According to NASA and National Climatic Data Center, the last decade was the warmest on record, and 2009 was tied for the second warmest year since 1880. Although 2005 was the warmest year on record, 2009 was only a small fraction of a degree cooler that 2005, putting 2009 in a virtual tie with a cluster of other years; 1998, 2002, 2003, 2006, and 2007 for the second warmest on record. It is true that "warmest" means only a few tenth of a degree Celsius at most, but in terms of global average temperature, these differences become quite significant. For instance, the global average temperature over a period since 1860 has marked an increase of between 0.3 °C and 0.6 °C. (How is this significant?)
The basic principle of Global Warming can be understood considering the Greenhouse Effect, which is a naturally occurring process that aids in heating the Earth's surface and the atmosphere. Basically, as the radiation energy from the Sun passes through the atmosphere, 26% of this energy is reflected back to space by clouds and other atmospheric particles. 19% of the energy is absorbed by clouds, gases, and particles in the atmosphere. Off the remaining 55% of the Sun's thermal energy passing through the atmosphere, 4% is reflected from the surface back to space and 51% is absorbed by the Earth's surface. However, ever since the Industrial Revolution began as early as the 18th century, the amount of carbon dioxide in the Earth's atmosphere has been increasing dramatically. Fossil fuel combustions resulted from factories, power plants, and automobiles release huge amounts of carbon dioxide and other gases, such as, methane, nitrous oxide, the chlorofluorocarbons and ozone (also known as the greenhouse gases) into the atmosphere. Directly influenced by human activities, the increase in the concentrations of the greenhouse gases trap more heat in the Earth's atmosphere, thus, disturbing the balance and causing an increase in the Earth's surface temperature.
(The other side of the story)
(Should we act?)
(What can we do?)