Twenty-five million more children will go hungry by the middle of this century as climate change leads to food shortages and soaring prices for staples such as rice, wheat, maize and soya beans, a report says today.

If global warming goes unchecked, all regions of the world will be affected, but the most vulnerable – south Asia and sub-Saharan Africa – will be hit hardest by failing crop yields, according to the report, prepared by the International Food Policy Research Institute (IFPRI) for the World Bank and Asian Development Bank.

The children of 2050 will have fewer calories to eat than those in 2000, the report says, and the effect would be to wipe out decades of progress in reducing child malnutrition.

The grim scenario is the first to gauge the effects of climate change on the world's food supply by combining climate and agricultural models.

Spikes in grain prices last year led to rioting and unrest across the developing world, from Haiti to Thailand. Leaders at the G20 summit in Pittsburgh last week committed $2bn (£1.25bn) to food security, and the United Nations is set to hold a summit on food security in November, its second since last year's riots.

But the UN secretary general, Ban Ki-moon, is pressing the World Bank and other institutions to do more. He said the industrialised world needs to step up investment in seed research and to offer more affordable crop insurance to the small farmers in developing countries. Though prices have stabilised, the world's food system is still in crisis, he said at the weekend.

"Ever more people are denied food because prices are stubbornly high, because purchasing power has fallen due to the economic crisis, or because rains have failed and reserve stocks of grain have been eaten," he said.

Even without global warming, rising populations meant the world was headed for food shortages and food price rises.

"The food price crisis of last year really was a wake-up call to a lot of people that we are going to have 50% more people on the surface of the Earth by 2050," said Gerald Nelson, the lead author of the report. "Meeting those demands for food coming out of population growth is going to be a huge challenge – even without climate change."

After several years in which development aid has been diverted away from rural areas, the report called for $7bn a year for crop research, and investment in irrigation and rural infrastructure to help farmers adjust to a warming climate. "Continuing the business-as-usual approach will almost certainly guarantee disastrous consequences," said Nelson.

The G20 industrialised nations last week began discussing how to invest some $20bn pledged for food security earlier this year.

Some regions of the world outlined in the report are already showing signs of vulnerability because of changing rainfall patterns and drought linked to climate change. Oxfam yesterday launched a $152m appeal on behalf of 23 million people hit by a severe drought and spiralling food prices in Kenya, Ethiopia, Somalia and Uganda. The charity called it the worst humanitarian crisis in Africa for a decade, and said many people in the region were suffering from malnutrition.

But southern Asia, which made great advances in agricultural production during the 20th century, was also singled out in the IFPRI report for being particularly at risk of food shortages. Some countries, such as Canada and Russia, will experience longer growing seasons because of climate change, but other factors – such as poor soil – mean that will not necessarily be translated into higher food production.

The report was prepared for negotiators currently trying to reach a global deal to fight climate change at the latest round of UN talks in Bangkok. It used climate models prepared by the National Centre for Atmospheric Research in Colorado and the Commonwealth Scientific and Industrial Research Organisation in Australia to arrive at estimates of how changes in growing seasons and rainfall patterns would affect farming in the developing world and elsewhere.

Without an ambitious injection of funds and new technology, wheat yields could fall by more than 30% in developing countries, setting off a catastrophic rise in prices. Wheat prices, with unmitigated climate change, could rise by 170%-194% by the middle of this century, the report said. Rice prices are projected to rise by 121% – and almost all of the increase will have to be passed on to the consumer, Nelson said.

The report did not take into account all the expected impacts of climate change – such as the loss of farmland due to rising sea levels, a rise in the number of insects and in plant disease, or changes in glacial melt. All these factors could increase the damage of climate change to agriculture.

Others who have examined the effects of climate change on agriculture have warned of the potential for conflict. In a new book, Plan B 4.0: Mobilising to Save Civilisation, published today, Lester Brown, founder of the Earth Policy Institute, warns that sharp declines in world harvests due to climate change could threaten the world order.

"I am convinced that food is indeed the weak link," he said.

Brown saw Asia as the epicentre of the crisis, with the latest science warning of a sea level rise of up to six feet by 2100. With even a 3ft rise, Bangladesh could lose half of its rice land to rising seas; Vietnam, the world's second largest producer of rice, could also see much of the Mekong Delta under water.

Wheat and rice production would also fall because of acute water shortages, caused by past over-pumping and the melting of the Himalayan glaciers, which currently store water that supplies the region's main rivers: the Indus, Ganges, and Yangtse.

Brown said: "The potential loss of these mountain glaciers in the Himalayas is the most massive projected threat to food security ever seen" .

Global shortfall

People in both the developing and developed worlds will have less to eat by 2050 if climate change is not seriously addressed, though the shortfall will be relatively slight in richer countries. Prices rises and shortages of food will drive down the average calories available:

• The calories available for each person in industrialised nations will fall from 3,450 in 2000 to about 3,200.

• In developing countries overall, the average will fall from 2,696 to 2,410 calories.

• In sub-Saharan Africa, people will on average have only 1,924 calories a day, compared with 2,316 in 2000.