In 2007, the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) concluded that global warming is unequivocal, and that serious impacts of global warming are already beginning to occur. The AR5 isn’t due until 2013, but in 2009, a non-IPCC report called the Copenhagen Diagnosis was released to provide an update on the AR4. The authors found more evidence that various impacts were occurring, many of them at an increasing rate.
This is the third in a series of posts on the arguments contrarians make against global warming. In Part 1, I discussed whether the global temperature had increased (short answer: yes). In Part 2, I discussed whether it had recently stopped increasing (short answer: probably not). I know I promised a post about the hockey stick, but that is turning out to be more difficult to write about than I thought it would be; so in the meantime, here is a piece about one impact of global warming.
Claim: Ice sheets, glaciers, and sea ice are not melting.
Fact: All of the above are melting.
The Gravity Recovery and Climate Experiment (GRACE) satellites, launched in 2002, are able to measure gravity so sensitively they can detect changes in the mass of ice sheets (huge layers of land ice). GRACE measurements show that since 2002, the Antarctic ice sheet has been losing mass at an accelerating rate.
About a third of this ice loss has come from the East Antarctic ice sheet, which is much more massive than the West Antarctic one and was until recently considered stable. Antarctica is now contributing about 0.7 mm/yr to sea level rise; its ice sheet contains enough water to raise sea levels by about 53 m (see Copenhagen Diagnosis, “Ice Sheets of Greenland and Antarctica”).
The Antarctic Peninsula has been warming particularly rapidly, and entire ice shelves have thinned or collapsed completely. Ice shelves are thick, floating platforms of ice formed when glaciers flow from the land onto the ocean surface. (Revised 18 March 2010:) They are being attacked both from within, as crevasses form at weak points in the shelves; from above, as meltwater from the surface seeps into crevasses and widens them; and from below, as warming oceans melt the ice on the bottom. (See Copenhagen Diagnosis, “Ice Shelves”.) All this makes the shelf thinner, weakening the “pinning points” where the shelf rests on solid ground, which in turn spreads the rifts and eventually results in a collapse.
In 1978, John Mercer identified the collapse of ice shelves on the Antarctic Peninsula as a warning sign of dangerous warming in West Antarctica. In the past 20 years Antarctica has lost seven ice shelves,including the 3,250 km2 Larsen B Ice Shelf in 2002, which had remained stable throughout the previous 12,000 years. The 17,400 km2 Wilkins Ice Shelf shrank by 3,700 km2 from 1990 to 2008, and a further 3,400 km2 from 2008 to 2009.
When ice shelves melt, they do not directly add to sea level rise. However, observations following ice shelf collapses have found that glaciers held behind them accelerated dramatically. This suggests that such collapses significantly speed up the melting process delivery of land ice to the ocean, helping erode the ice sheet’s stability. The weakening of ice shelves has also been observed along other parts of the West Antarctic coast (see Copenhagen Diagnosis, “Ice Shelves”). If part of the enormous Ross Ice Shelf, which holds back some of the main West Antarctic glaciers, were to warm a few degrees, then it too could begin to break up.
GRACE measurements show the Greenland ice sheet is also losing mass at an accelerating rate, doubling between 2002 and 2009. Earlier data obtained by a number of methods show that Greenland has been losing ice since the 1990s. (Prior to the 1990s, the trend varied up and down, but this was mostly down to changes in precipitation.) Greenland is contributing 0.7 mm/yr to sea level rise; its ice sheet contains enough water to raise sea levels by 7 m. (See Copenhagen Diagnosis, “Ice Sheets of Greenland and Antarctica”.)
Glaciers are also retreating. Contrarians like to point to particular glaciers that are growing, but these are exceptions to the rule. A bigger picture can be gained by considering measurements of glacier mass change from around the world. In 2008, a joint report by the United Nations Environment Programme (UNEP) and the World Glacier Monitoring Service (WGMS) concluded that globally averaged glacier mass measurements “indicate strong ice losses as early as the 1940s and 1950s, followed by a moderate ice loss between 1966 and 1985, and accelerating ice losses until present”. This graph shows the cumulative mass balance since 1980, including preliminary data for 2007/2008:
Glaciers are currently contributing about 1.2 mm/yr to sea level rise, 50% faster than in the 1990s. The world’s glaciers contain enough water to raise sea levels by 0.7 m. (See Copenhagen Diagnosis, “Glaciers and Ice-Caps”.)
(One thing I don’t have time to discuss here is an embarrassing error regarding Himalayan glaciers recently discovered in the IPCC AR4 WGII report. However, I may come back to this in a future post on the IPCC.)
And finally, we come to sea ice (ice floating on the ocean surface, not to be confused with ice sheets on land). Counter-intuitively, southern sea ice extent has increased since satellites began measuring it in 1979. We should not conclude from this that the Southern Ocean is cooling — in fact, it is warming. The reasons for the increasing ice are somewhat complex, but it is thought to be partly due to wind patterns altered by the ozone hole. The trend is expected to reverse in coming decades as the ozone hole repairs itself. (See Copenhagen Diagnosis, “Sea-Ice”.)
However, when it comes to climate change, southern sea ice is not as important as the ice sheets or Arctic sea ice. Unlike the Arctic Ocean, the Southern Ocean is already almost ice-free in summer. And a decline in winter sea ice would not significantly change the Antarctic climate (because that is the time of year when it gets the least heat from the Sun).
In the Arctic, summer (September) sea ice extent has shrunk by 35% since 1979. I saved this case for last because the observed decline far outstrips the worst-case projections in the IPCC AR4:
The five most recent minima have been the five lowest; all were well outside the projected range. 2009 came third lowest at 5.4 million km2 (compared with the 1979-2000 average of 7.0 million km2). The record-shattering 2007 minimum was just 4.3 million km2, opening for the first time the Northwest Passage shipping route from the Atlantic to the Pacific. And this phenomenon is not limited to Septembers. Arctic sea ice has declined at varying rates in every month of the year. Older and less reliable shipping records suggest that sea ice was also low in the 1930s and 1940s (when the Arctic was relatively warm); but then the melting was less widespread.
At the end of the 2009 summer, a record-breaking 81% of ice cover was less than two years old, making it easier to melt in future summers. Many cooler summers would be needed for the ice to recover.
(On a side note, how has the recent extreme negative Arctic Oscillation phase affected sea ice? Arctic sea ice this year had its fourth lowest extent for February; and off the east coast of Canada, sea ice was at an all-time February low. Counter-intuitively, though, the same weather pattern means that less old ice is transported out of the Arctic, so it could actually result in a higher minimum this September.)
Melting sea ice does not directly affect the sea level, but it does decrease the albedo (reflectivity) of the Arctic from 90% to 10%, accelerating the warming process. This explains why the Arctic is the fastest-warming place on Earth. According to the Copenhagen Diagnosis, ice-free Arctic summers in coming decades are very likely already inevitable.
What does all this mean for sea level rise? The IPCC’s 2007 projection of 18-59 cm by 2100 was based on thermal expansion only and did not take into account the contribution of the ice sheets. More recent (but still highly uncertain) estimates are around 0.5 to 1.5 m by 2100. A 1 m sea level rise would displace about 160 million people.
Because of the inertia in the response of both the ice sheets and oceans, sea level rise will continue for centuries even after the planet stops warming. It is still poorly known how stable ice sheets are, so we can’t be sure how long an ice sheet collapse might take; but the uncertainty is mostly in the direction of faster sea level rise. Scientists are particularly concerned about the West Antarctic ice sheet, which would add several metres to sea level if it collapsed. Entire island nations could disappear beneath the waves. (See Copenhagen Diagnosis, “Global Sea Level”.)
Coming up in future posts: a history of the “hockey stick” controversy; a discussion of CO2 and the greenhouse effect; a look at how humans have changed the carbon cycle; and more on sea level rise (though not necessarily in that order). Don’t go away!
Update 15 April 2010: Greenland’s ice loss is spreading.
Update 15 April 2010: Although the Arctic has remained relatively warm, Arctic sea ice did not reach its annual maximum until 31 March, the latest on record. The month of March had its fifth lowest sea ice extent, 15.1 million km2 (the record low of 2006 was 14.4 million km2 and the 1979-2000 average was 15.8 million km2). Thanks to the Arctic Oscillation, more old ice was retained during the winter, which may contribute to a higher minimum this summer.
Update 5 May 2010: As mentioned above, the Arctic is warming much more rapidly than the rest of the planet. Now a new study has found further evidence that this is mainly due to a change in albedo caused by less sea ice. If the “amplification” of Arctic temperature were due to changes in circulation, we would expect to observe it high in the atmosphere. But Arctic warming seems to be occurring mainly near the surface, as we would be expect if it were due to an albedo feedback.
Update 8 August 2010: A 260 km2 ice shelf has just broken off the Petermann Glacier in Greenland, the biggest such event in the Arctic in nearly half a century. Andreas Muenchow, a professor of ocean science at the University of Delaware, said that because records around the glacier had only been kept since 2003, “Nobody can claim this was caused by global warming. On the other hand nobody can claim that it wasn’t.” I expect the first half of that quote to be repeated on a lot of contrarian blogs.
Update 18 August 2010: Climate modelers Judith Curry and Jiping Liu have discovered that paradoxically, the growth of Antarctic sea ice is caused by the warming of the Southern Ocean. A warmer ocean means more water being evaporated from the surface and falling as snow off the coast of Antarctica. However, the modellers tentatively predict that further warming in the Antarctic would cause the trend to reverse during the 21st century.