Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly
Michael E. Mann, Zhihua Zhang, Scott Rutherford, Raymond S. Bradley, Malcolm K. Hughes, Drew Shindell, Caspar Ammann, Greg Faluvegi, and Fenbiao Ni Science 27 November 2009 326: 1256-1260
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Sir Galahad was hot—literally. Not because of poorly ventilated castles or armor that didn't breathe well, but because the legendary knight lived in a time during which, according to climatologists, the earth’s northern hemisphere experienced unusually warm temperatures. This warming, which lasted from approximately 950 to 1250 CE, is known as the Medieval Climate Anomaly (MCA), and new research revealing regional differences in the earth’s surface temperature during the MCA may help us better understand current global warming trends.
Determining the weather conditions of a thousand years ago isn’t easy. Daily temperature recording wasn’t commonplace until the mid-19th century, so today’s scientists rely on information that can be gathered from natural sources that have survived for hundreds, or even thousands, of years. These sources, called “proxies,” include tree rings, oxygen content in coral, and water molecule structure in ice from deep within the polar caps. Until recently, proxies have only allowed researchers to estimate past temperature change on a very broad scale, providing information about the earth as a whole, or in some cases, its hemispheres. However, a team headed by Penn State climatologist Michael Mann has been able to combine data gathered from an array of proxies to construct a more focused picture.
Mann’s group looked at two distinct periods: the MCA, and what’s known as the Little Ice Age (LIA), a period of uncharacteristic cold that spanned from about 1400 to 1700 CE. Unlike in previous studies, they were able to see differences in temperature patterns at a regional level, rather than global or hemispheric. Mann’s reconstruction for the MCA showed distinct warm patterns over much of North America, the North Atlantic, and some of Western Europe, all of which was expected. However, when they looked at the temperature of the eastern tropical Pacific area, they saw something surprising—it was cold.
In fact, this part of the Pacific was colder during the MCA than during the LIA, a finding that may initially seem counter-intuitive. Colder temperatures in the tropical Pacific are commonly referred to as “La Niña,” (the flip side of El Niño, when the tropical Pacific is particularly warm) and are often associated with increased rainfall in much of the United States, as well as increase snowfall in Canada. Mann’s findings suggest that the eastern tropical Pacific may act as a kind of global thermostat, adjusting to extreme swings in the earth’s temperature in an attempt to restore balance to the climate.
One thousand years ago, before cars and Styrofoam and other human-created sources of greenhouse gases, these swings were almost certainly caused by natural occurrences, like suppressed volcanic activity and changes in the intensity of the sun. Over the last 100 years the Northern Hemisphere has experienced a remarkable spike in surface temperature, presumably due at least in part to human activity. The question now facing climatologists is whether we can expect a response in the tropical Pacific comparable to that of the MCA, and what such a response would mean in terms of flooding, drought, and other potentially destructive weather patterns. Hopefully, Mann’s model will provide some answers.
