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Saturday, January 29, 2011

Temperatures of North Atlantic “are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming”

The 3.5 °F warming of Fram Strait water over the past century is "not just the latest in a series of natural multidecadal oscillations."

by Joseph Romm, Climate Progress, January 27, 2011

Study after study finds recent warming is unprecedented in magnitude and speed and cause.  The anti-science crowd keeps trying to debunk one or two old Hockey Sticks, but new ones crop up faster than a speeding puck.

I have pulled out the key graph — and it is one heck of a Hockey Stick.  It is derived from “planktic foraminifers in a sediment core”:
Temperature reconstructions of upper Atlantic Water in the eastern Fram Strait over the past ~2100 years

AW hockey top
AW hockey bottom
Thin lines are raw data, bold lines are three-point running means….  (C) Summer temperatures at 50-m water depth (red)….  Gray bars mark averages until 1835 CE and 1890 to 2007 CE. Blue line is the normalized Atlantic Water core temperature (AWCT) record … from the Arctic Ocean (1895 to 2002; 6-year averages)….  (D) Summer temperatures (purple) [calculated with a different method]
This astonishing warming in the past century is clearly not, as the anti-science crowd likes to say, some sort of recovery from the so-called Little Ice Age (see “A detailed look at the Little Ice Age“), which, in any case, is barely noticeable in this data.   The lead author, Robert Spielhagen of the Leibniz Institute of Marine Sciences, said, “Such a warming of the Atlantic water in the Fram Strait is significantly different from all climate variations in the last 2,000 years.”  The fact is, over 90% of human-caused warming is going into the oceans — and it is melting ice wherever it goes (see “Deep ocean heat is rapidly melting Antarctic ice“).
Air temperatures in Greenland have risen roughly 7 °F in the past several decades, thought to be due primarily to an increase in Earth’s greenhouse gases, according to CU-Boulder scientists. 
“We must assume that the accelerated decrease of the Arctic sea ice cover and the warming of the ocean and atmosphere of the Arctic measured in recent decades are in part related to an increased heat transfer from the Atlantic,” said Spielhagen.
Here are the abstract and conclusion:
The Arctic is responding more rapidly to global warming than most other areas on our planet. Northward-flowing Atlantic Water [AW] is the major means of heat advection toward the Arctic and strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ~150 years. Here, we present a multidecadal-scale record of ocean temperature variations during the past 2000 years, derived from marine sediments off Western Svalbard (79 °N). We find that early–21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming... 
Although we cannot quantify from our data the variability of previous AW inflow to the Arctic by volume, our temperature data series and the above observational link suggest that the modern warm AW inflow (averaged over two to three decades) is anomalous and unique in the past 2000 years and not just the latest in a series of natural multidecadal oscillations. Both effects — a temperature rise as well as a volume transport increase — introduce a larger heat input into the Arctic Ocean. Although there is no direct contact of the AAWL [Arctic Atlantic Water Layer] with the ocean surface in the Arctic, such an increased heat input has far-reaching consequences. The strong AW warming event in the Arctic Ocean in the 1990s caused a shoaling of the AW core and an enhanced heat flux to the surface , concurrent with decreasing sea ice. Recent oceanographic data from the Laptev Sea continental margin indicate the impact of warm AW-related water masses on the shallow (<50 m) shelf, a feature not observed before in a >80-year time series. The data also provide evidence for a significant heat flux to the overlying shelf waters. Even without any modification of the vertical heat transfer processes, the enhanced temperature contrast between the AW and the surface sea water freezing point (increased from ~5 to 7 K as identified here) leads to an increase in the vertical heat flux of ~40%. Any positive-feedback mechanism will magnify the effect of this flux increase on the ice cover. Complementing the strong feedback between ice and atmospheric temperatures, warming of the AW layer, unprecedented in the past 2000 years, is most likely another key element in the transition toward a future ice-free Arctic Ocean.
In September 2010, a first-of-its-kind analysis by an international team of 18 top scientists found “less ice covers the Arctic today than at any time in recent geologic history” and this ice loss is unexplainable by any of the known natural variabilities.”

In November 2010, Rear Admiral David Titley, the Oceanographer of the Navy and the Director of Navy’s Task Force Climate Change, told Congress, “the volume of ice as of last September has never been lower…in the last several thousand years.”

Arctic sea ice is in the last legs of its death spiral.

Using different data proxies for Arctic temperature itself (rather than the water entering the Arctic), the National Center for Atmospheric Research (NCAR), came to a roughly similar conclusion two years ago (see Human-caused Arctic warming overtakes 2,000 years of natural cooling, “seminal” study finds):
Arctic temperatures in the 1990s reached their warmest level of any decade in at least 2,000 years, new research indicates. The study, which incorporates geologic records and computer simulations, provides new evidence that the Arctic would be cooling if not for greenhouse gas emissions that are overpowering natural climate patterns.

As with a pride of lions, and a delusion of disinformers, perhaps the grouping should get its own name, like “a team of hockey sticks” (see “The Curious Case of the Hockey Stick that Didn’t Disappear“).
  1. GRL:  “We conclude that the 20th century warming of the incoming intermediate North Atlantic water has had no equivalent during the last thousand years.“
  2. JGR:  “The last decades of the past millennium are characterized again by warm temperatures that seem to be unprecedented in the context of the last 1600 years.”
  3. The Geophysical Research Letters paper, “Twentieth century warming in deep waters of the Gulf of St. Lawrence: A unique feature of the last millennium” concludes: “irrespective of the precise mechanisms responsible for the temperature variations reconstructed from core MD99‐2220, it is unquestionable that the last century has been marked there by a warming trend having no equivalent over the last millennium.”
The bottom line is that:

The rate of human-driven warming in the last century has exceeded the rate of the underlying natural trend by more than a factor of 10, possibly much more.  And warming this century on our current path of unrestricted greenhouse gas emissions is projected to cause a rate of warming that is another factor of 5 or more greater than that of the last century.  We are punching the climate beast — and she ain’t happy about it!


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