According to a major anti-poverty organization, unchecked global warming will exacerbate a crisis already being called "the defining human tragedy of this century": hunger.

Oxfam International says that global warming will bring crop failure, erratic storms, droughts, and a shift in growing seasons, thus increasing the pressure on millions of already-suffering people worldwide to produce the food they need to survive.

The report, "What Happened to the Seasons?" comes as leaders of the G8 nations prepare to meet in Italy this week to discuss food scarcity, climate change, and other issues.

Oxfam surveyed farmers around the world and found that changing growing seasons are already affecting the planned planting and harvesting of subsistence crops. For example, nations that depend on rice, the world's most common food, are among the hardest hit, as yields of the staple crop are predicted to decline by 10 percent for every 1 degree C rise in temperature in some poor countries.