As greenhouse gas levels rose, so did Earth's temperature, the scientists reported.

Earth has not endlessly warmed up, though, because these natural solar cycles ended, letting the planet cool down and prompting a corresponding drop in greenhouse gas emissions, the scientists reported.

But these previous periods of heating and cooling were not influenced by the burning of fossil fuels, and the current resulting trend toward higher global average temperatures, according to Margaret Torn of Lawrence Berkeley National Laboratory.

"It means the warming is happening faster, each decade is actually warming faster than it would have," Torn said in a telephone interview. "It's the pace of change that will be one of the big problems. It's how humans adapt and the cost that will depend on the rate of change of climate."

KEEPING THE HEAT IN

Global warming is caused in part by the emission of greenhouse gases, including carbon dioxide and methane. These gases are emitted naturally but are also the result of the burning of fossil fuels, especially coal and petroleum products. These gases swaddle Earth in a layer that lets the sun's warmth in but does not allow it to easily escape.

Over the past 30 years, Earth has warmed by 1.08 degrees Fahrenheit (0.6 degrees Celsius), NASA has reported. Over the past 100 years, it has warmed by 1.44 degrees F (0.8 degrees C), indicating a recent acceleration in warmth.

Current climate models predict temperature increases of 2.7 to 8.1 degrees F (1.5 to 4.5 degrees C), but Torn's team found that additional carbon dioxide caused by the natural solar cycle could push those estimates higher.

Taking this into account could mean temperature increases of 2.9 to 11 degrees F (1.6 to 6 degrees C), the scientists said. The higher temperatures are more likely, they said in a statement.

The European team estimated that global warming in the next century may be 15 percent to 78 percent higher than current estimates because these predictions fail to take into account the feedback mechanism involving carbon dioxide emissions.

Both articles are to be published on Friday in Geophysical Research Letters.

Separately, a group of leading US-based scientists whose research has linked global warming with an increase in hurricane intensity warned on Monday that humans must reduce their contribution by developing cleaner energy and transportation or bear the risk of stronger storms.

Last year's extraordinary hurricane season caused about US$55 billion of insured losses, and estimates of the total damage range as high as US$200 billion, said Dr. Evan Mills, a staff scientist at the Lawrence Berkeley National Laboratory.

"One is faced with repeating history, of putting up with US$200 billion worth of damage every so often," said Dr. Peter Webster, a professor at the Georgia Institute of Technology and author of a study last year that found the average strength and duration of hurricanes has doubled in the last 50 years.