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History of Climate Change in the Arctic |
[Last Glacial Maximum] [Dansgaard-Oeschger] [Younger Dryas] [Holocene] [Potential Future] Introduction As argued by Terry V. Callaghan, et al, in Changes in Arctic Terrestrial Ecosystems, Climate and UV Radiation, this group likewise shares the opinion that to understand the potential future impact of global warming on the Arctic region, the long term history of the region for must be examined. This section will assess the last 20,000 years of climate change in the Arctic environment covering the late-Quaternary period which extends back to the last glacial maximum and includes the Holocene period, or the last 11,400 years. In order to place global warming in the right context, historical and longer-term climate change events must be analyzed. Comparisons with historical trends examining the rates and magnitudes of historical climate as estimated by evidence from paleoclimatology records is therefore vital to the discourse on global warming. Evidence will be presented that indicates that the climate changes of the upcoming century in the Arctic are likely to be larger than the maximum changes of the Holocene, and further the regional rate of warming is expected to equal the most rapid warming of the late-Quaternary. The section will primarily present a historical viewpoint; however it will also discuss the estimated unprecedented level of the mean annual temperature in the Arctic, and conclude that climate models including anthropogenic forcings suggest that the future of the Arctic will include climate changes that are unprecedented in thousands of years of history. Last Glacial Maximum The Last Glacial Maximum (LGM), which lasted from about 24,000 to 21,000 years ago, was the period in which global ice extent was at its greatest (Arctic Climate Impact Assessment, 2004). Cores from LGM Arctic glacial ice sheets provide evidence that at this period average annual temperatures were 10-13°C colder than in the Holocene (Callaghan et al, 2004). Following the LGM, the most extreme transition on the geological record occurred from full glacial to full interglacial conditions, and by 10,000 years before present, the Arctic had temperatures comparable to the present (ACIA, 2004).
Dansgaard-Oeschger Dansgaard-Oeschger events, which are rapid climate fluctuations that marked the last glaciation, may have reached temperature oscillations with a range as high as 16°C. Evidence found in Greenland ice cores suggest that complete regional climate changes occurred in a few decades. A close relationship has been found between the thermohaline circulation and the rapid climate changes of the last glaciation, and evidence suggests that large-scale, global feedbacks were vital to the rapid temperature variability of D-O events (ACIA, 2004).
Ice cores from Greenland shows that in the distant past changes to the Conveyor circulation were associated with abrupt climate change. Younger Dryas The Younger Dryas was a brief climate period roughly 13,000 years before present in which a sudden turnaround from interglacial warming took place in less than 100 years. Mean annual temperatures drastically fell as most of Europe was 4-6°C cooler than now, and 10-12°C colder in the northern North Atlantic and Norwegian Sea. About 11,000 years ago, the Younger Dryas ended abruptly, with some regions increasing in temperature at a rate of more than 5°C in less than 100 years (Callaghan et al, 2004). The ACIA even suggests that much of the Northern Hemisphere could have seen temperature increases at a rate of 10°C per every 50 years. Holocene The Arctic region experienced warmer summers by about 1-2°C than the present in the early to mid-Holocene. After the general thermal maximum, the second half of the Holocene underwent a general trend of cooling culminating with the Little Ice Age (Overpeck et al, 1997). The Little Ice Age was a period that lasted from approximately the late 13th to 19th centuries in which, the most extreme periods saw the fall of mean annual temperatures by several degrees in some Artic regions (Callaghan et al, 2004). The general long term trend of the last millennium for the Northern Hemisphere is a “modest and irregular cooling from AD 1000 to around 1850 to 1900, followed by an abrupt 20th century cooling” (ACIA, 2004). With evidence of glaciers reaching post-Wisconsinan positions, the period from 400 to 100 years before present in the Arctic underwent a particularly cold period prior to the 20th century warming. Potential Future From this history, then, it becomes clear that natural climate variability over the last 20,000 years in the Arctic has been very active. The Arctic climate has had a tendency for rapid climate change, including drastic temperature increases in merely decades. Dating back to as early as the early Holocene, the warming of the second half of the twentieth century is without precedent. Predictions of future climate change for the next century suggest that the magnitude of change between full glacial conditions and present conditions will at minimum be comparable. In fact, the magnitude will likely be larger than the maximum changes previously seen in the Holocene. In addition, global increase in mean temperature is likely to happen at rates higher than those of the last deglaciation, and regionally, the rate of warming could eclipse even the most rapid warming of the late-Quartenary. Mean annual temperatures in the Arctic are also expected to reach unprecedented levels, and tundra coverage is likely to be lower than any time during the late-Quartenary (Callaghan et al, 2004). The coupling of the already seen intense natural variability with increasing anthropogenic forcings will provide unprecedented changes to the Arctic, which in turn is likely to have significant impacts on the larger global climate system.
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