northern land areas. Snow cover on land has decreased over the past several decades (Shi et 2013), especially in spring (Derksen and Brown, 2012). In percentage terms, the decline al., in end-of-season (June) snow cover has actually been more pronounced (-18% per decade) than the more publicized decline in end-of-melt-season (September) sea ice (-11% per decade). As with the loss of sea ice over the ocean, the loss of springtime snow cover enables the less reflective land surface to absorb greater amounts of incoming solar radiation, thereby contributing to warmer spring conditions in the northern high latitudes. Because land has a smaller heat capacity than the ocean, there is less seasonal lag in the warming relative to the loss of terrestrial snow cover compared to the loss of sea ice. 3.2. Increased atmospheric humidity and associated downwelling radiation The saturation vapour pressure (water vapor-holding capacity) of air increases with relative humidity (the ratio of its actual moisture content to its temperature. If air’s saturation value) remains approximately constant, the humidity can be expected air’s actual to increase in a warming climate. Water vapor, in turn, is a strong greenhouse gas. Increases in humidity can therefore be expected to result in additional trapping of the infrared radiation emitted by the Earth. The corresponding increase in downwelling radiation will then enhance the warming of the surface. Even before the recent decline of sea ice, Francis and Hunter (2006) showed that variability of downwelling radiation was associated with sea ice variations on interannual timescales. Since a loss of sea ice leads to increased atmospheric moisture, which then increases the downwelling radiation and warming of the surface, the Arctic is a prime candidate for a manifestation of the so-called and amplification of surface warming. Another reason why the “water vapor feedback” Arctic should be sensitive to the water vapour feedback is because its atmosphere is very
northern land areas. Snow cover on land has decreased over the past several decades (Shi et al., 2013), especially in spring (Derksen and Brown, 2012). In percentage terms, the decline in end-of-season (June) snow cover has actually been more pronounced (-18% per decade) than the more publicized decline in end-of-melt-season (September) sea ice (-11% per decade). As with the loss of sea ice over the ocean, the loss of springtime snow cover enables the less reflective land surface to absorb greater amounts of incoming solar radiation, thereby contributing to warmer spring conditions in the northern high latitudes. Because land has a smaller heat capacity than the ocean, there is less seasonal lag in the warming relative to the loss of terrestrial snow cover compared to the loss of sea ice. 3.2. Increased atmospheric humidity and associated downwelling radiation The saturation vapour pressure (water vapor-holding capacity) of air increases with temperature. If air’s relative humidity (the ratio of its actual moisture content to its saturation value) remains approximately constant, the air’s actual humidity can be expected to increase in a warming climate. Water vapor, in turn, is a strong greenhouse gas. Increases in humidity can therefore be expected to result in additional trapping of the infrared radiation emitted by the Earth. The corresponding increase in downwelling radiation will then enhance the warming of the surface. Even before the recent decline of sea ice, Francis and Hunter (2006) showed that variability of downwelling radiation was associated with sea ice variations on interannual timescales. Since a loss of sea ice leads to increased atmospheric moisture, which then increases the downwelling radiation and warming of the surface, the Arctic is a prime candidate for a manifestation of the so-called “water vapor feedback” and amplification of surface warming. Another reason why the Arctic should be sensitive to the water vapour feedback is because its atmosphere is very
Global%20_and_Planetary_Change_Walsh.docx