The tipping point in many scientists' view is the 2˚C rise that the EU has adopted as the maximum limit that mankind can risk. [1]
From - http://www.peterrussell.com/Earth/RunawayCC.php -
'Over the last hundred years, average global temperatures have increased by 0.75°C, one third of that rise occurring in the last twenty years. The 2007 report by The Intergovernmental Panel of Climate Change (IPCC) forecast that, by 2090, temperatures will have risen between 2 and 6 degrees.
Even a two degree rise in temperature would be disastrous. Changes in climate will lead to more intense storms, longer periods of drought, crop failures in many developing countries, the destruction of nearly all the coral reefs, the melting of much of the polar ice, the flooding of many low-lying urban areas, the possible collapse of the Amazonian rain forest, and the extinction of 20-30% of the planet's species. The IPCC projects that this could happen by 2050.
If the temperature were to rise by six degrees, the prognosis is extremely bleak. At this temperature, the entire planet will be ice-free. Sea levels will rise by 70 meters. Many species of tiny plankton will cease to exist, and the problem would echo up the food chain, bringing the extinction of many fish, sea mammals, and the largest whales. Much of the land will now be desert. Hurricanes of unimaginable ferocity will bring widespread ecological devastation. If, as is possible, the ozone layer were destroyed, the burning ultraviolet light could make life on land impossible. Evolution would have been set back a billion years. It would be a planetary catastrophe.
The predominant response to the horrendous dangers of climate change focus on reducing carbon emissions—both through significant reductions in fossil fuel consumption, and the rapid development of alternative sources of energy. The hope is to keep the warming to the lower end of the range—between one and two degrees. This is absolutely essential. However, there is growing reason to believe that, however much we may reduce our CO2 output, other factors will keep pushing the temperatures higher, and we may still find ourselves approaching the 6 degree mark, with its disastrous consequences.
Most climate models look at the direct effect of carbon emissions on global temperatures. What they do not include is the effect any warming might have on promoting further warming—what are called positive feedback loops. One such loop arises when warmer temperatures lead to an increased evaporation of water from the oceans. Water vapour is itself a powerful greenhouse gas, and this adds to the warming. A second feedback loop concerns the dwindling sea ice and snow cover. Exposed sea and land are darker than snow and ice, and absorb more sunlight, leading to a further rise in temperature.
The most dangerous feedback loop involves methane release. Methane is also a greenhouse gas, and one that is 21-times more potent than CO2. Billions of tons of methane lie frozen in the permafrost of the Arctic tundra. To make matters worse, the Arctic regions are warming three times faster than the rest of the planet, and are already 2 degrees warmer than they were in the 1980s. Consequently, large areas of the Siberian tundra are now beginning to thaw. In 2005 it was discovered that a million square kilometers—the size of France and Germany combined—in western Siberia had turned from permanently frozen peat bog into a mass of shallow lakes. Moreover, as the tundra thaws, it too changes color from white to brown, absorbing more of the sun's heat, and thus thawing even faster.
If this continues—and there is no reason to suppose it will not—billions of tons of methane will be released into the atmosphere leading to further rises in global temperatures—and even faster rises in the Arctic. The tundra will then thaw even faster, releasing even more methane. Within a short time—probably just a decade or two—a global tipping point will be reached at which global warming becomes unstoppable. It will then only be a matter of time before the temperature rises the six or so degrees that would bring planetary catastrophe.
Can we avoid runaway climate change? A recent panel study concluded that just another decade of business-as-usual carbon emissions will probably make it too late to prevent the triggering of runaway climate change. Even in the most hopeful of the IPCC scenarios, in which humanity moves away from material consumption towards service and information economies, and the introduction of clean, resource efficient technologies, temperatures are still set to rise by about 2°C by 2090. This would probably lead to a rise in Arctic temperatures by 4-6°C, and a runaway greenhouse effect would still be very likely.
Even if we were to stop all carbon emissions immediately, global temperatures would continue to rise by 0.1°C per decade, as we experience the full effects of the carbon already in the atmosphere. This is due environmental inertia—a mechanism by which the environment stores up part of the energy of generated by greenhouse gas emissions, only releasing it to the atmosphere later on. This will lead to a doubling of the current temperature increase by 2090, i.e. a rise of 1.5°C.
This is if we stopped all carbon emissions now. The likelihood of that is nill. On the contrary, carbon emissions are set to increase over the coming years. In China new coal-fired power stations are coming on-line at the rate of one-per-day. The USA and India are likewise turning to coal to compensate for reduced oil consumption. Governments are hampered by the need to maintain energy-consumptive economies. Consequently, their attempts to reduce carbon emissions fall far short of what is needed. The new EU guidelines on carbon emissions propose reducing carbon emissions by 20% by 2040, by when (by their own admission) temperatures would have risen by 2°C, and the point of no-return may well have been passed. Add to this the reluctance of people in the developed world to give up their comforts, and the desire of the remaining 80% of the world's population to have similar comforts, and there seems little chance of avoiding disaster.
It would seem that whatever we may do in the way of reducing fossil carbon emissions and developing alternative sources of energy, we are still facing the very real possibility of runaway climate change and a planetary catastrophe.
There is, however, another approach, one that requires looking at the problem a little differently. The root of the problem is that carbon that was buried in the ground for millions of years has suddenly (in geologic terms) been released into the atmosphere. Most current approaches to the problem seek to slow down the rate at which this is occurring. But if we are to survive, we also need to get the carbon already released out of the atmosphere (carbon capture) and back into the ground (carbon sequestration).'