In her November 11th lecture entitled Arctic Rivers, Climate, and Ocean Circulation,Dr. Marika Holland provided an overview of climate modeling efforts currently underway at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.  Dr. Holland’s research is focused specifically on loss of sea ice, alterations to the freshwater cycle, and connecting those factors to broader oceanographic processes in the Arctic and North Atlantic oceans. 

Dr. Holland began her talk with a summary of evidence for a melting Arctic, which includes an observed 10 percent per decade decrease in September sea ice since the 1970s; a recent reduction in the amount of “old” ice (5+ years old) present during fall months; and a sea ice minimum in 2007 that was 23 percent lower than the previous record set in 2005.  Coupled with this reduction in sea ice are indications that both ocean and ambient air temperatures are rising; “pulses” of warm water from the North Atlantic have been entering the Arctic Ocean and circulating throughout its basin in recent years, and scientists have correlated air temperature anomalies with years of minimal sea ice.  The bottom line, according to Dr. Holland and other climate modelers, is that a seasonally ice-free Arctic may be a reality in the near future.

To predict rates of change in Arctic sea ice and oceanographic processes, Dr. Holland and other climate modelers at NCAR employ “coupled” climate models with ocean, atmosphere, land, and sea ice components as inputs and solar radiation, greenhouse gas concentrations, and volcanic-derived aerosols as forcing mechanisms.  The trend simulated by these models was consistent with observed ice loss over the late 20th century, suggesting that they may predict future trends with similar accuracy.  Of particular note is that a seasonally ice-free Arctic may occur as early as 2040, and that an especially rapid rate of ice loss could occur between 2030 and 2040.

Positive feedback loops help explain the abrupt loss of ice projected between 2030 and 2040.  Dr. Holland noted that thinner ice conditions lead to a disproportionately large change in ice extent, in which extensive regions can be melted away.  Basal melting of ice is also enhanced by a warmer ocean – both those waters that enter the Arctic basin from the North Atlantic Ocean, and Arctic water itself absorbing an increasing amount of heat when not covered by ice. 

The freshwater cycle also undergoes changes in Dr. Holland’s climate models.  Broadly, a warmer climate accelerates the Arctic’s freshwater cycle, with increases in ice melt, liquid freshwater transport into the North Atlantic, and net precipitation over the land and ocean as temperatures rise into the 21st century.  Significantly, Dr. Holland noted that these changes are spatially variable across the Arctic basin; whereas other models have predicted widespread freshening of the North Atlantic Ocean and possible major impacts to the Meridional Overturning Circulation, Dr. Holland’s models predict that exported freshwater will largely bypass deep water formation areas east of Greenland and thus impact the Meridional Overturning Circulation less than previously thought.  In the absence of a large and sustained ice melt – such as the loss of the Greenland ice sheet – the MOC is likely to slow over the 21st century, but not collapse entirely.  In Dr. Holland’s estimation, then, scenes from “The Day After Tomorrow” are not likely to occur in our lifetimes.