Permafrost melts and Earth heats even faster

About 20% of our Earth's land surface is frozen all year long, a layer of ice and plant material tens of meters thick called permafrost. Or it was permanently frozen, until recently. The Arctic region is heating up at about twice the rate of the equatorial regions, so the southernmost, upper layers of permafrost have begun thawing. As this ice melts, the ground subsides, and often shallow lakes fill up the depressions with water from the melted ice (photo above). Bacteria then start to decompose the plant material.

But these bacteria are mostly at the bottom of lakes, where there is little oxygen. Because of the anaerobic conditions, they can only partly decompose the plant and animal remains they are eating. The decomposition, instead of going all the way to produce water and carbon dioxide, stops half way and produces methane instead. The methane gas, also known as "swamp gas," bubbles to the surface and escapes into the atmosphere.

This is a problem. Methane is 8 times more powerful a greenhouse gas than carbon dioxide. If enough methane escapes to affect the greenhouse warming, our planet heats even more. More methane is then released, and so it goes, into a runaway process. As discussed in the previous section, the permafrost melting constitutes a "tipping element." The global temperature at which an irreversible runaway starts is called the "tipping point" temperature.

It is not certain what the tipping temperature is for the permafrost system. We may have already passed it, or it could be 1 or 2 degrees higher than at present. In the previous section, it was noted that 1/3 to 1/2 of the permafrost was estimated to be within 1 to 1.5 degrees of melting at the present time. This fact suggests that a reasonable estimate would place the permafrost tipping temperature at about 0.75 degrees above the present mean global temperature. This is a conservative estimate, given that most models predict that the Arctic temperature will rise at about twice the rate of the mean.

The time that the transition will take depends on several factors. Even after it reaches zero degrees Centigrade, it takes heat to thaw ice. Once the process begins, the heat transfer from sun and air to permafrost layers is complicated by the growing plants and lakes dotting the landscape. It has been estimated by V. Alexeev that, including all the factors, the thawing permafrost could contribute an additional 0.32 degrees Centigrade to the global temperature by 2100.

Such estimates agree well with the measured ground temperatures as shown above. Note that, except for a brief period near 1999, the trend has been steadily upward in recent years. The horizontal dotted line is at zero degrees Centigrade, the melting point for ice.

The graph on the right shows the expected global temperature with the permafrost tipping element added. This is the same graph as was used to extrapolate the global temperature previously, except that now the red line shows the additional heating due to permafrost thawing. The tipping point for this element, relative to the 1988-99 average, is taken to be about 1 degree Centigrade. This may seem low, but remember that this refers to the increase in the global mean temperature, and most models predict that the Arctic temperature rise will be about twice that of the mean.

This graph suggests that the tipping point will occur about 2030, only 20 years from now. The increase shown in this figure is schematic, and not intended as a prediction of the actual magnitude of the temperature rise. No matter the magnitude of the rise, however, with the transition fully underway by 2050 the main effect is seen to be a significant increase in the rate of rise of the global temperature -- and the expected rate is already mind-boggling enough.

Recent observations by an international scientific team suggest that the problem may actually be worse than previously thought. A large section of the Arctic Ocean sea floor is showing signs of instability and is already venting vast quantities of methane. The current rate is equal to that coming out of all the rest of the world's oceans. Any increase in the Arctic Ocean temperatures will only increase the outflow. In combination with the loss of summer sea ice, these findings suggest that methane release from the Arctic may reach a tipping point well before 2050.

Finally, the forecast: Earth in 2050.

DETAILS

The photo at top (from treehugger.com.) shows a view of "thermokarst" lakes, illustrating the huge number of shallow lakes which form as permafrost melts.

The graph of temperature at different depths versus time is from the Green Institute website.

Estimates of permafrost melting rates are from the article by K. W. Anthony referenced in the previous section.

Recent measurements of melting of the permafrost on Arctic seafloor was reported in Science Daily, March 5, 2010.

Current estimates are that the total carbon released from Arctic permafrost melting and from the Arctic ocean bottom could be up to 25% of the total of Earth's carbon sink.