Geoscience Reference
In-Depth Information
390 ppm today. Models project that, unless forceful steps are taken to
reduce fossil fuel use, they will reach 700-900 ppm by 2100. According
to climate models, this will lead to a warming averaged over the globe
in the range of 3-5°C by 2100, with signifi cant further warming after
that. So unless there is either a major slowdown in economic growth or
strong steps to curb CO
2
emissions sharply, we can expect continued
accumulations of CO
2
emissions in the atmosphere—and the resulting
global warming with all its consequences.
Part II analyzes the impacts of climate change. The major con-
cerns are not temperature per se but the effects on human and na-
tural systems. A central concept in analyzing impacts is whether a
system can be managed. The nonagricultural sectors of high-income
countries are highly managed, and this feature will allow these sec-
tors to adapt to climate change at relatively low cost for at least a few
decades.
However, many human and natural systems are unmanaged or un-
manageable and are highly vulnerable to future climate change. While
some sectors or countries may benefi t from climate change, there are
likely to be signifi cant disruptions in areas that are closely tied to climate-
sensitive physical systems. The potential damages are likely to be most
heavily concentrated in low-income and tropical regions such as tropi-
cal Africa, Latin America, coastal states, and the Indian subcontinent.
Vulnerable systems include rain-fed agriculture, seasonal snow packs,
coastal communities, river runoffs, and natural ecosystems. There is
potential for serious impacts in these areas.
Scientists are particularly concerned about “tipping points” in the
earth's systems. These involve processes in which sudden or irreversible
changes occur as systems cross thresholds. Many of them operate at
such a large scale that they are effectively unmanageable by humans
with existing technologies. Four important global tipping points are the
rapid melting of large ice sheets (such as Greenland), large-scale changes
in ocean circulation such as the Gulf Stream, feedback processes where
warming produces more warming, and enhanced warming over the
long run. These tipping points are particularly dangerous because they
are not easily reversed once they are triggered.
