GardenDrumSouth-western USA headed for a Mega Drought

Southwest's drying climate 2050-2100CE

New climate modelling shows that the USA’s Southwest and Central Plains are likely headed for a megadrought in the 21st century that could last 35 years – worse than the one that forced Pueblo Indians to abandon the region one thousand years ago.

“Our results point to a remarkably drier future that falls far outside the contemporary experience of natural and human systems in Western North America, conditions that may present a substantial challenge to adaptation.”

The team of scientists from Cornell and Columbia Universities used tree ring data to map the regions’ climates over the past 1000 years. They then applied 17 different types of computer modelling to the data based on 2 different scenarios – no change to current greenhouse gas emission levels, and if moderate cuts were made. The study is published in Science Advances.

Fig. 1 Top: Multimodel mean summer (JJA) PDSI and standardized soil moisture (SM-30cm and SM-2m) over North America for 2050–2099 from 17 CMIP5 model projections using the RCP 8.5 emissions scenario.

Fig. 1 Top: Multimodel mean summer (JJA) PDSI and standardized soil moisture (SM-30cm and SM-2m) over North America for 2050–2099 from 17 CMIP5 model projections using the RCP 8.5 emissions scenario. Unprecedented 21st century drought risk in the American Southwest and Central Plains Benjamin I. Cook, Toby R. Ault, Jason E. Smerdon Science Advances 01 Feb 2015: Vol. 1 no. 1 e1400082 DOI: 10.1126/sciadv.1400082

The modelling is illustrated in the graphs above. The brown line is summer moisture balance over the past centuries taken from tree ring data, showing both drier and wetter times since 1000CE (either side of 0 on the graph’s baseline). The coloured shaded area shows the climate modelling predictions using the 17 different modelling systems.

The modelling shows that drier times are a certainty but also that the likelihood of a drought that lasts for decades – a ‘megadrought’ – was 70% more likely in the Central Plains and 80% more likely in the Southwest during the second half of the 21st century if high emissions continue, and that it would be even more severe than the ‘Medieval Climate Anomaly’ of 1100-1300CE.

What’s noticeably different about the megadrought prediction is that it wouldn’t be brought on by the La Niña effect of unusually cold waters in the Pacific, currently the main driver of western USA droughts. Instead it would occur as a direct response to increased greenhouse gasses in the atmosphere, causing a much faster drying-out of soils.

“the mean state of drought in the late 21st century over the Central Plains and Southwest will likely exceed even the most severe megadrought periods of the Medieval era in both high and moderate future emissions scenarios, representing an unprecedented fundamental climate shift with respect to the last millennium.”

Citation: Unprecedented 21st century drought risk in the American Southwest and Central PlainsBenjamin I. Cook, Toby R. Ault, Jason E. Smerdon. Science Advances 01 Feb 2015: Vol. 1 no. 1 e1400082 DOI: 10.1126/sciadv.1400082

You can read all the detail at Science Advances.

 

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