African countries are increasingly vulnerable to climate change and could struggle to feed and defend their people as temperatures rise, according to a major UN report.

The cost of developing drought-resistant crops, providing early-warning systems for floods, droughts and fires, and building seawalls, dykes, and wave breaks will be vast, says the UN Environment Programme's (UNEP) emissions gap report, launched this week at an African environment ministers' meeting in Warsaw.

It will cost Africa approximately $350bn a year to adapt its farming and infrastructure to climate change if governments fail to hold temperatures to less than 2C and allow them to rise to about 4C, according to the report.

The higher temperatures rise, the greater the financial and human challenge to adapt, says the report, which argues that present policies point to temperatures rising to 3-4C by 2100, a turn of events it claims would be catastrophic.

The report claims coral reefs will die, sea levels will rise, freshwater reserves will decline over wide areas, and rainfall in southern Africa will decrease by 30%. Countries that fail to adapt to even the minimum expected temperature increase of 2C will be in dire straits, the report says.

"Climate change in Africa is a reality," said Ephraim Kamuntu, Uganda's minister for water and environment. "We have to adapt or perish, but our capacity to respond is limited. The cost of waiting to do something is far greater than doing something now. How many super-typhoons do we need before we have to take action. This is a matter of survival; what are we waiting for?"

"The plight of Africa is not of our making," said Tosi Mpanu-Mpanu of the Democratic Republic of the Congo, lead negotiator for the Africa group of nations. "The developed countries have caused the problem and Africa are asking for the funds to help but so far they are not forthcoming.

"One billion Africans are in harm's way. We witness instability in rainfall, diseases spreading, sea level rise and floods. One of the effects of climate change is to send Africans further and further to seek water. This brings them into conflict with other Africans. We are faced with wars on African soil that are not created in Africa."

The report warns: "Even with a warming scenario of under 2C Africa's undernourished would increase 25-90%. Crop production would be reduced across much of the continent as optimal growing conditions are exceeded. The capacity of African communities to cope will be significantly challenged."

It suggests the magnitude of adaptation requirements could destabilise countries. In sub-Saharan Africa, water supply, infrastructure, and agriculture can be expected to incur the highest costs. In north Africa, infrastructure and adapting to extreme weather events are expected to prove costliest.