Friday, February 20, 2009

Mammoth Threat from Methane Melt.

In terms of the geological record, we are presently in the Holocene epoch, the previous Pleistocene period having given-way around 10,000 years ago. This practically coincides with the end of the last ice-age, and massive flooding which is thought to have resulted in the creation of the Black Sea by a spill-over from a surging Mediterranean Sea. There is much speculation as to what exactly caused this transition, during which many of the earth ecosystems changed, and the region presently labelled as northern Siberia was highly populous in large animals like mammoths, woolly rhinoceroses, yaks and hippos. In contrast to other northerly areas, which were subject to repeated climatic assaults, this Siberian region was largely protected from the erosive advance and retreats of ice-sheets. What did occur, however, was the accumulation of silt, dust and the creation of ice-based tundra, which in some parts were mostly forest, and in others mainly mosses thrived.

Now, it has been assumed that it was climate change that reshaped the region, but a new perspective has been offered by Sergey Zimov, who is the director of the Northeast Science Station at Cherskii, which is located in the Republic of Sakha (Yakutia), leading to the demise of the large animals there. He turns the argument on its head, and reasons that they were hunted to extinction and it was the loss of the animals that resulted in the ecological transformation there. To test this notion, Zimov's team of researchers are reintroducing animals such as bison, horses, and eventually tigers, among other species in an effort to reconstitute the Pleistocene ecology. If it is successful, the strategy will support another example of the way human activities can impact on the environment. Rather than this being a castigation I do sometimes muse that maybe part of the reason that humans are on earth is to change it, as part of some grand design.

The northern ecosystem originally stretched from France and on across the Bering Strait to Canada and from islands in the Arctic over as far as China. It was big. Over the million year duration of the Pleistocene epoch, the ebb and flow of the ice-sheets ploughed the lands of most of northern Europe and North America, while northern Siberia remained largely untouched. At the same time that the herds of herbivores disappeared in their millions, the grasslands on which they depended also vanished. Holocene vegetation is dominated by moss and shrubs, in contrast to the Pleistocene megafauna, and has insufficient transpiration of moisture to dry-out the soil. When soil is saturated with water, the decomposition of biomass is inhibited and so are the nutrients required to underpin plant growth.

In contrast, when mosses are decomposed, the land becomes overgrown with grasses within a short time of perhaps one to two years. These grasses then dry-out the soil through rapid transpiration and create a steppe-like ecosystem. When herbivorous populations are low, the grass remains uneaten and it builds-up on the surface of the soil, thus shading it and reducing its fertility. Thus mosses and shrubs with relatively low water and nutrient needs become dominant. When the mammoths thrived, in the winter the animals ate the grasses from the previous summer's growth, and throughout the year they kept the soil productive with their dung and also trampled down mosses and shrubs thus preventing them from taking hold.

We now get to the interesting part in relation to climate change, which may be influenced by what happens in northern Siberia, in particular to its permafrost. In total, these frozen soils, with an average carbon-content of around 2.5%, encapsulate around 500 Gt of potentially mobile carbon. This amounts to two and a half times the carbon in all the world's rainforests combined. It is known that the tundra permafrosts are melting, and when the soil does thaw, microbes will begin to convert that long-held soil carbon into carbon dioxide (if there is oxygen present) or to methane under anoxic conditions. Methane would in principle be worse, since it has around 100x the global warming potential of CO2, and could cause a potential feedback mechanism where consequently rising temperatures melts more of the permafrost and releases more GW gases, at least so the argument goes.

Can the restoration of Pleistocene ecology, in which grasses and their root systems stabilise the soil, put the brakes on this process? Indeed, can stabilising the soil prevent the permafrost from melting? Probably not. Millions of herbivores surely also emit large amounts of methane, which must be offset against the gain in albedo of the earth from maintaining these frozen lands, which reflects solar radiation back into space. There are many factors involved in the global warming scenario, all of them interconnected. However, looking toward nature to find solutions to our energy and climate problems is I believe the way to go.

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