Monday :: Jan 14, 2008

Antarctica and global warming

by Christina Hulbe

If you are a aficionado of the global warming "debate," you have probably read at one time or another that current trends in the Antarctic show that there is no such thing as global warming. This is, of course, not true. But the Antarctic is a vast region and it can be daunting to piece together the science stories that do get out into the mainstream press into one coherent picture. My goal here is to provide a broad overview of some key issues. We can fill in more details in the months ahead.

where the ice is
Antarctica can be divided into three major geographic regions: East Antarctica, West Antarctica, and the Antarctic Peninsula. The Transantarctic Mountains divide the continent into eastern (on the Indian Ocean side) and western (on the Pacific Ocean side) regions. The large East Antarctic Ice Sheet flows slowly through most of its interior (10's of meters per year at most), until the ice approaches the coast and is channeled through fast-flowing (100's of meters per year) outlet glaciers. The ice sheet surface is high, dry, and very cold. The West Antarctic Ice Sheet, as I have discussed here before, is a faster flowing ice mass that may be vulnerable to rapid change.

The Antarctic ice sheets store 90% of the ice on Earth and close to 70% of the planet's fresh water. The West Antarctic ice sheet contains enough ice to raise sea level between 5 and 6 meters, were it all to melt. The East Antarctic Ice Sheet holds about 10 times more. (The total ice volume is larger than this but once the ice is melted, you have to fill in the hole it left behind.) The relatively warm Antarctic Peninsula supports a series of ice caps and outlet glaciers that toether are estimated to contain less than half a meter of sea level equivalent.

The continent is surrounded, seasonally, by sea ice that freezes at the ocean surface. Just as in the Arctic, sea ice formation in the Antarctic is important to many parts of the Earth system, including ocean circulation and climate.

Here are some maps of Antarctic surface elevation, mean annual temperature, snow accumulation, and elevation of the bedrock beneath the ice sheet. The polar research group at the University of Illinois Urbana-Champaign maintain a wonderful Cryosphere Today website (now with a mobile web app...finally, a reason to buy an iPhone).

ice sheet mass balance
The mass balance of an ice sheet or glacier is the accounting between the amount of new ice (the water equivalent contained in the snow) that accumulates in a year and the amount of ice lost to melting or to the calving of icebergs. A positive mass balance means a growing ice mass, and thus a withdrawal from sea level while a negative mass balance means the opposite.

Suppose we start with a glacier or ice sheet that is in equilibrium with a given climate state. If climate cools, two things may happen: first, the area of the glacier subject to melting will likely decrease and second, the amount of snow the glacier receives over the course of the year may also decrease. In general, cooler air can hold less moisture than warmer air (this is why the cold interior of Antarctica is a desert). Whether or not the glacier grows in the cooling world depends on the relative magnitudes of the two changes.

If climate warms, the opposite may transpire: more melting and more precipitation are both possible. The extra precipitation may be either snow or rain, depending on the temperature at which it falls (only the former is good news if you want to grow a glacier). Whether or not the glacier grows depends on the relative magnitudes of the extra melting and any extra snow accumulation. There are a few growing glaciers around the world, and they are all doing so because the warming atmosphere is bringing them new snow at a greater rate than enhanced melting is taking it away.

In Greenland, we see both enhanced accumulation at high elevations and enhanced melting at low elevations. If the warming continues, the elevation of the transition from net mass gain to net mass loss (called the equilibrium line) moves higher and higher, and eventually enhanced melting will outpace enhanced accumulation.

mass balance trends
The Antarctic Peninsula (AP, map), a narrow, mountainous chain extending north toward Drake Passage, is warming at a phenomenal rate, about 0.5 degrees Celsius per decade. Temperatures on the AP are relatively mild, compared with the rest of the continent, and the recent warming has resulted in extensive surface melting, collapse of several ice shelves, and glacier retreat (abstract of a technical paper).

In contrast to the peninsula, the Antarctic interior is very cold (maps) and not likely to experience substantial melting any time soon, though there is evidence of melting in some limited areas and in particular, exceptional events. Just for fun, I dug a snow pit back in 2006 at a West Antarctic camp near the Transantarctic Mountains (near the outlet of Mercer Ice Stream) and found ice layers every few years, as far down as I cared to dig, which was about 20 years. The layers were perhaps refrozen melt from foehn winds coming down off the mountains. I discovered this year that topographic maps made in the 1960's show melt ponds on the glaciers upstream of that region.

A warmer, moister atmosphere may bring more snow to the Antarctic interior but meteorologists have not yet seen such effects. You may have read cherry-picked quotes about this from (the venerable, imho) David Bromwich at Ohio State. The more complete story is that at present, the effects of global warming on snow accumulation in Antarctica are expected to be very small and there are very few data (from weather stations) with which to look for that signal.

There are, in fact, changes in the mass balance of the Antarctic ice sheets, especially in west Antarctica, but they are primarily related to changes in the rate of ice flow and are spatially heterogenous.

Mass balance may be assessed in several ways: an accounting between annual net accumulation and annual discharge through outlet glaciers; satellite observation of change in surface height; or satellite observation of change in ice sheet mass (via gravitational attraction). Each method has its advantages and disadvantages, each has a different resolution and area coverage. Individual calculations tend to span different time intervals as well.

Overall, most estimates of Antarctic mass balance yield slightly negative values but slightly positive values are reported as well. A nice summary graphic can be found here. The Intergovernmental Panel on Climate Change reviewed all of the estimates in its Fourth Assessment Report (IPCC AR4) and concluded (pdf) that the range of estimates is from 50 gigatons per year of ice accumulation to 200 gigatons per year of ice loss over the time span 1993 to 2003. The review group declined to conduct an error analysis because differences among methodologies make this useless. Over the longer time period from 1961 to 2003, the IPCC AR4 reports a range from 100 gigatons per year ice accumulation to 200 gigatons per year of ice loss.

temperature trends
Just as with mass balance, the conclusions one might draw regarding temperature trends in the Antarctic depend in part on the window of observation. In this case, both the number of years included in the analysis and the season inspected make a difference in the conclusions one might draw (pdf of a technical paper, figures 9 through 13 are of particular interest). In a nutshell:

1. the Antarctic Peninsula is warming rapidly in every season of the year, over any time interval you care to pick, and

2. over the rest of the continent, temperature trends are spatially and seasonally variable in a manner consistent with the southern annular mode of atmospheric variability.

In order to make progress from here, we need to step back and think about the physics (always one of my favorite moves). What might we expect to see happen in Antarctica as a result of greenhouse gas-forced global warming? Do we see that signal?

southern annular mode of atmospheric variability
When looking for climate trends, we must distinguish between changes from one year to the next due to internal oscillations (such as the tropical El Nino Southern Oscillation) and changes due to external forcing. Outside of the tropics, the leading mode of variability in atmospheric circulation is an oscillation of atmospheric mass between mid- and high-latitudes in both the northern and southern hemispheres. Centered about the poles, these are often called annular modes. I've written a bit about the northern annular mode here. It should be noted that ENSO is a coupled ocean-atmosphere phenomenon while the annular modes are due only to atmospheric dynamics.

The annular modes describe variability in the atmospheric circulation that is not due to the changing seasons (this is termed anomalous atmospheric flow). They reveal themselves as distinctive patterns in sea level pressure, temperature, wind strength, and other weather phenomena. The convention is that lower-than-normal pressure conditions (relatively less mass) over the poles are termed a high index or positive state. The high index state corresponds to a strengthening of the polar vortex, with westerly wind anomalies around 60 degrees latitude.

State-of-the-art coupled climate models predict a shift toward a more uniformly positive state for the southern annular mode (SAM) under global warming. This is just what has been observed over the last few decades (link to a technical paper). In order to be clear about attribution, climate modelers use suites of model runs in which the effects various forcings (anthropogenic increases in greenhouse gasses, ozone depletion, volcanic particles, and so on) are evaluated individually and in combination with other forcings. The conclusion, in the case of the SAM, is that both anthropogenic greenhouse gas emissions and the decline in stratospheric ozone play a role.

effects of a more positive SAM
A shift to a more persistently positive SAM means a poleward shift of the westerly jet stream, an intensification of westerly winds over the circumpolar ocean (at about 60 degrees south latitude), and weaker westerlies farther north (link to a technical paper). The shifting winds are a result of changes in atmospheric pressure gradients. These changes, in turn, affect regional air temperature, ocean circulation and temperature, and sea ice processes. The circumpolar ocean circulation intensifies as the SAM becomes more positive.

The effect on ocean temperature of changing atmospheric circulation depends on latitude, and the connection is through wind-driven currents in the shallow part of the ocean (the top 100 meters or so). Latitudinal patterns in wind direction result in latitudinal patterns in the transport of shallow water that in some places cause water to "pile up" (termed convergence) and in some places cause divergence at the surface. Where there is convergence, water must move down, away from the surface (termed downwelling) and where there is divergence at the surface, water moves up from the depths (termed upwelling). The effect of the shift toward a more positive SAM in the southern ocean is enhanced warming in some locations and damped warming in others, as downwelling of warmed surface waters and upwelling of relatively cool deep waters adjust to the new wind patterns. The ocean is warming as a result of the greenhouse gas forcing, but the pattern of warming is determined by the change in the SAM (which itself is a result of global warming and ozone depletion).

Climate models show that the changes in wind-driven surface currents also affect Antarctic sea ice. That anomalous divergence noted above moves sea ice farther north (away from the continent) than would otherwise be the case. This, in turn, opens up new ocean surface to freeze and in the end, expands sea ice cover around the continent (link to a technical paper). Increased sea ice cover over the ocean further modifies the ocean temperature (because it is a barrier for heat exchange between ocean and atmosphere). Another important consequence of the enhanced sea ice transport is that sea ice closer to the continent becomes, on average, thinner than it would otherwise have been (once the initial, thin, sea ice cover is established, continued growth by freezing on the the lower surface is slow; another technical paper).

As noted above, the narrow, mountainous Antarctic Peninsula (AP) is warming at a very rapid rate. Analyses of regional weather station and troposphere temperature and circulation observations, together with medium-range weather forecast data and climate models, have been used to track what's happening there and, as it turns out, find a connection with the poleward-shifting and intensifying polar vortex. As you might expect by now, there's more than one culprit at work. In short, changes in storm tracks (which move energy through the atmosphere) and the increased sea ice transport away from the continent (two things here: the sea surface is relatively warm and growing new ice liberates energy to the atmosphere) work together to warm the AP. The strongest warming is on the western side of the peninsula but the increasingly strong westerlies have been pushing those air masses up, over the mountains and to the eastern side of the peninsula.

There are, of course, important biological effects of global warming in the Antarctic. This post has run on long enough (and I've only scratched the surface) but I'll leave you with one last link, to the remarkable PenguinScience site. If you click through to nothing else in this post, visit there. Make sure to check the education page there, where you can track the stories of an Adelie Penguin colony at Cape Royds, on Ross Island (I was there this past November for a visit).

summing up
So do we see the effects of global warming in Antarctica? The answer is an emphatic yes. But the signal is not so simple as "getting warmer" or "more snow" and for sea ice, it is a counterintuitive (relative to what we see transpiring in the Arctic) expansion of ice cover. Stay tuned, there is much more story to tell.

The British Antarctic Survey has a great statement on climate change and the Antarctic at its website.

Christina Hulbe :: 6:10 AM :: Comments (18) :: Digg It!