Geoscience Reference
In-Depth Information
that it is unlikely to undergo an abrupt transition or collapse over the
next century. Even in models that show a weakening of the Gulf Stream's
circulation, northwestern Europe continues to warm because the cooling
effect of the ocean current slowdown is smaller than the global warming
effect itself.
A third concern is a set of positive or reinforcing feedback interac-
tions between climate, the biosphere, and the carbon cycle. Some back-
ground on standard climate models will be helpful here. Many climate
model experiments consider a given path of industrial emissions of CO
2
and other GHGs. The CO
2
is gradually distributed through different res-
ervoirs, including the atmosphere, the oceans, and the biosphere (up-
take by natural vegetation, crops, and soils). In standard scenarios, CO
2
is added only from human sources such as burning fossil fuels.
A warmer climate and higher CO
2
concentrations bring important
feedback effects that may reinforce the effects of rising industrial emis-
sions. One type of feedback comes from the oceans. As a result of some
complicated ocean chemistry, the uptake of CO
2
by the oceans will be
reduced as the globe warms and the ocean becomes saturated with car-
bon. This ocean-CO
2
feedback is estimated to increase atmospheric con-
centrations of CO
2
over the twenty-fi rst century by about 20 percent
relative to a no-feedback scenario.
7
More reinforcing feedback is the impact of warming on the release
of locked-up carbon and methane (CH
4
). Methane is a powerful green-
house gas that is gradually transformed into the stable compound CO
2
.
Vast quantities of methane are stored in the form of methane hydrates,
which are methane molecules trapped in ice crystals. Most of the meth-
ane hydrates are stored in sediments in the oceans, while another large
quantity is frozen in the ground in cold regions in permafrost. Scien-
tists believe that warming would increase the releases of methane from
both these sources into the atmosphere, which could intensify the global
warming process; the timing of these releases is still an open question.
A fourth and fi nal mechanism involves the difference between the
medium-run and the very-long-run response of climate to human ac-
tivities. Today's climate models are basically designed to calculate the
