Environmental Engineering Reference
In-Depth Information
This report quantifies, insofar as possible, the outcomes of different
stabilization targets for greenhouse gas concentrations using analyses and
information drawn from the scientific literature. It does not recommend or
justify any particular stabilization target. It does provide important scientific
insights about the relationships among emissions, greenhouse gas concentra-
tions, temperatures, and impacts.
CLIMATE CHANGE DUE TO CARBON DIOXIDE
WILL PERSIST MANY CENTURIES
Carbon dioxide flows into and out of the ocean and biosphere in the
natural breathing of the planet, but the uptake of added human emissions
depends on the net change between flows, occurring over decades to mil-
lennia. This means that climate changes caused by carbon dioxide are
expected to persist for many centuries even if emissions were to be halted
at any point in time.
Such extreme persistence is unique to carbon dioxide among major
agents that warm the planet. Choices regarding emissions of other warming
agents, such as methane, black carbon on ice/snow, and aerosols, can affect
global warming over coming decades but have little effect on longer-term
warming of Earth over centuries and millennia. Thus, long-term effects are
primarily controlled by carbon dioxide (see Figure Syn.1).
The report concludes that the world is entering a new geologic epoch,
sometimes called the Anthropocene, in which human activities will largely
control the evolution of Earth's environment. Carbon emissions during this
century will essentially determine the magnitude of eventual impacts and
whether the Anthropocene is a short-term, relatively minor change from the
current climate or an extreme deviation that lasts thousands of years. The
higher the total, or cumulative, carbon dioxide emitted and the resulting
atmospheric concentration, the higher the peak warming that will be expe-
rienced and the longer the duration of that warming. Duration is critical;
longer warming periods allow more time for key, but slow, components of
the Earth system to act as amplifiers of impacts, for example, warming of
the deep ocean that releases carbon stored in deep-sea sediments. Warming
sustained over thousands of years could lead to even bigger impacts (see
Box Syn.1).
IMPACTS CAN BE LINKED TO GLOBAL MEAN TEMPERATURES
To date, climate stabilization goals have been most often discussed in
terms of stabilizing
atmospheric concentrations
of carbon dioxide (e.g., 350
