Environmental Engineering Reference
In-Depth Information
year, followed by a decrease of 3% per year down to a selected total reduc-
tion of 50, 80, or 100%. The rate of decrease of 3% per year used here is
derived from scenario analysis described in the next section. This section
together with the next section aim to probe what plausible rates of emis-
sions reduction based upon scenario studies imply for the future evolution
of carbon dioxide concentrations. The rate of possible emissions reductions
of carbon dioxide depends upon factors including e.g., commitments to
existing infrastructure and development of alternatives, see Section 2.2. It is
interesting to note that even in the case of the phaseout of ozone-depleting
substances under the Montreal Protocol, emissions reductions were about
10% per year initially but stalled at a total reduction of about 80% of the
peak, with some continuing emissions of certain gases occurring due for
example, the challenge of finding alternatives for fire-fighting applications
(see IPCC, 2005).
Figure 2.2 shows that carbon emission reductions of 50% do not lead to
long-term stabilization of carbon dioxide, nor of climate, in either of these
models, as has also been shown in previous studies (e.g., Weaver et al.,
2007). It is noteworthy that the Bern model has weaker carbon-climate
feedbacks than the UVIC model; nevertheless both models show the need
for emissions reductions of at least 80% for carbon dioxide stabilization
even for a few decades, while longer-term stabilization requires nearly
100% reduction. Very similar results were obtained in other test cases run
for this study considering peaking at higher values, or decreasing at rates
from 1 to 4% per year (see also Meehl et al., 2007; Weaver et al., 2007).
Figure 2.3 shows sample calculations evaluated in Meehl et al. (2007) us-
ing three different models for various stabilization levels. Figure 2.3 shows
that stabilization levels of 450, 550, 750, or 1,000 ppmv require eventual
emission reductions of 80% or more (relative to whatever peak emission
occurs) in all of the models evaluated. Thus current representations of the
carbon cycle and carbon-climate feedbacks show that anthropogenic emis-
sions must approach zero eventually if carbon dioxide concentrations are
to be stabilized in the long term (Matthews and Caldeira, 2008). This is a
fundamental physical property of the carbon cycle and is independent of the
emission pathway or selected carbon dioxide stabilization target. Box 2.1
discusses how emissions of non-CO
2
greenhouse gases could affect attain-
ment of stabilization targets.
Figures 2.2 and 2.3 illustrate a fundamental change in understanding
stabilization of climate change that has been prompted by the scientific
literature of the past two years or so (see Jones et al., 2006; Matthews and
Caldeira, 2008). Early work on stabilization using relatively simple models
