Climate System Forcings
At this point, it is necessary to discuss the difference between climate change and climate variability as often these terms are used interchangeably, which can confuse our discussion about the Earth's projected future climate. Climate variability refers to relatively short-term variations (months to years) in the natural climate system, such as the El Niño Southern Oscillation Cycle. Variability implies shifts about some mean point. Climate change (as used in the Intergovernmental Panel on Climate Change volumes) refers to long-term changes from decades to centuries that are associated with changes in concentrations of greenhouse gases. These changes are often viewed as unidirectional -- at least over relatively long time scales. Global climatic change involves unidirectional changes in climatic features over the entire globe and may either be amplified or lessened by climate variability. As an example, long-term climate change in the Pacific Basin may be amplified or lessened by the El Niño Southern Oscillation.
Fluctuations in Solar Energy
The amount of energy radiated by the sun fluctuates. For 500 years, astronomers have observed visible changes on the sun’s surface such as sunspot activity. This sunspot record indicates that there is an approximately 11-year cycle in the number of sunspots visible on the sun’s surface. Furthermore, when sunspots are in abundance, an increase in solar emissions from the darker sunspots and the sun’s polar regions is observed. When sunspot activity is minimal, these solar emissions are less intense. This correlation between changes in sunspot activity and solar emissions has been confirmed by satellite observations over two complete sunspot cycles. Solar radiation that hits the top of the Earth’s atmosphere has been measured to vary by 2.5 watts over the 11-year sunspot cycle. Such a change in incoming solar radiation could result in a variation of Earth’s temperature by 0.1o C in response to the 11-year variation in the intensity of the sun’s radiation. The linkages between sunspot variation, the sun’s strength, and the Earth's climate are uncertain at a decadal (every ten years) to century (every 100 years) time scale. It is interesting to note that there exists a significant correlation between the northern hemisphere land temperature record for the last 100 years and the length of sunspot cycle (Figure 1). In other words, decreased sunspot cycle length matches very well with increase in northern hemisphere temperature.
Figure 1. Correlation between the variation of the sunspot cycle length (solid dark line, left hand scale) and Northern Hemisphere land temperatures anomalies (red line, right hand scale) from 1861 to 1989. The temperature anomalies are the deviations in temperatures relative to the period of 1951 to 1980. Note that as the sunspot cycle length decreases, the temperature increases.