Global warming - just hot air?

We know from weather station records that Earth's surface temperature has increased on average by 0.6 degrees C in the last 100 years. The 1990s have been the warmest decade on record. Over the same period, global sea level has increased by 10-20 cm. We know also that planet Earth has an atmosphere that creates a natural greenhouse effect, keeping our surface warmer than it would otherwise be. Human activities are substantially increasing the atmospheric concentrations of greenhouse gases, principally carbon dioxide and methane, to levels far above those that have existed for the past 200,000 years.