Is Global Warming Caused by Humans?

Whether the world is currently undergoing a warming period that is largely caused by human activity is a question over which there has been considerable disagreement. While there may be moderate agreement that the earth has warmed measurably in recent history, there seems to be less consensus that human activity has played a significant role in the warming process.

The recent 2007 report from the Intergovernmental Panel on Climate Change (IPCC), a respected organization that attempts to reach agreement among hundreds of scientists, diplomats, and politicians, showed that thermometer records from around the world indicate that the average global temperature has increased from about 56 degrees Fahrenheit to 58 degrees since 1850. The assertion is that temperatures on earth are higher now that they have been in the past 400 years.

The argument that human activity has caused the temperature rise depends primarily on the concept of the "greenhouse effect." A real greenhouse creates warmth by gathering incident heat rays from the sun and trapping them, not allowing all of the infrared (IR) rays to escape. Similarly, the earth's greenhouse effect is caused by the gasses water vapor, carbon dioxide, methane, and a few minor gasses trapping incident IR rays from the sun and not allowing them all to escape from the atmosphere. In theory and in demonstrated practice, the more greenhouse gasses there are in the atmosphere the more IR rays are retained, and the greater are the measured temperatures on earth.

The IPCC research produced a graph that plotted CO2 in the atmosphere from about 800 AD to the present. The graph shows that for centuries the concentration of CO2 remained stable at around 280 parts per million (ppm) and that since about 1850 the concentration has increased 36% to about 380 ppm. Significantly, the period of increase of this important greenhouse gas coincides with increased industrialization worldwide that has been spurred by the increased use of fossil fuels such as coal and petroleum products. Burning of fossil fuels creates CO2 as a byproduct. This period of increased greenhouse gas production also coincides with the period during which worldwide temperatures have increased 2 degrees. The resulting conclusion is that human activity has played a role in the observed global warming.

On the other hand those who argue that the present perceived warming trend is not caused by human activity make numerous points, two of them significant. First, the implication that increased levels of CO2 and other greenhouse gasses cause warming may be incorrect. It appears from close examination of the measurements that as the earth gets warmer CO2 levels increase, not the other way around. That is, warming causes increased CO2 concentrations and therefore human activity is not the cause of higher CO2.

Secondly, there have been many periods of warming and cooling during earth's natural history, and nearly all of them occurred before humans existed on earth. Warming periods on earth have numerous natural causes, increased volcanic activity being the most common. Recent history has not seen abnormal volcanic activity but it has seen increased levels of solar activity. Several studies claim that solar activity over the past century is at the highest level seen in the past 8,000 years. Since our Sun is huge and its output is huge, the conclusion is that increased solar activity is more likely to be the cause of the observed global warming trend than anything that humans could contribute.

The debate over whether human activity is a large factor in global warming will likely continue for a long time. Perhaps additional evidence or more sophisticated computer models of our complex climate system will bring the debate to a conclusion in the future.