The Arctic heats up
Content provided by: UCAR / University Corporation for Atmospheric Research
The Arctic heats up
NCAR researchers study climate change in Earth’s northern realms
September 24, 2009 | The Arctic is on thin ice—literally as well as figuratively.
New research shows that temperatures were warmer there in the 1990s than any
decade in at least 2,000 years. Arctic sea ice has been dwindling for the past
few decades, with 2009 levels well below normal. Greenland's melting ice sheets
have the potential to dramatically raise sea levels, and thawing permafrost
threatens to release massive amounts of methane into the atmosphere.
Although many UCAR/NCAR labs and programs touch on the
Arctic in their research, the core of the organization's Arctic research is in
ESSL/CGD. Researchers there are observing, measuring, and modeling the Arctic's
changing snow, ice, land, and water, in hopes of understanding what the warm
21st century has in store for the top of the world.
Arctic sea ice outlook
Atmospheric circulation patterns in August helped spread out
sea ice, slowing ice loss in most regions of the Arctic, according to the
National Snow and Ice Data Center. Even so, sea ice levels for 2009 were well
below average once more. The two years with the greatest summer loss of Arctic
ice in the 30-year-long satellite record are 2007 (first place) and 2008
(second place). By early September of this year, sea ice extent had dropped
below the minimum for 2005 (which was previously in third place), and melting
was expected to continue until late in the month.
With the Arctic Ocean becoming more accessible, there is
increasing interest from the shipping industry, oil and gas companies,
northern governments, and the polar community in general in short-term
predictions of sea ice levels.
In CGD, Marika Holland is researching how to most
effectively forecast sea ice on a short-term, seasonal timescale in the Arctic's
changing environment. "There are seasonal forecasting methods in place for
Arctic sea ice, but it's questionable whether they will continue to work in the
Arctic regime we're now experiencing," Marika says.
These seasonal forecasting methods have traditionally been
based on statistical methods that look at historical trends. In a new study,
Marika and colleagues used NCAR's Community Climate System Model (CCSM) to
assess the inherent predictability of sea ice, and whether this predictability
will change as the Arctic warms. "We wanted to take a step back," Marika says. "Should
we even expect that, given knowledge of January conditions, we can predict summertime
conditions?"
Their results suggest that climate change does influence
predictability and that forecasters will likely need to modify the tools they
use for seasonal forecasts, relying more on physically-based models rather than
statistical methods.
Sea ice loss and future climate
Also in CGD, a team led by Clara Deser has used an
atmospheric general circulation model to explore how projected losses in Arctic
sea ice may affect climate. The results were published in the Journal of
Climate in September.
The goal of the study was to isolate the direct impacts of
Arctic sea ice loss without feedbacks from other components of the climate
system, notably the oceans. Scientists in the United Kingdom and Norway have
undertaken similar efforts, but the CGD study is the first to apply a
high-resolution, integrated model to this research question.
Running experiments with the CCSM coupled to the Community Land
Model (CAM), the researchers found that loss of summer Arctic sea ice will
result in considerable (more than 9°F, or 5°C) warming by 2100 over adjacent
land regions north of 65° latitude, particularly during autumn and winter. The
modeling experiment also forecasts increased snowfall over land, especially
over Siberia and northern Canada, as ice loss sends moisture from the ocean
into the atmosphere.
Another important finding is that although sea ice loss
peaks in September, the atmosphere's full response is delayed until November or
December, according to the model. "This is because the ocean is more efficient
at losing heat to the atmosphere in winter when air temperatures are lowest and
winds strongest," Clara explains.
The researchers' next step is to incorporate an interactive
ocean component into its atmospheric model to study how ocean temperatures,
salinity, and currents respond to sea ice loss. This new version of the model
will be used for the next IPCC assessment, due out in 2013.
Greenland's ice sheets and sea level rise
It's not just sea ice that's melting. New research led by
Aixue Hu in CGD looks at Greenland's melting ice sheets, focusing on how they
may raise sea levels.
Aixue and his team used the CCSM to study different
scenarios that depend on how fast the island's ice melts in the coming century.
Their results, published in Geophysical Research Letters last May, conclude
that the island's melting ice sheets may drive more water than previously
thought this century toward the northeastern coasts of the United States and
Canada, threatening coastal cities such as Halifax, Boston, and New York.
"If the Greenland melt continues to accelerate, we could see
significant impacts this century from the resulting sea level rise," Aixue
says. "Major northeastern cities are directly in the path of the greatest
rise."
modeling, shows that sea level rise may be an additional 4 inches (10
centimeters) higher by populated areas in northeastern North America
than previously thought. Extreme northeastern North America and
Greenland may experience even higher sea level rise. (Image courtesy Geophysical Research Letters, modified by UCAR.) [ENLARGE]
According to the study, if Greenland's ice melts at moderate
to high rates, ocean circulation by 2100 may shift and cause sea levels off the
northeast coast of North America to rise by about 12 to 20 inches (about 30 to
50 centimeters) more than in other coastal areas. The research builds on recent
reports that have found that sea level rise associated with global warming
could adversely affect North America, and its findings suggest that the
situation is more threatening than previously believed.
Unlike water in a bathtub, water in the oceans does not
spread out evenly. The northeast coast of North America is especially
vulnerable to the effects of Greenland's ice melt because of circulations that
transport warm water from the tropical Atlantic Ocean northward, where it cools
and descends to create a dense layer of cold water. As a result, sea level is
currently about 28 inches (71 cm) lower in the North Atlantic than the North
Pacific, which lacks such a dense layer. (See the debut issue of UCAR Magazine
for more on regional variations in sea-level rise.)
A 2,000-year Arctic climate record
Temperatures in the Arctic were warmer in the 1990s than any
decade in at least 2,000 years, according to research by CGD's Dave Schneider
published in Science in early September. The study, which incorporates
geologic records and computer simulations, provides new evidence that the
Arctic would be cooling were it not for greenhouse gas emissions that are
overpowering natural climate patterns.
The study was led by Northern Arizona University's Darrell
Kaufman, who collaborated with CGD's paleoclimate group, where Dave recently
finished his postdoctoral appointment, to develop a synthesis of Arctic
climate records and compare them with model simulations. The researchers used
data from sediments in Arctic lakes, glacial ice, and tree rings to reconstruct
temperatures for the last 2,000 years, extending far beyond the 400 years of
Arctic temperature records previously available at that level of detail.
research from CGD’s paleoclimate group shows that the Arctic reversed a
long-term summer cooling trend and began warming rapidly in recent
decades. The blue line shows estimates of summertime Arctic land
temperatures over the last 2,000 years, based on proxy records from
lake sediments, ice cores, and tree rings. The green line shows the
long-term cooling trend. The red line shows the recent warming based on
actual observations. (Image courtesy Science, modified by UCAR.) [ENLARGE]
The study is the first to quantify a pervasive cooling
across the Arctic on a decade-by-decade basis related to a cyclic wobble in
Earth's tilt relative to the Sun. Over the last 7,000 years, the timing of
Earth's closest pass by the Sun has shifted from September to January. This has
gradually reduced the intensity of sunlight reaching the Arctic in summertime,
as Earth is farther from the Sun.
The analysis shows that summer temperatures in the Arctic
cooled at an average rate of about 0.36°F (0.2°C) per thousand years, in step
with the reduced energy from the Sun. The orbital cycle that produced the
cooling continues today, but was overwhelmed in the 20th century by
human-induced warming, resulting in summer temperatures in the Arctic by the
year 2000 that were about 2.5°F (1.4°C) higher than would have been expected
from the continued cyclic cooling.
"This study provides us with a long-term record that reveals
how greenhouse gases from human activities are overwhelming the Arctic's
natural climate system," Dave says. "It's particularly important because the
Arctic, perhaps more than any other region on Earth, is facing dramatic impacts
from climate change."
The impermanence of permafrost
A study published in June 2008 by CGD's Dave Lawrence, working
with colleagues at the National Snow and Ice Data Center, showed that the rate
of climate warming over northern Alaska, Canada, and Russia could more than
triple during periods of rapid sea ice loss. The research raised concerns about
thawing permafrost (permanently frozen soil), which releases carbon as well as
methane, a greenhouse gas that is 23 times more effective than carbon dioxide
at trapping heat in the atmosphere.
Dave and postdoctoral researcher Sean Swenson are currently
laying groundwork to study in more depth how the carbon cycle will respond to
Arctic warming and permafrost thaw. They're developing a dynamic wetlands
scheme to model how changes driven by terrain, climate, and permafrost thaw control
the distribution and extent of wetlands.
Such a model is critical because one of the keys to
predicting the carbon cycle's response to thawing permafrost is predicting how
the changing climate and thawing permafrost will affect the Arctic's hydrology.
When dead plant materials break down under wet soil conditions, methane is
produced, whereas in dry conditions, enhanced decomposition tends to produce
carbon dioxide.
"Since methane is a much stronger greenhouse gas, you will
see a much different climate impact if the Arctic land surface becomes wetter,
either due to increased precipitation or changing landforms due to permafrost
thaw, than if it becomes drier," Dave says. Mapping these regional variations
in precipitation and permafrost thaw will be critical to predicting the future
of the Arctic's climate cycle.
Polar amplification:
Why the Arctic warms quicker
The reason that Earth's far northern latitudes are so vulnerable
to climate change is a phenomenon called polar amplification, which causes the
polar regions, particularly the North Pole, to experience enhanced warming
compared to other parts of the globe. By some estimates, the Arctic is warming
twice as fast as the rest of the world.
A number of factors contribute to polar amplification, one
of the most important being the feedback effect caused by ice albedo. Polar ice
has a high albedo, meaning that it reflects more incoming solar radiation than
darker land or water surfaces. As ice melts in the Arctic, the exposed land and
water absorb radiation, in turn amplifying the warming effect. Other influences,
such as natural variability in Arctic temperatures and the transport of heat to
the Arctic via the ocean, may also play a role in polar amplification.
