What scares the scientists

If we do not make every effort to combat climate change, global temperatures could rise high enough to trigger events like the mass melting of Arctic permafrost, or huge wildfires in Amazonia. These would release more greenhouse gases, accelerating global warming. We know that such catastrophic climate change has happened in the distant past. To stop it from happening again, we must act now, before events move beyond our control.

MELTING THE TUNDRA

Vast areas of the far north are frozen below ground. In summer this permafrost partly thaws, forming huge swamps full of decaying vegetation, as here in Siberia. The decay process releases methane, a greenhouse gas that is 25 times more powerful than carbon dioxide, but a lot of this methane is frozen into the permafrost. As temperatures rise and more permafrost melts each summer, more methane is released, adding to the greenhouse effect.

BURNING RAINFORESTS

Tropical rainforests do not normally suffer wildfires, but as temperatures rise, they are drying out and burning. Even if the trees do not burn, they can die from drought. Trees pump water into the air through their leaves, so fewer trees means less rain. This process could destroy the richest ecosystem on Earth, and as the trees burn or decay, all the carbon in their timber will turn to carbon dioxide, raising temperatures still further.

ACID OCEANS

As oceans get warmer and become more acid through absorbing carbon dioxide, many marine organisms will start to die off. If temperatures rise to 3.6°F (2°C) above pre-industrial levels, up to 97 percent of the world's coral reefs could suffer "coral bleaching"and die. Acidified water may make it impossible for organisms such as crabs, clams, and microscopic plankton to build their shells, and this would destroy the food supply of other animals such as fish. The result could be a mass extinction involving many types of marine life.

Methane hydrate in a sample of ocean-floor mud

OCEANIC METHANE

Methane seeping from decaying plankton on the ocean floor can combine with very cold water to create a type of ice called methane hydrate. It normally stays frozen, but if the water temperature rises, the hydrate can melt. This releases methane, which then bubbles up through the water into the atmosphere, adding to the greenhouse effect. It will take centuries for the ocean depths to get warm enough for this to happen, but some scientists think that it may have helped cause extreme global warming in the past.

MASSIVE SEA LEVEL RISE

Most scientists think that sea levels will rise by less than four feet (one meter) by 2100. But if the immensely thick ice sheets of Antarctica and Greenland start to collapse in a big way, sea levels could rise by a lot more than this, and ultimately by up to 82 ft (25 m). Even a 23-ft (7-m) rise—the effect of the Greenland ice sheet collapsing—would swamp coastal cities such as London, New York, Tokyo, Shanghai, and Calcutta.

COLLAPSING ICE SHEETS

The fringes of the Greenland and west Antarctic ice sheets are already melting faster than ever recorded before, with huge slabs of ice breaking off and floating away as icebergs. This loss of ice around the fringes will make it easier for blocks of continental ice to slip toward the ocean, lubricated by meltwater seeping down through fissures in the ice. This could allow the ice sheets to collapse, making sea levels rise dramatically.

THE BOTTOM LINE

If we allow global warming to run out of control, then it could eventually wipe out most of life on Earth. This happened about 250 million years ago, when some 96 percent of all species at that time became extinct. Scientists believe that massive volcanic activity caused colossal carbon dioxide release and global warming, reinforced by methane release from oceans. During this event the average temperature appears to have risen by 10.8-14.4°F (6-8°C). Global temperatures could easily increase by 7.2°F (4°C) by 2100—and possibly more.

LIVING ON THE EDGE The Inuit hunter in this 19th-century lithograph is waiting for a seal to pop up through a breathing hole. Some Inuits still use the same hunting method today. Like Inuit hunters, people who live in harsh climates have developed ways of life that suit their environments. Climate change could make their skills useless, and their cultures could fall apart.

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