The process by which less sunlight is reflected and more is absorbed by forest canopies, heating the surface, cancels out the positive effects from the trees taking in carbon, conclude scientists from Lawrence Livermore National Laboratory, Universite Montpellier II, and the Carnegie Institution.
Their research shows that, by the year 2100, forests in mid-latitudes and high latitudes will make some places up to 10 degrees Fahrenheit warmer than they would have been if the forests did not exist.
"Although it was previously known that trees could have an overall warming effect in the boreal forests north of 50 degrees, this is the first study to show that temperate forests could lead to net global warming," said Livermore's Seran Gibbard, lead author of the study.
The scientists say theirs is the first study to investigate the combined climate and carbon cycle effects of large-scale deforestation in a fully interactive three dimensional climate carbon model.
"This is the first comprehensive assessment of the deforestation problem," said co-author Govindasamy Bala, an atmospheric scientist with Lawrence Livermore's climate and carbon cycle modeling group.
Forests affect climate in three different ways, the scientists explain. They help to keep the planet cool in two of those ways. They absorb the greenhouse gas carbon dioxide from the atmosphere and that helps to keep the planet cool, and they evaporate water to the atmosphere and increase cloudiness, which also helps keep the planet cool.
"Our study shows that tropical forests are very beneficial to the climate because they take up carbon and increase cloudiness, which in turn helps cool the planet," Bala said.
But forests also are dark and absorb sunlight, warming the Earth.
"The darkening of the surface by new forest canopies in the high latitude boreal regions allows absorption of more sunlight that helps to warm the surface," said Bala. "In fact, planting more trees in high latitudes could be counterproductive from a climate perspective."
Climate change mitigation strategies that promote planting trees have taken only the first effect into account.
The models calculated the carbon-climate interactions and took into account the physical climate effect and the partitioning of the carbon dioxide release from deforestation among land, atmosphere and ocean.
The story is different for the tropical forests. In tropical regions, forests help keep the Earth cool by not only absorbing carbon dioxide, but by evaporating plenty of water as well.
The UN Convention on Climate Change, which sets an overall framework for the efforts of 189 governments to tackle global warming, explains in a newly issued document on the role of forests that, "Some forests act as 'sinks' by absorbing carbon from the air, while forests whose carbon flows are in balance act as 'reservoirs.' Deforestation and changes in land use make the world's forests a net source of carbon dioxide."
Under the climate convention, the Kyoto Protocol, which entered into force in February 2005, allows the industrialized countries that ratified it to offset their emissions by increasing the amount of greenhouse gases removed from the atmosphere by creating carbon sinks in the land use, land-use change and forestry sector.
Eligible activities include afforestation, reforestation and deforestation and forest management, cropland management, grazing land management and revegetation.
Greenhouse gases removed from the atmosphere through eligible sink activities generate credits known as removal units. Any greenhouse gas emissions from eligible activities, in turn, must be offset by greater emission cuts or removals elsewhere.
But the treaty documents do not differentiate between forests planted in temperate regions and those planted in tropical latitudes.
"Should we give carbon credit to the planting of forests? Probably not for countries in mid and high latitudes," Bala said. "But the tropical forests present a win-win because they cool the planet by evaporative cooling and the uptake of carbon."
Study co-author Ken Caldeira of the Carnegie Institution warned that proposals to grow more forests to cool the planet should be met with caution.
"I like forests. They provide good habitats for plants and animals, and tropical forests are good for climate, so we should be particularly careful to preserve them," Caldeira said. "But in terms of climate change, we should focus our efforts on things that can really make a difference, like energy efficiency and developing new sources of clean energy."
"It is only by transforming our energy system and preserving natural habitat, such as forests, that we can maintain a healthy environment," he said. "To prevent climate change, we must focus on effective strategies and not just feel-good strategies."
The study discovered that a global replacement of current vegetation by trees would lead to a global warming of 2.4 degrees Fahrenheit (1.3 degrees Celsius).
Global replacement with grassland led to cooling of about 0.7 degrees F (.38 degrees C).
The researchers also found that planting trees between 30 and 50 degrees latitude worldwide -- an area covering the United States, Europe, most of the Middle East, northern China, Korea, and Japan -- saw the global mean surface air temperature increase by 0.7 degrees Fahrenheit.
Regional warming in North America and Eurasia was as high as eight degrees F (4.4 degrees C).
Earlier studies also found that planting trees in the boreal forest regions, which cover the upper half of the Northern Hemisphere, caused a warming of surface temperatures.
The research, also authored by Thomas Phillips and Michael Wickett of Lawrence Livermore, appears online in the current issue of the journal "Geophysical Research Letters."
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