Climate Update: Of Ice and Men

If ice is the canary in the climate mine, the canary is melting.

Arctic Sea Ice Trend

Much of the Arctic Ocean is covered by ice that waxes and wanes with the seasons. In the winter, the extent of the ice grows and in the summer it shrinks, reaching a minimum in September.

With global warming, the extent of sea ice in the Arctic has been shrinking — by about 34 percent over the last 30 years (see graphic below). In total the average area of September sea ice has decreased over this period by more than 700,000 square miles — size-wise, that's bigger than the state of Alaska.

September ice extent from 1979 to 2009 shows a continued decline. Since 1979, the rate of sea ice decline for the month of September has now increased to 11.2 percent per decade. Credit: National Snow and Ice Data Center

In 2007 warm temperatures, cloudless skies, and winds combined to make that year’s summertime melting especially severe. The sea ice extent in September 2007 was by far the lowest on record. A modest recovery in 2008 made that year the second lowest on record.

As we entered the summer of 2009, the question was: whither Arctic Sea ice? Would it continue the upward trend begun in 2008, or would it dip back down perhaps establishing a new record? We now have the answer. The National Snow and Ice Data Center (NSIDC) reports that 2009 showed another uptick in sea ice extent relative to 2007, but continued the long-term downward trend. The year 2009 now ranks third for the lowest summer sea ice extent, coming in behind 2008 and 2007 in the 30-year satellite record. The 2009 minimum of 5.10 million square kilometers (1.97 million square miles), while larger than the previous two years, is still well below the average for the past 30 years.

Ice Sheet Melting Picks Up the Pace

Against this backdrop a paper published online today in Geophysical Research Letters by Isabella Velicogna of the University of California, Irvine, shows that not only are both the Antarctic and Greenland ice sheets melting, but their rate of melting is accelerating over time.

Using satellite gravity data collected from GRACE, the Gravity Recovery and Climate Experiment (April 2002 – February 2009), Velicogna found that:

• the annual mass loss from the Greenland ice sheet increased from 137 gigatons in 2002-2003 to 286 gigatons in 2007-2009;
• in Antarctica the annual mass loss increased from 104 gigatons in 2002-2006 to 246 gigatons in 2006-2009.

In both cases the rate of loss increased by more than 100 gigatons/yr. In case you were wondering, 100 gigatons are 100 billion tons or about 25 trillion gallons of water — that’s enough water to fill about 40 million Olympic sized pools.

CO2, Climate and Ice Coupling

So what was the extent of sea ice and polar glaciers the last time atmospheric concentrations matched today’s level of about 390 ppm? A new technique that extends the coupled climate/carbon dioxide (CO2) record from 800,000 years ago to 20 million years ago has an answer. Aradhna Tripati of the University of California, Los Angeles, and colleagues were able to extend the atmospheric CO2 record by developing a technique that derives atmospheric CO2 from the boron-to-calcium ratios found in foraminifera shells collected from sediments in the Pacific Ocean.

Their new record, published in ScienceExpress last week, shows that the last time CO2 levels were sustained at levels similar to today’s was between 15 and 20 million years ago, a time when:

• temperatures were roughly 3 to 6 degrees Celsius (5 to 11 degrees Fahrenheit) warmer than today,
• there was very little ice on Greenland or Antarctica and no permanent sea ice in the Arctic, and
• sea level was about 25 to 40 meters (80-130 feet) higher.

Makes you wonder about setting 450 ppm as the target for stabilizing CO2 concentrations — over the long term, will that be low enough?