In 2009 the inventor of the renowned hockey stick graph Michael Mann and his colleagues published a paper in the journal Science where they attempted to refute the global significance of the Medieval Warm Period. The idea behind the paper was to show that the warmth in some areas was offset by cold in other parts of the world. To show this the authors searched out places that were colder than normal 1000 years ago.

The problem was the Atlantic region, which had an excellent amount of data to support the Medieval warmth. Here the temperatures stood at least at today's levels. Therefore Michael Mann searched around for other regions where far less data was available and found the Central Eurasian region would do just fine. The scarcity of available data left lots of room for interpretation. This is how the authors plotted a huge region of cold over a large swath of Central Eurasia during the Medieval Warm Period, which supposedly offset the inconvenient Atlantic warmth.

Mann map
© Mann et al 2009 / Journal of Science
Figure 1: Temperature anomalies during the Medieval Warm Period 1000 years ago as to Mann et al. (2009). In Central Eurasia the authors interpreted a pronounced cold zone.
A Chinese team of scientists led by He YuXin of the University of Hong Kong, however, took a very close look at Mann's liberally interpreted data and his postulated Central Eurasian cold zone. Using sediment cores extracted from two different lakes and using the so-called alkenone method, the Chinese scientists reconstructed the temperature development over the past 2000 years for the northern Tibetan Plateau, which according to Mann was significantly colder 1000 years ago.

The surprise was big when the new, hard data showed the opposite was in fact true. It turns out that the region of the theoretical cold in the northern Tibetan Plateau during the Medieval Warm Period was indeed not colder but was warmer than today, see Science Bulletin.

In their paper, YuXin and his colleagues made yet another interesting discovery. In the abstract they write: 'Further, our temperature reconstructions, within age uncertainty, can be well correlated with solar irradiance changes, suggesting a possible link between solar forcing and natural climate variability, at least on the northern Tibetan Plateau.'

The surprisingly good synchronicity between sun and climate in the region of study is clearly visible from the study's chart:

Solar activity temp Tibet
© YuXin et al, 2013.
Figure 2: Good agreement between the temperature development at the northern Tibetan Plateau (lower curve) and solar activity over the past 2500 years.