Environmental Engineering Reference
In-Depth Information
The study presents best estimates for climate change and its impacts at
varying warming levels and associated stabilization targets, and it also briefly
describes the level of understanding of the processes involved (e.g., physical,
biological, etc.) in such changes. Presentation of best estimates provides a
clear view of the best current understanding that may otherwise be poorly
communicated, and this is one aim of this study. However, we also balance
descriptions of best estimates with their corresponding uncertainties, along
with appropriate coverage of issues of uncertain risks. Climate changes
and their impacts are discussed on a global basis, and specific regional ex-
amples within America and American territories are also presented for the
purpose of illustration. We summarize as appropriate the following factors:
(1) the extent to which multiple studies are available, and the robustness
of findings across work by a range of authors, (2) the scientific confidence
in understanding of the underlying processes, and (3) studies that already
attribute a contribution of observed changes to anthropogenic effects where
available (see Section 1.2). Where attribution is already possible for current
levels of climate change, confidence in further future changes is generally
strengthened.
Many climate changes or impacts currently are understood only in a
qualitative manner, and thus are not quantifiable as a function of stabiliza-
tion target. The report assesses and identifies these unquantified factors to
the extent practical based upon the available literature. It should be empha-
sized that these should not be considered negligible; indeed some of these
could be very important, or even dominant, in evaluating future risks due
to anthropogenic climate change.
Many studies involve the use of “pattern scaling” whereby it is assumed
that the spatial pattern of future climate change computed for one level of
perturbation (i.e., radiative forcing) may be scaled to derive values for an-
other test case such as one with stronger forcing and larger perturbations.
A variety of studies have shown that such methods generally simulate the
results of atmosphere-ocean general circulation models rather well (see Sec-
tion 4.2), although results are generally less robust for precipitation than for
warming (see Section 4.3), and near regions where strong feedbacks such
as sea ice retreat take place. We employ pattern scaling for many of the
estimates in this report.
Earth's history shows that climate changes can occur on time scales
ranging from decades to centuries to millennia. All of these time scales
are considered here. As there is abundant evidence that climate is already
changing in part due to human activities, a focus of the report is on the
next few decades to century, where the climate changes under increased
