Ask Dr. Bill
More answers to our global warming questions
Is global warming a cyclical trend?
Many global warming skeptics try to obscure the danger of global warming by noting that Earth's climate has experienced wide swings in the past. True, but not relevant.
Over the past 2 million years, our climate has been characterized by ice ages lasting about 100,000 years, punctuated by relatively short (10,000- to 20,000-year) warm periods. These climate swings were caused by changes in the Earth’s orbit around the sun amplified by natural changes in the cycle of carbon dioxide and other greenhouse gases.
After decades of research we can now say with scientific certainty that the current warming is due primarily to greenhouse gas emissions and other human activities.
Carbon dioxide concentrations are higher now than they have been for the past 650,000 years, a period that spans several ice age cycles. And, based on various measurements, we can also trace the extra atmospheric carbon dioxide to the burning of fossil fuels.
As CO2 and other greenhouse gases continue to accumulate in the atmosphere, the planet will continue to warm. Without action to cut our global warming emissions, we face a growing climate catastrophe.
For more on this question:
Global Warming's Increasingly Visible Impacts [PDF]
Are Humans Responsible for Global Warming? [PDF]
Global Warming Myths and Facts
Some say global warming could trigger another ice age. How is this possible?
Scientists believe that global warming could trigger cooling in the North Atlantic, possibly resulting in a new "ice age" across parts of northern Europe. The basis for these forecasts is found in the ocean's thermohaline circulation; so called because it is driven by heat (thermo) and ocean salinity (haline).
This circulation acts like a huge conveyor belt bringing warm waters from the tropical Atlantic ocean northward in the Gulf Stream and returning cold water from the North Atlantic back to the tropics in the deep ocean.
As the waters of the Gulf Steam flow north, they transmit heat in the form of evaporation, and become cooler and saltier. This makes the water denser and heavier.
The ocean's system of currents takes 1,000 years to go full cycle. Warm water is chilled in the far North Atlantic and sinks. The cold, salty current flows south near the bottom. PHOTO: Argonne National Laboratory.
Near Greenland the water becomes so dense that it sinks to the deep ocean. The force of the sinking water is the pump that keeps the ocean's conveyor belt flowing.
This circulation is extremely important to the climate of northern Europe. Much of the heat from the Gulf Stream gets blown over Europe and helps moderate temperatures there. If the Gulf Stream were to shut down, western European temperatures would plummet.
How do we know this? Because we know from fossil records and ice core measurements that it has happened in the past – most notably about 10,000 years ago, right at the time the world was coming out of its last Ice Age.
This is what we think happened: As the world was warming, a huge amount of freshwater from melting ice rapidly flowed into the North Atlantic. This freshwater diluted the salty water of the Gulf Stream and stopped it from sinking into the deep ocean. This, in turn, caused the Gulf Stream to shut down and plunged Europe back into an Ice Age that lasted a couple thousand years.
Today, we know that global warming is causing the Greenland ice sheet to melt. If this melting becomes sufficiently rapid, it could shut down the conveyor belt just like it did some 10,000 years ago. In fact, there is some evidence that the thermohaline circulation in the North Atlantic has already slowed.
However, while it is conceivable that global warming could cause the shutdown of the Gulf Stream, the chances of it happening any time soon are pretty remote. The rate of melting, at least for now, doesn’t seem rapid enough. But, we still have a lot to learn about the ocean thermohaline circulation, so we cannot be sure and we need to keep our eyes on the North Atlantic.
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05/11/2006 - Global warming is confusing. Dr. Bill answers your questions.
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