Tungurahua volcano spews gas and ash south of Quito in
Ecuador earlier thid month. REUTERS/Carlos Campana
Small volcanic eruptions help explain a hiatus in global
warming this century by dimming sunlight and offsetting a rise
in emissions of heat-trapping gases to record highs, according
to a new study.
Eruptions of at least 17 volcanoes since 2000, including
Nabro in Eritrea, Kasatochi in Alaska and Merapi in
Indonesia, ejected sulphur whose sun-blocking effect had been
largely ignored until now by climate scientists, it said.
The pace of rising world surface temperatures has slowed
since an exceptionally warm 1998, heartening those who doubt
that an urgent, trillion-dollar shift to renewable energies
from fossil fuels is needed to counter global warming.
Explaining the hiatus could bolster support for a U.N.
climate deal, due to be agreed by almost 200 governments at a
summit in Paris in late 2015 to avert ever more floods,
droughts, heatwaves and rising sea levels.
"This is a complex detective story," said Benjamin Santer of
the Lawrence Livermore National Laboratory in California,
lead author of the study in the journal Nature Geoscience
that gives the most detailed account yet of the cooling
impact of volcanoes.
"Volcanoes are part of the answer but there's no factor that
is solely responsible for the hiatus," he told Reuters of the
study by a team of U.S. and Canadian experts.
Volcanoes are a wild card for climate change - they cannot be
predicted and big eruptions, most recently of Mount Pinatubo
in the Philippines in 1991, can dim global sunshine for
Santer said other factors such as a decline in the sun's
output, linked to a natural cycle of sunspots, or rising
Chinese emissions of sun-blocking pollution could also help
explain the recent slowdown in warming.
The study suggested that volcanoes accounted for up to 15
percent of the difference between predicted and observed
warming this century. All things being equal, temperatures
should rise because greenhouse gas emissions have hit
"Volcanoes give us only a temporary respite from the
relentless warming pressure of continued increases in carbon
dioxide," said Piers Forster, Professor of Climate Change at
the University of Leeds.
A study by the UN Intergovernmental Panel on Climate Change
last year suggested that natural variations in the climate,
such as an extra uptake of heat by the oceans, could help
explain the warming slowdown at the planet's surface.
The IPCC projected a resumption of warming in coming years
and said that "substantial and sustained" cuts in greenhouse
gas emissions were needed to counter climate change.
It also raised the probability that human activities were the
main cause of warming since 1950 to at least 95 percent from
90 in 2007. Despite the hiatus, temperatures have continued
to rise - 13 of the 14 warmest years on record have been this
century, according to the World Meteorological Organisation.