Scientists have good reasons to contemplate climate in global terms, but this planetary way of looking at seemingly minor changes in worldwide average temperatures leads the rest of us down a slippery slope.
People live in one place or another - it's regional climate that matters. Perfectly accurate global averages tell us how planet Earth is doing but almost nothing about such things as heat-waves and freezes and floods and droughts that can devastate people, their health, their food supplies and the economies of their countries.
Climate researchers are aware of this, of course, but downscaling global models to the finer details of regional climate is not as straightforward as you might think.
With that problem in mind, a team of leading paleoclimate specialists reconstructed the two most significant natural climate episodes of the last 1,500 years - the so-called Medieval Warm Period from the 900s to 1300s, and the LIttle Ice Age from the 1300s to 1800s. Using ice cores, sediments, tree rings and other "proxy" records of these periods, they analyzed their size and shape, the mechanisms at work and then they looked to see if climate models capture them.
These were large-scale, but regional events, they report in the journal Science. They were driven by subtle changes in solar output and volcanic activity - both higher during the warm period and lower during the LIttle Ice Age. These solar changes led to shifts in atmospheric circulation over the North Atlantic and to changes in sea surface temperatures in the Pacific.
Curiously, when much of the Northern Hemisphere was feeling the warmth of the Medieval Climate Anomaly, the Tropics was responding like a thermostat - the equatorial Pacific was unusually cool, a pattern akin to La Niña, the cool phase of the El Niño cycle. During the LIttle Ice Age, in contrast, the equatorial Pacific is somewhat warmer. In this reconstruction of temperatures during the Medieval Warm Period, notice the warm North Atlantic and the cool tropical Pacific.
The large global climate models do not successfully simulate this Pacific Ocean response, the researchers report, although it is captured by models specifically designed to simulate the El Niño cycle.
Lead author of the study, paleoclimate scientist Michael Mann of Pennsylvania State University, who discusses the research in this Science podcast, sees "important implications" for future climate change. "For example," he said, "if the tropical Pacific thermostat response suggested by our analyses of past changes applies to anthropogenic climate change, this holds profound implications for regional climate change effects such as future drought patterns."
A "Pacific thermostat response" to global warming would not be good news for people who live in the drought-plagued southwestern U.S., because a Pacific that looks like La Niña makes matters worse.