Bending of shrubs could attenuate effect of shrub expansion on climate change

The Arctic: The “refrigerator” of the Earth.

The temperature at the Earth's surface depends upon many factors but one of the most critical one is the amount of solar radiation that is reflected back to the atmosphere rather than being absorbed at the surface. The amount of reflected radiation depends upon the colour of the receiving material: the darker the object is, the more radiation is absorbed and the hotter the object is (we all know that wearing a white T-shirt on a hot summer's day will keep us cooler than wearing a black T-shirt of the same material). This process, called the "albedo" of a surface, is important at high latitudes where much of the surface is covered by snow. Fresh snow reflects on average 85% of the solar radiation reaching the surface, therefore keeping the Earth cool. However, when vegetation, of which albedo can be just 10%, protrudes above the snow, the albedo of the area as a whole will depend on both vegetation and snow albedo. As a consequence, increasing vegetation cover in the Arctic can directly contribute to global warming by lowering the albedo of what is sometimes called the “refrigerator” of the Earth.

Shrub expansion warming the “refrigerator”?

Recent studies have found that shrubs are spreading in altitude, latitude or in height and coverage. Many of those shrubs are tall and protrude above the snowpack, therefore darkening the surface and lowering the albedo. However, many of these shrubs bend under the weight of snow and are therefore “hidden” in the snowpack throughout winter. Where these shrubs bend, the albedo of the surface is the same as where there is only snow and therefore the surface stays “cool”. However, little is known about the conditions required to bend shrubs: How significant is the structure (branch height, thickness) of the shrubs? How much snowfall is required to bend a branch? Do shrubs bend more when close to 0oC because the snow is wet and heavy? Yet, when global climate models analyze the present climatic conditions on the planet in order to forecast future changes, they need to know how much of the vegetation is protruding above the snowpack and how much is hidden.

A model to look at the effect of shrub expansion on albedo

A researcher at the Finnish Meteorological Institute teamed with researchers from the UK and Canada to improve our understanding of the conditions required for shrubs to bend. They combined the few, mostly qualitative, observations available with a biomechanical model which represents branches as cantilever columns to provide the first approximation of bending mechanisms. Using structural parameters of shrubs measured at two sites in Canada (one near Whitehorse, Yukon and another near Inuvik, Northwest Territories) to model the shrubs, the model managed to answer these three questions for the two sites tested: Is the shrub bent or protruding above the snowpack? If the latter, how much of it is protruding?

How is this affecting albedo?

While the model can not tell us how much shrubs will affect the albedo of the Arctic tundra in the future - it is aimed at understanding the processes affecting albedo in the present - the researchers believe that it can be used as one of the many tools used in models forecasting future conditions at the Earth's surface. Measurements are currently being conducted by the team in the Yukon and in Lapland to continue our understanding of shrub bending processes and further improve the model to make it applicable in wider regions.

More information:

Dr. Cécile Ménard, tel. +358 505709620, Cecile.Bauduin-Menard@fmi.fi