According to the IPCC Fourth Annual Assessment Report (AR4), the scientific consensus is that global warming is indeed an established scientific fact. The evidence is undeniable, and includes:
1. Increasing air temperatures
2. Increasing ocean water temperatures
3. Tree ring characteristics
4. Ice core sample chemical characteristics
5. Ice cap retreat
While climate variability has certainly been noted over past centuries, with supporting evidence over millennia, the rate of warming is believed to be unprecedented. The increase in the global mean air temperature as compared to the 1951 to 1980 average is depicted in Figure 1, and the increase in average oceanic heat content for depths of the 0-700 meter layer is depicted in Figure 2. The increase in oceanic heat content is notable because it takes a very large amount of energy to heat such a large volume of water.
Global warming is caused by what is known as the “greenhouse effect,” where greenhouse gases such as carbon dioxide and methane absorb radiation emitted by the Earth, and re-emit that radiation back towards the surface, causing a warming effect. The process is very similar to how car windows work on a sunny day - the glass allows energy from the sun to heat the interior of a car but traps the heat before it escapes from the car. Greenhouse gases are responsible for the fact that the mean temperature of the Earth is 15 degrees (Celsius) instead of minus 18 degrees (Celsius). However, the increase in anthropogenic greenhouse gases since the industrial revolution is unprecedented in the Earth’s history. According to the IPCC AR4 (2007), “most of the observed increase in global average temperatures is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.
Greenhouse gas forcing alone would likely have resulted in a greater warming than observed if there had not been an offsetting cooling effect from aerosol and other forcings. It is extremely unlikely (less than five percent) that the global pattern of warming can be explained without external forcing, and very unlikely that it is due to known natural external causes alone.”
As the Earth continues to warm, the air can hold more water vapor, which is a far more effective greenhouse gas than carbon dioxide. Water vapor can accelerate the warming process further.
The IPCC has further concluded, based on consistent and scientifically defensible evidence, that “warming of the last half-century cannot be explained without external radiative forcing” (IPCC AR4, Working Group I, Section 9.7). So in essence, it is most unlikely that recent warming can be explained by natural variability alone.
A Look at the Future
Scientists use a collection of global climate models (GCMs) to estimate the implications of global warming through the end of the century. While any single model demonstrates considerable error, the collection or ensemble of models has reasonably captured the climate change history of the past century. What is even more remarkable is that if greenhouse gas levels are held to pre-industrial levels in the GCMs, these models show a much colder climate between the Nineteenth Century and the present day than was actually observed. The same models are used for forward projections through the end of the century, under expected greenhouse gas concentrations.
Global climate model ensembles project a best estimate of a further two to four degree (Celsius) increase in the mean temperature of the Earth by the end of the century. This seems minor on an intuitive level. However the resulting impacts are of significant concern.
Sea-level rise presents the most significant threat for coastal areas as a result of melting glaciers and thermal expansion of ocean waters. Apart from this threat, changing weather patterns will impose drought and inland flood threats for many areas. As a general principle of climate change, some modification to the mean of meteorological extreme value distributions can be expected, but with a more concerning increase in tail thickness (or variability). This change could amplify the effects of existing natural variability modes, such as the El Niño Southern Oscillation (ENSO), which already cause severe disruption due to flood, drought and hurricane frequency. The meteorological consequences of global warming are expected to be most severe in high latitudes and particularly in the polar regions.