Global warming

Global warming is the raising of average atmospheric and ocean temperature that has been observed in recent decades. Though climate change has been a natural phenomenon that has occurred several times through geologic time, the term "global warming" is used most often to refer to recent warming believed to be the result of human activity. Causes of this recent warming have been strongly debated in the last decades; most scientists, however, reached a consensus on its anthropogenic origin. An extended discussion of global warming is given in the Fourth Assessment Report by the International Panel on Climate Change (IPCC).[1] Several steps have been taken to mitigate global warming, such as the Kyoto Protocol, an international treaty intended to reduce greenhouse gas emissions.

Historical observations of temperature

For example, the Earth's average air temperature has increased hundreds of times over the last 900,000 years and then decreased again each time. These cycles of approximately 1,500 years can be seen in proxy records such as the Vostok Ice Core.[2]

Since the beginning of written historical records in Ancient Rome, there has been a warm period, followed by the cool period of the Dark Ages, followed by the Medieval Climate Optimum (when Greenland was colonized), a Little Ice Age (when European settlers abandoned Greenland), and since around 1850 a warming trend.

In the historical ice-core records, variations in carbon dioxide (CO₂) levels correlate closely with the ups and downs of air temperature, lagging behind by about 800 ± 200 years. Most scientists believe that the variations in CO₂ are driven by the variations in air temperature in the historical record because of natural variations in Earth's axis tilt and orbit around the Sun, called Milankovitch cycles. These slight changes in Earth's movement cause the onset of the warming. This warming leads to higher CO₂ level, which in turn cause further warming (positive feedback). Measurements of present-day warming show CO₂ leads temperature.

The Sun's role on climate

When the Sun boasts a maximum of spots, cycle after cycle, Earth tends to be warmer than when its face is clear. [3] A lengthy period of cold weather coincided with the Maunder Minimum when hardly any sunspots were observed. While the Sun has played a role in climate change, recent observations show it is not a major cause of recent warming trends since the 1980s.[4][5]