The western United States has been haunted by wildfires and major droughts the past several years, some of the worst experienced in several decades. However, if climatology is any indication, and it usually is, the potential for significantly worse droughts is actually a regular part of the western United States climatology.
NATURE’S CLIMATOLOGIST, TREES!
Trees are natures living climatologists. You can learn a lot about your climate by studying the width of a tree ring. Typically, when a tree ring is thick, that means conditions are favorable for growth. As such, when drought conditions are not in place, you can find that trees have thick tree rings. When drought conditions are in place, the tree rings are very narrow because trees can not grow as much in those conditions.
The western United States, because of the normally dry weather conditions anyway, have trees that can survive in this climate. As such, we are lucky to have trees that have survived and grown for hundreds, even thousands of years. As a result, we have a lot of reliable climate data available to study when droughts have developed and how long they lasted.
With this data, we have learned that major droughts have developed over the western United States several times and they have been far worse than anything the region has experienced in the past 100 years.
MEGA DROUGHTS OF THE PAST
So how bad where droughts of the past? Well, we know from Native American history that one drought in 1250 to 1290 that one drought forced the Puebloans out of the Chaco Canyon to other more favorable locations for water due to the dry weather conditions. Other similar droughts developed in the early to mid 800’s, mid 900’s to early 1100s, mid to late 1100s, the early 1300s, early 1400s, and the late 1600s. Each of these droughts last at least 20 years, some as long as 50 years!
CAN IT HAPPEN AGAIN? YES!
The tree rings and even the river beds can tell us that mega-droughts can happen, but they don’t tell us why they happen. There is some disagreement on what causes a mega-drought. Some point to solar activity, volcanoes, or changes in the sea surface temperature anomalies. Other’s suggest that there is no exact trigger, but simply a feedback mechanism in the atmosphere that happened randomly.
When I study the data, what stands out to me is that the precipitation anomalies strongly match what I would expect for a negative Pacific Decadal Phase. So perhaps, the evolution of a prolonged negative PDO phase is one of the triggers that could lead to a mega-drought supportive environment. In fact, a study by McCabe, Palecki, and Betancourt (March 2004, Pacific and Atlantic Ocean Influences on Multidecadal Drought Frequency in the United States, found support for just that influence. A negative PDO would feature below normal sea surface temperature anomalies over the coastal waters of western North America while a positive PDO would feature above normal sea surface temperature anomalies.
Impacts from human-influenced climate change from CO2 increase has to also be considered a trigger, especially if the increase of CO2 disrupts sea surface circulations as air temperatures rise.
IMPACTS OF A MEGA-DROUGHT
When not if, a mega-drought return to the western United States, the impacts frankly may change the landscape of the United States. Consider that for the first time in human history, there are now millions of people living in the western United States, especially in California. Water restrictions may be rough now, but 25 years of little to no rain may just break the infrastructure of the region and cause a mass migration to areas of greater rainfall like the Pacific Northwest and locations further east. After all, we already have seen this happen in human history on a smaller but no less significant scale.