ecosystems.
In the humid tropics, which are home to rainforests such as the Amazon, the lack of rainfall is not strong enough to diminish vegetation. This suggests that plants in those regions might have a “greater resistance” to drought conditions, the authors write.
Boreal forests in the far-northern hemisphere and tundra ecosystems also had a “minor response” to these events. The authors say this is because their vegetation productivity is more dependent on temperature than on the presence or absence of rainfall.
The drought with the most severe vegetation impacts occurred in Mongolia from 2000 to 2011 and reduced vegetation “greenness” by almost 30%.
For Karger, it is difficult to pinpoint the strongest multi-year drought ever because it depends on what aspect is considered: the drought that had the largest extent or the one that lasted the longest. He continues:
“With our database we can easily answer any of these questions, it’s just a matter of what we sought for, since we provide that open source and open data”.
§ Drivers of droughts
The research reveals that multi-year droughts have increased in size, temperature, dryness and duration.
The global land area affected by this kind of drought increased at a rate of 49,279km2 per year during that time – equivalent to a size larger than Switzerland per year.
The factors behind the intensification of multi-year droughts are increased potential evapotranspiration, decreased rainfall and rising temperatures, the study says.
The researchers note that during multi-year drought events, the “precipitation deficit” – the difference in the amount of rain compared to a baseline over a certain period and region – has surged over time.
For the 10 most severe multi-year droughts, the precipitation deficit has increased, on average, by 7mm per year over nearly four decades.
At the same time, the temperature during these events has increased by 0.26-0.35C per decade.
The study attributes the higher temperatures during multi-year droughts to climate change, noting that the warming “align[s] well” with global changes. It also notes that the years with the largest areas under multi-year drought have followed the El Niño events of 1998, 2010 and 2015.
Dr Maral Habibi, a researcher at the University of Graz, in Austria, and who was not involved in the study, tells Carbon Brief:
“The study clearly illustrates how rising temperatures amplify drought through increased evapotranspiration, precipitation deficits and vicious feedback loops (such as reduced cloud cover exacerbating heat).”
§ ‘More regular’ multi-year droughts
The research says that the most severe multi-year droughts identified in the study “represent valuable case studies to prepare for similar events that may occur more regularly in the 21st century”.
It also says that analysing droughts at a global level, rather than focusing on a single drought event, “paves a more realistic way to develop adequate and fair mitigation strategies”.
Dr Ruth Cerezo-Mota, a researcher at the National Autonomous University of Mexico (UNAM) who was also not involved in the study, tells Carbon Brief that the world needs more data, including high-quality and continuous observations, and more investment in science to “understand these dynamic processes”.
Habibi agrees on the need for “enhanced monitoring tools and predictive climate models”. She adds that “investments in AI-driven drought forecasting and cross-border water resource management are also vital” to “mitigate and adapt to the challenges of a warming, drying world”.