Land-use change caused historical cooling trend

Changes in land use from natural vegetation to agriculture caused atmospheric cooling over the land of up to 2°F (1.1°C) between 1000 and 1900 AD, according to US scientists.

Using climate model simulations with pre-industrial levels of greenhouse gases, researchers from the Lawrence Livermore National Laboratory operated by the University of California in the United States, have found that the previously recognised historical cooling trend could have been the result of human activity. The research also revealed a slight increase in sea ice volumes in association with the temperature decrease.

As forests tend to look dark from the sky, and agricultural lands appear much lighter, changes from forest to crops have resulted in more sunlight being reflected back into space. According to the researchers, the cooling effect has occurred particularly in regions such as the eastern and mid-western US, where huge tracts of land have been converted to crops.

“This is an example of inadvertent geo-engineering – we changed the reflectivity of the Earth and have probably caused a global cooling in the past,” said Ken Caldeira, one of the researchers on the project, and Co-Director for the Department of Energy’s Centre for Research on ocean carbon sequestration.

However, greenhouse gas emissions in the last century are likely to have overcome the cooling trends that took place prior to this time, say the researchers, pointing out that carbon sequestration through the planting of new forests, which has been advocated by a number of countries, including the US, would actually add to warming through increased absorption of sunlight.

“The estimated temperature change in the continental United States as a result of change from forests to agriculture is up to a two degree Fahrenheit cooling,” said Caleidra. “So, when we talk about global warming, we can no longer take for granted that this global warming is starting from some natural climate state, undisturbed by human activities.”

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