The earlier summit will bring together officials from more than 150 countries, including 70 heads of state and government, making it the largest-ever meeting on climate change by world leaders. The aim is to secure political commitment before negotiations begin in Bali in December.

"Bali must advance a negotiating agenda to combat climate change on all fronts, including adaptation, mitigation, clean technologies, deforestation and resource mobilization," says UN secretary-general Ban Ki-moon. "Bali must be the political response to the recent scientific reports by the Intergovernmental Panel on Climate Change (IPCC). All countries must do what they can to reach agreement by 2009, and to have it in force by the expiry of the current Kyoto Protocol commitment period in 2012."

The New York summit is particularly timely as this week the full fourth-assessment IPCC report by working group II  on climate change impacts, adaptation and vulnerability is released, with key authors speaking about their findings at the UK's Royal Geographical Society in London. The full report includes results not previously reported in the summary for policymakers released in April.

It states, for example, that if warming is not kept below 2 °C – which "will require the strongest of mitigation efforts and currently looks very unlikely to be achieved" – then substantial global impact will occur. This could include species extinctions and put millions of people at risk from drought, hunger and flooding.

What's more, some future effects of global warming that are predicted by the IPCC's second and third assessments in 1995 and 2001 are already being observed – much faster than expected. And since the completion of the fourth assessment in April, the mosquito-borne disease Chikungunya has been discovered in northern Italy. The outbreak, in August and September, was the first European recording of the disease, which generally occurs in tropical areas.

In any case, the IPCC has found that there is already about 1 °C of warming in the climate system, which is likely to impact heavily on developing countries. The full report concludes that Africa, Asian megadeltas, small islands and the Arctic will be hit the hardest by climate change. By 2020, for example, between 75 and 250 million people in Africa will face increased water shortages. And in South Africa, revenue from crops is likely to fall by as much as 90% by the end of the century.

"What's particularly galling is that people in Africa, who are likely to be worst affected by climate change, are those least responsible," said Gareth Thomas of the UK's Department for International Development at the report launch in London. "In Europe, carbon dioxide emissions for the European Union 15 [nations] are over 8 tonnes a head. In America they are over 20. Yet in Kenya, carbon dioxide emissions are just one-fifth of a tonne per person."

The most affected sectors, meanwhile, will be water (especially in the dry tropics), agriculture (particularly in low latitudes), human health in countries with low adaptive capacity, and ecosystems such as coral reefs, sea-ice biomes, mangrove swamps, salt-marshes, tundra, boreal and mountain systems.

That means that adaptation to cope with the climate change that is already "built in" to the system and mitigation to reduce additional climate change are both crucial and urgent. Any mitigation measures taken now will not start to "bite" until around 2050, but should nevertheless help prevent temperature increases that are too large to adapt to. "Achieving - soon - the right combination of adaptation and mitigation, is the most urgent task of the policymakers," said the IPCC.

Let's hope that the UN meetings next week, and in December, come up with a viable and achievable agreement that will cut greenhouse gas emissions rapidly.