Millions of people could face poverty, disease and hunger as a result of rising temperatures and changing rainfall expected to hit poor countries the hardest, the World Health Organization warned Monday.

Malaria, diarrhea, malnutrition and floods cause an estimated 150,000 deaths annually, with Asia accounting for more than half, said regional WHO Director Shigeru Omi.

Malaria-carrying mosquitoes represent the clearest sign that global warming has begun to impact human health, he said, adding they are now found in cooler climates such as South Korea and the highlands of Papua New Guinea.

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Warmer weather means that mosquitoes' breeding cycles are shortening, allowing them to multiply at a much faster rate, posing an even greater threat of disease, he told reporters in Manila.

The exceptionally high number cases in Asia of dengue fever, which is also spread by mosquitoes, could be due to rising temperatures and rainfall, but Omi said more study is needed to establish the connection between climate change and that disease.

"Without urgent action through changes in human lifestyle, the effects of this phenomenon on the global climate system could be abrupt or even irreversible, sparing no country and causing more frequent and more intense heat waves, rain storms, tropical cyclones and surges in sea level," he said.

In the Marshall Islands and South Pacific island nations, rising sea levels have already penetrated low-lying areas, submerging arable land and causing migrations to New Zealand or Australia, he said.

Omi said poorer countries with meager resources and weak health systems will be hit hardest because malnutrition is already widespread, with the young, women and the elderly at particular risk.

He said unusual, unexpected climate patterns — too much rain or too little — will have an impact on food production, especially irrigated crops such as rice, and can cause unemployment, economic upheavals and political unrest.

Dr. John Ehrenberg, WHO adviser on malaria and other parasitic diseases, said unchecked human development has contributed to the problem. That includes deforestation and an unprecedented level of human migration. As people move, so do diseases.

Omi said governments need to strengthen current systems providing clean water, immunizations, disease surveillance, mosquito control and disaster preparedness.