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Most d eveloped co untries are i n t he ea rly stag es of im plementing policies tosta bilize and
ultimately reduce greenhouse gas emissions a nd the attendant g lobal w arming. The sc ientific
consensus about the ris ks of further significant increases i n at mospheric gr eenhouse g as
concentrations grows s teadily stronger and more widely endorsed. This consensus underlies a
strong impetus for governmental actions that prepare the ground for meeting possibly stringent
CO2 emission constraints in the decades ahead, specifically global emission levels comparable to
or below those of today, despite a considerable increase in energy production and use. Developing
countries w ill ne ed toli mit the growth of greenhouse gas e missions w hile t heir en ergy
consumption incre ases dramatically. For example, if atm ospheric concentration of CO2 is not
allowed to exceed twice its pre-industrial value, then CO2 emissions in the 21st century will need
to be held to half the cumulative total expected under a “business as usual” trajectory, and the
annual emission rate would eventually need to fall well below the 2000 value. While our focus is
on global warming because of its o verwhelming international implications, we recognize that
reduction in other emissions from fossil fuel combustion would have important regional and local
benefits for clean air.
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