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.
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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.”
