The worst case scenario is that there is a run away greenhouse and temperatures rise hundreds of degrees. This has happened on Venus. But this is not going to happen.
However, according to the IPCC, billions of people will be affected.[1][2] Key findings of the IPCC report include:
* 75-250 million people across Africa could face water shortages by 2020;
* Crop yields could increase by 20% in East and Southeast Asia, but decrease by up to 30% in Central and South Asia;
*Agriculture fed by rainfall could drop by 50% in some African countries by 2020;
*20-30% of all plant and animal species at increased risk of extinction if temperatures rise between 1.5-2.5C;
*Glaciers and snow cover expected to decline, reducing water availability in countries supplied by melt water.