A blue Arctic Ocean and a climate tipping point

In a paper soon to be published in the Geophysical Review Letters (GRL), I reported (together with some colleagues at NASA) that the Arctic perennial ice cover this year underwent a phenomenal decline. It was about 38 percent less extensive in 2007 than the long-term average and 27 percent less than the previous record low observed in 2005. The perennial ice is sea ice that survives the summer melt and consists mainly of multiyear ice floes that are the mainstay of the Arctic sea ice cover. The decline in perennial ice from 2006 to 2007 is equal if not greater than that from 1979 (when the satellite time series started) to 2006. Such a big change in a single year is thus intriguing if not puzzling. During the satellite era, the perennial ice cover went through periods of large variability and there were times when unusually low perennial ice cover in a year would be followed by unusually extensive perennial ice cover the following year (e.g., 1995 and 1996). Interannual variations in atmospheric effects are known and have been attributed to the Arctic Oscillation (AO), which was highly correlated with the ice cover until the late 1990s. The AO has since became neutral while the perennial ice stayed at low levels with a record low in 2002, another record low in 2005 and a relatively low value in 2006 preceding this year’s big anomaly.

Our analysis of ancillary surface data indicates that much of the Arctic region went through anomalously warm temperatures in winter and spring of 2007, especially in February and April. Relatively warm temperatures in winter and spring keep the ice from gaining the normal thickness that would enable ice to survive the summer melt. Moreover, there were strong southerly winds that may have carried warm air from low latitudes and caused enhanced melting and advection of ice to the north. Also, strong cyclones in the region in August may have caused ice breakup and further ice decline. In a 2002 article, published also in GRL, I predicted that we would lose the perennial ice cover within this century, using statistics based on satellite data that was then showing a nine percent per decade decline. With the perennial ice cover so low in 2007 and the rate of decline currently at 11.4 percent per decade, similar analysis would yield an earlier disappearance. The accelerated decline is likely caused by a process called the “ice-albedo feedback,” explained as follows. Because sea ice has high reflectivity (albedo) compared to that of liquid water, much of the solar heat energy gets reflected back to outer space in sea ice covered areas while such energy gets absorbed in ice free areas of the ocean. Thus, as the ice retreats, the net amount of heat absorbed by the Arctic Ocean increases, causing a warmer ocean. A warmer Arctic ocean, in turn, causes a shorter ice growth period and therefore thinner ice that is vulnerable to total melt in the spring and summer. This leads to less perennial ice and the “feedback” process is repeated. Because of this phenomenon, modelers have estimated that a global warming would be amplified in the Arctic by a factor of about three to five.

A question of interest is, have we reached the “tipping point” or the “point of no-return” in which the Arctic will forever be different? Or is there a chance for the perennial ice to recover? For the latter to happen, we need some sustained periods of relatively cold temperatures that will allow a large fraction of the Arctic sea ice cover to get thick enough to be able to survive the melt season. Unfortunately, what we have been observing is just the opposite. Surface temperature inside the Arctic Circle (>66.5oC) has been increasing at 0.6oC per decade while sea surface temperatures in ice-free regions have been increasing at an average of 0.7oC per decade. The sea surface temperature in the Arctic Basin in 2007 was the highest observed during the satellite era. Concurrently, the open water area in the Arctic has been increasing at the rate of 23 percent per decade and was abnormally high in 2007. This is a manifestation that the impact of ice-albedo feedback is already significant and the tipping point for ice is likely reached.

A blue Arctic Ocean appears imminent, but a ray of hope comes from climate modeling results from the NASA Goddard Institute for Space Sciences which indicate that if we act now and do our responsibilities to limit emissions of greenhouse gases to the atmosphere, we could keep global temperatures from reaching critical levels. Except for the Arctic ice, such endeavor could keep many climate parameters from reaching undesirable tipping points. This should not be a difficult decision for us since planet Earth is the only entity in the vast universe known to have intelligent life and is the only home available for our kids and the future generations.

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Joey Comiso is a senior scientist at NASA Goddard Space Flight Center. He received his BS and PhD in physics at UP and UCLA, respectively, and was the chief scientist in several NASA aircraft missions in the Arctic and Antarctic regions, including a successful flight over of a nuclear submarine near the North Pole. This year, he was chosen as the Most Distinguished UP Alumnus by the UP Alumni Association of America. He was also a contributing author and an expert reviewer of the IPCC report, put together by many scientists around the world, which, together with former Vice President Al Gore, won the Nobel Peace Prize in 2007. E-mail him at [email protected]