Introduction - Climate Stabilization Targets - page 52

human forcing of the climate system represent a transient climate change,
linked to transient climate response (or TCR, see Section 3.2), and reflects a
changing climate in which relatively fast-responding variables such as water
vapor and cloudiness change but slower processes such as ocean warming
and ice melt may lag behind. This implies that only a portion of the climate
change is “realized” during a period of increasing greenhouse gas concen-
trations. If greenhouse gases are stabilized following a period of changing
radiative forcing, the climate system response within a few centuries can
be expected to reflect the equilibrium climate sensitivity (see Section 3.2).
On very long time scales of multi-millennia, changes in factors such as ice
sheets and vegetation can lead to different and generally stronger climate
changes under stabilization (see Chapter 6).
The report considers a range of warming levels, which can be related
to stabilization targets ranging from 350 to near 2,000 ppmv of total green-
house gas forcing expressed as equivalent carbon dioxide concentration (see
Box 1.1 for definition). We note that 1,000 ppmv is a level that a number of
studies suggest may be reached by 2100 for a “business as usual” scenario
with little or no mitigation of emissions. Even higher concentrations have
precedent in the Earth's paleo history millions of years ago (see Chapter 6),
and illustrate how the Earth may be changed if, for example, feedbacks from
the Earth system trigger large releases of carbon from peat.
This report is organized as follows:
The next part of this introductory chapter summarizes the attribution
of currently observed climate changes and impacts, as a starting point for
discussion of the future.
Chapter 2 describes the relationship between greenhouse gas emissions
and concentrations for a range of gases and cases. Aerosols and other forc-
ings are also discussed.
Chapter 3 presents the current understanding of how the climate changes
due to greenhouse gas concentrations on a range of time scales including

BOX 1.1 CARBON DIOXIDE-EQUIVALENT (CO 2 2 -EQ) CONCENTRATIONS
Greenhouse gases differ in their warming influence (radiative forcing) on the global climate sys-
tem due to their different radiative properties and lifetimes in the atmosphere. These warming
influences may be expressed through a common metric based on the radiative forcing of CO 2 .

• CO 2 -equivalent concentration is the concentration of CO 2 that would cause the same

amount of radiative forcing as a given mixture of CO 2 and other forcing components.