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If humans didn't cause global warming and cooling in the past, is that evidence they also aren't now?

A closer look at the argument that because the world warmed and cooled naturally in the past, current global warming or climate change isn't the result of human activity or CO2.

By Moises Velasquez-ManoffStaff writer of The Christian Science Monitor / December 20, 2009

A receding glacier in Southern Greenland is one of many that has retreated (melted) dramatically since the end of the Little Ice Age (about 1850).



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.

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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).

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