Climate change is "the greatest humanitarian challenge the world faces" as heatwaves, floods and forest fires become more severe.

    Climate change is already responsible for 300,000 deaths a year and is affecting 300 million people, according to the first comprehensive study of the human impact of global warming.

    It projects that increasingly severe heatwaves, floods, storms and forest fires will be responsible for as many as 500,000 deaths a year by 2030, making it the greatest humanitarian challenge the world faces.

    Economic losses due to climate change today amount to more than $125 billion a year - more than the all present world aid. The report comes from former UN secretary general Kofi Annan's thinktank, the Global Humanitarian Forum. By 2030, the report says, climate change could cost $600 billion a year.

    Civil unrest may also increase because of weather-related events, the report says: "Four billion people are vulnerable now and 500m are now at extreme risk. Weather-related disasters ... bring hunger, disease, poverty and lost livelihoods. They pose a threat to social and political stability."

    If emissions are not brought under control, within 25 years, the report states:

• 310 million more people will suffer adverse health consequences related to temperature increases.

• 20 million more people will fall into poverty.

• 75 million extra people will be displaced by climate change.

    Climate change is expected to have the most severe impact on water supplies. "Shortages in future are likely to threaten food production, reduce sanitation, hinder economic development and damage ecosystems. It causes more violent swings between floods and droughts. Hundreds of millions of people are expected to become water stressed by climate change by the 2030."

    The study says it is impossible to be certain who will be displaced by 2030, but that tens of millions of people "will be driven from their homelands by weather disasters or gradual environmental degradation. The problem is most severe in Africa, Bangladesh, Egypt, coastal zones and forest areas."

    The study compares for the first time the number of people affected by climate change in rich and poor countries. Nearly 98% of the people seriously affected, 99% of all deaths from weather-related disasters and 90% of the total economic losses are now borne by developing countries. The populations most at risk it says, are in sub-Saharan Africa, the Middle East, south Asia and the small island states of the Pacific.

    But of the 12 countries considered least at risk, including Britain, all but one are industrially developed. Together they have made nearly $72 billion available to adapt themselves to climate change but have pledged only $400 million to help poor countries. "This is less than one state in Germany is spending on improving its flood defences," says the report.

    The study comes as diplomats from 192 countries prepare to meet in Bonn next week for UN climate change talks aimed at reaching a global agreement to reduce greenhouse gas emissions in December in Copenhagen. "The world is at a crossroads. We can no longer afford to ignore the human impact of climate change. This is a call to the negotiators to come to the most ambitious agreement ever negotiated or to continue to accept mass starvation, mass sickness and mass migration on an ever growing scale," said Kofi Annan, who launched the report today in London.

    Annan blamed politicians for the current impasse in the negotiations and widespread ignorance in many countries. "Weak leadership, as evident today, is alarming. If leaders cannot assume responsibility they will fail humanity. Agreement is in the interests of every human being."

    Barabra Stocking, head of Oxfam said: "Adaptation efforts need to be scaled up dramatically. The world's poorest are the hardest hit, but they have done the least to cause it.

    Nobel peace prizewinner Wangari Maathai, said: "Climate change is life or death. It is the new global battlefield. It is being presented as if it is the problem of the developed world. But it's the developed world that has precipitated global warming."

    Calculations for the report are based on data provided by the World Bank, the World Health organisation, the UN, the Potsdam Institute For Climate Impact Research, and others, including leading insurance companies and Oxfam. However, the authors accept that the estimates are uncertain and could be higher or lower. The paper was reviewed by 10 of the world's leading experts including Rajendra Pachauri, head of the UN's Intergovernmental Panel On Climate Change, Jeffrey Sachs, of Columbia University and Margareta Wahlström, assistant UN secretary general for disaster risk reduction.