The report said global warming, widely blamed on human activities, will cause species to extinct, seas to rise, water shortages to spread and droughts and floods to become more frequent. With poor countries to be worst hit, all regions from Africa to Pacific islands will suffer from the adverse effects of climate change.

"Approximately twenty to thirty percent of plant and animal species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5 degrees Celsius," said the report, drafted by experts from the UN's Intergovernmental Panel on Climate Change (IPCC).

The IPCC had said in its previous report issued in February that global average temperature had already increased by 0.74 degrees Celsius in the past century, while it could rise by another 1.1 to 6.4 degrees Celsius this century.

According to Friday's report, many ecosystems are likely to be overburdened this century by an unprecedented combination of climate change and its associated disturbances like flooding, drought and wildfire. Other man-made consequences like land use change, pollution and over-exploitation of resources could make the situation even worse.

The overall picture drawn by the report showed that drought-affected areas will likely increase in extent, while frequent heavy rainfalls will augment flood risk. During this century, water supplies stored in glaciers and snow cover are projected to decline, leaving one-sixth of the world population at risk.

For the first time, the nearly 1,500-page report broke down its findings into regions.

Take Asia as an example. More than 1 billion people there will be affected by less freshwater by the 2050s.

"Freshwater availability in Central, South, East and Southeast Asia particularly in large river basins is projected to decrease due to climate change," the report said.

Ironically, some Asian areas will first be hit by flood ahead of drought due to the melting of glacier in the Himalayas, an obvious proof of climate change.

While 1 billion people are thirsty for water, the coastal areas throughout the continent will be at great risk due to increased flooding from the sea and in some mega-deltas flooding from the rivers, the report said.

Besides water shortage, Asia will also face the risk of hunger, particularly in Central and South Asia, by the middle of this century.

Experts estimated that crop yields could increase up to 20 percent in East and Southeast Asia while it could decrease up to 30 percent in Central and South Asia by 2050s.

However, the places that will be worst affected by global warming are poor countries.

"Poor people are the most vulnerable and will be the worst hit by the impacts of climate change. This becomes a global responsibility," said Rajendra Pachauri, chairman of IPCC.

Africa, a continent mainly composed of less developed countries, will be hardest hit by the adverse effects of climate change, according to the report.

"New studies confirm that Africa is one of the most vulnerable continents to climate variability and change because of multiple stresses and low adaptive capacity," the report summary said.

By 2020, between 75 and 250 million people in Africa are projected to be exposed to an increase of water shortages. As a result of climate change, the area suitable for agriculture, the length of growing seasons and yield potential are expected to decrease, which will further adversely affect food production in the continent as it was already suffering from malnutrition.

In some countries, yields from rain-fed agriculture could be reduced by up to 50 percent by 2020, the report warned.

The poverty-inflicted continent may also have to spend at least five to ten percent of gross domestic products to cover the cost of adaptation since sea-level rise will affect low-lying coastal areas with large populations towards the end of this century.

The report is part of the IPCC's first review since 2001 of the evidence for climate change resulting from human activities. It is the second of four reports from the IPCC this year. With the first report in February elaborating how global warming is happening, the second one is focused on what the effects of global warming will be.

As the summary for policymakers was unanimously adopted by government delegates, it may serve as a policy guide in future international negotiations on climate change.