Ancient Rings Reveal Drought's True Cause

New research using bristlecone pine tree rings has uncovered surprising insights into the causes of drought in the American West over the past two millennia. Contrary to common belief, severe droughts have historically occurred during cooler periods, not solely during heatwaves. A comprehensive 2,000-year record from Colorado shows that cool droughts were nearly as frequent and just as impactful as warm ones.

The most severe drought in this 2,000-year reconstruction occurred in the second century, with two periods lasting 66 and 16 years. During this time, summer nights in the mountains remained cooler than average. The water deficit from this second-century drought significantly exceeded the capacity of Lake Mead.

While recent droughts, including the current one affecting the Colorado River, have been accompanied by high temperatures, the study emphasizes that a simple lack of precipitation and snow is sufficient to cause extreme water shortages. The current heat is worsening the situation, but the historical data indicates that severe water loss can happen even without unusually warm conditions.

The temperature record also highlights the unusual nature of present-day warmth. The warmest sustained period recorded spans from 1933 to 2012, with most of the warmest summers occurring in the last three decades. Researchers are now working to understand how temperature and precipitation interact to influence river flow, a crucial step for future water management in the Southwest and other dry regions.