GREENHOUSE effect sceptics may have lost their final excuse. The Sun has been dethroned as the dominant source of climate change, leaving the finger of blame pointing at humans.

A correlation between the sunspot cycle and temperatures in the northern hemisphere seemed to account for most of the warming seen up until 1985. But new results reveal that for the past 15 years something other than the Sun-probably greenhouse emissions-has pushed temperatures higher.

In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. The mysterious-and unexplained-relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period.

Now Lassen and astrophysicist Peter Thejll have updated the research and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 °C since 1980. "The curves diverge after 1980," says Thejll, "and it's a startlingly large deviation. Something else is acting on the climate."

Although they can't be sure, they suspect that emissions from the burning of fossil fuels are responsible. "It has the fingerprints of the greenhouse effect," says Thejll. Other climatologists agree. "It sounds like an actual piece of evidence for greenhouse warming," says Richard Betts of Britain's Hadley Centre for Climate Prediction and Research in Bracknell, Berkshire. "Any natural effect would swamp the small early changes, so you'd expect to see the larger changes more recently."

Others, however, remain sceptical about this line of research. Tom Wigley at the National Center for Atmospheric Research in Boulder, Colorado, who in 1992 criticised Lassen's initial research, points out that since then no one has provided a convincing physical explanation for the correlation between the sunspot cycle and temperature. Wigley accepts that solar effects may have dominated until about 1950, but certainly not as late as 1980.

Lassen and Thejll recognise that the link between the solar cycle and climate is controversial. But they hope their new findings will move climate-change researchers towards a more balanced view. "It became political," says Thejll. "We're now seeing that the Sun plays a role, and something in addition to the Sun. Maybe that will help people see there is room for both."

Predicted global warming compared with actual temperature rises

Robert Adler