Africa and much of south Asia faces extreme risk from climate change even as top carbon polluters, by comparison, will be shielded from its ravages, according a ranking of 166 nations obtained by AFP Wednesday.

Somalia, Haiti and Afghanistan top the Climate Change Vulnerability Index, calculated from dozens of variables measuring the capacity of a country to cope with the consequences of global warming.

"We wanted to look at what is going to impact human populations," explained Fiona Place, senior environmental risk analyst at Maplecroft, a British-based firm that provides global risk intelligence for businesses.

Even if the world agrees at make-or-break climate talks in December to slash CO2 emissions, many of those impacts -- rising sea levels, increased disease, flooding and drought -- are already inevitable, UN scientists say.

Of the 28 nations deemed at "extreme risk", 22 are on the African continent.

Sri Lanka and low-lying Bangladesh are similarly threatened, with Pakistan not far behind.

At the other end of the spectrum, Norway, Finland, Japan, Canada and New Zealand are best protected, due to some combination of wealth, good governance, well-managed ecosystems and high resource security.

The United States and Australia -- the largest per capita emitters of CO2 among developed nations -- are comfortably within the top 15 countries least at risk, according to the index.

With the exception of Chile and Israel, the rest of the 41 nations in the "low risk" category of the ranking are European or from the Arab Peninsula.

Japan's enviable position is due to its highly-developed infrastructure, its stable political and economic system, and its overall food and water security, explained Place.

Even if it imports much of its needs, it does so from dozens of countries, spreading the risk.

"Japan is also relatively rich in biodiversity, including well-managed forests. Human induced soil erosion is not a critical issue," she said.

"That's in contrast to, say, Ethiopia" -- or dozens of other poor nations -- "where there's a high population density and soil erosion is a real issue, impacting the ability to grow crops," she said.

One weak point, however, is the high concentration of populations along the coast, exposed to the risk of rising sea levels.

"Japan does need to take very seriously the issue of climate change vulnerability.

The index is based on 33 distinct criteria grouped into six sub-indices: economy, government institutions, poverty and development, ecosystems, resource security, and population density in relation to infrastructure.