Medieval Warm Period Warmer Than Today

At first reading the message one takes from this research just published in the journal Science is that Michael Mann of Penn State University has shown that the Medieval Warm Period and the Little Ice Age do in fact exist. This is in contrast to his earlier work that produced the now infamous hockey stick graph that showed a constant temperature over the past thousand years or so and a recent dramatic upturn. In fact, the graph produced in the Science article showing the temperature anomaly from 500 - 2000 AD (no data from the post-2001 temperature standstill here) doesn’t resemble a hockey stick at all!This research has been pushed at journalists with a press release and a dedicated podcast just in case anyone didn’t get its main points.

It involves using data from temperature proxies; tree rings, ice cores, sediments and corals to reconstrct past temperatures. The data were calibrated using the overlap with direct temperature measurements made since 1850 or thereabouts. This time the researchers extracted spatial information from the data to obtain regional temperature time-series as opposed to an amalgamated one for the entire earth or the north and south hemispheres.

Looking at the geographical locations of the data sources it must be noted how regionally limited the data is for long-term observations. This is especially true when it comes to reconstructing the Medieval Warm Period (950 - 1250) when there are only two data points in (Northern) Europe, three in Asia and not many more anywhere else. The situation is a little better of the Little Ice Age (1400 - 1700) but this is chiefly due to better coverage in Northern Europe and Northern Siberia, the west coast of the United States and China.

According to the researchers, (who prefer the term Medieval Climate Anomaly or MCA), their analysis shows that some parts of the world were warmer in the MCA than today. “The reconstructed MCA pattern is characterised by warmth over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic and parts of North America, which appears to substantially exceed that of the modern late 20th century (1961 - 1990) baseline and is comparable to or exceeds that of the past one-to-two decades in some regions.” The North Pacific is relatively warm. However, Central Eurasia, NW North America, and possibly the South Atlantic were according to this work  “anomalously” cooler.

Likewise the Little Ice Age reconstruction also shows a patchy distribution of warm and cold spots with general cooling over the Northern Hemisphere but with some places, parts of the Middle East, the central North Atlantic, Africa, parts of the United States, parts of Eurasia and extra-tropical Pacific Ocean, displaying warmth comparable to the present day.

The researchers say that the distribution of warm and cold regions looks like that seen in the El Nino phenomenon, “dynamical connections” they say, though one can take this connection too far, as some in the media have done. El Nino is a short-term effect whereas the MWP and the LIA are centuries-long major climatic effects.

Changes in solar output acting in concert with volcanoes are suggested as mechanisms to explain the MCA and the LIA. This leads me to wonder that if the sun can explain the MCA, which was as warm as it is today, can it do the same a thousand years later? The findings of a recent conference (Space Climate Symposium held in Finland) on solar-terrestrial effects comes to mind in this context.  Averaged over the entire globe the consensus is that solar variability contributes only 10-20% of the temperature variations observed, although some say it is 50%. Many at the conference agreed that solar variability can have a very strong regional effect – in fact stronger than any anthropogenic signal, especially over North America, Europe and Siberia. If this is the case then it throws this latest research into confusion.

The researchers conclude that knowing how changes in the past have affected el Nino could provide information that could resolve uncertainties in climate change prediction in helping to choose between climate models. Overall however, I’m left wondering how substantial a contribution this research actually is.

It is interesting to compare this paper with one published in Nature a few months ago that looked at the MWP and the LIA from the vantage point of the tropics.

It has been remarked that the data for the elevated temperatures in the late 20th century are based on non-tropical and high-elevation sites, (Jansen at al in Climate Change: The Physical Science Basis (eds Solomon, S. et al) 466 - 482 (Cambridge University Press 2007) and Mann et al Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proc. Nat. Acad. Sci. USA 105, 13235 - 13257 (2008).

Delia Oppo (Wood’s Hole Oceanographic Institute) used core sampling to determine sea surface temperatures (SST) that were decadal resolved for the past 2000 years. They measured the Indo Pacific Warm Pool which is the largest reservoir of warm surface water on the Earth and the main source of heat for the global atmosphere.

They found that SST for the MCA was comparable with current values suggesting that the MWP did not have regional cold spots. Oppo et al added, “our results for this time period are really in stark contrast to the Northern Hemisphere reconstructions."

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