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 sub-tropical 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 heatwaves, 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 (which include 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% by 2020. Climate change would also lead to increased water stress, which by 2020 could affect 75-250 million people in Africa alone.

Overall, temperature increases are projected to increase by the year 2100 within a range of 1.1C to 6.4C. 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 1.8C, and 4C 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 increased morbidity and mortality as a result of heatwaves, 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 (GHGs). Several scenarios have been assessed by the IPCC, and one that would limit future temperature increase to between 2.0-2.4C 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% of global GDP in 2030. Moreover, there are enormous co-benefits to mitigation: lower emissions of GHGs 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% 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.

The need for international action, therefore, stems from two important observations arising out of the IPCC's work. First, if we do not mitigate emissions of GHGs, the negative effects of climate change will be difficult to reverse, implying great hardship and possibly danger to mankind and other species.

Second, the benefits of mitigating emissions of GHGs are so overwhelming that this, combined with the prospect of the harm resulting from inaction, makes it imperative for the world to devise an international response and a plan of action. Given the challenge facing us, the magnitude and nature of which were clearly brought out by the IPCC, the Copenhagen conference later this year must produce a multi-lateral agreement that deals adequately with climate change.

Copyright: Project Syndicate 2009.