Welcome to CGD

expand   collapse

 

Role of sea ice in the climate system

Approximately 15% of the worlds oceans are covered by sea ice at some point during the year. While this is a relatively small fraction of the globe, sea ice is an important part of the global climate system through its influence on the surface energy budget and global hydrological cycle. Investigating the role of sea ice in the climate system is the focus of CGD's Marika Holland.

Sea ice has a high albedo, reflecting from about 60-80% of the impinging sunlight. It also insulates the overlying atmosphere from the relatively warm ocean, modifying the atmosphere-ocean exchange of heat and water. There are important feedbacks associated with these properties of sea ice that influence projected climate sensitivity. The net effect is such that changes in sea ice contribute to a projected amplification of climate warming in the Arctic region. (Accompanying figure) However, the sea ice is complex and the quantitative influence of the myriad feedbacks associated with sea ice is unclear. The possibility of threshold behavior also contributes to the uncertainty of how the ice cover may evolve in the future. These uncertainties contribute to a spread in model projections of Arctic surface air temperature change that is larger than anywhere else on the globe. Interestingly, NCAR's Community Climate System Model has considerable Arctic warming in future climate simulations and a strong polar amplification signal.

Marika Holland has been studying the polar processes that influence the Arctic amplification signal among different climate models. In collaborative work with Cecilia Bitz from the University of Washington, Holland found that models with thinner control climate sea ice, larger increases in ocean heat transport, and larger increases in polar cloud cover, tended to have higher polar amplification (Holland and Bitz, 2003). Additional work (Holland et al., 2006) suggests that sea ice model parameterizations, particularly the sub-gridscale representation of the ice cover, modifies simulated sea ice related feedbacks. By including a representation of the high spatial variability of sea ice, thin regions within the ice pack are resolved. These more easily melt away in response to a warming perturbation, producing open water and removing the highly reflective ice cover. This enhances the positive albedo feedback and can increase polar warming.

Another important influence of sea ice on the climate system is that it effects ocean circulation. When sea ice forms from ocean waters, it rejects most of the salt back to the ocean. Sea ice melt results in a release of fresh water. The motion of the ice cover thus redistributes fresh water in the system, which modifies the ocean salinity and density structure. Observations suggest that the Arctic freshwater cycle is changing, with sea ice conditions being one important component of these changes. In collaborative work with Mark Serreze, Joel Finnis, and Andy Barrett at the University of Colorado, Holland is exploring the impacts and processes of Arctic freshwater cycling on the local, pan-Arctic and global scales. This includes the study of processes that modify Arctic freshwater budgets in the terrestrial, atmospheric and oceanic environments, the integration of these processes to produce the exchange of freshwater between the Arctic and North Atlantic, and ultimately the influence of these processes on the thermohaline circulation (THC) and global climate. The observational record and climate model simulations, suggest an increased freshwater exchange between the Arctic and the North Atlantic in the recent past and into the foreseeable future. Holland's work shows that while liquid freshwater discharge from the Arctic increases in future climate model projections, this is partially compensated for by a decrease in the solid (sea ice) freshwater transport. The influence of these transports on deepwater formation within the north Atlantic is regionally dependent and, in some locations, the reduced ice transport appears to dominate and stabilize deepwater formation in the warming climate (Holland et al., 2006b).