A key climate change reduction strategy recommended by the United Nations won't have much effect on global temperatures, according to a couple of Canadian scientists.

Afforestation involves planting trees over croplands that aren't very productive in order to absorb more carbon dioxide from the air. High emissions of carbon dioxide have been linked to climate change, especially rising average global temperatures.

But even if 100 per cent of the area planted with crops now was gradually replaced with forests, wherever possible, over the next 50 years, warming would only be reduced 0.45 degrees Celsius between 2081 and 2100, said a study by Vivek Arora, an Environment Canada researcher based at the Canadian Centre for Climate Modelling and Analysis in Victoria, B.C., and Alvaro Montenegro, an earth sciences professor at St. Francis Xavier University in Antigonish, N.S.

If 50 per cent of the area was afforested, warming would be reduced by just 0.25 degrees.

"That says a lot about the smaller efforts," Montenegro said on Monday, a day after the research was published in Nature Geoscience.

The United Nations lists afforestation as one way developing countries can earn emission reduction credits that can be sold to industrialized countries to meet their obligations under the Kyoto Protocol, an international agreement to reduce greenhouse gas emissions by 5.2 per cent from 1990 levels by 2008 to 2012.

Arora and Montenegro used a mathematical model of the Earth's climate, land surface and oceans to calculate the effect of replacing cropland with trees where trees could naturally grow. For example, they excluded areas like the Canadian prairies that are naturally grasslands.

They found that while forests do absorb large amounts of carbon dioxide, they are darker than crops, so they absorb more sunlight. That results in net warming, especially in areas further away from the equator.

Afforestation effective in tropics

The study found that in the tropics, afforestation and not cutting down trees in the first place "are effective forest management strategies from a climate perspective" because warming reductions per forested hectare were three times higher in the tropics than in boreal and northern temperate regions.

While afforestation doesn't do much to slow warming globally, Montenegro noted that it does have other positive environmental effects, such as restoring wildlife habitat, boosting biodiversity, preventing erosion and reducing the acidification of the oceans caused by high carbon dioxide levels. Ocean acidification appears to be harmful to marine life, partly because acidic water dissolves the shells of creatures such as corals and shellfish.

Montenegro said in the past, emission credits and offsets for planting trees included only the amount of carbon dioxide absorbed by the trees, and not the extra heat they absorb.

"Now…we have a handle on that darkening and its climatic effects," he said. "We should be including those values."

Hank Margolis, a professor of forest ecosystem science at the University of Laval and a program leader for the Canadian Carbon Program, said the paper was important from both a scientific and policy point of view.

But he noted that planting forests can provide many societal benefits in addition to environmental benefits such as forests products, including biofuels. "If forest products are used in a way that allows their carbon to not be re-emitted into the atmosphere, then the climate benefits of tree planting may be even greater than reported in this paper," he said in a statement to the Canadian Science Media Centre.