Causes of Climate Change

Most climate scientists agree that human enhancement of the greenhouse effect has caused Earth's mean surface temperatures to increase over the past century.  Although the greenhouse effect is a natural phenomenon, human activity has led to an increase in the concentration of greenhouse gases in the atmosphere, leading to unnatural increases in global temperatures over long periods of time.

Source: IPCC, Climate Change 2007 Synthesis Report

The following gases contribute to the greenhouse effect:

• Water vapor
• Carbon dioxide
• Methane
• Nitrous oxide
• Halocarbons

Water vapor is the most abundant of these gases, but emission of water vapor is not a major driver of climate change.  Instead, it acts as positive feedback: an increase in mean global temperatures may cause an increase in the amount of water vapor in the atmosphere at a given time, which may further enhance the greenhouse effect (NASA).

Ice core and modern data indicates that sharp increases in atmospheric carbon dioxide, methane, and nitrous oxide have occurred over the past hundred years. The graphs on the right show the concentration of these greenhouse gases over the past 10,000 years compared to changes that have occurred since the year 1800, as well as the changes in the radiative forcing of each gas.  While these greenhouse gases can be produced through natural processes, human activities have caused greater emissions and diminished the uptake of these gases.

Source: IPCC, Climate Change 2007 Synthesis Report

According to the Intergovernmental Panel on Climate Change (IPCC), increased carbon dioxide emissions are likely the greatest driving force in climate change.  The chart on the upper left shows the share of different anthropogenic greenhouse gases in total emissions that were recorded globally for 2004.  These emissions are caused largely by worldwide processes in industry, agriculture, energy supply, transport, and deforestation, as indicated by the chart on the lower left (IPCC 2007).