Megadrought American West
Atmosphere Climate Change Daily Earth Science Repost

Hot and dry in the Western USA, a megadrought in progress

The western USA is hot, dry, and parched after two decades of drought. But the situation may get worse before it gets better. The science behind current research lies in counting tree rings. Dry years show up as patterns of thinner, compacted tree rings, so historic droughts appear in the natural records of tree growth.  The rings tell us that the last megadrought in western America ended before the Pilgrims landed in 1620. 

Tree ring data takes us back 1200 years to the first recorded megadrought in the 800’s during the Medieval Era. This long period of severe drought was a harbinger to repeated dry spells in the mid-1100s, the 1200s, and the late 1500s. These droughts tie into the collapse of the North American Anasazi culture, and the downfall of civilizations in Mexico.

The bad news from historical data is that megadroughts last for decades. The current drought started in the year 2000, and the period between 2000 and 2018 was the second driest 19-year period on record since the 800’s.

Natural causes

The very fact that four previous megadroughts are identified in tree rings points to events sparked by natural causes. When investigating earth processes, geologists use a mantra, “the present is the key to the past.” But the hydrologists and ecologists counting tree rings are relying on the opposite advice, “the past is the key to the present”.  The tree data tells us that megadroughts will occur again. But the hydrologic data tells us that we may be entering the fifth event now. 

A continuing drought is not a guarantee; it is simply a probability. The climate is complex, and long-term ocean trends like La Niña tend to correlate with drought conditions in the American West. The cooler ocean waters during La Niña push storms, and hence precipitation, northward thus bypassing western America. Reversal of ocean surface temperatures during El Niño lets the wetter weather return bringing drought relief.

Human amplification

While it is true that natural causes drive the occurrence of megadroughts, that does not mean human influences are not also at work. Anthropocene climate change and global warming are human-induced changes that amplify the severity of this megadrought. 

The word drought denotes a prolonged period with less water than average. The term megadrought simply changes the understanding of “prolonged” to encompass decades as opposed to years. There are two paths to a drought. One is that a region receives less precipitation than normal. The second is when hotter temperatures or dryer atmospheric conditions cause more water loss through evapotranspiration. 

Changing weather patterns that bring less precipitation to the American West, historically pushed this area into drought. This process is a natural one, and the less precipitation that falls, then the more severe the drought. However, the severity of the current drought is also a function of average annual temperature increases. 

Global warming has increased the average global temperature by 0.8 degrees Celsius over the long-term trend. But this number averages out both larger and smaller regional trends. The average annual temperatures in the American West have increased by 1.2 degrees Celsius over the norm. Higher temperatures induce more moisture loss, so global warming works to amplify the severity of the drought. Researchers attribute up to 50 percent of the severity of the current drought to human activity.


Ogallala Aquifer groundwater, sustaining life (Source: ArcheanWeb) – Also:

Flash drought? (Source: ArcheanWeb) – Also:

Hot, dry, and windy: Australia’s wildfires explode (Source: ArcheanWeb) – Also:


Global Warming Making U.S. West Megadrought Worst in Modern Age (BY SETH BORENSTEIN/ AP  APRIL 17, 2020; Rime) – Also:

Megadrought Conditions Not Seen for 400+ Years Have Returned to the West, Scientists Say (By Kevin Stark; KQED) – Also:

The American West May Be Entering a ‘Megadrought’ Worse Than Any in Historical Record (By Brian Handwerk; Smithsonian Magazine) – Also:

Feature Image: Corn shows the affect of drought in Texas on Aug. 20, 2013 (Modified) – By USDA photo by Bob Nichols –, Public Domain,

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.