Summary - Climate Stabilization Targets - page 14

warming) and are clearly relevant to policy aimed at controlling emissions
and reducing the risk of dangerous impacts. The approaches used here
thereby provide additional policy-relevant information that would be lost
in an analysis that only related impacts to CO 2 -equivalent concentration
levels.
KEY FINDINGS
There are three key findings of this report, which correspond to the
structure of this summary:
1. Climate change in the very long term: Future stabilization targets cor-
respond to altered states of Earth's climate that would be nearly irreversible
for many thousands of years, even long after anthropogenic greenhouse gas
emissions ceased. The capacity to adapt to slow changes is generally greater
than for near-term rapid climate change, but different stabilization levels can
lock the Earth and many future generations of humans into large impacts
that can occur very slowly over time, such as the melting of the polar ice
sheets; similarly, some stabilization levels could prevent such changes.
2. Climate change in the next few decades and centuries: Understand-
ing the implications of future stabilization targets requires paying attention
to the expected climate change and to the emissions required to achieve
stabilization. Because of time lags inherent in Earth's climate, the observed
climate changes as greenhouse gas emissions increase reflect only about
half of the eventual total warming that would occur for stabilization at the
same concentrations. Moreover, emissions reductions larger than about 80%
(relative to whatever peak global emission rate may be reached) are required
to approximately stabilize carbon dioxide concentrations for a century or so
at any chosen target level (e.g., 450 ppmv, 550 ppmv, 650 ppmv, 750 ppmv,
etc.). 4 Even greater reductions in emissions would be required to maintain
stabilized concentrations in the longer term. It should be emphasized that
this finding is not linked to any particular policy choice about time of sta-
bilization or stabilization concentration, but applies broadly, and is due to
the fundamental physics of the carbon cycle presented in Chapter 2.
3. Climate changes, impacts, and choices among stabilization targets:
A number of key climate changes and impacts for the next few decades and
4 In this report the mixing ratio for any compound, CO 2 for example, is expressed in either
ppm (parts per million, i.e., the number of molecules of CO 2 for every million molecules of
air) or in ppmv (parts per million volume, the ratio of CO 2 to air calculated in volumes) but
used equivalently.