For example, levels of carbon dioxide (CO2), which is released primarily by burning fossil fuels such as coal, oil, and natural gas, have dramatically increased since pre-industrial times.

Greenhouse gases such as CO2 act like the glass in a greenhouse or in your car windows: they allow sunlight in, but prevent some heat from escaping back into space. Thus, increased greenhouse gases cause Earth’s average temperature to rise, which in turn causes changes in precipitation frequency and intensity.

Warming of the ocean and the melting of glacial ice is also causing a global rise in sea level. Significant sea level rise can eliminate beaches and cause flooding, which is particularly problematic for low-lying countries. For example, the 11,000 residents of Tuvalu, a small island nation in the Pacific, need to evacuate their island due to rising sea level.

Source: http://www.earth-policy.org/index.php?/plan_b_updates/2001/update2