The most recent Fourth Assessment Report of the Intergovernmental Panel on Climate Change has confirmed findings of its earlier assessments that climate change is occurring because of increases in global atmospheric concentrations of greenhouse gases resulting from human activities since about 1750. These gases trap solar radiation that is reflected from the earth’s surface and that would otherwise pass through the atmosphere and back into space. The trapped radiation adds warmth to the atmosphere between these gases and the earth’s surface The principal effects of this climate change are increasing global mean temperature, increasing average sea level and decreasing northern hemisphere snow cover (see www.ipcc.ch).
Human activities are contributing to the increasing of atmospheric concentrations of greenhouse gases mainly through the emissions of carbon dioxide from our uses of fossil fuels; and to a lesser extent, through the emissions of methane, nitrous oxide and halocarbon chemicals that result mainly from our industrial and agricultural activities.
Prior to the industrial revolution, our emissions of carbon dioxide where only a few hundred million tons per year and atmospheric concentration of carbon dioxide was relatively stable at about 270 parts per million (ppm). By 2005 our carbon dioxide emissions from human activities had increased to about 27 billion tons per year, and atmospheric concentration had increased to 379 ppm.
It is generally agreed by climate scientists that to reverse the trend of global climate change humans need to reduce their global emissions of carbon dioxide by at least 50 percent and that the time to achieve this is very limited. If current trends continue it is expected that atmospheric concentration of carbon dioxide will increase to about 450 ppm by 2035. If concentration rises above 450 ppm, scientific opinion is that global climate change may not be reversible and significant negative alteration of human activity and global ecosystems would occur. If the current trend continues to 2100, atmospheric concentration of carbon dioxide is expected to reach 1100 ppm; a level at which most human activity and other species would likely cease to exist.