The Global Humanitarian Forum also estimates that global warming seriously affects 325 million people and causes $125 billion in economic losses each year.
Annan says people in the world's poorest countries are most affected by changes in the weather and environment.
The Forum report on climate change released Friday used existing data on weather-related disasters and population trends to draw its conclusions.
Meanwhile, Africa's environment ministers called for more money and support from rich nations ahead of a landmark climate conference, saying the continent contributes little to global warming but suffers disproportionately from its effects
The ministers, meeting in Nairobi, said they will ask for funding from rich nations at December's U.N. conference in Copenhagen of 190 countries.
They did not give a figure, but the U.N. says Africa needs at least $1 billion a year to manage the effects of climate change such as sinking islands, changing farming techniques and even relocating people from areas affected by extreme weather.
Buyelwa Sonjica, South Africa's water affairs and environment minister, said she wants "stronger leadership from the developed world ... I am not sure it is there yet."
In recent years, Africa has begun to experience the effects of a swiftly warming planet, exacerbating an already existing litany of woes on the world's poorest continent.
Malaria, which is prevalent in warm lowland areas of Africa and kills millions, has started to be recorded in what were the continent's cooler highland areas. Countries have reported more instances of extreme weather. Climate scientists are predicting that some African mountains will lose all their snow cover, and a staple crop like wheat may disappear in the 2080s.
December is the target date for concluding a new treaty to replace the Kyoto Protocol, a 1997 agreement that aimed to reduce carbon dioxide and other global-warming emissions by industrial nations.
The U.S. and China are the world's largest polluters, accounting for about half the world's carbon emissions. But neither country was part of the Kyoto accord, which called on 37 countries to cut carbon emissions by a total of 5 percent below 1990 levels.
The United States refused to sign Kyoto, citing the costs to the economy and lack of participation by China, India and other fast-developing countries. But some of those countries have said rich countries are not aggressive enough in cutting their own emissions. U.S. emissions now are 16 percent above what they were two decades ago.
Earlier this month, former British Prime Minister Tony Blair said the key to a new global climate change agreement will be a deal between the United States and China.
Blair also said climate change negotiators must find a way to integrate the United States, which has fallen far behind on controlling greenhouse gas emissions, into an agreement with Europe and other wealthy countries that have been working to reduce pollution for years.
Blair said the heart of that deal will be determining the responsibilities of both the developing countries and the industrial world. That, he said, can only happen once the United States has an understanding with China.
Global temperatures have risen 0.22 degrees since 1990, according to one U.S. government estimate. The U.N.'s chief panel on climate change estimates that the risk of increased severe weather will rise if the global average temperature increases between 1.8 degrees Fahrenheit and 3.6 degrees above 1990 levels.
Scientists attribute at least some of the past century's 1-degree rise in global temperatures to the atmospheric accumulation of carbon dioxide and other heat-trapping gases, byproducts of power plants, automobiles and other fossil fuel-burning sources.
Experts project that within 11 years some African countries may see farm harvests drop by up to 50 percent because water will be scarce and the continent relies on rain for its agricultural production. In the same period, they say, between 75 million and 250 million Africans are expected to suffer increased water shortages because of climate change.