The research adds to the growing body of information that the Earth is being warmed by a man-made greenhouse effect.

According to Herman Zimmerman of the United States National Science Foundation: "The balance of evidence now firmly supports an important human influence on the global climate system."

The scientists, from the University of Massachusetts at Amherst, were able to estimate temperatures over more than half of the Earth. They say that variations in the brightness of the sun as well as gas and dust from volcanoes have played an important role in climate variation over the period studied.

But they believe that the greatest recent effect comes from heat trapped in the lower atmosphere by the build up of carbon dioxide, a well-known 'greenhouse gas'.

During the past century, industry has increased the levels of carbon dioxide in the atmosphere by 25% over the pre-industrial level.

Some believe that if the amount of carbon dioxide in the atmosphere were to increase at its current rate the global effects would be severe.

Floods like these in Bangladesh could be more common in the future

Rising sea level, more frequent extreme weather events, heat waves and droughts would be more common.

The evidence that the Earth is getting warmer comes from many sources - the growth of tree-rings in Mongolia and Canada are getting wider showing that they are growing more each year; the growing season for crops in Australia is getting longer; permafrost in Siberia and Canada is melting.

The distribution of climatic regions would change but scientists cannot predict how.