Geology Reference
In-Depth Information
1
0
-3°C, and that there was a risk of “serious, large scale system disruption” once av-
erage global temperature increase exceeded 2°C. Many experts think that this level of
temperature increase will take place at some point during this century when atmospheric
carbon dioxide levels reach around 400 parts per million. When this happens, Gaia could
move though a series of irreversible tipping points, such as the melting of the Green-
land ice cap, the re-configuration of the global ocean circulation, the disappearance of
the Amazon forest, the emission of methane from permafrost and undersea methane hy-
drates, and the release of carbon dioxide from soils. Add to this the insidious effects
of climate change on biotic communities around the world, and the enormity of what
we are doing to our planet becomes shockingly apparent. What are the implications of
crossing each of these tipping points?
In the far north the ice glints in the summer sun, and the Arctic feels warmer than it
has for thousands of years. The 2-kilometre thick Greenland ice cap is already melting,
at a rate of about 10 metres per year, ten times faster than was previously contemplated.
By 2006, 40% of the sea ice around the North Pole had melted over a period of only 33
years. The melting is happening faster than previously imagined partly because of the
infamous ice-albedo feedback. A small initial warming in the long Arctic summer melts
some ice, exposing the dark surface of either land or sea, which warms the region even
further, thereby promoting even more warming, and so on. The extra heat absorbed by
the dark ocean exposed to date is equivalent to the warming due to all the carbon dioxide
we have added to the atmosphere to date. Thus, by melting the Arctic sea ice, we almost
double the earth's heat load at a stroke. Much of the heat that would have warmed the
earth's surface has gone into melting sea ice, and once this has gone any given spot of
Arctic ocean will warm eighty times faster than before.
Confidence in the predictive accuracy of the IPCC models has been further under-
mined by its serious underestimate of Arctic sea ice melting. The IPCC predicted that
60% of the summer sea ice would have vanished by 2040-50, but this degree of melting
had in fact occurred by 2008. The situation is now so severe that scientists worry that the
far north has reached an irreversible tipping point that will lead to no summer sea ice in
the far north by 2023-33. As the sea ice vanishes, the entire region warms, increasing the
melting of the Greenland ice cap. But there is another positive feedback particular to the
Greenland ice cap that also contributes to its demise—the ice cap loses height as it melts
and so progressively encounters warmer air above it. If all of the ice on the Greenland
ice cap melted, sea levels around the world would rise by seven metres within 1,000
years, with catastrophic impacts on civilisation, which is centred mostly in vulnerable
coastal cities. And it won't take much of a temperature increase to make this happen, as
the tipping point seems to be 2.7°C—well within the IPCC predictions. Moreover, once
the melting has started in earnest, nothing can stop it, not even a drastic fall in carbon
