Environmental Engineering Reference
In-Depth Information
equivalent level of atmospheric carbon dioxide, also known as the CO 2 -
equivalent concentration.
APPROACH
The goal and implications of stabilizing climate change are most often
discussed in terms of stabilizing atmospheric concentrations of CO 2 . This
report takes a different approach by (1) using global temperature change as
the frame of reference and (2) focusing in part on the relationship between
accumulated carbon emissions and global mean temperature change.
The motivation for this approach is both practical and conceptual. Avail-
able data and modeling suggest that the magnitudes of many key impacts
can be quantified for given amounts of global warming through scaling of
local to global warming and through coupled linkages to warming (such as
alterations in the water cycle that scale with warming). But although pub-
lished analyses of future climate impacts can be tied to specific warming
levels in particular studies, this information often cannot readily be linked
to CO 2 -equivalent concentrations (because, for example, of lack of informa-
tion on aerosol forcing used in many future climate impact studies based
on emission scenarios).
Moreover, using warming as the frame of reference provides a picture
of impacts and their associated uncertainties in a warming world—uncer-
tainties that are distinct from the uncertainties in the relationship of CO 2 -
equivalent concentrations to warming. Use of warming as a metric of change
also permits coverage of the transient climate changes and impacts while
concentrations increase, as well as the lock-in to further changes after sta-
bilization. Further, the approach taken here facilitates cataloging ranges of
impacts that should be expected for 1°C, 2°C, 3°C, or other levels of warm-
ing. The reader can thus consider how much warming s/he considers to be
an appropriate target. Information is also provided to translate warming into
best estimates of associated CO 2 -equivalent target concentrations with these
best estimates accompanied by estimated likely uncertainty ranges derived
from uncertainty in climate sensitivity.
Furthermore, this report also describes the cumulative carbon framework,
a perspective that has recently received considerable attention. Rather than
CO 2 -equivalent concentration levels, this approach considers the amount
of carbon emissions accumulated over time and the implications of differ-
ent accumulated emissions targets. Models consistently suggest a persistent
temperature response to a given level of cumulative carbon emissions. Ac-
cumulated carbon emission targets link to impacts through temperature (or
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