Predicting the consequences of global warming is one of the really difficult tasks for the world’s climate researchers. First, because the natural processes that cause precipitation, storms, increases in sea level and other expected effects of global warming are dependent on many different factors. Second, because it is difficult to predict the size of the emissions of greenhouse gases in the coming decades, as this is determined to a great extent by political decisions and technological breakthroughs.

Many of the effects of global warming have been well-documented, and observations from real life are very much consistent with earlier predictions. It is the precise extent that is difficult to predict. Among the effects that can be predicted are:

More drought and more flooding:

When the weather gets warmer, evaporation from both land and sea increases. This can cause drought in areas of the world where the increased evaporation is not compensated for by more precipitation. The extra water vapour in the atmosphere has to fall again as extra precipitation, which can cause flooding other places in the world.

Less ice and snow:

Glaciers the world over are shrinking rapidly at present. The trend is for the ice to melt faster than estimated in the IPCC’s latest report. In areas that are dependent on melt water from mountain areas, this can cause drought and a lack of drinking water. According to the IPCC, up to a sixth of the world’s population lives in areas that will be affected by this.

More extreme weather incidents:

The warmer climate will most probably cause more heatwaves, more cases of violent rainfall and also possibly an increase in the number and/or severity of storms.

Rising sea level:

The sea level rises for two reasons. Partly because of the melting ice and snow, and partly because of the thermic expansion of the sea. Thermic expansion takes a long time, but even an increase in temperature of two degrees celsius is expected in time to cause a rise in the water level of almost a metre.

In order to get an idea of the extent of the consequences, researchers typically work with scenarios that show various possible developments.
http://en.cop15.dk/climate+facts/what+consequences+can+we+expectdrought_china_sx