By Environment Correspondent Alister Doyle

OSLO (Reuters) - Plants help to slow climate change by emitting gases as temperatures rise that lead to the formation of a sunshade of clouds over the planet, scientists said on Sunday.

The tiny sun-dimming effect could offset about one percent of warming worldwide and up to 30 percent locally such as over vast northern forests in Siberia, Canada or the Nordic nations, they wrote in the journal Nature Geoscience.

While proportionally small, some scientists said the study provided further evidence of the importance of protecting forests, which help to slow climate change by absorbing greenhouse gases as they grow and to preserve wildlife.

Observations of forests from 11 sites around the world showed that plants emitted tiny particles that float on the wind as temperatures warm and act as seeds for water droplets that create clouds, they wrote.

Clouds' white tops in turn reflect sunlight back into space and offset warming, they wrote.

The study focused on forests in Europe, North America, Russia and southern Africa. The effect is believed to be smaller over far hotter tropical forests such as in the Amazon or the Congo basin.

"It's a small effect - one percent is not much," said lead author Pauli Paasonen of the University of Helsinki and the International Institute for Applied Systems Analysis in Austria.

"If temperatures were to increase by 1 degree without this effect, they'd rise 0.99 degrees with it," he told Reuters of a study that included researchers in the United States, Canada, Germany, the Netherlands, South Africa, Hungary and Sweden.

SUNSHADE

Many other tiny aerosols, such as human pollution from factories, cars and power plants, also have a sun-dimming effect that may be slowing the pace of climate change, blamed mainly on emissions of greenhouse gases such as carbon dioxide.

But there has been uncertainty about the role of nature, and of plants' emissions of gases such as monoterpenes.

"Everyone knows the scent of the forest," Ari Asmi, a University of Helsinki researcher who also worked on the study, said in a statement. "That scent is made up of these gases."

It is unclear why plants emit more monoterpenes at higher temperatures - it may be a side-effect of trees' natural air conditioning to reduce heat.

"Forests are providing an additional cooling. This is another reason why we should conserve and protect forests," said Dominick Spracklen, an expert on plants and climate change at the University of Leeds who was not involved in the study.

But the damaging effects of warming on forests, such as more wildfires or insect pests, may exceed tiny benefits of more clouds that would only come from healthy forests, he said.

Spracklen said plants' cooling effect was tantalizing evidence for people who believe the planet somehow acts as a self-regulating organism for life, sometimes known as the Gaia hypothesis.

One idea launched in 1987 was that warmer temperatures spur the growth of more algae in the upper oceans. These tiny plants would in turn release more of the chemical dimethyl sulphide that seeds clouds to reflect sunlight.

"No one has yet proved that this effect exists," he said.

The U.N. panel of leading climate scientists says that human emissions of greenhouse gases are driving up world temperatures and will lead to ever more floods, droughts, heatwaves and rising sea levels.

It says that it is at least 90 percent certain that human activities, rather than natural variations in the climate, are to blame for most of the warming in the past half-century.

(For the study: http://www.nature.com/doifinder/10.1038/ngeo1800)

(Reporting By Alister Doyle, Environment Correspondent; editing by Mike Collett-White)