Oxfam International released a report Monday as leaders of the Group of Eight wealthiest nations prepare to meet in Italy this week, with an agenda to include both food security and climate change.

It says that as the weather changes, millions of people in areas suffering food scarcity will have to give up traditional crops, possibly leading to social upheavals such as mass migrations and possible conflict over water resources.

Rich countries in temperate climate zones, such as northern Europe and parts of the United States, will benefit from warmer weather and more rainfall, but far more people in hotter, poorer countries will face more erratic and expensive food supplies, said the British-based nonprofit group.

The report, "What Happened to the Seasons?" was meant to add urgency to the G-8 meeting and to a broader group of 17 countries, the Major Economies Forum, which convenes later in the week to try to unblock negotiations on a new climate change agreement due to be completed in December.

Oxfam said it prepared a study for the Institute of Development Studies by surveying farmers around the world, who report that changing seasonal patterns were already affecting their ability to plan the sowing and harvesting of crops. The results, it said, were "strikingly consistent across entire geographies."

Farmers have begun changing their crops in the tropics, where a 1 degree Celsius (1.8 Fahrenheit) temperature shift can make traditional crops unsustainable. Unpredictable rainfall makes their choices of new crops a gamble, the report said.

Among the worst hit are nations that grow rice, the world's most common food. Yields are predicted to drop an average 10 percent for every 1 degree C rise in temperature in countries like the Philippines, where production could fall 50-70 percent as early as 2020. At the same time, China will grow more rice as the area of warm temperatures spreads, it said.

Corn is another staple that will be widely affected by climate change since it is particularly vulnerable to water stress, it said. Corn is the main source of food for 250 million people in east Africa and is used as animal feed around the world.

Negotiators at U.N. climate talks have been tasked with setting up an adaptation fund to help poor countries deal with the affects of climate change. U.N. estimates suggest as much as $200 billion a year may be needed by 2030 for developing water resources in increasingly arid regions, shifting agriculture to more suitable crops, building sea walls to protect coastal cities from rising sea levels and helping fishermen whose stocks would be affected by acidification of the ocean.

The Oxfam report said steps can be taken to bolster the world's food supply.

"The world's agricultural potential is less than 60 percent exploited: there is still enough land to feed everyone, even with population levels at the 9.2 billion currently predicted by the United Nations for 2050," it said. Modern agricultural methods, irrigation and fertilizers could dramatically lift yields.