The Intergovernmental Panel on Climate Change is composed of three working groups. The second group is responsible for reporting on the impacts of climate change on the natural and human environment. Its most recent report (the fourth assessment report) makes for difficult reading and not simply because of its disturbing predictions. The prose is at best terse, sometimes to the point of being cryptic. Futhermore, while it describes the probable impacts of climate change, it does not clearly identify when these impacts will come about, neither on a temporal scale nor on a thermal scale. Thankfully, Mark Lynas has done an admirable job of partly filling this gap in his book Six Degrees.
The basis of Lynas's book is scholarship done at the Earth Sciences Library at Oxford University. Lynas systematically scoured articles in peer reviewed journals and classified their predictions of impacts according to temperature increases degree by degree. The literature estimates impacts for temperature increases ranging from less than one degree Celsius to five degrees Celsius. Occasionally impacts are discussed for more than five degrees. This provided a convenient structure for Lynas's book: each of his six substantive chapters describes the effects of progressively greater temperature increases. Lynas makes no attempt to predict when these increases might occur as this depends on the public policies we adopt.
Broadly speaking, the impacts of increases up to two degrees Celsius are in the manageable range, though they will certainly be extremely problematic for regions most vulnerable to climate change. Indeed, we have already experienced enough unusual weather events consistent with climate change to say that the impacts of climate change are upon us. The 2003 heat wave that dominated Europe for three months, for example, is estimated to have killed 22,000 to 35,000 people. Lynas describes this event in his "Two Degrees" chapter, suggesting that such heat waves would become common in such a world. It is noteworthy, that reasonable assumptions about our future carbon emissions would make a two or three degree increase likely.
As the temperature rises into the third degree, predictions of cataclysmic consequences become common in the scientific literature. Dangerous feedbacks will begin to play an important role in increasing the Earth's surface temperature. For example, the Amazon and other rain forests are likely to begin burning away, causing a loss of moisture. As the forest floor dries out, it will begin releasing significant amounts of greenhouse gasses locked in the peat and soil. The permanent ice covering Greenland is also likely to begin melting at an accelerated pace. This will raise sea level, of course, but it will also lower the elevation of the ice pack and dump increasing quantities of ice into the surrounding water, thus raising the local temperature in Greenland and accelerating the melting process in another dangerous feedback. Food production will be severely disrupted by flooding and droughts around the world.
By the fourth degree increase, the nightmare truly begins. Both the Ross and Ronne ice shelves of the Antarctic could become unstable. Were one or both to collapse (as did the Wordie, Larsen A, and Larsen B ice shelves) the rate of glacier melt from the Antarctic mainland would increase dramatically leading to a rapid rise in sea level. By the fifth degree increase "an entirely new planet is coming into being....The remaining ice sheets are eventually eliminated from both poles. Rain forests have already burned up and disappeared, rising sea levels have inundated coastal cities and are beginning to penetrate far inland into continental interiors. Humans are herded into shrinking 'zones of habitability' by the twin crises of drought and flood."
As early as the third degree of warming, methane hydrates will begin to be released from the Arctic Ocean floor and from melting permafrost. Methane hydrates are greenhouse gasses that are far more potent than carbon dioxide. By the fifth degree of increase, the quantities of methane hydrates released into the atmosphere are likely to be staggering and will trigger a feedback that might make the planet entirely uninhabitable.
Lynas does not find many predictions in the scientific literature about the consequences of a sixth degree of warming; however, it is recognized as a possibility, particularly if there are enough strong feedbacks to push the planet to an new equilibrium that is far warmer than what we now experience. In Storms of My Grandchildren, NASA climatologist James Hansen raises the possibility of "the Venus Syndrome" in which the greenhouse effect extinguishes all life on Earth. Lynas thinks it is unlikely that the changes to the climate will extinguish human life, but were the planet to reach the high end of the range of warming predictions, such a possibility is not negligible.
The methodology behind Lynas's book is sound and the presentation of his scholarship is illuminating. I doubt that there is a better general summation of the scientific literature as it pertains to the effects of climate change. Six Degrees is a cogent account of the future we likely face. Despite its dispassionate tone, it is a clarion call for action to mitigate the disaster that is likely to unfold during the lifetimes of the younger members of our world.