What are the effect of global warming?

Actually, there are many effects, and many different theories. At the current rate of temperature increase, ice melting in Antarctica and Greenland might raise global sea-levels as much as a meter (slightly over three feet) in as soon as 200 years (2 centuries). Not exactly a crisis.

However, some scientists have speculated that rising temperatures might cause a large increase in methane released from melting permafrost in places like Canada and Russia. Methane is a much more harmful greenhouse gas than CO2, since it absorbs/re-emits more radiation and isn't removed from the atmosphere by any natural process (as CO2 is by plants during photosynthesis). This could cause a massive increase in average global temperatures in a much shorter time, which could result in rising seas and climate change.

Since the majority of humanity lives within a stone's throw of the ocean, the results of rising seas could be disastrous. Ports could become inoperable, causing famine and resource shortages. Whole populations would be displaced, resulting in crime, poverty, and other social problems that always accompany large numbers of refugees.

Climate change could be disastrous as well. Regions accustomed to high precipitation might experience crippling droughts. Areas used to mild weather could be hit by killing storms (think Katrina times 10). Some experts believe that an increase in global temperatures could disrupt the Gulf Stream, a huge, warm-water current running from the Gulf of Mexico to the North Atlantic. If that were to happen, it would effectively transform much of Northern Europe into a frozen, lifeless wasteland.