(11 am. – promoted by ek hornbeck)
The BBC has the summary:
Arctic temperatures are now higher than at any time in the last 2,000 years, research reveals.
Changes to the Earth’s orbit drove centuries of cooling, but temperatures rose fast in the last 100 years as human greenhouse gas emissions rose.
Scientists took evidence from ice cores, tree rings and lake sediments.
Writing in the journal Science, they say this confirms that the Arctic is very sensitive both to changes in solar heating and to greenhouse warming.
The full article is firewalled at Science, but it’s long enough to allow some significant quotes. The lead author is Darrell S. Kaufman of the School of Earth Sciences and Environmental Sustainability at Northern Arizona University.
The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.
By “orbitally driven reduction in summer isolation”, they mean the Milankovitch cycles, which are described by the National Climatic Data Center:
The Milankovitch or astronomical theory of climate change is an explanation for changes in the seasons which result from changes in the earth’s orbit around the sun. The theory is named for Serbian astronomer Milutin Milankovitch, who calculated the slow changes in the earth’s orbit by careful measurements of the position of the stars, and through equations using the gravitational pull of other planets and stars. He determined that the earth “wobbles” in its orbit. The earth’s “tilt” is what causes seasons, and changes in the tilt of the earth change the strength of the seasons. The seasons can also be accentuated or modified by the eccentricity (degree of roundness) of the orbital path around the sun, and the precession effect, the position of the solstices in the annual orbit.
Using these three orbital variations, Milankovitch was able to formulate a comprehensive mathematical model that calculated latitudinal differences in insolation and the corresponding surface temperature for 600,000 years prior to the year 1800. He then attempted to correlate these changes with the growth and retreat of the Ice Ages….
But, for about 50 years, Milankovitch’s theory was largely ignored. Then, in 1976, a study published in the journal Science examined deep-sea sediment cores and found that Milankovitch’s theory did in fact correspond to periods of climate change (Hays et al. 1976). Specifically, the authors were able to extract the record of temperature change going back 450,000 years and found that major variations in climate were closely associated with changes in the geometry (eccentricity, obliquity, and precession) of Earth’s orbit. Indeed, ice ages had occurred when the Earth was going through different stages of orbital variation.
Since this study, the National Research Council of the U.S. National Academy of Sciences has embraced the Milankovitch Cycle model.
Kaufman and his six co-authors describe their methodology, and this is an excerpt:
This study places the warming of the instrumental period against the backdrop of the past 2000 years, well beyond the 400-year scope of the last Arctic-wide synthesis of high-resolution paleoclimate data. Our synthesis is based on a new compilation of proxy records from Arctic lakes, combined with complementary ice core and tree ring records, to form a new 2000-year-long, decadally resolved paleoclimate reconstruction for the Arctic. Lakes are distributed across the Arctic, and they contain the most accessible proxy records that consistently extend through the late Holocene. The synthesis is restricted to records longer than 1000 years because we aim to explore the long-term pattern of temperature variability at decadal scale. These records extend beyond the most recent (preindustrial) major climate perturbation-the Little Ice Age-when most of the Arctic experienced the coldest sustained temperatures of the past 8000 years. In some locations, warm intervals before the Little Ice Age have been recognized during the early part of the period from 2000 to 1000 years ago, as well as during the Middle Ages. The spatial coherence of the warming during these intervals is not yet clear, but this is critical for understanding the underlying causes of change. Climate change is amplified in the Arctic, and warming during these historical intervals might be more reliably detected where the temperature change exceeds the sensitivity limits of the proxies.
The conclusions are much simpler to describe:
Among the most striking features of our composite temperature reconstruction is a cooling from 1 C.E. to 1900 C.E.
Strong warming in the 20th century contrasts sharply with the preceding cooling trend. An Arctic summer temperature of
0.5°C (relative to the period 19611990) might have been expected by the mid-20th century on the basis of a simple forward projection of the linear trend in the proxy data for the period from 1 C.E. to 1900 C.E. Instead, our reconstruction indicates that temperatures increased to +0.2°C by 1950. This shift correlates with the rise in global average temperature, which coincided with the onset of major anthropogenic changes in global atmospheric composition, the absence of major volcanic eruptions, and changes in solar irradiance.
And the bottom line?
The warming during the 20th century (and first decade of the 21st century) contrasts sharply with the millennial-scale cooling, with the last half-century being the warmest of the past two millennia. Our synthesis, together with the instrumental record, suggests that the most recent 10-year interval (1999-2008) was the warmest of the past 200 decades. Temperatures were about 1.4°C higher than the projected value based on the linear cooling trend and were even more anomalous than previously documented.
Once again, the most striking result is that climate impacts are even more intense than had been predicted or expected. As the BBC reported in March:
The worst-case scenarios on climate change envisaged by the UN two years ago are already being realised, say scientists at an international meeting.
In a statement in Copenhagen on their six key messages to political leaders, they say there is a increasing risk of abrupt or irreversible climate shifts.
Even modest temperature rises will affect millions of people, particularly in the developing world, they warn.
But, they say, most tools needed to cut carbon dioxide emissions already exist.
If it’s not already too late, December may be the last chance for the world’s governments to get it right.