Geoscience Reference
In-Depth Information
woodland back to tundra dominated by a little plant called the Dryas. Wally Broecker of
the Lamont Doherty Geological Observatory has worked out what may have happened. As
the ice sheet over North America receded, a vast lake of fresh meltwater, far bigger than
the present Great Lakes, built up in central Canada. At first, it drained over a great ridge of
rock into the Mississippi. As the ice receded, it suddenly opened a far lower passage down
the St Lawrence River to the east. The vast lake of cold fresh water drained into the North
Atlantic almost instantaneously. So much water was involved that it caused an immediate
sea level rise of 30 metres. It diluted the salty surface water of the North Atlantic and put
a virtual stop to the conveyor belt of ocean circulation. Thus there was no warming current
into the North Atlantic and Arctic conditions returned. A thousand years later, the ocean
circulation resumed as quickly as it had vanished and a temperate climate returned.
The North Atlantic deep water, together with cold bottom water from the Antarctic, finds
its way at depth as far as the Indian and Pacific oceans. The deep current continues into the
North Pacific, slowly accumulating nutrients as it goes, before it rises again to the surface.
Global greenhouse
Some of the gases in the Earth's atmosphere act rather like the glass of a greenhouse, letting
sunlight in to warm the ground but then preventing the resulting infrared heat radiation
from escaping. Were it not for the greenhouse effect, average global temperatures would be
around 15 degrees Celsius lower than they are, making life almost impossible. The princip-
al greenhouse gas is carbon dioxide but others, including methane, play an important role.
So does water vapour, an effect that is sometimes forgotten. Over hundreds of millions of
years, an approximate balance has been struck with plants removing carbon dioxide from
the atmosphere through photosynthesis and animals returning it by respiration. Vast quant-
ities of carbon have been buried in sediments such as limestone, chalk, and coal. Volcanic
eruptions have released carbon from inside the Earth.
In recent years, concern has grown over what should be termed the enhanced greenhouse
effect, the very significant rise in greenhouse gas levels in the atmosphere resulting from
human activity. The burning of fossil fuels such as coal and oil are prime culprits, but so
are agricultural practices which produce methane, and deforestation which releases carbon
dioxide from timber and soils as well as reducing the plant cover to absorb it again. Climate
models suggest that these activities could result in a global temperature rise of several de-
grees over the next century, accompanied by greater extremes in weather and a possible sea
level rise.
