The Medieval Warm Period (MWP) is a seemingly warmer-than-average 500 hundred years (very roughly 800 to 1300 AD) centered around the North Atlantic. During this period of slightly higher temperatures, the Vikings first colonized Greenland (the 980s), and established an ultimately ill-fated colony in today's Newfoundland, which they called Vinland.
Following the MWP came what's called the Little Ice Age (LIA), a dip in temperatures centered around the north Atlantic that ended in the mid-1800s.
(Here's a graphic of the MWP and the LIA from Wikimedia Commons with the multiple proxy data sources listed.)
These days, the MWP is often cited as evidence that, because the world was warmer in the past, the warming of recent decades is 1) hardly anomalous, 2) can't be the result of higher concentrations of carbon dioxide since the world warmed when CO2 was at pre-industrial levels, and 3) can't possibly be the result of human activity since humans were clearly not responsible for warming in medieval times.
These arguments bear some examination.
First, scientists think that the medieval warming period was a local phenomenon centered on the north Atlantic and a few other regions, and not global.
In other words, the whole world did not grow warmer during medieval times. Some areas appear to have been cooler. And while some areas seem to have experienced warmth comparable to today's, much evidence indicates that the global average was still lower than today's.
That was the conclusion of the Intergovernmental Panel in Climate Change's 2007 report (Chapter 6) [PDF]:
The evidence currently available indicates that NH mean temperatures during medieval times (950–1100) were indeed warm in a 2-kyr context and even warmer in relation to the less sparse but still limited evidence of widespread average cool conditions in the 17th century (Osborn and Briffa, 2006).
However, the evidence is not sufficient to support a conclusion that hemispheric mean temperatures were as warm, or the extent of warm regions as expansive, as those in the 20th century as a whole, during any period in medieval times (Jones et al., 2001; Bradley et al., 2003a,b; Osborn and Briffa, 2006).
This more recent 2009 study [PDF] in the journal Science says: "The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally."
To recap: During the MWP, temperatures went up in some places, but not all places. In some places, they may have been cooler. Today, however, temperatures are rising much more uniformly around the world.
One obvious source of confusion is the name. Given the substantial uncertainty that the phenomenon was worldwide, "medieval warm period" is somewhat misleading. That's partly because of location bias: Scientists first discovered and began studying it in Europe, which did warm during medieval times.
(An in-depth discussion of the MWP, including history, can be found in this 2003 review [PDF] in Science, as well as in Chapter 6 [PDF] of the IPCC's 2007 report.)
But if scientists had begun studying the MWP elsewhere – in parts of central Eurasia and northwestern North America for instance, which appear to have been anomalously cool during medieval times – the MWP might be called something different entirely.
And, in fact, for some time, many have argued that the MWP should be rechristened "The Medieval Climate Anomaly," which is truer to its "warmer here, cooler there" nature.
Another problem: The relative paucity of data from the southern hemisphere adds to scientists' uncertainty over whether it was truly a global phenomenon. (See the graphic of data collection sites [PDF] on page 470 of the IPCC report chapter on Paleoclimate for a quick idea of what data comes from where. )
It may well be that the southern hemisphere was cooler than average, but scientists don't have enough information.
This 2002 study, for example, finds that during medieval times — and again during the Little Ice Age, for that matter — temperatures on the Antarctic Peninsula dropped. (They also rose, indicating inherent climate instability there.)
Corals from the central tropical Pacific indicate that temperatures were somewhat cooler there during medieval times as well, although the authors concede that the considerable noise caused by the El Niño/La Niña cycle puts this conclusion on somewhat shaky ground.
In general, though, scientists think that La Niña-like conditions prevailed in the Pacific during medieval times — cooler in the eastern Pacific and warmer in the western — leading to intense drought in places like the US Southwest. Incidentally, that's when the settled native American peoples of the Southwest, the Anasazi, deserted their homeland in the Four Corners region and headed east and south.
On the East Coast, meanwhile, this USGS study finds that between 450 and 1000 AD, temperatures rose for a sustained period of time in Chesapeake Bay. "However, late 19th and 20th century temperature extremes in Chesapeake Bay associated with NAO [North Atlantic Oscillation] climate variability exceeded those of the prior 2000 years, including the interval 450-1000 AD, by 2-3°C, suggesting anomalous recent behavior of the climate system."
Earth's climate is a complex and dynamic system. Scientists don't perfectly understand how it works, although with each passing year their understanding improves. They can't completely explain what caused the medieval climate anomaly. And it's also true that there's no reason to think something like the MCA won't happen again — that some regions will experience warming comparable to today's, due solely to natural cycles and variability.
But none of that means that humans can't affect climate by increasing the concentration of greenhouse gases in the atmosphere, a matter of physics. One question worth contemplating is, next time an MCA-type event occurs, how will it look with human impact — or forcing, as scientists call it — added to the already considerable extremes?
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