Sergei Zimov

Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle

ABSTRACT Thawing permafrost and the resulting microbial decomposition of previously frozen organic carbon (C) is one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate. In this article we present an overview of the global permafrost C pool and of the processes that might transfer this C into the atmosphere, as well as the associated ecosystem changes that occur with thawing. We show that accounting for C stored deep in the permafrost more than doubles previous high-latitude inventory estimates, with this new estimate equivalent to twice the atmospheric C pool. The thawing of permafrost with warming occurs both gradually and catastrophically, exposing organic C to microbial decomposition. Other aspects of ecosystem dynamics can be altered by climate change along with thawing permafrost, such as growing season length, plant growth rates and species composition, and ecosystem energy exchange. However, these processes do not appear to be able to compensate for C release from thawing permafrost, making it likely that the net effect of widespread permafrost thawing will be a positive feedback to a warming climate.

Steppe-Tundra Transition: A Herbivore-Driven Biome Shift at the End of the Pleistocene

A simulation model, recent experiments, and the literature provide consistent evidence that megafauna extinctions caused by human hunting could have played as great a role as climate in shifting from a vegetation mosaic with abundant grass-dominated steppe to a mosaic dominated by moss tundra in Beringia at the end of the Pleistocene. General circulation models suggest that the Pleistocene environment of Beringia was colder than at the present with broadly similar wind patterns and precipitation but wetter soils. These and other observations suggest that the steppelike vegetation and dry soils of Beringia in the late Pleistocene were not a direct consequence of an arid macroclimate. Trampling and grazing by mammalian grazers in tundra cause a shift in dominance from mosses to grasses. Grasses reduce soil moisture more effectively than mosses through high rates of evapotranspiration. Results of a simulation model based on plant competition for water and light and plant sensitivity to grazers and nutrient supply predict that either of two vegetation types, grass-dominated steppe or moss-dominated tundra, could exist in Beringia under both current and Pleistocene climates. The model suggests that moss-dominated tundra is favored when grazing is reduced below levels that are in equilibrium with climate and vegetation. Together these results indicate that mammalian grazers have a sufficiently large effect on vegetation and soil moisture that their extinction could have contributed substantially to the shift from predominance of steppe to tundra at the Pleistocene-Holocene boundary. Our hypothesis suggests a mechanism by which the steppe ecosystem could be restored to portions of its former range. We also suggest that mammalian impacts on vegetation are sufficiently large that future vegetation cannot be predicted from climate scenarios without considering the role of mammals.

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release w ...

Expeditions to the Russian Arctic to Survey Black Carbon in Snow

Snow is the most reflective natural surface on Earth, with an albedo (the ratio of reflected to incident light) typically between 70% and 85%. Because the albedo of snow is so high, it can be reduced by small amounts of dark impurities. Black carbon (BC) in amounts of a few tens of parts per billion (ppb) can reduce the albedo by a few percent depending on the snow grain size [Warren and Wiscombe, 1985; Clarke and Noone, 1985]. An albedo reduction of a few percent is not detectable by eye and is below the accuracy of satellite observations. Nonetheless, such a reduction is significant for climate. For a typical incident solar flux of 240 watts per square meter at the snow surface in the Arctic during spring and summer, an albedo change of 1% modifies the absorbed energy flux by an amount comparable to current anthropogenic greenhouse gas forcing. As a result, higher levels of BC could cause the snow to melt sooner in the spring, uncovering darker underlying surfaces (tundra and sea ice) and resulting in a positive feedback on climate [Hansen and Nazarenko, 2004].