A few years ago, Melinda and I visited a group of rice farmers in Bihar province of India, one of the most flood-prone regions in the country. All of them were extremely poor and depended on the rice they grew to feed and support their families. When the monsoon rains arrived each year, the rivers would swell, threatening to flood their farms and ruin their crops. Still, they were willing to bet everything on the chance that their farm would be spared. It was a gamble they often lost. Their crops ruined, they would flee to the cities in search of odd jobs to feed their families. By the next year, however, they would return - often poorer than when they left - ready to plant again.

Our visit was a powerful reminder that for the world's poorest farmers, life is a high-wire act - without safety nets. They don't have access to improved seeds, fertilizer, irrigation systems and other beneficial technologies, as farmers in rich countries do - and no crop insurance, either, to protect themselves against losses. Just one stroke of bad fortune - a drought, a flood or an illness - is enough for them to tumble deeper into poverty and hunger.

Now, climate change is set to add a fresh layer of risk to their lives. Rising temperatures in the decades ahead will lead to major disruptions in agriculture, particularly in tropical zones. Crops won't grow because of too little rain or too much rain.