Start the car, turn on a light, or do just about anything, and you add carbon dioxide to the atmosphere. If youre an average resident of an industrial country, your contribution adds up to more than 5,5 tons of carbon a year. The coal, oil, and natural gas that drive the industrial worlds economy all contain carbon inhaled by plants hundreds of millions of years ago carbon that now is returning to the atmosphere through smokestacks and exhaust pipes, joining emissions from forest burned to clear land in poorer countries.
Carbon dioxide is foremost in an array of gases from human activity that increase the atmospheres ability to trap heat. This greenhouse warming of the atmosphere is already taking hold. Melting glaciers, earlier springs, and a steady rise in global average temperature are just some of its harbingers.
Each year humanity dumps roughly 8,8 billion tons of carbon into the atmosphere, 6,5 billion tons from fossil fuels and 1,5 billion from deforestation. But less than half that total, 3,2 billion tons remains in the atmosphere to warm the planet. Where is the missing carbon? Its really a mystery. Forest is apparently not the only place where nature is breathing deep and helping save us from ourselves. Forests, grasslands, and the waters of the oceans must be acting as carbon sinks. They steal back roughly half of the carbon dioxide we emit, slowing its buildup in the atmosphere and delaying the effects on climate. But the problem is that scientists cant be sure that the process will not be reversed, and as the globe continues to warm, it might even change to a curse if forests and other ecosystems change from carbon sinks to sources, releasing more carbon into the atmosphere than they absorb.
This is not just a matter of intellectual curiosity. Scorching summers, fiercer storms, altered rainfall patterns, and shifting species are some of the milder changes that global warming might bring. And humanity is on course to add more and more to atmospheric carbon dioxide. At that level, all kinds of terrible things could happen, and probably some of them will. Coral reefs could vanish, deserts could spread, currents that ferry heat from the tropics to northern regions could change course, perhaps chilling the British Isles and Scandinavia while the rest of the globe keeps warming.
Tens of billions of tons of carbon a year pass between land and the atmosphere: given off by living things as they breathe and decay and taken up by green plants, which produce oxygen. A similar traffic in carbon, between marine plants and animals, takes place within the waters of the ocean. And nearly a hundred billion tons of carbon diffuses back and forth between ocean and atmosphere.
But these processes are different. Because plants give off oxygen when they absorb carbon dioxide, a plant sink would lead to a corresponding oxygen increase. But when carbon dioxide dissolves in the ocean, no oxygen is added to the atmosphere. Plants prefer taking in gas that contains carbon 12, a lighter form of the carbon atom. The rejected gas, containing carbon 13, builds up in the atmosphere. The ocean, though, does not discriminate, leaving the carbon ratio unchanged. So researchers have come to the conclusion that while the ocean is soaking up almost half the globes missing carbon 2 billion tons of it the sink in the Northern Hemisphere appears to be the work of land plants. Their appetite for carbon dioxide surges and ebbs, but they remove, on average, more than 2 billion tons of carbon a year.
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Right now global warming, ironically, may be helping forestall even more warming, by speeding the growth of carbon-absorbing trees in the north. But northern ecosystems could soon turn against us. The trees grow more slowly in warmer years because of moisture stress. Trees are being killed by insects, and are becoming less able to fight off infestation. The vast tracts of dead trees will ultimately send their carbon back to the atmosphere when decay or fire consumes them. If that happened, the boreal forest as we know it would be no more.
Climate experts keep a worried eye also on the permafrost because vast reserves of peat and other carbon-rich organic material are frozen into it a global trove of carbon estimated at 200 billion tons. For hundreds, perhaps and thousands, of years low temperatures entombed it. Now scientists predict that if, due to warming in the Arctic, the permafrost warmed up and dried out, most of that carbon could be released. The atmospheric level of carbon dioxide could jump by more than 25 percent above current levels.
Reading
15. Read the text and say how to balance the global level of carbon dioxide?
