Earth is a greener planet than it was just 20 years ago, and scientists say that higher temperatures associated with global warming are helping plants to grow, including many nuisance plants.

Researchers say their analysis of satellite and weather data shows that increased sunshine, rainfall and higher temperatures have extended growing seasons and increased vegetation during the past two decades by 3.4 billion metric tons, a jump of 6 percent.

The study, funded by NASA and the Department of Energy, is one of several using satellite images to track plant growth and climatic trends, but is the first to look comprehensively at the effects of rising temperatures on vegetation worldwide.

The findings were published in today's issue of Science.

Researchers found that rainfall, sunshine and the length of growing seasons increased from 1982 to 1999, creating optimal conditions for plant growth in parts of the world where shortages of water, sun and heat have historically inhibited vegetation.

Arid regions saw a 20 percent increase in rainfall. Temperatures in Alaska and other chilly areas shot up by about 4 degrees Fahrenheit, and days of sunshine in the Amazon rain forest increased by one during each year of the study, the researchers say.

"There have been fairly dramatic changes around the world, in terms of the conditions important to all plants," said Ramakrishna R. Nemani, the University of Montana forestry professor who is lead author of the study.

The authors note that the 18-year period covered by their study includes two of the warmest decades since record keeping began.

During the same period, atmospheric carbon dioxide levels increased by 9 percent and the world's population increased by 36 percent.

But the authors stopped short of blaming higher temperatures on global warming, a widely held but contentious belief that the 1-degree rise in Earth's overall temperature is caused by a man-made buildup of carbon dioxide and other greenhouse gases.

"All of these changes were such that it's impossible to predict what lies down the road," Nemani said.

Nemani said the most immediate impact is likely to be seen in more fragile ecosystems, such as the Amazon rain forest.

But he said further studies must be conducted to determine what parts of the planet and types of plant life are most affected.

Other experts said that if the warming trend continues, it could have major consequences for a variety of plants and ecosystems. One of them could be a weedier world, said Lewis H. Ziska, a weed expert with the U.S. Agricultural Research Service in Beltsville who is studying the effects of climatic changes on unwanted plants that cause crop damage. "It raises the question of what types of plants are going to become dominant, and what does it mean for species diversity," Ziska said after reviewing the study.

He said weeds are usually the first plants to prosper when man disturbs a habitat, such as when a forest is cleared for a housing development.

If global changes are creating a more hospitable environment for plants, it could exacerbate such trends, creating hardier weeds and other unwanted vegetation. "The plants that are likely to thrive are what we consider weeds because they have a short life cycle, flower quickly and produce lots of seeds," he said.

Richard A. Houghton, an atmospheric ecologist at the Woods Hole Research Center in Woods Hole, Mass., who also reviewed the study, added that more plant life translates into more nutrients flowing into waterways, such as Chesapeake Bay.

"It would probably be a more alarming report if it showed a decrease in [overall plant] productivity, but increased productivity isn't so great for every area," said Houghton. "If you think about an area like Chesapeake Bay, plant productivity means more nutrients flowing into the waterways."

Nutrients encourage the growth of algae that deplete the bay's oxygen.

Houghton said the study fails to solve a puzzle that has baffled atmospheric scientists for years. As carbon dioxide has increased, so has the capacity of "carbon sinks," areas of the globe that absorb disproportionate amounts of the gas. The increased plant growth noted by the study doesn't appear to be enough to account for the additional carbon dioxide being absorbed, he said. So where the carbon dioxide goes remains a mystery.