Geoscience Reference
In-Depth Information
council calculated that the tolerable maximum increase above the 1900
temperature would be about 2°C.
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The second rationale for the 2°C target is based on ecological argu-
ments. According to an advisory group to the World Meteorological
Organization in 1990, a global warming of 2°C would be “an upper limit
beyond which the risks of grave damage to ecosystems, and of nonlinear
responses, are expected to increase rapidly.” There was relatively little
support for this statement at that time. Several of these ecological prob-
lems are discussed earlier in this topic (see the impact analysis in Part II).
They do appear to worsen with more rapid climate change, but there is
no clear threshold at any specifi c level of warming.
The IPCC Fourth Assessment Report looked at dangerous results
summary of the impacts at different thresholds:
• At 1°C: increased water shortages; increased coral bleaching; in-
creased coastal fl ooding; increasing amphibian extinctions.
• At 2°C, in addition to the above: about 20-30 percent of species
at increasingly high risk of extinctions; increasing burden of dis-
eases.
• At 3°C, in addition to the above: decrease in productivity of food
crops; long-term commitment to several meters of sea-level rise
due to melting ice sheets; substantial burden on health systems.
• At 5°C, in addition to the above: major extinctions around the
globe; major decreases in food productivity; 30 percent loss in
coastal wetlands; major coastal fl ooding and inundation; recon-
fi guration of the world's coastlines; major change in ocean circu-
lation.
These projections defi nitely paint a disturbing picture. But they enter in
an increasingly severe manner and do not suddenly occur at a single
threshold temperature.
The third rationale for limiting the temperature increase to 2°C is
based on the notion that higher temperatures might trigger major insta-
bilities and tipping points (see Chapter 5). I concluded that research on
