VANCOUVER -- The planet is growing greener because climate changes have made it easier for plants to get the water, sunlight and temperatures they need, according to a study released yesterday.

Ramakrishna Nemani, a forestry professor at the University of Montana and the study's lead author, said researchers examined almost 20 years worth of weather data and satellite information to look at how the climate has changed and what impact that has had on plant growth.

Creating a world map that looked at the factors that limit plant growth -- freezing temperatures, cloud cover, drought -- researchers were able to interpret climate changes.

"In most places on Earth, those limiting factors have eased," said Dr. Nemani. "In Canada, the temperatures have gotten warmer; in India, where they needed more rain, they got more rain, and in the Amazon, where they needed more clear days, they got more clear days."

The scientists were extremely cautious about making any predictions about what their findings could mean for long-term trends in global warming.

They were also unsure whether the changes they observed are linked to greenhouse gas emissions or to natural variability in the climate.

Past studies have pointed to increasing levels of carbon dioxide as the cause an increase in vegetation. Levels of the gas have risen steadily in the last 20 years. But this study indicates climate changes are a larger part of the equation than just carbon dioxide levels and forest regrowth.

Previous studies linking plant growth to increased carbon dioxide led to the belief that plant growth would continue to increase as more of the gas is released.

"This study says there are other things that are changing simultaneously: Precipitation patterns are changing; cloud patterns are changing. For us to understand the impact of global change, we need to account for all these things," Dr. Nemani said. "If the climate is the major driver, then we're not sure what's going to happen in the next decade."