What is Global Warming?

The Intergovernmental Panel on Climate Change (IPCC) has reviewed a large volume of historical and globally collected scientific data of changes in surface and air temperatures, rise in sea level, change in the thickness of ice sheets, glacier melting and the decrease snow cover in the northern hemisphere, and the changes in the constituents of the earth’s atmosphere. The IPCC in its Fourth Assessment Report published in 2007 has concluded that a global warming trend is unequivocal. Global mean surface temperatures have risen by 0.74°C ± 0.18°C when estimated by a linear trend over the last 100 years (1906–2005). The rate of warming over the last 50 years is almost double that over the last 100 years (0.13°C ± 0.03°C vs. 0.07°C ± 0.02°C per decade).

Since temperature is causally connected with the wind and water cycles, the slow yet continuous global warming trend has already produced observable climate change. Among some of the noticeable surface and atmospheric changes are the shrinking of Arctic Sea ice cover by about 2.7 ± 0.6% per decade since 1978, increased precipitation in latitudes north of 30^oN, greater incidence of droughts in the tropics, and increased tropical cyclone activity.

The earth’s mean temperature is determined by the energy received from the sun and the properties of the earth’s atmosphere. The atmosphere influences the amount of heat that is reflected off the earth into space. Thus, changes to the constituents of the atmosphere affect the global energy budget. Atmospheric constituents such as carbon dioxide, methane, water and nitrous oxide absorb the energy reflected off the earth’s surface and generate heat, and for this reason are called greenhouse gases (GHG). The concentration of these GHGs has increased significantly in the last 250 years. The concentration of carbon dioxide has increased by 64% compared to pre-industrial times and continues to rise at the rate of approximately 0.5% per year; similarly, the concentration of methane has doubled and that of nitrous oxide has increased by about 18%. Among the factors that less significantly affect the global energy budget but are not well-understood are the effect of aerosol particles in the atmosphere, the contrails from global aviation and human-induced changes to land cover. Based on the scientific data, IPCC has concluded that the changes in the concentration of GHGs in the atmosphere as the primary cause of global warming.

Two-thirds of the emissions of carbon dioxide from human activities arise from the use of fossil fuel (coal, oil and natural gas) in the industrial era. The other third of the emissions result from changes in land use such as deforestation. Natural processes of absorption of carbon dioxide by the oceans and the biosphere have removed about only 50% of these emissions, resulting in the observed net increase in the concentration of carbon dioxide in the atmosphere. Effects of methane and nitrous oxide are second only to carbon dioxide.

The actual scenarios for future trends in GHG emissions are governed by variations in demographic, economic and technological factors. Even if concentrations of all GHGs and aerosols are kept constant, a warming trend of 0.1^oC per decade is expected to occur. If the carbon dioxide levels doubled pre-industrial levels, the global average temperature is predicted to rise by 1.5 to 4.5^oC. Global warming if unchecked will increase the intensities of extreme weather conditions and cause sea levels to rise, negatively impacting water resources, human health, agricultural output, industry and social organization. Therefore, limiting carbon emissions is the fundamental to arresting global warming.