Not Alike

A reader has suggested that modern warming, and the “medieval warm period” (MWP), are alike. In fact he states:

What you need to show, to keep your position logical, is that there is some difference between MWP and present day warming other than the hypothesized cause. Because the hypothesized cause is what is up for examination.

You need to show that either in amount of warming, geographical spread or something else, the two are quite different. Otherwise, the argument similar effects similar causes is very strong.

I strongly disagree; the argument “similar effects similar causes” is very weak. But let’s compare modern warming to the MWP anyway.

Rather than examining the temperature reconstruction of Mann, Bradley, & Hughes (the classic “hockey stick,” which the reader suggests is untrustworthy) or the later reconstruction of Mann & Jones, I’ll look at the reconstruction of Moberg et al. (2005, Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data, Nature, 443, 613-617, doi:10.1038/nature03265), because it shows a much stronger MWP (and “little ice age,” or LIA) than any other reconstruction I’ve got. I guess that’s why they call it “highly variable.” Also, it conveniently spans the entire time period from year 1 to 1979, so we’ll have the medieval period “covered.”

Both the warmth of the MWP and coolness of the LIA are apparent. Moberg’s reconstruction is for northern hemisphere temperature only, and only goes to 1979, so to compare it to modern warming we’ll need more up-to-date northern hemisphere data. I’ll use data from GISS, but I can’t use the GISS data as is because its “reference period” (for computing temperature anomaly) is 1951 to 1980, while the Moberg reconstruction uses a reference period 1961 to 1990; that will make the GISS data seem too warm by comparison. But I can easily “cool” the GISS data by computing temperature anomaly relative to the reference period 1961 to 1990, and bring them onto the same scale:

We can already see a notable difference between modern warming and the MWP: modern temperatures are hotter. The warmest single year in the northern hemisphere is 2005, with temperature anomaly (relative to the 1961-1990 reference period) 0.90 deg.C, while the warmest single year in the Moberg reconstruction is 1105, at temperature anomaly 0.37 deg.C. So the warmest modern year is 0.53 deg.C hotter than the warmest medieval year, according to Moberg.

The eye suggests another possible difference: modern warming is steeper than medieval warming. The “modern global warming era” covers about the last 30 years. How does the modern warming rate compare to what was observed in medieval times?

I computed the rate of warming for every possible time span, from each year to 30 years later, in the Moberg reconstruction. The fastest warming occured from 867 to 897, at a rate of 0.0183 deg.C/yr. I also computed the rate for every similar-length time span in the thermometer record; the fastest warming is from 1976 to 2006 (which is the most recent, by the way, since 2007 isn’t over yet) at a rate of 0.0300 deg.C/yr. That’s quite a lot bigger than 0.0183; in fact it’s 64% bigger.

So, modern warming has reached temperatures in the northern hemisphere which are 0.53 deg.C hotter than any in medieval times, and the modern warming rate is 64% greater than the fastest rate in medieval times. The biggest 30-year warming trend in medieval times amounted to 0.55 deg.C, while modern times have seen 0.9 deg.C warming over the same time span (again, 64% bigger). These differences are especially significant, because the Moberg reconstruction gives us nearly 2000 years to work with, so there are a lot more time spans to test, hence a lot more opportunities for the data to reach a greater extreme. Yet despite the vastly greater number of data points to test in the Moberg record, the hottest temperatures are occuring right now, and the fastest warming (by far) is occuring now. The conclusion is unavoidable: modern warming is NOT like the medieval warm period. Not even close.

Even if they were similar, the argument “similar effects similar causes” is very weak. But they aren’t. What do you think of the argument “different effects different causes”?