Climate models, cited in Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report, projected that global temperatures could rise by as much as 10.4°F (5.8°C) by the end of the twenty-first century. To find out what would happen if the world warmed by that much, journalist Mark Lynas read thousands of scientific papers dealing with how climate change affected life on Earth in the past. The result of his work was a book entitled Six Degrees.
An increase of one degree would bring about conditions like those of the Holocene Maximum, about 6,000 years ago. Then, much of the United States experienced drought worse than the Dust Bowl of the 1930s. Arctic ice would melt, giving way to ocean water, which will absorb more heat than ice and lead to further warming. Species could die out in sensitive areas such as the rain forests of northeast Australia. Low-lying island nations would be threatened by rising sea levels.
An increase of two degrees would cause severe droughts and water shortages in China's interior. As carbon dioxide accumulates, the Earth's oceans would become more acidic and less able to support life. As sea creatures that absorb carbon dioxide die off, more carbon dioxide stays in the atmosphere. Europe would suffer frequent heat waves as severe as the deadly one in 2003, which was called a "once-in-a-thousand-years" event. Because temperatures rise faster at higher altitudes, mountain glaciers would continue to shrink and Greenland's ice sheet would start to disappear. A two-degree rise could put hundreds of thousands of plant and animal species at risk of extinction.
An increase of three degrees would breed "super hurricanes" more powerful than anything humans have experienced so far. El Niño, and the extreme weather that it brings, could become a near-permanent occurrence. Higher temperatures would bring drought to southern Africa, forcing those who live there to flee—and fight for survival against neighbors who refuse to take them in. Asia's monsoons would become more intense but less predictable. At the same time, higher temperatures would melt Asia's massive glaciers, depriving large parts of the continent the water needed to grow crops and generate electric power. More and more crops would reach their "thermal tolerance threshold," beyond which they cannot grow. As the soil warms, bacteria will break down the huge amounts of carbon stored inside, returning it to the atmosphere as carbon dioxide. Rain forests in the Amazon basin would give way to grassland, perhaps as the result of a gigantic wildfire, and perhaps turn into a Sahara-type desert.
If temperatures rise by four degrees, the Arctic Ocean would become ice-free for the first time in several million years. The West Antarctic ice sheet could collapse into the ocean, raising ocean levels by 15 feet (4.6 m) or more. Coastal areas such as greater New York would flood, and the millions who live there would be forced inland. Land where crops can be grown would become scarcer, and mass starvation would be difficult to avoid. Higher temperatures would melt the permafrost in Siberia, Alaska, and northern Canada, releasing hundreds of billions of tons of greenhouse gases that had been trapped inside by below-freezing temperatures for millions of years. Southern Europe could become so hot that those who live there would be turned into climate refugees.
An increase of five degrees would empty most of the planet's underground reservoirs of water, making it more difficult yet to grow crops. Competition for the world's remaining arable land could lead China to invade Russia and the United States to invade Canada. Increasingly, humans would be concentrated toward the poles, and the Earth's population could fall to one billion or less. Conditions could resemble those of about 55 million years ago, when carbon dioxide levels topped 1,000 parts per million, oceans were acidic, and there were extremes of wet and dry. During that time, a massive die-off of sea creatures occurred. Scientists believe the die-off might have been the result of a huge eruption of a combination of methane and water loosened from the ocean depths. Even today, vast amounts of this substance remain trapped on the continental shelves underneath the oceans.
Left unchecked, global warming could lead to conditions similar to those of the end of the Permian period, about 250 million years ago. Then, a cataclysmic event wiped out nearly all life on Earth. Scientists are unsure what caused it, but one possibility is a greenhouse event that raised global temperatures by six degrees. Oceans were almost inhospitable to life, ferocious hurricanes raged, and erupting volcanoes released large quantities of carbon into the atmosphere. At plus-six degrees humans, too, are at risk of extinction. Lynas raised the possibility of "the ultimate nightmare scenario," super-eruptions of underwater methane that would be 10,000 times as powerful as all of the world's nuclear weapons combined.
Source: Mark Lynas, Six Degrees: Our Future on a Hotter Planet. London: Fourth Estate, 2007.
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exhibiting a nearby tipping point."11Another team of scientists, led by James Hansen, expressed similar concern over the irreversible loss of Arctic ice:
If we stay our present course, using fossil fuels to feed a growing appetite for energy--intensive life styles, we will soon leave the climate of the Holocene, the world of prior human history. The eventual response to doubling pre--industrial atmospheric CO2 likely would be a nearly ice--free planet. . . . Ocean and ice sheet inertias provide a buffer delaying full response by centuries, but there is a danger that human-made forcings could drive the climate system beyond tipping points such that change proceeds out of our control.12
Mark Lynas, who has read much of the scientific literature, suggests that global warming might become unstoppable once temperatures rise to 7.1°F (4°C) above pre-industrial levels. At that point, vast amounts of greenhouse gases under the frozen soil of the Arctic will escape into the atmosphere, causing even more warming. The end result of runaway global warming is a frightening prospect. Lynas believes that higher water temperatures could release methane gas currently trapped on the ocean floor. The highly combustible gas could become a massive fireball that would be far more devastating than the most powerful nuclear bomb. Al Gore described another possible dire outcome in a column in the New York Times:
Consider this tale of two planets. Earth and Venus are almost exactly the same size, and have almost exactly the same amount of carbon. The difference is that most of the carbon on Earth is in the ground—having been deposited there by various forms of life over the last 600 million years—and most of the carbon on Venus is in the atmosphere.
As a result, while the average temperature on Earth is a pleasant 59 degrees, the average temperature on Venus is 867 degrees.13
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