The severe drought in Texas this year has fueled speculation that alleged human-caused global warming has somehow caused “unprecedented” conditions. But real research data show that the current drought is not unprecedented and is part of a natural cycle. There have been much more severe and persistent droughts in the past before humans began emitting signification amounts of carbon dioxide into the atmosphere. This post focuses on research from the University of Arizona and the Lamont-Doherty Earth Observatory of Columbia University.
From the University of Arizona and Arizona State University we have “A 1,200-year perspective of 21st century drought in southwestern North America.”
The Abstract reads in part:
A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, AD 900–1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.
This paper goes on to discuss the role of El Niño-La Niña cycles and sea-surface temperature, but the paper does not really address cause of the droughts. The theme of this paper is that past droughts are associated with warm periods and that continued warming may portend more severe droughts in our future. However, the authors partly contradict themselves by saying that the more severe droughts of the Medieval period occurred when the temperatures were cooler than the current warm period.
It seems we have a complex interplay of natural cycles which are not completely understood.
From Cornell, we have “The characteristics and likely causes of the Medieval megadroughts in North America.” and a very interesting graph:
This graph shows that while the current drought is severe, it is much less severe than droughts during the Medieval Warm Period, a time before humans were emitting much carbon dioxide into the atmosphere.
The paper presents three conclusions:
1) The similarity of the spatial patterns suggests that the physical processes that caused the modern droughts also caused the medieval megadroughts.
2) The global atmosphere ocean conditions that currently cause modern droughts for a few years at a time were the prevailing ocean climate during the medieval period.
2) Despite the shift in the mean tropical ocean climate ENSO variability continued as now but oscillating about a colder mean state.
The authors also present an archaeological speculation:
The medieval megadroughts may also have left their signature on the human environment of the West. The great cliff cities in the four corners region of the West such as at Chaco Canyon and Mesa Verde were all abandoned towards the end of the drought. These societies were based on irrigated agriculture. Although there remains much debate about why these highly organized Indian societies collapsed, archaeologists are revisiting the idea that decades of dry conditions were part of the reason.
With both papers we see that data collection is one thing, interpretations are another.