LONDON (Reuters) - The 20th century was the hottest in more than 500 years, scientists said Wednesday.

 The earth's temperature has increased by about 1.8 degrees Fahrenheit since the 1500s, they said. In the Northern  Hemisphere it rose even faster: up 2 degrees Fahrenheit in the last 500 years and 1.1 degrees in the 20th century alone.

 ``The 20th century was the warmest for the last five, and the one which was most rapidly changing,'' Henry Pollack of the  University of Michigan said in a statement.

 Pollack and his colleague Shaopeng Huang and Po-Yu Shen of the University of Western Ontario reconstructed past trends in  climate change by using data on subsurface temperature gathered from holes drilled into the ground at 616 sites on every  continent except Antarctica.

 Their research is published in the latest edition of the science journal Nature.

 Highly sensitive thermometers in the holes show just how much temperature has changed over the years because signals from  surface temperature travel below the earth and are preserved in rock and soil. Temperatures of the past 1,000 years are  recorded to a depth of 500 meters (yards) down.

 ``So the upper 500 meters is an archive -- a historical record of temperature changes that have occurred in the last thousand  years,'' Pollack said.

 ``Like any historical archive, there are of course missing pages, and the ink has run in a few places. But in principle, if you drill  a borehole anywhere on a continent, you can observe a temperature profile and reconstruct what has happened at that  location,'' he added.

 By averaging the temperatures taken from the boreholes, the researchers reconstructed a picture of past climates. Pollack and  his team had previously examined data from 358 borehole sites around the globe.

 Their findings are consistent with other ways of estimating past temperature such as studying ice cores, lake sediment and  coral growth.

 ``All the methods generally show a very unusual 20th century, and ours does, too,'' Pollack said.