over the past half-century, Earth's entire surface or the lithosphere has warmed up significantly and is gaining energy at approximately the same rate as the atmosphere and cryosphere (the portion of Earth's surface where water is in solid form such as sea ice, snow cover, glaciers, ice caps and permafrost). This was discovered during a recent study carried out by researchers from the St Francis Xavier University, Canada, and University of Michigan (u-m), Ann Arbor, usa. "Our findings remove the last doubt that warming of the planet is anything other than a global phenomenon,' said Henry Pollack, one of the u-m researchers.

Until recently, the story of global warming has been built up primarily on the basis of temperature measurements of the atmosphere, cryosphere and the oceans. These measurements provide enough information to reconstruct a rough temperature history of Earth for the past 140 years. However, such measurements have left out one major component of the climate system: the surface of the continents that cover almost 29 per cent of the planet's surface.

Now, Pollack and his colleagues have completed the picture by determining how much the continental rocks have warmed in recent centuries. The scientists based their analysis on temperature readings taken by lowering sensitive thermometers into holes drilled from Earth's surface into rock formations on six continents (Africa, Asia, Europe, North America, South America and Australia). These readings can reveal how temperatures have changed in the past because the heat that surface rocks absorb from the atmosphere travels slowly downward into subsurface rocks, leaving a distinct signature in the rocks. Signals from short-term daily or seasonal variations penetrate only a few metres, and the Earth quickly "forgets' them. However, temperature changes that take place over hundreds of years are preserved in deeper rock.

The researchers' calculations, based on data from 616 bore holes, found evidence of an increase in the heat content of the continents over the past 500 years, with more than half of that heat gain occurring during the 20th century and nearly one-third of it since 1950. "The magnitude of global warming we estimated is very similar to that which has come from the numerous studies of the ocean, atmosphere and ice,' said Pollack. "We believe it makes a persuasive case that the warming has been truly global and steps should be immediately taken to contain it.'