Environmental Engineering Reference
In-Depth Information
of acceptable risk associated with both quantifiable and non-quantifiable
impacts and changes.
SUPPORTING EVIDENCE
1. Cumulative Carbon Dioxide Emissions and
Climate Change over Millennia
Climate changes that occur because of carbon dioxide increases are ex-
pected to persist for thousands of years 5 even if emissions were to be halted
at any point in time. Recent scientific literature has shown that the con-
tribution to global warming caused by anthropogenic CO 2 can be directly
related to the cumulative emissions of carbon dioxide.
For example, our best estimate (see Figure S.2) is that 1,000 gigatonnes
of anthropogenic carbon (GtC) (Box S.1) emissions lead to about 1.75°C
increase in global average temperature, 6 implying that approximately 1,150
gigatonnes of carbon (or 4,200 Gt CO 2 ) would lead to a global mean warm-
ing of 2°C (the stated aspirational goal of the “group of eight” nations). Based
on current understanding, this warming is expected to be nearly irreversible
for more than 1,000 years (Figure S.3). Figure S.2 shows best estimates and
likely uncertainty ranges for cumulative carbon emissions leading to a range
of warming levels, along with cumulative emissions to date (about 500 GtC).
Carbon dioxide alone accounted for about 55 percent of the total CO 2 -
equivalent concentration of the sum of all greenhouse gases in 2005. The
contribution of carbon dioxide increases to between 75 and 85 percent of
total CO 2 -equivalent by the end of this century based on a range of current
emission scenarios. Some anthropogenic carbon dioxide is removed by the
oceans and biosphere in decades to centuries, but the slow time scales of
the long-term uptake of carbon in the ocean means that some is expected
to persist in the atmosphere for many thousands of years. This behavior is
unique to carbon dioxide among major radiative forcing agents. Choices
regarding continued emissions or mitigation of other warming agents such
5 Approaches to “geoengineer” future climate, e.g., to actively remove carbon from the
atmosphere or reflect sunlight to space using particulate matter or mirrors are topics of ac-
tive research. If effective, these may be able to reduce or reverse global warming that would
otherwise be effectively irreversible. This study does not evaluate geoengineering options, and
statements throughout this report regarding the commitment to climate change over centuries
and millennia from near-term emissions should be read as assuming no geoengineering. Re-
forestation or other methods of sequestration of carbon are also not considered.
6 The quasi-linear response of temperature to cumulative carbon is discussed in detail in
Section 3.4.
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