Global warming is already leading to more violent storms and less predictable weather patterns. According to the Pew Center on Global Warming, since 1995, only two years have not had above average hurricane activity. The overall number of tropical storms has not increased, but there are more storms strong enough to be called hurricanes. We will probably continue to get bigger storms, which will do more damage to coastal areas.
Global warming will destabilise the weather in other ways. In all likelihood, it will change worldwide weather patterns, leading to droughts in some areas and flooding in others. The Intergovernmental Panel on Climate Change (IPCC) indicates significant changes in the climates of all 5 continents. Although some of these changes may be positive in the short term, in the long term rising heatwaves and unstable weather will most likely have a negative effect everywhere.
The increased warming will cause the polar ice caps to melt more quickly. This could flood the coastlines of the world and decrease the salinity of the oceans. This could disturb the ocean currents which regulate temperature around the world, leading to drastic changes in local climates. The IPCC predicts a decrease of about 25% in the current that regulates ocean temperature by 2100.
Global warming will also mess with seasonal water cycles. Many rivers are powered by mountain glaciers which grow in the winter and thaw in the summer, producing seasonal flows. As these glaciers melt, these rivers could decrease or even dry up.
Flora and Fauna
Global warming will have drastic effects on local ecosystems. Most plants and animals are adapted for a certain environment. Generally, each species does well when the temperature is in a certain range, and the seasons work in a regular way. As things like temperature and seasonal precipitation shift, less robust plants and animals are not going to be able to adapt quickly enough. This will result in widespread extinction.
Effects on Humans
It is hard to predict exactly how severely global warming will affect individual locations. The combination of violent storms, rapid changes in local climates, disruption of the water cycle and extinction of plants and animals will probably cause local food shortages and disruption of infrastructure in some areas. The panic and anxiety over global warming will damage the world economy, as will the population squeeze when people in coastal areas are forced to move inland by rising water levels. No matter how you spin it, it's going to take a toll on the economy--at least in the short term.
Then again, with enough planning we may be able to make the best of it. According to some predictions, some parts of the world will become more fertile even while others become more arid. There will probably be enough food and water to go around if we are smart about distribution. The question is, will we come together as a planet to make the best of a tough situation?