The Runaway Greenhouse Effect

Extracted from Stormy Weather: 101 Solutions to Climate Change
by Guy Dauncey and Patrick Mazza
New Society Publishers, Gabriola Island, 2001, ISBN 0-86571-421-5

The greenhouse effect is not a static process. It only seems that way because the past 10,000 years have been so stable. But this is not the way the world's climate works. If you think the enhanced greenhouse effect is troublesome, wait until the runaway greenhouse effect kicks in. As the saying goes, "You ain't seen nothin' yet."

Every previous interglacial period in the past 1.8 million years (the Pleistocene Epoch) ended with the temperature climbing to 2-3° C above the pre-industrial level and then crashing down to the next ice age. Prior to the Pleistocene, Earth's temperature was permanently higher. During the Cretaceous Period, 65 million-135 million years ago, it was 6-8° C higher – and there were monkeys and alligators on Ellesmere Island near the North Pole. London was a tropical mangrove swamp.

What caused the interglacial temperatures to crash during the Pleistocene? Scientists don't know. But they do know that climate transitions can occur suddenly in as little as one or two decades. The scary thing is that we may be about to find out what causes these changes if we don't act quickly to stabilize the climate. We're grown-up planetary citizens now, not squabbling nations. We've got to accept this responsibility before it is too late.

Far more dangerous than the enhanced greenhouse effect, the runaway greenhouse effect includes feedback from the various climate change impacts, which intensifies things in a vicious cycle. There are seven acknowledged climate crunchers, any or all of which might trigger such a runaway effect, or at least play a contributing role:

The Tundra Thaws

The Siberian and Canadian permafrost stores billions of tonnes of frozen carbon dioxide (CO2 ) and methane close to the surface – a third of the Earth's stored soil carbon. When the tundra warms by 3.6°F (2*C), the gases escape into the atmosphere. The Arctic temperature has already increased by 4-7° F, so the process has already begun. Methane is 62 times more potent a greenhouse gas than CO2 over 20 years, accelerating the rate of warming as the Arctic thaws.

The Ice Sheets Melt

As long as the Arctic summer ice exists, its albedo reflects 80% of the sun's light back into space. As soon as it goes, the sun's light and heat will penetrate the water, adding to ocean warming. The same applies to Greenland's ice cap. About 125,000 years ago, at the very end of the last interglacial period, half of Greenland's entire ice cap melted away, raising global sea levels by 5 meters. The possibility also exists that the west Antarctic ice sheet might collapse, creating a 5-6 meter rise in sea levels.

Estuaries and Farmland Flood

When there is serious flooding from rising sea levels, the water drowns huge areas of vegetation, which then rots anaerobically and releases large quantities of methane, further intensifying the greenhouse effect.

The Methane Hydrates Melt

Methane hydrates are huge blobs of pressurized frozen methane that are locked under the ocean floor along the world's continental shelves. They contain ten trillion tonnes of methane; twice as much as the world's entire fossil fuel reserves. As the ocean warms, the deposits could begin to break free, releasing an enormous burst of greenhouse gases . . . .

The Phytoplankton Stop Working

The plankton in the world's oceans are absorbing 25% of the CO2 from human activities. Plankton thrive on the annual upwelling from the cool, deep, nutrient-rich waters below, which is triggered by the temperature difference between the surface and the deep ocean. If the warming ocean waters start to circulate less efficiently, fewer inorganic nutrients will be brought up from the bottom, cutting off a key food supply for plankton. This happens during El Nino events, when it is responsible for the collapse of many Pacific fisheries. Without plankton, the ocean food chain would crash and the oceans would stop absorbing CO2, accelerating global warming and causing ocean circulation to slow even more, in another feedback loop. It would also scupper the whales, which eat the krill that eat the plankton.

The Forests Die Back

As the world warms, the world's forests will begin to feel the stress. British scientists fear that by 2050, the world's forests and terrestrial vegetation will cease being a carbon sink and become a carbon source. If we continue on our current trajectory, the entire Amazon rainforest will begin to dry out around 2040 due to changes in ocean temperature that alter the normal storm tracks, drying the soil and causing massive fires.

These are terrible possibilities to express in so few brief words. After 2050, scientists estimate that as it dies, vegetation will release two billion tonnes of carbon a year instead of absorbing 2-3 billion tonnes, sending CO2 levels soaring. This is all the more alarming because the projections come from the UK Hadley Center's climate model, which many scientists believe to be the world's best at projecting ocean changes and their consequences.

The Ocean Conveyor Belt Stops

Northern Europe is at the same latitude as Labrador but it is warmed by the Gulf Stream, part of the global thermohaline current (the "conveyor belt") that pushes warm water around the world. The current starts off Iceland, where it is propelled into motion by heavier salt water falling to the bottom of the ocean. By the time the cold water has been south to the tropics and returned, it is warm enough to create the warm westerlies that Europe enjoys. So far, so good.

The Gulf Stream has been flowing for 10,000 years, but geological evidence suggests that the current can switch off at very short notice. The trigger is an increase in fresh water from flooding rivers and melting ice in the Arctic, which dilutes the salt water and stops it from sinking. If the current switches off, temperatures in northern Europe could fall by 10°C within ten years and London would experience the winter cold that now grips Irkutsk in Siberia. This could happen when CO2 concentrations reach 750 ppm, which will happen within 100 years if we don't get things under control. Since 1980, the salinity of water flowing south has dropped by 0.01 grams of salt per kg of sea water, after being constant for at least 100 years. One of the deep water currents flowing south from the Greenland Sea has already stopped and gone into reverse, and in 2000, scientists discovered that west Greenland's melting glaciers were sending a Nile's worth of fresh water into the ocean in this very area.

This is no picnic. Any or all of these events could flip the world into a new ice age or lock it into a permanently sweltering climate. We have a choice. We can wait to find out what will happen, or start reducing our emissions now, before it is too late.

* Resources: The Great Climate Flip-Flop: Sudden climate transitions during the Quaternary: Full footnotes available in Stormy Weather

[From WS February/March 2002]

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