Current and Potential Effects of Climate Change on Marine Mammals  Created by: Gaia Meigs-Friend   I. Introduction        Climate change may be the most important environmental threat to  marine mammals.  Polar marine mammals may be effected most of all.  Climate change, also known as global warming, involves excess trapping of heat in the lower atmosphere in response to increases in atmospheric  "greenhouse gases," particularly carbon dioxide (CO2).  There is general consensus in the scientific community that climate change is occurring and will continue to occur (MacGarvin and Simmonds 1996).  Climate change will not  be reversed quickly, if at all (IPCC 2001).       A number of predictions have been made about the disturbances to the  seas and oceans likely to result from climate change.  A doubling of CO2 in  the atmosphere could result in a 3-4 Celsius temperature increase in high latitude seas over the nest 50 years (MacGarvin and Simmonds 1996).  Recent studies have shown that there has been a 40% decline in Arctic sea ice thickness during late summer to early autumn in recent decades and also some decline in winter sea ice thickness.   The area of the sea ice has also decreased 14% since 1978 (USGCRP Alaska).  Predictions are for this sea ice extent, thickness, and persistence to decrease further (IPCC 2001), including an extension of the ice-free season from 41 to 100 days (ECO) and some predictions even have year-round ice completely disappearing by 2100 (US GCRP Alaska).  The Antarctic ice sheet is predicted to gain mass while the Greenland ice sheet is predicted to lose mass.  In general, Antarctic sea ice is predicted to decrease in some areas and increase in others, but overall, rising temperatures could decrease Antarctic sea ice by more than 40% over the next century (Pacific Institute).  Most models show a weakening of the ocean current circulation and potential (perhaps permanent) shut down in either hemisphere beyond 2100 due to the influx of fresh water from an increase in precipitation (IPCC 2001).       Animals in the polar regions could experience some of the greatest impacts from climate change.  Much of this fauna is composed of marine mammal species including whales, walruses, seals, and polar bears.  Increases in water temperatures could cause all marine mammal species to shift their ranges towards the poles in search of water temperatures that they are accustomed to.  Unfortunately, the ranges of the polar species are already as close to the poles as possible and they will not be able to move to cooler waters (MacGarvin and Simmonds 1996).        Reductions in ice cover in the Arctic will have significant impacts on phytoplankton that live under the ice.  These plankton are the base of many marine food webs, including ones with fish and  marine mammals as top predators.  The sea ice is also an important habitat for other marine mammals and reductions or loss of it could have serious impacts on several aspects of these species' life histories (MacGarvin and Simmonds 1996).  Changes in currents could also effect plankton abundance (EIA).        Other Arctic phytoplankton depend on a stratification of marine waters that occurs for a specific period of time.  For this period of time, a thin surface layer of less dense water is separated from the deeper water.  This separates the phytoplankton from the deeper water and helps them absorb the stronger light at the surface.  All the nutrients are trapped in the deeper water, however, so there is an optimal amount of time for the plankton that this stratification can exist.  Increased precipitation and melting of ice would lead to a greater input of fresh water to the area.  This fresh water could increase both the period and strength of stratification and the depth of the surface layer.  A longer period of stratification will benefit small phytoplankton species at the expense of larger ones.  With an increase in the ratio of small to large plankton, consumers will get a reduced amount of energy per mouthful which will reduce their biomass.  Also, there will be a general reduction in productivity due to the increase in depth of the surface layer and strengthening of the gradient.  The increase in depth will mean that the plankton will have a greater chance of being in sub optimal light and the strengthened gradient means more difficulty for nutrients to enter the surface layer (MacGarvin and Simmonds 1996).          Changes in Planktonic abundance and community structure could have effects all the way to the top of the food chain.  Both the fish and zooplankton that marine mammals consume eat these phytoplankton.  Some important zooplankton eat specific phytoplankton species and may be affected by changes in these species abundance.  These changes are important to larval fish that feed selectively on particular phytoplankton species (MacGarvin and Simmonds 1996).

This Web page created for Dr. Rob Jackson's Ecology and Global Change course,
BIN 217, taught at Duke University.
Feel free to contact me at: gm14@duke.edu