Medieval Warm Period and Hockey Stick Revisited

Were global temperatures during the beginning of the last millennium as warm as today? New study says ”yes.“ But what does it mean?

The global instrumental temperature record dates back only about 150 years. Reconstructing temperature before then requires the use of temperature proxies such as tree rings and variations of temperature-sensitive isotopes in ice cores and sediments. It’s a tricky, difficult business, and yet the past is often the key to the future and so climate scientists have been working for decades to suss out the past climate.

The Hockey Stick Controversy

One climate interval that has fascinated climate scientists and become a battleground in the global warming debate is the so-called Medieval Warm Period (MWP) from about 900 to 1200 AD.

For many years, climate scientists believed that MWP temperatures were as warm as or even warmer than current temperatures. Since the climate during the MWP was clearly not influenced by emissions from fossil fuel burning, this inference has been used by some global warming naysayers as evidence that the current warming must be of natural origin and thus of no concern. I do not agree with this assessment, but more on this later.

In the late 1990s more detailed statistical analyses of climate proxies suggested that the MWP’s warm temperatures were not global in extent but rather largely limited to the Northern Hemisphere. Most famous was the analysis of Michael Mann, now of Penn State, and colleagues, published in 1998 in Nature, which produced the famous hockey stick graph. It portrays fairly constant temperatures over the past 1,000 years until the 1900s when they spike upward forming the shape of a hockey stick.

Michael Mann’s “hockey stick” graph depicts variations of the Earth’s surface temperature over the last millennium. Adopted by IPCC in 2001.

The Mann analysis has been attacked for statistical errors by’s Stephen McIntyre and Ross McKitrick of the University of Guelph and others. But a number of independent analyses like that of Timothy Osborne and Keith Briffa of the University of East Anglia published in Science in 2006 reinforced the idea that the MWP was not a global event, and from a global point of view a hockey stick was a reasonable portrayal of the climate over the last 1,000 years or so.

That same year the U.S. National Academy of Sciences formed a committee to look at the issue. Its findings [pdf] supported the hockey stick representation of the climate, but it was a rubbery stick with a good deal of wiggle:

“The committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. …

Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records … from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain. …

It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. …

Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. …

Even less confidence can be placed in the original conclusions by Mann et al. (1999) that ‘the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium‘ because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales.”

New Analysis of Sea Surface Temperatures Puts Bend in Hockey Stick

Virtually all the proxies used to reconstruct temperatures over the past millennium — the proxies that yielded the hockey stick — have come from land-based sites. But what about the ocean? With oceans covering some 70 percent of the Earth’s surface, how can we infer global temperatures without using sea surface temperatures?

These were just the questions asked by Delia Oppo of the Woods Hole Oceanographic Institution and colleagues, and they decided to do something about it. They analyzed sediment cores lying beneath the Indonesian Seas in the so-called tropical Indo-Pacific Warm Pool. Using the ratio of magnesium to calcium in the sediments as a proxy for sea surface temperature (SST), they found that “reconstructed SST was … within error of modern values from about AD 1000 to AD 1250, towards the end of the Medieval Warm Period.” In other words, temperatures during the MWP were comparable to today’s temperatures, putting a significant bend in Mann’s hockey stick stick just above the handle.

Does This Mean We Should Skip Copenhagen and Burn All the Coal We Want?

Hardly. Understanding past climate trends is obviously important, but the fact that MWP temperatures may have been as warm as today’s does not invalidate our basic understanding of the current warming trend. Consider the arguments a skeptic might make:

  1. The MWP was a natural event, so the current warming must be natural. Of course climate has changed in the past due to natural causes, but it does not follow that the current warming also must be due to natural causes even if MWP temperatures were comparable to today’s. Regardless of the cause of the MWP warming, the preponderance of the evidence is that the current warming cannot be explained by natural causes and is due to greenhouse warming from emissions of carbon dioxide and other heat-trapping gases.
  2. Life and humanity survived during the MWP and so we needn’t worry about the current warming. Another logical fallacy. The world is quite different now compared to 1000 AD. Today some 6.7 billion people are bound together by a global economy and critically dependent on communication, transportation, and energy infrastructure for survival. The impact of environmental change, even if limited to a single region, has the potential to profoundly affect people worldwide. We ignore the threat of such change at our own peril.

So I find the results of Oppo and colleagues to be important and scientifically interesting. But this scientific finding about climate 1,000 years ago should not make us take our eye off the real problem: dealing with the very real threat of global warming.

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