Open Mind

The temperature history of the earth as reconstructed by thermometers only goes back a little over a century. We’d like to have global temperature history back many hundreds or even thousands of years. We can’t rely on a single location, we can’t rely on a single century, and we can’t rely on thermometers. We have to use other data which help us estimate temperature, but aren’t direct temperature measurements. Such data are called proxy data.

There are lots of different proxies for temperature. One of the most useful is tree rings; their width and their density both give clues about temperature. Ice cores and sediment cores can be analyzed to help reconstruct temperature. Borehole temperatures give us terrific information about the past several centuries of temperature change. Studying the advance and retreat of glaciers also provides a proxy for temperature. Historical records can be very useful; even without thermometers, people noted the comings and goings of heat waves and cold spells, and the freezing and melting of lakes and rivers.

Multi-Proxy Reconstructions

No single proxy is accurate enough to paint the overall earth temperature history, and no single location can be considered indicative of the earth as a whole. The logical thing to do is to reconstruct temperature not from a single proxy, but to use all the available information in a multi-proxy reconstruction to estimate how earth’s temperature has varied over the globe in the past. The geographic distribution of temperature is averaged to estimate regional, or even global, average temperature anomaly.

This is exactly what has been done, and I’d like to show you a reconstruction of earth’s global average temperature for the last 1800 years. It’s a multi-proxy reconstruction by two researchers named Mann and Jones (Mann, M.E., and Jones, P.D. 2003, Geophysical Research Letters, Vol. 30, No. 15, pg. 1820), covering the years 200 to 1980, in figure 1 (click on the graph for a larger view).

Figure 1. Reconstructed global average temperature, A.D. 200
to 1980.

There are lots of ups and downs. To get a better picture of the long-term changes, I’ll slow down the fluctuations in two ways. First, I’ll compute 20-year averages, not the smoothest way to isolate long-term changes, but one of the simplest and most reliable. I’ll also compute a smooth-fit curve using a wavelet fit on a 20-year timescale. Figure 2 (wavelet fit in green, 20-year averages in blue) shows that both methods give essentially the same result:

Figure 2. 20-year averages and wavelet fit.

Now we can see that the recent part (up to 1980) has shown a faster sustained rise than any comparable period in the last 1800 years. Also, the most recent 20-year average temperature estimate is hotter than any other.

But it’s not a lot hotter than 20-year average temperatures in the past, and the rate hasn’t been sustained for that long or gone that far. And the hottest individual year in the Mann and Jones reconstruction is 671, over 1300 years ago! This graph alone is suggestive, but not impressive; we should be concerned, but not alarmed.

That’s because this graph only goes up to 1980. That leaves out the last 25 years, the most interesting part. The reconstruction by Mann and Jones doesn’t cover the modern period, but thermometers do, so I’ll add the actual measured annual average global temperature anomaly from 1980 to the present. Figure 3 shows the same data, plus the HADCRU thermometer data as little red squares (I had to expand the temperature axis to make the new data fit).

Figure 3. Additional data (thermometers) from 1980 to 2005.

That’s what this global warming fuss is all about.

This is a good illustration of one way to be deceptive about past temperatures: leave out the data that disagrees with your desired conclusion! I recently encountered a lengthy document prepared to dispute the reality of global warming. It contains a graph illustrating a number of different temperature reconstructions:

Figure 4. Temperature reconstructions as displayed by Monckton.

Note that the caption indicates that “In three of the studies (Esper, Briffa, and Moberg) the mediaeval warm period is shown to have been as warm as, or warmer than, the current warm period.” This is because most of the temperature reconstructions only go as far as about 1980. This leaves out the hottest part: 1980 to the present! When we include the last 25 years, it’s evident that according to all reconstructions, the mediaeval warm period is not as hot as it is today (there’s more about this piece here)

Hemispheric Reconstructions

The Mann & Jones estimate isn’t the first multi-proxy reconstruction of past temperature. One of the “breakthrough” papers on this topic was a 600-year reconstruction, for the northern hemisphere only (which is not the whole globe, but is a rather good proxy for it), by Mann, Bradley, and Hughes (Mann, M.E., Bradley, R.S., and Hughes, M.K. 1998, Nature, vol. 392, pg. 779). It features a rather cluttered and hard-to-read graph (figure 5). Even with the clutter, it’s plain that for the last 600 years in the northern hemisphere, temperatures were reasonably stable until today.

Figure 5. 600-year reconstruction of Mann et al. 1998.

By the time the Intergovernmental Panel on Climate Change (IPCC) published its third assessment report (TAR) in 2001, the same three researchers had extended the northern hemisphere reconstruction back 1,000 years. Figure 6 reproduces the version published in the IPCC TAR (from Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999, Geophysical Research Letters, vol. 26, pp. 759-762). The shape of earth’s temperature history somewhat resembles a hockey stick, with the long slow decline the handle and the recent sharp rise the blade. Therefore this graph has been dubbed the “hockey stick.” This particular chart is probably the most famous graph in all of climate science.

Figure 6. The hockey stick: from IPCC Third Assessment Report.

Most climate reconstructions cover only the northern hemisphere; because there’s far less data for the southern hemisphere than for the northern, hemispheric reconstructions are more reliable than global. In general, the northern hemisphere tends to show more temperature variation than the globe as a whole; because the oceans have so much “thermal inertia,” and the southern hemisphere has far more ocean than the northern, southern hemisphere temperatures respond more slowly to changes than northern.

Let’s take a look at two such reconstructions, which represent the range of results found so far, looking for variation in past temperature. One is by Mann & Jones (Mann, M.E., and Jones, P.D. 2003, Geophysical Research Letters, Vol. 30, No. 15, pg. 1820), covering the years 200 to 1980; the other is from Moberg et al. (Moberg, A., D.M. Sonechkin, K. Holmgren, N.M. Datsenko and W. KarlĂ©n, 2005, Nature, Vol. 433, No. 7026, pp. 613-617) for the years 1 to 1980. In figure 7 I’ve plotted 10-year averages for both reconstructions.

Figure 7. Northern hemisphere temperature reconstructions.

Clearly the reconstruction by Moberg et al. shows much more past variation than that by Mann & Jones. In the Moberg et al. series we can plainly see a warm period around the year 1000 which is called the “medeival warm period.” It’s a matter of dispute whether this period was a global phenomenon, or confined to the northern hemisphere. We can also see that there was a cooler period from about 1400 to about 1800, that is often called the “little ice age.”

Both these reconstructions end in 1980, so figure 8 shows both reconstructions together with thermometer data (for the northern hemisphere) since 1980. Whichever reconstruction turns out to be more correct, the modern period is still hotter, and shows faster temperature increase, than any time during at least the last two thousand years.

Figure 8. Northern hemisphere temperature reconstructions
together with thermometer data since 1980.

The original hockey stick and its successors have incited great effort on the part of global warming denialists, to discredit them. But all the efforts to debunk the proxy reconstructions, have themselves been debunked. And, many other investigators have estimated proxy reconstructions of earth’s temperature history, both hemispherically and globally, on scales of a thousand years or more. The details always differ, but the bottom line is always the same: that sharp rise on the far right-hand side.

In fact, the National Academy of Sciences has issued a report on the state of the science of temperature estimates from proxy reconstruction. They urge caution interpreting results, and estimating the error ranges of past numbers, but their bottom line is that cutting-edge proxy reconstructions are most likely a correct reflection of the fact that temperatures now are higher than they’ve been in a very long time.