Read this article about climate change in the New York Times.
If we switch quickly from fossil fuels, global average temperatures might only rise 2 to 5 degrees Fahrenheit higher in the next 70 years. Sea levels by 2100 will rise by about 20 feet, but most polar ice will survive. Stronger monsoons will turn the Sahara lush and watery, and intense droughts will parch the American Southwest. Many species will adapt by migrating toward the poles. The Deep South's oak-hickory-black gum forests will move to upstate New York, southern Europe will resemble North Africa, and hippos, elephants and African animals will move to newly-subtropical Germany and France. The temperature rise will be strong, but occur over a century or more. Recovery could take about 50,000 years. At the end of this period, most life will return to its modern patterns, and die-offs will be uncommon.
But if we burn all the remaining coal, oil, and gas within the next century, we will create catastrophic change...
The Arctic Ocean became a warm, brackish sea surrounded by redwood forests. Antarctica will be covered in beech forests, and so much runoff will fill the Antarctic Ocean that it will turn brown with silt. Sea levels will rise over 200 vertical feet, and our grandchildren will lack the fossil fuels to cope with the problem. Most sea creatures as we know them (tuna, herring, sardines, cod, haddock, salmon) will die, replaced by warm-water species which lack food value. Polar regions will become rain-soaked, while the equator will become a desert.
In this worst-case scenario, the temperature rise will be swift and steep, with temperatures reaching their maximum in 50 to 75 years. Recovery will take at least 150,000 years, and at the end there will be another massive die-off as heat-adapted creatures go extinct in favor of new colder-adapted ones.