While everyone had now started to recognise the threat posed by climate change, no one was taking effective leadership to tackle it and no one could tell precisely when and where it would hit hardest, it added.

"The most recent international moves towards combating global warming represent a recognition ... that if the emission of greenhouse gases ... is allowed to continue unchecked, the effects will be catastrophic -- on the level of nuclear war," the IISS report said.

"Even if the international community succeeds in adopting comprehensive and effective measures to mitigate climate change, there will still be unavoidable impacts from global warming on the environment, economies and human security," it added.

Scientists say global average temperatures will rise by between 1.8 and 4.0 degrees Celsius this century due to burning fossil fuels for power and transport.

The IISS report said the effects would cause a host of problems including rising sea levels, forced migration, freak storms, droughts, floods, extinctions, wildfires, disease epidemics, crop failures and famines.

The impact was already being felt -- particularly in conflicts in Kenya and Sudan -- and more was expected in places from Asia to Latin America as dwindling resources led to competition between haves and have nots.

"We can all see that climate change is a threat to global security, and you can judge some of the more obvious causes and areas," said IISS transnational threat specialist Nigel Inkster. "What is much harder to do is see how to cope with them."

The report, an annual survey of the impact of world events on global security, said conflicts and state collapses due to climate change would reduce the world's ability to tackle the causes and to reduce the effects of global warming.

State failures would increase the gap between rich and poor and heighten racial and ethnic tensions which in turn would produce fertile breeding grounds for more conflict.

Urban areas would not be exempt from the fallout as falling crop yields due to reduced water and rising temperatures would push food prices higher, IISS said.

Overall, it said 65 countries were likely to lose over 15 percent of their agricultural output by 2100 at a time when the world's population was expected to head from six billion now to nine billion people.