    Latest Posts
|
 Is Antarctica losing or gaining ice?The amount of ice surrounding Antarctica is now at the highest level ever measured for this time of the year, since satellites first began to monitor it almost 30 years ago. All of the IPCC’s models of Antarctica in the twenty-first century forecast a gain in ice, as a warmer surrounding ocean evaporates more water, which subsequently falls in the form of snow when it hits the continent. Other studies, such as Peter Doran’s in Nature in 2003, show actual cooling in recent decades. It’s simply too cold for rain in Antarctica, and it’ll stay that way for a very long time. The bottom line is that there is more ice than ever surrounding Antarctica (source: Patrick Michaels). What the science says...Overall, Antarctic land ice is falling. Antarctic sea ice is growing despite a warming Southern Ocean. It's important to distinguish between Antarctic land ice and sea ice which are two separate phenomenon. Antarctic Land IceMeasuring changes in Antarctic land ice mass has been a difficult process due to the ice sheet's size and complexity. However, over the last few years, the Gravity Recovery and Climate Experiment (GRACE) satellites have been able to comprehensively survey the entire ice sheet. Using measurements of time-variable gravity, Velicogna 2007 determined mass variations of the entire Antarctic ice sheet from 2002 to 2005. They found the overall mass of the ice sheet decreased significantly, at a rate of 152 ± 80 cubic kilometers of ice per year (equivalent to 0.4 ± 0.2 millimeters of global sea-level rise per year). Most of this mass loss came from the West Antarctic Ice Sheet. Figure 1 displays Antarctica's ice mass from 2002 to 2005 - the red crosses is their best estimate with the dotted line the linear trend.
Also illuminating is Figure 2 which contrasts the mass changes in West Antarctica (red) compared to East Antarctica (green):
Most of the Antarctic mass loss comes from Western Antarctica with a mass loss of 148 ± 21 km3/year. The mass loss from East Antarctica is 0 ± 56 km3/year. Because of its relatively large uncertainty, it's uncertain whether East Antarctica is in mass balance or not. Why is Western Antarctica losing ice mass while East Antarctica is relatively steady. The hole in the ozone layer above the South Pole causes cooling in the stratosphere. This increases circular winds around the continent preventing warmer air from reaching east Antarctica and the Antarctic plateau. The flip side of this is the Antarctic Peninsula in Western Antarctica has "experienced some of the fastest warming on Earth, nearly 3°C over the last half-century". Antarctic Sea IceAntarctic sea ice has shown long term growth since satellites began measurements in 1979. This is an observation that has been often cited by skeptics as proof against global warming. However, in all the skeptic articles I've read, not one has raised the crucial question: why is Antarctic sea ice increasing?
The implicit assumption is that if Antarctic sea ice is growing, it must be cooling around Antarctica. This is decidely not the case. In fact, the Southern Ocean has been warming faster than other oceans in the world. The average global ocean temperature trend has been 0.1°C per decade from 1955 to 1995. In contrast, the Southern Ocean has been warming at 0.17°C per decade. Not only is the Southern Ocean warming, it is warming faster than the global trend.
So this raises the big question: if the Southern Ocean is warming, why is Antarctic sea ice increasing? The paper Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions (Zhang 2007) attempts to answer this question. The paper uses a coupled ocean/sea ice model to find the predominant reason that sea ice is increasing is due to a decrease in upward ocean heat transport. Eg - less heat is being carried up by ocean convection to melt sea ice. The reason for this is a complex chain of events. When surface temperature increases, the upper ocean warms and ice growth decreases. This leads to a decrease in salt rejection from new ice. The salinity of the upper ocean falls. Lower salinity and warmer water results in lower water density in the upper ocean. With fresher, less dense upper water, there is now increased stratification of ocean layers which weakens convective overturning. Less ocean heat is transported upwards. This leads to a decrease in ice melting from ocean heat. Hence we observe an increase in net ice production - sea ice increases. While all that is a bit of a mouthful, it's actually a simplification of the process as there are various feedbacks along the process. Warming air increases upper ocean temperature which affects air temperature through air-sea interactions. Warming temperature leads to increased precipitation which increases sea ice growth. More sea ice means less atmospheric heat can penetrate waters. The bottom line is the answer to Antarctic sea ice isn't simple - the Southern Ocean is a complex system with a number of factors likely contributing. One factor certainly isn't a contributor - the simplistic explanation that it must be cooling around Antarctica is not the case. Warming is happening - how it affects specific areas is complicated.
Further readingTamino compares and analyses the long term trends in sea ice data from the Northern and Southern Hemisphere in Sea Ice, North and South, Then and Now. Comments 1 to 10:
Post a CommentFoul language, trolling, personal attacks or non-relevant links will be deleted. You need to be logged in to post a comment. Login via the left margin or if you're new, register here.
|
| © Copyright 2009 John Cook | |
 |
|
The skeptic argument...