Loss Of Sea iCE

Sea ice is formed by the freezing of seawater and so is to be found at high latitudes in both the Northern and Southern hemispheres, especially in their respective winters and springs. Sea ice is important to the climate system because it has a higher albedo than seawater (and therefore reflects back more solar radiation to space), because it affects the way the atmosphere and ocean exchange heat and water vapor, and because when sea ice forms, salt is extruded into the ocean, and when it melts, freshwater is added to the ocean. As the oceans warm, it would seem natural for the extent of sea ice to diminish, and indeed there is evidence (both anecdotal and quantitative) that sea ice already has diminished. If the sea-ice extent were to continue to diminish, then the consequences could be quite significant, from changes in physical effects already mentioned to the opening of the famed Northwest Passage through the Arctic Ocean for shipping, to diminished habitats for some of the most charismatic life forms on the planet, such as polar bears and penguins. What has happened in the past, and what will happen?

Before the era of satellites, observations of sea ice were taken somewhat intermittently from ships and aircraft and from the coast, but the observations were naturally hard to synthesize into a coherent record of total sea-ice extent. Since the 1970s, satellites that detect passive microwaves emitted from the surface have provided a much more comprehensive record. Objects near the surface emit radiation over a whole range of wavelengths, and although most of it is in the infrared range, microwave radiation is also emitted, albeit at relatively low energy levels. Unlike infrared radiation, microwave emission is determined not so much by the temperature of an object as by its physical composition—its crystalline structure, for example. Of relevance to us, sea ice emits more microwave radiation than does seawater. Furthermore, clouds emit very little microwave radiation, and the radiation emitted from the surface passes through clouds. For these reasons, passive microwave detection is a good way to measure sea ice, year round, day or night, and in cloudy or clear sky conditions.

Over the past couple of decades, sea-ice area has certainly diminished, particularly in the Northern Hemisphere (figure 7.8). The biggest changes have been in the summer months, where the change is about twice that of the annual variation, so the changes have been significant but not overwhelming. The total ice area in the Northern Hemisphere varies between about 6 X 106 km2 (in summer) and 14 X 106 km2 (in winter), so the decrease corresponds to about a 10% drop of the summer value. However, in the year 2007, the ice cover fell by about 25% over previous years, with a subsequent partial recovery, so it is possible that the decline in Arctic sea-ice cover is accelerating, but there is insufficient data to be definitive.

Perhaps the biggest fear associated with diminished sea ice is that it will enable the climate system to reach a so-called tipping point, beyond which changes in climate are large and almost irreversible. As ice diminishes, less solar radiation is reflected to space and so more is absorbed at Earth's surface, further warming the climate, and so on. The effect is most pronounced in summer, when sea ice is already at a minimum, because in winter the amount of incident solar radiation is already small. It is hard to be definitive about whether, as global warming progresses, such an effect could lead to a rapid diminution and even total disappearance of Arctic sea ice (where effects have been largest so far) in summer. Most model calculations do not suggest that a disappearance is likely to happen in the near future, but if the level of CO2 were, for example, to quadruple (which it easily will if we freely burn all the coal, oil, and shale buried in Earth) and the

Figure 7.8. Sea ice cover for the Northern Hemisphere from satellite data. Perennial ice excludes seasonal ice cover, and multi-year ice accounts only for ice that has existed for more than one season. Source: Adapted from Comiso, 2002, and Comiso et al., 2008.

climate system were to warm by several degrees Celsius (which would then be quite likely), disappearance cannot be ruled out. Indeed, given that such warming would, as we have discussed, then likely persist for centuries, there is a distinct possibility that the ice sheets on land would also melt, with still more catastrophic consequences.

0 0

Post a comment