Groentje-18
20 juni 2007, 00:27
S IT WET OR DRY? GREENLAND'S SNOW SPEAKS VOLUMES Dr. Heidi Cullen (http://climate.weather.com/mission/), Climate Expert
http://image.weather.com/web/multimedia/images/onedegree/blogHome_cullen.jpg
2006 was not a good year for the Greenland ice sheet. Greenland, setting its ironic name aside for a second, is hemorrhaging the freshwater that's been locked up on its surface in the form of snow and ice. Daily satellite observations going back to 1988 show the number of days during which snow melted on certain areas of Greenland's ice sheet (mostly the western, southeastern and northeastern coasts) has increased by more 10 days (compared to the long-term average). While more melting means more water pouring from the ice sheet into the surrounding ocean, it means less of the sun's radiation being reflected back into the atmosphere -- in other words, more melting.
The daily satellite data set is a pretty impressive thing in and of itself. The sensor can see through clouds and doesn't need sunlight to make measurements, so researchers get multiple daily observations enabling them to analyze trends in the duration of snowmelt and extent over specific areas of Greenland. As you can see in this snapshot, the parts of Greenland in purple have seen more than 60 days of melting.
http://image.weather.com/web/multimedia/images/onedegree/heidi_52907.jpg
Photo: Nasa Goddard Space Flight Center (http://www.nasa.gov/centers/goddard/news/topstory/2007/snowmelt_greenland.html)
One of the really interesting ramifications of these findings boils down to one important difference. Is the snow wet or is it dry? Ask any kid and they'll tell you all about the best type of snow to make a snowball. Ask a climatologist, and they'll tell you about the differing physics of wet and dry snow. Wet snow absorbs more of the sun's radiation, reflecting only 50-60 percent back into the atmosphere. Dry snow reflects about 85 percent of the sun's radiation. So watch out for all that wet snow, it absorbs three to four times as much energy as dry snow, and that only adds on to Greenland's further greening.
http://climate.weather.com/blog/9_12658.html
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In the paper, Hansen et al. present evidence for their view that greenhouse gases that have already been emitted place the planet at the edge of "dramatic climate change that could run out of control." They base their conclusions on paleoclimate data and recent evidence of accelerating melting in the vast ice sheets in Greenland and parts of Antarctica.
The study states that the IPCC's assumptions that led to their sea level estimate don't match the historical record and fail to align with current observations of ice sheet behavior in part because they assume that ice sheets respond to warming in a linear fashion.
In making the case for a nonlinear response, Hansen et al. cite the "albedo flip" property of ice and water, which Dr. Cullen discussed in a recent Forecast Earth blog post (http://climate.weather.com/blog/9_12658.html). In summary, the flip occurs when a climate forcing, such as greenhouse gases or increased solar radiation, causes the ice and snow to melt into darker and slushier ice and snow and water. When this happens, the melted and now darker ice and snow reflect less solar energy than they did in a more solidly frozen state.
This melting feedback in turn accelerates warming.
Or, to put it in the most alarming way possible as Hansen and his colleagues prefer: "A climate forcing that ‘flips' the albedo of a sufficient portion of an ice sheet can spark a cataclysm."
http://image.weather.com/web/multimedia/images/onedegree/blogHome_cullen.jpg
2006 was not a good year for the Greenland ice sheet. Greenland, setting its ironic name aside for a second, is hemorrhaging the freshwater that's been locked up on its surface in the form of snow and ice. Daily satellite observations going back to 1988 show the number of days during which snow melted on certain areas of Greenland's ice sheet (mostly the western, southeastern and northeastern coasts) has increased by more 10 days (compared to the long-term average). While more melting means more water pouring from the ice sheet into the surrounding ocean, it means less of the sun's radiation being reflected back into the atmosphere -- in other words, more melting.
The daily satellite data set is a pretty impressive thing in and of itself. The sensor can see through clouds and doesn't need sunlight to make measurements, so researchers get multiple daily observations enabling them to analyze trends in the duration of snowmelt and extent over specific areas of Greenland. As you can see in this snapshot, the parts of Greenland in purple have seen more than 60 days of melting.
http://image.weather.com/web/multimedia/images/onedegree/heidi_52907.jpg
Photo: Nasa Goddard Space Flight Center (http://www.nasa.gov/centers/goddard/news/topstory/2007/snowmelt_greenland.html)
One of the really interesting ramifications of these findings boils down to one important difference. Is the snow wet or is it dry? Ask any kid and they'll tell you all about the best type of snow to make a snowball. Ask a climatologist, and they'll tell you about the differing physics of wet and dry snow. Wet snow absorbs more of the sun's radiation, reflecting only 50-60 percent back into the atmosphere. Dry snow reflects about 85 percent of the sun's radiation. So watch out for all that wet snow, it absorbs three to four times as much energy as dry snow, and that only adds on to Greenland's further greening.
http://climate.weather.com/blog/9_12658.html
------------------------------------------------
In the paper, Hansen et al. present evidence for their view that greenhouse gases that have already been emitted place the planet at the edge of "dramatic climate change that could run out of control." They base their conclusions on paleoclimate data and recent evidence of accelerating melting in the vast ice sheets in Greenland and parts of Antarctica.
The study states that the IPCC's assumptions that led to their sea level estimate don't match the historical record and fail to align with current observations of ice sheet behavior in part because they assume that ice sheets respond to warming in a linear fashion.
In making the case for a nonlinear response, Hansen et al. cite the "albedo flip" property of ice and water, which Dr. Cullen discussed in a recent Forecast Earth blog post (http://climate.weather.com/blog/9_12658.html). In summary, the flip occurs when a climate forcing, such as greenhouse gases or increased solar radiation, causes the ice and snow to melt into darker and slushier ice and snow and water. When this happens, the melted and now darker ice and snow reflect less solar energy than they did in a more solidly frozen state.
This melting feedback in turn accelerates warming.
Or, to put it in the most alarming way possible as Hansen and his colleagues prefer: "A climate forcing that ‘flips' the albedo of a sufficient portion of an ice sheet can spark a cataclysm."