What historic megadroughts in the western US tell us about our climate future

The Uvas Reservoir near Silicon Valley in California, photographed about one year ago. Ian Abbot/ Flickr, CC BY-NC

Let’s start with some old news. Earth’s climate is warming, human activities are causing it, and as long as we continue to dump greenhouse gas emissions into the atmosphere, the impacts on climate and society will only intensify.

These impacts will be felt regionally and be linked to a chain of consequences relating the way we live and operate on planet Earth. Understanding how these regional climatic impacts will unfold in the future is therefore important for adaptation planning and assessing the risks of climate mitigation efforts. Many scientists are working to characterize the regional changes that can be expected from future greenhouse gas emissions — a complicated endeavor given the myriad natural and social systems connected to climate.

It was these interests that motivated me and my colleagues, Benjamin Cook at NASA and Toby Ault of Cornell University, to study how climate change will affect droughts in the future and how those changes relate to drought variability in the past. In a paper published in Science Advances last week, we found that future droughts driven by human-induced global warming could surpass even the driest periods in North America over the past 1,000 years.

More, worse and longer

Scientists have known for at least a decade that the American Southwest and Central Plains are expected to dry throughout the 21st century as a consequence of continued increases in greenhouse gas emissions. This trend toward aridity will be the result of both changing precipitation patterns and higher temperatures, the latter of which will draw more moisture from the soil and plants through evaporation.

Beyond the physics of these changes, the projected drying trends are bad news for regions already affected by water scarcity. Recent droughts throughout the American West have been stark reminders of our current and growing drought vulnerabilities. Depleted water resources have continued to impact our economy, food supply, ecosystem services and recreation, to name only a few. These contemporary experiences make the simple message from the future all the more disconcerting: when it comes to drought in the American West, particularly in the Southwest and Central Plains, expect more, worse and longer events.

Epic droughts in history

The most recent western droughts are nevertheless not the full story. Paleoclimatology, or the study of past climates from natural climatic archives such as tree rings, ice cores, or lake sediments, tells us that there have been periods of aridity in the American West that dwarf anything the region has seen in hundreds of years.

Prolonged droughts over the past 1,000 years are thought to have contributed to the fall of civilizations in the Southwest US, including Chaco Canyon, pictured here. Robert Wilson/Flickr, CC BY-NC

Megadroughts, so called because of their severity, spatial extent, and the multiple decades that each event persisted, are known to have existed throughout western North American between about 800-1400 CE. These events had widespread impacts across the ecology and landscape of the West and have been tied, at least in part, to the collapse of the Ancient Puebloan civilizations of Chaco Canyon, Mesa Verde and related settlements.

Suffice to say that the “mega” prefix for these events is well deserved, and for a long time, megadroughts have been seen as the worst of the worst possibility for drought conditions in the Southwest and Central Plains.

Sobering

It was our combined knowledge of the past and future that motivated us to compare the projected 21st-century states of aridity in the western US to the megadrought periods over the last millennium. Our findings surprised us all.

We stitched 1,000 years of paleoclimatic estimates of soil moisture variability derived from tree rings, together with an ensemble of state-of-the-art climate model simulations for the 20th and 21st centuries. When we compared the future projections of drought to the past we found that they were more severe and persistent than at any time during the last thousand years, even if we considered only the driest megadrought periods.

A representation of the summer moisture in the U.S. Central Plains and Southwest. The brown line represents the variation in dryness since the year 1000, based on data from the North American Drought Atlas; the lower the line on the graph, the drier the conditions. Colored lines to the right side of the graph represent what climate models see ahead: a trend toward dryness not seen in the previous millennium. Cook et al., Science Advances, 2005, Author provided

This is a sobering result and one that illustrates the urgency and magnitude of the challenges that stand before us. If there is good news in these findings, it is that each of the future emissions scenarios is based on choices - choices that we have yet to make. These choices become more limited and more difficult the longer we delay our efforts to seriously address climate change. The findings from our current study nevertheless are one more reminder that choosing to do nothing but pursue a business-as-usual strategy may confront us with even more daunting and devastating challenges in the future.

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