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Unexpected Evergreen Expansion In Siberian Forest During The Global Warming Hiatus

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Journal of Climate & University of Virginia

Unexpected evergreen expansion in the Siberian forest under warming hiatus

Abstract: Siberia has experienced a pronounced warming over the past several decades, which has induced an increase in the extent of evergreen conifer forest. However, the potential slowing of the trend of increasing surface air temperature (SAT) has produced intense debate since the late 1990s. During this warming hiatus, the Siberia region experienced a significant cooling during the winter season around ten times that of the North Hemisphere (NH) as a whole. This potentially stresses evergreen conifer forests because cooler winters can cause cold-temperature damage and, hence, increase the mortality in young evergreen conifer forests. In this study, the response of Siberian forest composition during the warming hiatus was investigated using satellite observations coupled with model simulations. Observations indicated that from 2001 to 2012, the apparent area of evergreen conifer forest has increased by 10%, while that of the deciduous conifer forest has decreased by 40%. The transition from deciduous to evergreen conifer forest usually occurs through mixed forest or woody savannas as a buffer. To verify the response of evergreen conifer forest, model experiments were performed using an individual-based forest model. Hysteresis of forest change seen in the model simulations indicates that changes in forest composition dynamics under temperature oscillations induced by internal climate variability may not reverse this composition change. As a result, the evergreen conifer forest expansion under climate warming is expected to be a continuing process despite the occurrence of a warming hiatus, exerting far-reaching implications for climate-change-induced albedo shifts in the Siberian forest.

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see also — Cold war-era spy satellite images reveal significant greening of Siberian Tundra

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A July 1966 U.S. spy satellite photo of Arctic tundra in the western Taymyr Peninsula and a July 2009 commercial satellite image of the same location shows considerable expansion of vegetation during the 43-year interval. Credit: University of Virginia

During the Cold War, the United States and the Soviet Union routinely spied on each other using high-altitude reconnaissance aircraft and space satellites.

After the dissolution of the Soviet Union, the U.S. declassified tens of thousands of images obtained from its two major spy satellite programs, Corona and Gambit. Many of these highly detailed photographs, taken from 1960 through 1984, are of the massive and relatively little-studied western Siberian tundra. The government was looking for military installations and nuclear arsenals, but it found mostly undeveloped, wild terrain.

It occurred to University of Virginia environmental scientists that the imagery is a storehouse of information for better understanding how vegetation in tundra regions may be altering as a result of climate change and other factors.

“These spy images are a gold mine as a reference point,” environmental sciences professor Howie Epstein said.

He oversaw a study comparing old spy photographs from 1960 into the 1980s with environmental images of the same terrain made in more recent years from commercial satellite sensors. “We are able to look at the exact same locations, in close detail, across several decades,” he said.

Epstein and his graduate student, Gerald Frost, who conducted the study as part of his Ph.D. dissertation, tracked 11 sites in Siberia through half a century of imagery, and were able to distinguish the expansion of tall shrubs such as alder, willow, birch and dwarf pine. They found that tall shrubs and trees had expanded their range in some areas by up to 26 percent since the 1960s, though the overall expansion was less dramatic.

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