Bylot Ice Cap on Bylot Island, one of the Canadian Arctic Islands, August 14, 1975 (USGS)
The Arctic: Cooling No More.
A group of climatologists at Northern Arizona University are confirming that 2000 years ago, the Earth’s Arctic region had already entered a prolonged cooling phase. The phase continued up through the Middle Ages and on past the so-called Little Ice Age (1400 – 1800 C.E.). However, that all started to change (in the positive direction) between 1850 and 1900 C.E.–roughly in parallel with the onset and rise of the Industrial Revolution in the U.S. and Europe. And, by 1950, the warming trend had picked up in earnest.
The results of their 2000-year reconstruction of Arctic temperatures also showed clearly that four of the five warmest decades occurred in the period between 1950 and 2000. This buttresses the mounting evidence (such as that from the International Polar Year studies) of recent climate change and would suggest some newer mechanism at work impacting global temperatures.
In the published study by Darrell S. Kaufman et al (Recent Warming Reverses Long-Term Arctic Cooling, Science, 4 Sept. 2009)*, the researchers also note that their findings were in agreement with another 2000-year “transient climate” simulation using the Community Climate System Model (CCS3).
The reconstruction and the CCS3 Model show the same “temperature sensitivity” to changes in a factor known as insolation (stands for: incident solar radiation) which is a measure of the amount of solar radiation received on a given surface area at a given time (usually measured in Watts/m² or kW/m², per day). The researchers believe this congruence lends support to the hypothesis that this cooling trend was caused by an “orbitally-driven reduction in summer insolation.” This refers to the Earth’s precession of solstices as it moves through its elliptical orbit around the sun (precession refers to the variation, relative to the fixed stars, in the way the polar axis “wobbles” as the Earth rotates, see: Milankovitch cycles). Strangely, orbitally-driven insolation, according to the climatologists, continues to decrease. And yet, the expected summer cooling in the Arctic has not happened; instead we have warming. This suggests that the cause of the recent warming is not orbitally-driven, but the result of some other cause, or set of causes or climate feedbacks (such as that resulting from loss of summer albedo, via ice loss, in the Arctic).
Scientists reconstructed their Arctic temperature model through using what are known as “proxy records” which contain naturally preserved indicators of likely temperatures (and which are necessary since no one was recording Arctic temperatures two thousand years ago). The three main proxy records used here were: ice cores (from Greenland glaciers), lake bottom sediments, and tree rings (whose width varies along with seasonal temperature). These proxies–sampled from locations 60° N or higher–are used as paleoclimate “archives” and give scientists the best possible glimpse into our planet’s climatological past. Using three proxies instead of one or two gives them a more accurate picture of the paleoclimate, as, for example, only three of the tree ring records used extended back farther that 750 C.E. But all proxy records used in their data analysis and model synthesis covered 1000 years or more.
Tree growth bands (“rings”) are an important proxy record for temperature variations
Despite the usefulness of these paleoclimate proxies, each has its own “sensitivity limit” and only understanding these limits will help climatologists arrive at a more complete understanding of why and how, for example, shorter, warming intervals occur (such as the period known as the Medieval Warming Period, 1000 CE – 1380 CE). Further, these records (such as the radioisotope records found in lake sediment cores) do not necessarily resolve (or cycle through) definitively in the course of a year, thus, the scientists used a ten-year scale (decadal) to minimize the influence of “small age uncertainties” on their reconstruction.
Evidence for the earlier cooling trend may be responsible for recent claims by climate skeptics that we are in a “cooling period”, not a warming one.
Retreat of the Helheim Glacier, Greenland ANSA
According to the NAU researchers, the warming that occurred in the 20th Century and early 21st Century, “contrasts sharply” with the millennial-scale cooling trend; the last half-century being the warmest in the record, and, the decade from 1999 – 2008 being the warmest decade of the last 200 decades. Over all average temperatures were 1.4° C (about 2.5° F) warmer than the projected value based on the linear cooling trend.
In general, whatever climate influences exist on the Earth (such as in the tropical zones), are amplified in the Arctic, which in turn impact the global climate system and its many climate “drivers”. Thus the Arctic is perhaps the best place to study the short and long-term trends of our planet’s dynamic climate system.
Supporting On-line Material (PDF) for this paper can be found here: SOM / Arctic Cooling Reversal