LONDON—The sun’s changing energy levels are not to blame for
recent global warming and, if anything, solar variations over the past 20 years
should have had a cooling effect, scientists said on Wednesday.
Their findings add to a growing body of evidence that human
activity, not natural causes, lies behind rising average world temperatures,
which are expected to reach their second highest level this year since records
began in the 1860s.
There is little doubt that solar variability has influenced
the Earth’s climate in the past and may well have been a factor in the first
half of the last century, but British and Swiss researchers said it could not
explain recent warming.
"Over the past 20 years, all the trends in the sun that could
have had an influence on Earth’s climate have been in the opposite direction to
that required to explain the observed rise in global mean temperatures," they
wrote in the Proceedings of the Royal Society.
Most scientists say emissions of greenhouse gases, mainly
from burning fossil fuels in power plants, factories and cars, are the prime
cause of the current warming trend.
A dwindling group pins the blame on natural variations in the
climate system, or a gradual rise in the sun’s energy output.
In order to unpick that possible link, Mike Lockwood of
Britain’s Rutherford Appleton Laboratory and Claus Froehlich of the World
Radiation Centre in Davos, Switzerland, studied factors that could have forced
climate change in recent decades, including variations in total solar irradiance
and cosmic rays.
The data was smoothed to take account of the 11-year sunspot
cycle, which affects the amount of heat the sun emits but does not impact the
Earth’s surface air temperature, due to the way the oceans absorb and retain
heat.
They concluded that the rapid rise in global mean
temperatures seen since the late 1980s could not be ascribed to solar
variability, whatever mechanism was invoked.
Britain’s Royal Society — one of the world’s oldest
scientific academies, founded in 1660 — said the new research was an important
rebuff to climate change skeptics.
"At present there is a small minority which is seeking to
deliberately confuse the public on the causes of climate change. They are often
misrepresenting the science, when the reality is that the evidence is getting
stronger every day," it said in a statement.
The 10 warmest years in the past 150 years have all been
since 1990 and a United Nations climate panel, drawing on the work of 2,500
scientists, said this year it was "very likely" human activities were the main
cause.
The panel gave a "best estimate" that temperatures would rise 1.8 to 4.0
degrees Celsius (3.2 to 7.8 Fahrenheit) this century.
