As I blogged about previously, the Arctic is very sensitive to climate change.  There are new estimates of Arctic temperatures over 2000 years that support a long term cooling trend in the Arctic that can be ascribed to precession, the eccentricity in the Earth’s orbit, but suggest that since roughly 1900 another effect has become more important.

The Earth’s orbit is very slightly elliptical, which means that the distance between the Earth and the Sun varies over the course of the year.   At the present time the Earth reaches the point in its orbit where it is closest to the Sun in early January. Conversely currently it reaches the point when it is most distant from the Sun in July. Every year these times shift 25 minutes, so the dates of the closest and furthest approach of the Earth to the Sun shift by about 1 day every 58 years. This is due to precession and the entire precession cycle takes about 21,000 years.  Over the last 7,000 years, the Earth’s closest approach to the Sun has moved from September to January which has reduced the intensity of sunlight reaching the Arctic in the summer.

An article just published in the current edition of Science (“Recent Warming Reverses Long-Term Arctic Cooling“, Darrell S. Kaufman et al., Science, Sep 4, 2009) used 2000 years of ice cores, tree rings, and lake sediments from 23 sites to estimate Arctic summer temperatures.  The article’s conclusion is that on average, the Arctic cooled at a rate of 0.2^oC per thousand years until about 1900, since when it has warmed much more rapidly, by about 1.2^oC.  The long term cooling of the Arctic over the the 1900 years analyzed by the study has been ascribed to the effect of precession, but the more rapid temperature change since 1900 is due to something else. (Image Science)