Geoscience Reference
In-Depth Information
Climate change
The steady annual rise in carbon dioxide levels has been carefully recorded from an isolated
mountain peak in Hawaii since 1958. More than 130 years of meticulous weather read-
ings around the world confirm an average global warming of about half a degree, with the
effect particularly pronounced over the last 30 years. But natural climate records go back
much, much further. Tree rings record periods of drought and severe frosts, as well as the
frequency of wild fires, over their lifespan. Extrapolating overlapping sequences in pre-
served timber can reveal climate conditions back to 50,000 years ago. Coral growth rings
reveal sea surface temperatures over a similar span. Pollen grains in sediments record shifts
in vegetation patterns over 7 million years. Landscapes reveal past glaciation and changes
in sea level over billions of years. But some of the best records come from cores drilled
from ice and from ocean sediments. Ice cores reveal not only rates of snow accumulation
and trapped volcanic dust, but bubbles in the ice represent samples of the ancient atmo-
sphere trapped in the snow. Isotopes of hydrogen, carbon, and oxygen can also indicate
global temperature at the time. The ice record from Antarctica and Greenland now goes
back over 400,000 years. Marine sediments all around the planet have been sampled by the
ocean drilling programme and can carry records up to 180 million years old. Isotope ratios
in microfossils trapped in the sediments can reveal temperature, salinity, atmospheric car-
bon dioxide levels, ocean circulation, and the extent of polar ice caps. All these different
records reveal that climate change is a fact of life and that long periods in the past have
been considerably warmer than the climate we experience today.
Web of life
The most insubstantial of the Earth's layers has had perhaps the most profound effect on
the planet: life. Without life, the Earth might be a runaway greenhouse world like Venus,
or possibly a cold desert like Mars. There would certainly not be the temperate climate
and oxygen-rich atmosphere in which we flourish. We've already heard how the first al-
gae kept pace with the warming Sun by eating the carbon dioxide blanket that insulated
the young Earth. The independent scientist James Lovelock suggests that such feedback
mechanisms have managed the terrestrial climate for more than 2 billion years. He uses the
term Gaia, after the ancient Greek earth goddess, as a name for this system. He does not
pretend that there is anything conscious or deliberate about this control; Gaia does not have
divine powers. But life, principally in the form of bacteria and algae, does play a key role
in the homeostatic process that keeps the planet habitable. A simple computer model called
'Daisyworld' shows how two or more competing species can set up a negative feedback
system that controls the environment within habitable limits. Lovelock suspects that the
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