Climate change is the most serious environmental challenge facing society in the 21st century. The basic science is clear: the International Panel on Climate Change concluded that there is more than 90% probability that human activities are causing the observed changes in the Earth’s climate in recent decades. The average temperature of the Earth's surface has increased so far by about 0.8 degrees Celsius since the Industrial Revolution, and the frequency of extreme weather events such as droughts, floods and intense hurricanes is also increasing, most likely as a consequence of this temperature change. There are scientific uncertainties that remain to be worked out, connected with issues such as the feedback effects of clouds and aerosols. Nevertheless, the consensus among experts is that the risk of causing dangerous changes to the climate system increases rapidly if the average temperature rises more than two or three degrees Celsius. Society faces an enormous challenge to effectively reduce greenhouse gas emissions to avoid such dangerous interference with the climate system. This goal can only be achieved by taking simultaneously measures such as significantly increasing energy efficiency in the transportation, building, industrial and other sectors, using renewable energy sources such as solar, wind, geothermal and biomass, and possibly developing and using safer nuclear energy power plants.