As the Cryosphere Relentlessly Meltsby Bill Chameides | January 26th, 2011
posted by Erica Rowell (Editor)
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This image uses colors to represent the Earth’s albedo (reflectivity) over land surfaces. The red areas which show the brightest, most reflective regions are most concentrated in the Arctic where much of the world’s ice is found. (NASA)
Ice: an important and shrinking component of the Earth’s climate system.
Less than two percent of our planet’s water is found in the form of ice or snow — with most found on land (in the form of ice sheets and glaciers) and the rest as sea ice. Two percent is a small amount, but ice’s role in the climate is huge.
Sea Level: The more ice on land, the less is in the ocean and therefore the lower the sea level. It is estimated that if all the world’s glaciers melted, sea level on average would be about 230 feet higher — placing cities like New York and London underwater.
Reflection of Sunlight Ice-Albedo Feedback: Of course melting sea ice doesn’t contribute to sea level rise (since the ice is already, well, in the sea), but together with ice sheets and glaciers it cools the planet. Ice’s white surface reflects significantly more sunlight back to space than do other surface features like soil, forests, and water. That’s why the albedo over the Arctic is larger than that for most other parts of the globe (see figure above; click on it for larger image). (The exception is very arid regions like the Sahara where desert sands can be as reflective as some ice surfaces.) The implications for the climate are obvious: melt ice and you warm the planet.
Currently, the Earth reflects about 30 percent of the Sun’s radiation that reaches the planet; this number expressed as a ratio (i.e., 0.3) is referred to as the Earth’s reflectivity or (if you want to sound like a climate scientist) its albedo. It is estimated that if all the Earth were covered with ice, the albedo would rise to 0.84, leading to a very cold world indeed. Alternatively, if all the ice on Earth were melted, we would have a much warmer planet.
For this reason, melting ice gives rise to a potentially important amplifier or positive feedback for any driver of global warming, such as increases in carbon dioxide (CO2) or other greenhouse gases. Here’s an example:
- Global temperatures increase because of CO2,
- Some ice melts,
- Earth’s albedo decreases,
- More energy is absorbed from the sun,
- Global temperatures increase more,
- Some more ice melts,
- And so on …
This is known in the jargon as the ice-albedo-feedback.
Given the above, the fate of the world’s ice as the globe warms is key to understanding how the climate works in general, and more specifically, and arguably importantly, how the climate might respond to ever larger amounts of heat-trapping greenhouse gases in our atmosphere. And in that light, the findings of two papers that appeared in the scientific literature last week are not reassuring if you’re a fan of the current climate conditions.
- In a paper published in the journal Environmental Research Letters, Marco Tedesco of the City College of New York and co-authors used satellite data, ground observations and modeling to assess conditions of the Greenland ice sheet from 1970 to 2010. They found that 2010 saw “new records … for surface melting and albedo, runoff, the number of days when bare ice is exposed and surface mass balance of the Greenland ice sheet, especially over its west and southwest regions.” They speculate that 2010’s record melting rates were triggered by high surface temperatures in the spring and summer. (For more on the authors’ paper, visit their blog.)
- In a paper published in the journal Nature Geoscience, author Mark Flanner of the University of Michigan et al used satellite data along with ground observations to estimate the change in the albedo from ice cover between 1979 and 2008. The authors estimate that “cyrospheric cooling declined by 0.45 [watts per meter squared] Wm-2 from 1979 to 2008, with nearly equal contributions from changes in land snow cover and sea ice.” To put that into perspective≤ consider this: the amount of warming from the increase in CO2 alone since the Industrial Revolution has been about 1.66 Wm-2, the effect seen here from the melting of the Northern Hemisphere ice since 1979 is equivalent to about 25 to 30 percent of that.
And so, as the politicians and nations debate what to do about global warming, you may notice that the world, it is turning, while the cryosphere relentlessly melts..filed under: climate change, faculty, global warming, Planetary Watch
and: albedo, climate, climate models, climate science, cryosphere, feedback, glaciers, ice, positive feedback, sea ice, sea level rise