sea ice in recent decades has declined even faster than predicted by most
models of climate change. Many scientists have suspected that the trend now
underway is a combination of global warming and natural climate variability.
new study finds that a substantial chunk of summer sea ice loss in recent
decades was due to natural variability in the atmosphere over the Arctic Ocean.
The study, from the University of Washington, the University of California
Santa Barbara and federal scientists, is published March 13 in Nature Climate
forcing is still dominant -- it's still the key player," said first author
Qinghua Ding, a climate scientist at the University of California Santa Barbara
who holds an affiliate position at the UW, where he began the work as a
research scientist in the UW's Applied Physics Laboratory. "But we found
that natural variability has helped to accelerate this melting, especially over
the past 20 years."
paper builds on previous work by Ding and other UW scientists that found
changes in the tropical Pacific Ocean have in recent decades created a
"hot spot" over Greenland and the Canadian Arctic that has boosted
warming in that region.
hot spot is a large region of higher pressure where air is squeezed together so
it becomes warmer and can hold more moisture, both of which bring more heat to
the sea ice below. The new paper focuses specifically on what this atmospheric
circulation means for Arctic sea ice in September, when the ocean reaches its maximum
area of open water.
idea that natural or internal variability has contributed substantially to the
Arctic sea ice loss is not entirely new," said second author Axel
Schweiger, a University of Washington polar scientist who tracks Arctic sea ice.
"This study provides the mechanism and uses a new approach to illuminate
the processes that are responsible for these changes."
designed a new sea ice model experiment that combines forcing due to climate
change with observed weather in recent decades. The model shows that a shift in
wind patterns is responsible for about 60 percent of sea ice loss in the Arctic
Ocean since 1979. Some of this shift is related to climate change, but the
study finds that 30-50 percent of the observed sea ice loss since 1979 is due
to natural variations in this large-scale atmospheric pattern.
we've found is that a good fraction of the decrease in September sea ice melt
in the past several decades is most likely natural variability. That's not
really a surprise," said co-author David Battisti, a UW professor of
method is really innovative, and it nails down how much of the observed sea ice
trend we've seen in recent decades in the Arctic is due to natural variability
and how much is due to greenhouse gases."
long-term natural variability is ultimately thought to be driven by the
tropical Pacific Ocean. conditions that set off ripple effects, and atmospheric
waves snake around the globe to create areas of higher and lower air pressure.
apart the natural and human-caused parts of sea ice decline will help to
predict future sea ice conditions in Arctic summer. Forecasting sea ice
conditions is relevant for shipping, climate science, Arctic biology and even
tourism. It also helps to understand why sea ice declines may be faster in some
decades than others.
the long term, say 50 to 100 years, the natural internal variability will be
overwhelmed by increasing greenhouse gases," Ding said. "But to
predict what will happen in the next few decades, we need to understand both
will happen next is unknown. The tropical Pacific Ocean could stay in its
current phase or it could enter an opposite phase, causing a low-pressure
center to develop over Arctic seas that would temporarily slow the long-term
loss of sea ice due to increased greenhouse gases.
are a long way from having skill in predicting natural variability on decadal
time scales," Ding said.