Environmental Engineering Reference
In-Depth Information
Summary
Society faces important choices in the coming century regarding fu-
ture greenhouse gas emissions and the resulting effects on Earth's climate,
ecosystems, and people. Atmospheric concentrations of several important
greenhouse gases have increased markedly since the start of the 20th century
because of human activities, and the increased concentrations of these gases
very likely 1 account for most of the globally averaged warming of the past 50
years. Carbon dioxide is responsible for more than half of the current impact
of human emissions of greenhouse gases on Earth's climate, or radiative forc-
ing, 2 and its influence is projected to grow. Its atmospheric concentration
has increased by more than 35 percent since 1750, and is now higher than
at any time in at least 800,000 years. Looking to the future, the concentration
of carbon dioxide could undergo a further doubling or tripling by the end of
the century, greatly amplifying the human impact on climate.
Because of the long atmospheric lifetime of carbon dioxide and the time
lags in the climate system (particularly slow processes in the ocean, see Sec-
tion 3.2), human choices in the near-term have long-term ramifications on
Earth's climate not only for the rest of the century but also for the next sev-
1 In this report, uncertainty ranges indicated as likely correspond to >66% probability (2 out
of 3 chance), while very likely is used for >90% (9 out of 10 chance). Assessed uncertainty
intervals are not always symmetric about the corresponding best estimate, and include statisti-
cal information and expert judgment.
2 Radiative forcing (RF) refers to the radiative flux change evaluated at the tropopause (which
has been adjusted for stratospheric changes, see Ramaswamy et al., 2007). Greenhouse gases
such as carbon dioxide, methane, and nitrous oxide exert a warming influence on climate,
and differ in their radiative forcing of the global climate system due mainly to their different
radiative properties and abundances in the atmosphere. Some greenhouse gas changes (e.g.,
stratospheric ozone depletion) and aerosols produce negative radiative forcing. The net RF is
the sum of positive and negative terms, and each term is defined as the change relative to 1750.
These warming influences may be expressed as CO 2 -equivalent concentrations , corresponding
to 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.
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