A study published in the journal Science Advances, is the first to scientifically establish that climate change exacerbates the threat of drought, estimating there is up to a 50 per cent risk of ‘megadrought’ hitting the southwest of the US this century as a direct result of climate change – in comparison to 15 per cent without it. The American Southwest and Central Plains, along with California will be worst hit, and will potentially dry out as a consequence of increasing greenhouse gas levels in the atmosphere in an unprecedented fundamental climate shift in the past 1,000 years.
A megadrought is defined as a prolonged drought that lasts two decades or longer, They have occurred intermittently over the past few decades and are expected to become a far more common occurrence in the near-future across the world, as climate change continues, global population increases and water gets ever more scarce. The consequences are expected to be wide-ranging – from detrimental effects on the world’s economy, to risking food supply, to increasing conflicts as the battle for increasingly scarce resources intensifies.
Professor William Park, speaking to the Independent, warns ‘many of the already drought-prone parts of the planet will see megadroughts during this century that are far worse than anything those regions have seen in the past several thousand years at least.’
Scientists have compared earlier droughts in the 12th and 13th Century – the ‘Medieval Climate Anomaly’ (MCA) megadrought, the driest ones hitting North America on record – with climate simulations for the upcoming decades. Worrying conclusions were reached. After 2050, the Southwest and Central Plains will likely be hit by a drought epoch worse than any seen for the past millennium (including the MCA), and is projected to hit even in moderate future emissions scenarios.
‘These mega-droughts during the 1100s and 1200s persisted for 20, 30, 40, 50 years at a time, and they were droughts that no-one in the history of the United States has ever experienced,’ said Dr Ben Cook from NASA’s Goddard Institute for Space Studies, talking to the BBC. ‘The droughts that people do know about, like the 1930s dustbowl or the 1950s drought or even the ongoing drought in California and the Southwest today – these are all naturally occurring droughts that are expected to last only a few years or perhaps a decade. Imagine instead the current California drought going on for another 20 years.’
The cause of this drying was assigned to two factors: reduced precipitation and increased evaporation, both of which were driven by rising temperatures, leading to more parched soils and creating major added stress on both natural ecosystems and agriculture.
This also comes at a time when human populations in the region, and their associated water resources demands, have been increasing rapidly, and that isn’t expected to slow in the foreseeable future. Moreover, recent years have witnessed the widespread depletion of non-renewable groundwater reservoirs in the area, which have historically mitigated the impacts of the naturally occurring droughts, further increasing water pressure in the area.
The research took reconstructions of past climate conditions based on tree ring data – with the rings narrower in drier years – and compared these with 17 state-of-the-art general circulation models. The use of the palaeo-information, Dr Cook theorised, allowed the full extent of the natural variability that exists in the climate system to be captured, making the model more reliable.
‘In both the Southwest and Central Plains, we’re talking about levels of risk of 80 per cent of a 35-year-long drought by the end of the century, if climate change goes unmitigated,’ said the study’s co-author Dr Toby Ault from Cornell University. ‘And that’s a really important point – we’re not necessarily locked into these high levels of mega-drought risk if we take actions to slow the effects of rising greenhouse gases on global temperatures.’
However, Dr Ault did offer some hope for future redemption – that while it was a challenge, it is one that modern America could possibly rise to: ‘The records we have of past mega-droughts are based on tree-ring width estimates [which] means the events weren’t so bad as to kill off all the trees. I am optimistic that we can cope with the threat of mega-drought in the future because it doesn't mean no water – it just means significantly less water than we’re used to having from the 20th century.’