Geoscience Reference
In-Depth Information
that about one-fi fth of the world's coral reefs have already been lost
as a result of habitat destruction, pollution, overfi shing, warming, and
ocean acidifi cation. The main threat to corals in coming decades is the
increasing carbon concentration in the oceans caused by the rising CO
2
in the atmosphere. This is the phenomenon of ocean acidifi cation (dis-
cussed in detail in Chapter 9).
At today's CO
2
concentrations, coral reefs will probably go into long-
term decline. According to a report by a technical group of scientists
from the U.K. Royal Society, when CO
2
concentrations reach 450 ppm
(which is likely to occur within three decades), coral reefs “will be in
rapid and terminal decline world-wide from both temperature-induced
bleaching and ocean acidifi cation.”
9
There have been a few systematic surveys of tipping points in earth
systems. A particularly interesting one by Lenton and colleagues exam-
list includes the examples given above, plus shifts in monsoons, dieback
of the Brazilian rain forest, and a few others. The most important tip-
ping points, in their view, have a threshold temperature tipping value
of 3°C or higher (such as the destruction of the Amazon rain forest) or
have a time scale of at least 300 years (the Greenland Ice Sheet and the
West Antarctic Ice Sheet). Their review fi nds no critical tipping ele-
ments with a time horizon less than 300 years until global tempera-
tures have increased by at least 3°C. However, at 3°C, we encounter the
danger zone for several important tipping elements. This conclusion is
tentative, however, because of the inherent diffi culty of assessing the
danger and timing of tipping events. For those interested in pursuing
this point, a detailed discussion is available in the endnotes.
11
THE POTENTIAL MELTING OF THE GREENLAND ICE SHEET
It will be useful to analyze a specifi c tipping point, the Greenland
Ice Sheet (GIS), to illustrate the mechanisms and why they are of great
concern. This discussion provides a taste of what climate science is
grappling with at the frontier of knowledge.
The GIS covers 1.7 million square kilometers—roughly the size of
western Europe. It is the planet's second largest ice sheet, after the Ant-
