How does Ljungqvist's reconstruction compare to others?
What the science says...
Fredrik Ljungqvist created a 2000-year temperature history of the extra-tropical portion of the Northern Hemisphere (30-90°N) based on 30 proxy records. Certain "skeptics" have argued that his reconstruction shows greater natural variability than previous reconstructions, and that it shows the peak of the Medieval Warm Period (MWP) hotter than today's surface air temperatures.
Ljungqvist Compared to other Reconstructions
However, Ljungqvist's reconstruction is not substantially different from the many other millennial northern hemisphere temperature reconstructions, as the author himself states in his paper:
“Our temperature reconstruction agrees well with the reconstructions by Moberg et al. (2005) and Mann et al. (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. AD 300–800, despite significant differences in both data coverage and methodology.”
Indeed by plotting Ljungqvist's data along with Moberg et al. (2005), Mann et al. (2008), and the surface temperature record, we can confirm that the three reconstructions are very similar (Figure 1).
Figure 1: Moberg et al. 2005 NH (blue), Mann et al. 2008 EIV NH (red), and Ljungqvist 2010 NH (green). Courtesy of Robert Way and John Cook.
MWP Peak vs. Current Temperature
Contrary to "skeptic" claims that his reconstruction shows the peak of the MWP as hotter than today's temperatures, Ljungqvist says the following when combining his proxy reconstruction with recent instrumental temperature data:
“Since AD 1990, though, average temperatures in the extra-tropical Northern Hemisphere exceed those of any other warm decades the last two millennia, even the peak of the Medieval Warm Period”
Figure 2: Ljungqvist (2010) 30-90°N decadal averages (black) vs. HadCRUT land-ocean 30-90°N decadal averages (red). Courtesy of Robert Way.
What Reconstructions Tell Us
The NIPCC also claims that if the MWP was as hot as today (which it wasn't), that means that current global warming and climate change could be natural. It's true, hypothetically, the current warming could be natural, if there were a natural mechanism causing it. However, there is no such known mechanism. There is a measured energy imbalance caused by the increase in atmospheric greenhouse gases. We know that this energy must cause the planet to warm, and how much it warms depends on the climate sensitivity to the energy imbalance.
In fact, the hotter the MWP, the more sensitive the climate is to these energy imbalances. So arguing for a hot MWP is actually arguing that greenhouse gases must be causing significant global warming - the NIPCC has it exactly backwards.
Despite the different methodologies and data coverage used in Ljungqvist (2010), his reconstruction is consistent with previous peer-reviewed northern hemisphere temperature reconstructions, and like all previous peer-reviewed reconstructions, concludes that current temperatures are higher than the peak of the MWP. Claiming that the MWP was hotter than today is also counter-productive for "skeptics", because a hotter MWP means climate sensitivity is high.
Intermediate rebuttal written by dana1981
Update July 2015:
Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial
Last updated on 9 July 2015 by pattimer. View Archives