Antarctica's Bright Future

The Antarctic ice sheets are extensive; stretching out for 14 million square km, they contain nearly 90% of all the freshwater on the planet. Melting all of the ice would lead to an increase in sea level of 60 m (NSIDC). Taking into account that Antarctica is one of the coldest places on Earth with a balmy average summer temperature of -20 ºC and a slightly cooler winter temperature of -60 ºC, this seems unlikely to happen. But then there was global warming — so should we get worried and start moving inland?

Contrary to the Arctic, which is a sea surrounded by land, Antarctica is land surrounded by sea. This means that precipitation plays an important role in ice formation, which in the Arctic ocean is less so, because most precipitation occurs on or near land. Interestingly, this snow could also mean that Antarctica might stay cool despite warming from climate change.

One of the effects of global warming is that precipitation over the Antarctic ice sheets is projected to increase (also see the 4th Assessment Report of the IPCC on Antarctica). A recent study done by Picard et al. (2012) looked at the importance of fresh snow on the albedo. It turns out that it is in fact very important.

Fresh, pristinely, white snow reflects about 85% of the sunlight. As I wrote in my previous blog post, this means that any pollution that makes the snow greyer reduces the albedo, so the snow will trap more heat and is thus more susceptible to melt. Soot can have this effect. It is stronger in the Arctic than on Antarctica, because most of the sources of pollution are on the Northern Hemisphere.

Snow is sharply faceted when it is fresh and becomes rounder and larger as it ages. The bigger the crystals, the darker the snow becomes. This is illustrated in the picture below, which is taken with an electron microscope.

Left: Fresh snow is sharper and more reflective. 

Right: As snow ages, it becomes smoother and larger, and less reflective. 

(Source: Electron and Confocal Microscopy Laboratory, USDA via NOAA)

Old snow thus reduces the albedo. In the study conducted by Picard et al., satellite data and model outputs were used to see how strong this effect is, and whether the projected increase in precipitation would lower the albedo of the Antarctic ice sheets.

They found that there is a very consistent cycle of snow grain size. In winter, the crystals are small and therefore bright. When it becomes warmer in the Antarctic summer (December in the Southern Hemisphere) the grains start to grow. In summers with high precipitation, the growing of the grain sizes is less, and the ice stays lighter, leading to increases in albedo of about 0.02-0.03 compared to normal summers. This could lead to a drop in surface temperature of 0.5 ºC in summer, and 0.3 ºC on a yearly average. Moreover, the additional snow could offset some of the loss in ice.

Picard et al. point out that there is also much research that still needs to be done when it comes to the interaction between snow and the climate system. Many climate models do not incorporate (all of) the characteristics of snow and its behaviour. This study shows that something seemingly small such as snow grain size actually plays an important role in albedo, and thus in the climate.

It is interesting to compare these results with studies done on Greenland, which has the largest land ice sheets in the Northern hemisphere. Contrary to Antarctica, Greenland's ice has been shown to darken in the last years (also see the Arctic Report by the NOAA). A recent analysis by Box et al. (2012) identified three reasons for this. First of all, there have been more warm air currents over the ice sheets that have increased the snow grain growth, so more of the snow crystals have become large and smooth, which has reduced the albedo. Secondly, more solar heat has reached the surface, warming it, and decreased the albedo through that process. Thirdly, there has been less snowfall, so there has been little fresh snow to raise the albedo (contrary to what is happening in Antarctica).

Combined, these processes have started to reduce the albedo in Greenland quite severely. In the picture below, the albedo over Greenland has been indicated, based on satellite data. In particular near the edges of the ice sheet, the albedo has dwindled strongly. This has brought up concerns as to the melting of the Greenland ice sheet, which like that of Antarctica, stores a large amount of freshwater and could raise sea levels by 6 metres (NSIDC).

The changes in albedo over the Greenland ice sheet in the summer of 2011 compared to the average of 2000-2006. The darker the area is, the greater the reduction in albedo. This is a combination of increase in snow grain size, surface melting and soot. 

(Source: NOAA)

To conclude, where Antarctic ice sheets are maintaining and/or increasing their albedo, the ice sheets of Greenland are only becoming greyer and lower in albedo. In this case, we might literally say that Greenland's future is a lot less bright than that of Antarctica.

Further Reading

• Arctic vs Antarctic (NSIDC) on the differences between the North and South Pole in terms of sea ice
• About Antarctica, by the British Antarctic Survey, on the characteristics of Antarctica
• Highlights of the Arctic Report Card, by the National Oceanic and Atmospheric Administration (NOAA), on the darkening of Greenland
• Snowfall Brightens Antarctic Future, an analysis of the research of Picard et al. by Charles Zender for Nature Climate Change

References

• Box, J.E., Fettweis, X., Stroeve, J.C., Tedesco, M., Hall, D.K., and K. Steffen (2012), Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821, doi:10.5194/tc-6-821-2012
• Picard, G., Domine, F., Krinner, G., Arnaud, L., and E. Lefebvre (2012), Inhibition of the positive snow-albedo feedback by precipitation in interior Antarctica, Nature Climate Change, 2, 795, doi:10.1038/nclimate1590
• Zender, C.S. (2012), Snowfall brightens Antarctic future, Nature Climate Change, 2, 770.