First, the governments of some developing countries, led by India, have insisted that relatively poor countries should not have to reduce their greenhouse gas emissions. They maintain that developed nations long exploited cheap energy to build their present wealth. These countries should therefore pay the price for global warming. Such thinking is not without merit, but it ignores the political challenges faced by governments in developed nations. It is also disingenuous: developing countries now account for such a large share of the world's carbon emissions that no cuts made by wealthy nations alone could sufficiently mitigate climate change.
Second, pledges currently made by participating governments would still lead to warming of nearly three degrees Celsius, well beyond the two degree limit suggested by scientists. Third, while that limit is the most ambitious goal we can now hope to achieve, it may not be enough to avoid catastrophic and irreversible changes in regional environments. Studies of past climates - many described on this site - have revealed the disturbing vulnerability of ecosystems and civilizations to even minor fluctuations in average global temperatures.
Finally, fourth, a poll recently carried out in twenty countries has revealed sharply declining popular support for ambitious climate change targets at COP21. This disheartening shift in public opinion may reflect new fears about terrorist attacks launched by ISIS, which appears to be the more pressing problem. Yet some recent scientific articles conclude that the Syrian civil war, and in turn the rise of ISIS, may have been partially enabled by climate change.
Nevertheless, the urgency of action has never been more clear. This year will be, by far, the warmest ever recorded, and October was an especially hot month even in that context. This is a map of global temperatures in October, relative to worldwide averages between 1951 and 1980:
After I gave my lecture, it occurred to me that, during COP21, this vision of the coming century should be widely shared by climate change researchers. It may be our best response to the headwinds that now challenge a comprehensive climate agreement.
I began my lecture by explaining that, according to model simulations in the latest IPCC Assessment Report, this is how global temperatures and precipitation patterns will likely change by 2081-2100, relative to averages in 1986-2005:
The impact of these changes will be further exacerbated by an increase in the regional frequency of weather extremes. Hurricanes, typhoons, and cyclones may well become more severe, if not necessarily more common, and heatwaves will be far more frequent. The following maps demonstrate that, in many places, both droughts and floods will also be much more common in 2080-2099 than they were in 1986-2005. They depict changes according to very high warming scenarios.
However, this prediction may be underselling the threat, because it does not account for the catastrophic ice loss in Greenland and Antarctica. Scientists have recently discovered that Greenland is melting more quickly and more thoroughly than they had expected. The complete melting of the Greenland ice sheet will probably take thousands of years, but it will eventually raise sea levels by some 23 feet.
The story in Antarctica is far more complex. Scientists recently announced that the West Antarctic Ice Sheet has started an irreversible but relatively slow-moving collapse. This conclusion was not included in the fifth IPCC assessment report, but it will eventually raise global sea levels by a further 12 feet. Nevertheless, a new NASA study has found that an increase in Antarctic snow accumulation that began 10,000 years ago is adding enough ice to the continent to more than outweigh losses from the West Antarctic Ice Sheet. Within 20 to 30 years, however, Antarctic ice losses should start exceeding accumulation.
Even the IPCC's conservative sea level predictions are concerning. These maps compare sea levels in 1986-2005 to their projected averages in 2081-2100. Atmospheric and oceanic circulation ensure that sea levels are not uniform, and some of the most severe changes will affect the coasts of Africa.
Ocean acidification will have catastrophic consequences for many (but not all) kinds of marine life. It will depresses metabolic rates for animals such as jumbo squid, and dampen immune responses in animals such as blue mussel. By killing algae, it will threaten reef ecosystems by bleaching coral and reducing oxygen levels. The following maps show oceanic pH levels in 1850 (top) and, according to forecasts, 2100 (bottom). The lower the number (and warmer the colour), the higher the pH level (and more acidic the water).
One of these tipping points would lead to the breakdown of the vast ocean current called the Atlantic Meridional Overturning Circulation (AMOC), which helps govern the world’s climate. When ice forms in the Arctic and northern Atlantic Oceans, water around it gets saltier and therefore heavier. When it sinks, water from farther south moves north to replace it. This brings heat far into the northern hemisphere. When ice melts on a huge scale, cold freshwater drains into the northern Atlantic and slows down the current.
Take a look at the map of October temperatures, at the top of this article. That cold spot near Greenland reflects meltwater from the Greenland ice sheet. The AMOC is already slowing down. Could the collapse of the Greenland ice sheet shut it down completely, and, ironically, bring bitter cold to Europe and North America?
More worrisome still is the risk of widespread melting of the permafrost, which currently stores some 1.4 trillion tons of methane. As a greenhouse gas, methane is 84 times more potent than Carbon Dioxide. The permafrost is already starting to melt on a large scale. In 2006, melting Siberian permafrost released 3.8 million tons of methane into atmosphere, but in 2013, that number increased to 17 million tons.
There is a feedback loop at work here, because melting permafrost warms the atmosphere, especially in the far north, which then leads to more melting. More dramatic melting of the world's permafrost would lead to more warming than is predicted by even the IPCC's worst-case scenarios. The map below depicts the global distribution of permafrost, and shows the scale of the potential problem:
The IPCC has published the following map of the ongoing environmental and social consequences of climate change today. If warming temperatures have already had profound global consequences, what will our future have in store?