March 17, 2010

Problems with the Permafrost?

Filed under: Arctic, Climate Changes

You’ve heard it a thousand times before – greenhouse gases are causing the Earth to warm, there is more warming in the Arctic than other parts of the planet, and the permafrost is melting away. Remind the world that permafrost holds carbon and methane that can be released into the atmosphere, throw in some pictures of a drunken forest (below), claim that the permafrost melting is some type of global warming time bomb, and you will be embraced by the global warming alarmists. Do a web search on the subject of global warming and permafrost melting for 1,000s of additional ideas.


Figure 1. “Drunken forest” undoubtedly caused by widespread melting of the permafrost?

We have covered the permafrost issue before, and over and over, this story seems to be far more complex than one might expect. A recent article in Global Change Biology is yet another addition to the complicated warming = melting of permafrost issue.

The article was produced four scientists with Wageningen University in The Netherlands, the University of Zurich, Switzerland, and the Russian Academy of Sciences, Siberian Division in Yakutsk.

Blok et al. start off explaining “Climate change has caused rapid environmental changes at northern high latitudes. Atmospheric warming is expected to continue in the future, especially in the Arctic region. Climate models predict a mean annual temperature rise of 5°C in the Arctic by the end of this century. A rise in temperature may have important consequences for the stability of permafrost soils, which are thought to store twice as much carbon as is currently present in the atmosphere. Siberian permafrost soils in particular contain a significant reservoir of easily decomposable organic carbon. Given that the decomposition of organic matter is largely controlled by permafrost conditions, there are fears that if the permafrost thaws, much of the carbon stored will be released to the atmosphere. Thawing permafrost might thus trigger important feedback effects between further climate change and soil carbon release.”

We’ve heard this all before.

Blok et al. then start throwing some doubt into the picture as they note “It is unclear how permafrost will respond to a warmer climate: a recent discovery of ancient permafrost that survived several warm geological periods suggests that vegetation cover may help protect permafrost from climate warming.” Furthermore, they remind us “However, higher air temperature does not necessarily lead to higher soil temperature: it has been demonstrated that increases in air temperature sometimes lead to vegetation changes that offset the effect of air warming on soil temperature.”

One of the expected changes in the Arctic is an expansion of dwarf birch (a.k.a., Betula nana, or more simply, B. nana); the plant grows to about three to four feet tall with shiny red-copper colored bark and leaves than are rounded with a bluntly toothed margin. The plants shade the ground, alter snow cover, and ultimately change land-surface properties that might protect permafrost from higher summer temperatures.


Figure 2. Dwarf birch (Betula nana or B. nana) common in the Arctic

Blok et al. headed to northeastern Siberia at a site where “Regional climate data (Chokurdakh airport weather station, 1999–2006) show mean annual air temperatures of -10.5°C and average July temperatures of 10.4°C.” In case you cannot think in degrees Celsius, -10.5°C for average annual temperature equates to 13°F – their study site is far from paradise! They explain “We selected circular plots of 10m diameter, located in the two different sites. In total, there were 20 plots: 10 plots per site. The two sites were chosen because of their difference in relative cover of plant functional types; together the two sites cover most of the terrain types in the area.”

To the bottom line we go! As seen in the figure below, they found that as the plots were covered by more and more dwarf birch, the active layer thickness decreased. The active layer is the not-frozen (in summer) soil layer above the permafrost, and as seen in a different light, the plot shows that the permafrost is thicker in plots with greater coverage of dwarf birch.

Figure 3. Active layer thickness (ALT) plotted against Betula nana cover for control plots at the two sites in 2007 and 2008 (from Blok et al., 2010).

Blok et al. comment “However, under multiple scenarios of climate change it is expected that tundra biomass will increase, mainly because of B. nana and combined with the observed negative relationship in natural vegetation, our experimental results suggest that increased shrub biomass may slow down the expected future increase in permafrost thaw with climate warming.” Furthermore, “Similar findings were observed in a model study, where permafrost thaw was found to be less under a shrub canopy than under unvegetated ground.”

Next up, Blok et al. note “Global temperature data show that the mean annual air temperature in northeast Siberia increased by 1.5–2°C between 2001 and 2007, compared with the 1951–1980 average. This is much higher than the observed 0.5°C average global surface temperature rise during this period. Permafrost temperature records, however, do not show a general warming trend during the last decade, despite large increases in surface air temperature. Data from several Siberian Arctic permafrost stations do not show a discernible trend between 1991 and 2000. Our results suggest that an expansion of deciduous shrubs in the Arctic triggered by climate warming may buffer permafrost from warming resulting from higher air temperatures.”

Next, we learn “Failure to fully understand the effect of climate change and related vegetation shifts on permafrost thermodynamics is hampering predictions on future permafrost thaw. We have presented the first experimental evidence that the expansion of deciduous shrubs in the Arctic triggered by climate warming may reduce summer permafrost thaw. This vegetation change may partly offset the permafrost degradation expected to result from the air temperature rise predicted for the coming decades.”

Blok et al. conclude “These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.”

Enough said!

Reference:

Blok, D., M.M.P.D. Heijmans, G. Schaepman-Strub, A.V. Kononov, T.C. Maximov, and F. Berendse. 2010. Shrub expansion may reduce summer permafrost thaw in Siberian tundra. Global Change Biology, 16, 1296–1305.




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