Summer Suboptimum

Summer Suboptimum
July 1, 2000

Like moths to flame, a few climate scientists and many enviropoliticos are irresistibly drawn to pronouncements of record high temperatures.

We have heard all too often reports that this particular day/month/season/year/decade/millennium is now officially the warmest in the last x thousand/million years. And whose fault do you think that is?

But one recent study has a somewhat different take on the issue. In the Journal of Geophysical Research, Russian scientists M. Naurzbaev and E. Vaganov reported on their reconstruction of a “superlong” tree-ring record from the Russian high arctic.

They studied tree growth from a total of 118 trees, 27 living and 91 dead, in the Taymir and Putoran regions well north of the Arctic Circle.

Though not exactly a vacation hotspot, the area boasts a wealth of well-preserved dead tree specimens in subfossil forms as well as preserved wood buried in bogs. It was these samples that allowed the scientists to complete an unusually long reconstruction of the region’s climate over 2,000 years.

After analyzing tree-ring width, the key step in this kind of study is comparing contemporary tree growth with observed temperatures.

Using tree rings, Naurzbaev and Vaganov were able to explain a sizeable percentage (from more than half to two-thirds) of the variation in both early summer and mean annual temperatures, as measured at the Khatanga meteorological station from 1933 to 1989. This explanation allows them to develop equations that can then reproduce past temperatures based on early tree-growth calculations.

It is apparent that the 20th-century warming is comparable to other warm periods between 300 and 600 A.D. and to the “Medieval Warm Period” of 900 to 1200 A.D, which incidentally contained half of the 10 warmest and 10 coldest years.

When examined over 20-year periods, three of the warmest regimes occurred between 900 and 1200 A.D., and only one transpired in the last century.

In their summary, the authors noted that:

“The warming in the middle of the twentieth century . . . has analogs in the past. So the warming at the border of the millennia shows a close amplitude and was longer. Historical evidence on the climate of this Medieval Warm Period says it was a larger climate warming than the present one [and that] temperature variations in high latitudes for the instrumental record (1850–1990) do not go far beyond limits of natural variations revealed during two millennia.”

This study provides a useful reminder that natural climate variations have been rather large, at least on regional levels—a fact we should bear in mind as we look at patterns of observed climate change and attempt to determine their source.


Reference

Naurzbaev, M.M. and E.A. Vaganov, 2000. Variation of early summer and annual temperature in east Taymir and Putoran (Siberia) over the last two millennia inferred from tree rings. Journal of Geophysical Research, 105, D6, 7317-7326.