If there is a major increase in atmospheric methane leading to a feedback cycle that reinforces itself, we are looking at 6 degrees celsius warming, minimum, by the year 2100, with no slowdown in warming afterward. In other words, a return to Permian/Triassic boundary temperature conditions (251 million years ago, when high temperatures led to the worst mass extinction in earth's history) in the course of less than 100 years. What does this mean for our future?
According to journalist Mark Lynas:
At 1 degree warming, within 10 years droughts would intensify across the American West, leading to sharp increases in food prices. Europe's Alps would crumble in catastrophic rockslides, killing thousands at the minimum. The American Southeast would be hit by torrential rainstorms. Southeast Asia and the Pacific would be hit by massive hurricanes, sending millions of homeless, hungry refugees to places like the U.S.
At 2 degrees warming, within 40 years the Yangtze and Yellow Rivers would run dry, leading to catastrophic famine in China. As the oceans became warmer and more acidic (and they would become more acidic more rapidly than in past warming periods because of the unprecedented rate of increased atmospheric CO2), photosynthesizing plankton--responsible for more than half the conversion of solar energy into glucose--would begin a long, slow decline toward extinction, as would shellfish and coral, which is essential to the survival of many fish. This would have a ripple effect up the food chain, causing mass extinctions that would wipe out most sources of meat. Europe would become much hotter, with catastrophic heat waves that killed hundreds of thousands every summer.
At 3 degrees warming, within 30 years the Indus and Colorado Rivers would run dry, while the Amazon would slow to a trickle. Fresh water would be increasingly hard to come by. The Eastern United States, including New York City and nearly all of Florida, would be underwater. The Midwest and Australia would be taken over by desert, the American South would have substantially more winter snow, frequent thunderstorms would batter the West Coast of South America (which would also see massive declines in fish populations), and massive ice storms could hit the upper east coast. The Arctic would become ice-free year-round. Temperate forests would grow in Siberia, Antarctica, and Alaska. Super-hurricanes would hit the American East Coast, the Pacific Coast of Asia, and the Mediterranean Coasts. The savannahs of Africa would become deserts, and grain crop production worldwide would decline by more than 25%, causing massive famine.
At 4 degrees warming, a level not seen since the Eocene Epoch (56 million years ago) within 40 years North Africa, Southeast Asia, and the large cities of Australia's coast would end up underwater. The Antarctic would become ice-free year-round. Grains not genetically engineered would stop growing.
At 5 degrees warming, a level not seen since the Cretaceous Period (65 million years ago) within 90 years the Gulf Coast, most of Brazil, much of Southern Europe, and all but the central and eastern parts of Africa will end up underwater. The Great Plains and Central Australia would become great inland lakes. The tropical rain forests of the Amazon, Africa, and Asia would burn up, with deadly consequences for air quality, and perennial wildfires would sweep across what remained of South America. Massive floods would batter Alaska, Siberia, China, and the British Isles. Most of the rest of the world would become giant deserts. People would be unlikely to be able to survive outside the tropical rain forests of Eastern Europe and the temperate forests of Northern Europe and Antarctica. At this point, global warming would almost certainly become irreversible.
At 6 degrees warming, a level not seen since the Permian-Triassic extinction event, 241 million years ago, within 15 years atmospheric oxygen levels would fall by more than 25% as ocean warming released clouds of poisonous hydrogen sulfide gas into the atmosphere. Humans would be unable to breathe without technological assistance.
At 8 degrees or more, a level not seen since the Mesoproterozoic Eon, 1.6 billion years ago, the bulk of the atmosphere would be composed of CO2, methane, and hydrogen sulfide. Tornadoes of fire and rains of sulfuric acid would be daily events, while nights would bring blasts of freezing winds. The only terrestrial life surviving without technological assistance would be lichens, fungi, microbes, and mutant arthropods.
Methane is a much more potent greenhouse gas than carbon dioxide. Our only hope to stabilize warming over the next century at 1-3 degrees (rather than 5-6 degrees or more) is to take advantage of the current solar cooling cycle by immediately cutting greenhouse gas emissions to stabilize CO2 levels at or below the current level. We may also need to look at technological solutions to lower the CO2 levels in the air presently. If we do this very, very quickly, we may be able to cool the oceans enough to halt or slow the methane releases and prevent triggering an autocatalytic cycle that will be irreversible.
http://www.amazon.com/Six-Degrees-Future-Hotter-Planet/dp/1426203853
