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Megadroughts in past 2000 years worse, longer, than current droughts

What hockeystick eh?

A new atlas shows droughts of the past were worse than those today — and they cannot have been caused by man-made CO2. Despite the claims of “unprecedented” droughts, the worst droughts in Europe and the US were a thousand years ago. Cook et al 2015[1] put together an old world drought atlas from tree rings data as a proxy for summer wetness and dryness across Europe. They compare the severity and timing of European droughts with the North American Drought Atlas (NADA) released in 2004. Yes, it’s a tree ring study with all the caveats about how trees are responding to several factors at once etc etc. But at least the modern era is measured with the same proxy as used in the old eras.

Something else is causing droughts, something modern models don’t include:

“megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes.”

The worst megadrought in the California and Nevada regions was from 832 to 1074 CE (golly, 242 years). The worst drought in  north-central Europe was from 1437 to 1473 CE, lasting 37 years.

Climate models don’t predict any of the droughts below, and all of them occurred before 99% of our emissions were released.

Authors compare results from the new atlas and its counterparts across three time spans: the generally warm Medieval Climate Anomaly (1000-1200); the Little Ice Age (1550-1750); and the modern period (1850-2012).

The atlases together show persistently drier-than-average conditions across north-central Europe over the past 1,000 years, and a history of megadroughts in the Northern Hemisphere that lasted longer during the Medieval Climate Anomaly than they did during the 20th century. But there is little understanding as to why, the authors write. Climate models have had difficulty reproducing megadroughts of the past, indicating something may be missing in their representation of the climate system, Cook said.

Droughts, Rainfall, Europe, Last Millenia, 1000AD - 2000AD, Atlas.

Figure 3A Maps from a new 2,000-year drought atlas show rainfall conditions over the whole continent, and much of the Mediterranean. A chart for 1741 shows severe drought (brown areas) running from Ireland into central Europe and beyond. A chart for the year 1315 shows the opposite problem—too much rain (dark green areas), which made farming almost impossible. (Cook et al., Science Advances, 2015)

A large part of the Northern Hemisphere is included in the study.

Droughts, North America, Year 1000 - 2000, last millenia, Atlas

Figure 4B

The worst droughts:

Besides the MCA, Fig. 3B also reveals the occurrence of a mid–15th-century megadrought in north-central Europe. The most intense drought phase lasted for 37 years from 1437 to 1473 CE (−1.84 ± 0.20), with only two isolated years of positive scPDSI. The timing of this megadrought is similar to that of the worst drought reconstructed to have occurred over the past 1000 years in the southeastern United States (27). This suggests the existence of some common hydroclimate forcing across the North Atlantic, perhaps related to Atlantic Ocean sea surface temperature variations and/or the North Atlantic Oscillation (31, 32). Finally, a third megadrought occurred from 1779 to 1827 (−1.34 ± 0.16). This period has a subperiod of “major long-duration drought” (33) from 1798 to 1808 (−1.89 ± 0.38) in England and Wales identified from early instrumental and historical climate information. It is also the driest period within the longer epoch (1779–1827) of persistently drier-than-average conditions over north-central Europe.

More generally, Fig. 3B reveals the existence of large-amplitude decadal to centennial hydroclimate variability over Europe and shows that, like North America, megadroughts in the Old World were not restricted to just the MCA period. In comparison, hydroclimate variability over the 20th century, although large, does not appear unprecedented in amplitude or trend. Isolating signals of recent GHG-induced hydroclimate change from this complex record of natural variability will be challenging.

H/t to Science Daily


[1^] Cook, et al (2015) Old World megadroughts and pluvials during the Common Era, Science Advances 06 Nov 2015: Vol. 1, no. 10, e1500561    DOI: 10.1126/sciadv.1500561


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