Today, international action on climate change is urgent and essential. Indeed, there can no longer be any debate about the need to act, because the Intergovernmental Panel on Climate Change (IPCC), of which I am chairman, has established climate change as an unequivocal reality beyond scientific doubt.
For instance, changes are taking place in precipitation patterns, with a trend toward higher precipitation levels in the world’s upper latitudes and lower precipitation in some subtropical and tropical regions, as well as in the Mediterranean area.
The number of extreme precipitation events is also increasing — and are increasingly widespread. Moreover, the frequency and intensity of heat waves, floods and droughts are on the rise.
This change in the amount and pattern of rainfall has serious implications for many economic activities, as well as for countries’ preparedness to handle emergencies such as large-scale coastal flooding or heavy snowfall.
Some parts of the world are more vulnerable than others to these changes. The Arctic region, in particular, has been warming at twice the rate of the rest of the globe. Coral reefs, mega-deltas (including cities like Shanghai, Kolkata and Dhaka) and small island states are also extremely vulnerable to rising sea levels.
Other negative effects of climate change include possible reductions in crop yields. In some African countries, for example, yields could decline by as much as 50 percent by 2020. Climate change would also lead to increased water stress, which by 2020 could affect between 75 million and 250 million people in Africa alone.
Overall, temperature increases are projected within the range of 1.1ºC to 6.4ºC by the year 2100. In order to focus on this set of outcomes, the IPCC has come up with a best estimate at the lower end of this range of 1.8ºC, and 4ºC at the upper end. Even at the lower estimate, the consequences of climate change could be severe in several parts of the world, including an increase in water stress, serious effects on ecosystems and food security, and threats to life and property as a result of coastal flooding.
There also may be serious direct consequences for human health if climate change is not checked, particularly increases in morbidity and mortality as a result of heat waves, floods and droughts. Moreover, the distribution of some diseases would change, making human populations more vulnerable.
Because the impact of climate change is global, it is essential that the world as a whole take specific measures to adapt. But it is already clear that the capacity of some communities to adapt will quickly be exceeded if climate change goes unmitigated.
To help these most vulnerable communities, it is essential for the world to devise a plan of action to limit the emission of greenhouse gases. Several scenarios have been assessed by the IPCC, and one that would limit a future temperature increase to between 2ºC and 2.4ºC would require that emissions peak no later than 2015 and decline thereafter. The rate of decline would then determine the extent to which the worst effects of climate change can be avoided.
The IPCC also found that the cost of such a strict effort at mitigation would not exceed 3 percent of global GDP in 2030. Moreover, there are enormous co-benefits to mitigation: Lower emissions of greenhouse gases would be accompanied by lower air pollution and increased energy security, agricultural output and employment. If these co-benefits were taken fully into account, that price tag of 3 percent of GDP in 2030 would be substantially lower, perhaps even negative. The world could actually enhance economic output and welfare by pursuing a path of mitigation.