Climate change futures: health, ecological and economic dimensions

(see McCarthy et al. 2001). As glaciologist Richard
Alley put it succinctly, addressing skeptical staffers from
Congress at a 2004 conference convened by the
American Association for the Advancement of Science,
“Get over it, climate is warming.”
Figure 1.2 The Warming Tropical Seas
Humans affect climate primarily through the combustion
of fossil fuels, which pump carbon dioxide into the
atmosphere. Atmospheric levels of carbon dioxide
have consistently tracked global temperatures. For the
past 420,000 years, as recorded in ice cores taken
from Vostok in Antarctica, CO 2 levels in the lower
atmosphere have remained within an envelope ranging
from 180 parts per million (ppm) to 280 ppm
(Petit et al. 1999). CO 2 levels are now 380 ppm,
most likely their highest level in 55 million years
(Seffan et al. 2004a).
These levels have been reached despite the fact that
ocean and terrestrial “carbon sinks” have absorbed
enormous amounts of CO 2 since the industrial revolution
(see Mann et al. 1998). The combustion of fossil
fuels (oil, coal and natural gas) generates 6 billion
tons of carbon annually (one ton for each person on
Earth). Meanwhile, warmer and more acidic oceans
(from the excess CO 2
already absorbed) take up less
heat and less CO 2
(Kortzinger et al. 2004).
In essence, humans are tinkering with the operating
systems that control energy distribution on the planet.
As fossil fuels are burned, their excess by-products
accumulate in the lower atmosphere, reinforcing the
thin blanket that insulates the earth. Warming of the
atmosphere heats the oceans, melts ice and increases
atmospheric water vapor — shifting weather patterns
across the globe.
The world’s oceans dominate global climate. Because
of their staggering capacity to store heat, the oceans
are the main drivers of weather and the stabilizers of
climate. The relationship between the oceans and the
atmosphere is marked by exchanges of heat on scales
that range from the immediate to hundreds of years.
The oceans have been warming along with the rest of
the globe, down to two miles below the surface
(Levitus et al. 2000). It appears that the deep ocean is
the repository for the global warming of the 20th century,
having absorbed 84% of the warming (Barnett et
al. 2005).
Intermittently, the ocean releases some of its huge store
of heat into the atmosphere. Thus a certain degree of
warming already in place has yet to become manifest.
Satellite images showing warmed Atlantic and Pacific Oceans and
an especially warm Gulf of Mexico in early August 2005. The
tropical oceans have become warmer and saltier (from increased
evaporation; Curry et al. 2003), and warmer sea surfaces evaporate
rapidly and provide moisture that fuels hurricanes.
Image: NOAA
In several ways the oceans have already begun to
respond. Warming of the atmosphere and the deep
oceans is altering the world water cycle (Dai et al.
1998; WMO 2004; Trenberth 2005). Ocean warming
(and land surface warming) increase evaporation,
and a warmer atmosphere holds more water vapor
[6% more for each 1°C (1.8°F) warming; Trenberth
and Karl 2003]. Water vapor has increased over the
US some 15% over the past two decades (Trenberth et
al. 2003).
One of the less appreciated natural heat-trapping
gases is water vapor. Greater evaporation leads to
greater warming. Higher humidity also fuels more
intense, tropical-like downpours, while warming, evaporation
and parching of some of Earth’s surface creates
low-pressure pockets that pull in winds and weather.
The increased energy in the system intensifies
weather extremes from several perspectives and the
accelerated hydrological (water) cycle is associated
with the increasingly erratic and severe weather patterns
occurring today. As models projected (Trenberth
2005), some areas are becoming drier due to more
heat and evaporation, while others are experiencing
recurrent flooding. And when heavy rains hit parched
regions they are poorly absorbed and this can lead to
flooding. Part of the increased vapor comes from melting
of the cryosphere — ice cover in polar and Alpine
regions. Over this century, Arctic temperatures are
expected to increase twice as fast as in the world at
large (ACIA 2005), a chief reason being that more heat
is absorbed by the expanding open ocean and less is
reflected from the shrinking ice cover.
17 | THE CLIMATE CONTEXT TODAY