The Earth’s environment is in a state of constant flux: We expect temperatures to vary from day to day. However, we are ever concerned and dependent upon long-term changes—and with good reason. Not many scientists deny that the Earth has gone through a warming period during the past 100 years, but there is far less of a consensus of how and why this warming takes place.

Conventional Viewpoints about Global Warming

For many years now, scientists have been linking global temperature rise with increased concentrations of greenhouse gases, most often with carbon dioxide (CO2). The evidence is hard to deny: escalation of carbon dioxide concentrations have been paired with temperature rise from direct and indirect (i.e. ice core data, tree rings, etc.) evidence

At a bare minimum, scientists expect global temperatures to rise by about 1 degree Celsius in the next 100 years. However, some predict even higher temperatures. According to the Intergovernmental Panel on Climate Change (IPCC) the surface temperature could rise by between 1.4 to 5.8 degrees Celsius by the end of the century. And while numbers like these prima facie might not seem so alarming, there can be many unpredictable and even drastic changes in weather patterns and global sea level. However, estimates such as these often don’t take into account another factor that could produce climate change, namely, solar variability.

Solar Variation and Climate Change As we discussed in Part II, our Sun undergoes a natural periodic change in solar energy output. This change, called the solar cycle, is predictable and the entire period of fluctuation is about 11 years. But new research indicates that the overall radiation that the Sun emits has increased by 0.05 percent per decade since the late 1970s. Richard Willson, a Columbia-affiliated researcher and Principal Investigator of NASA’s ACRIM experiments, has pieced together six overlapping satellite experiments that monitored total solar irradiance (TSI). Willson thinks that the 0.05 percent increase could cause “significant climate change” if it were sustained over many decades. In a press release from the Earth Institute at Columbia University Willson noted that “Historical records of solar activity indicate that solar radiation has been increasing since the late 19th century.” If this is true, then 20th century warming trends reported by the IPCC (noted above) might be due in part to increases in longer-term solar energy output.

A Historical Perspective on Solar Variation

During the period of the 15th to 18th centuries, the Earth entered a cooling period that has been dubbed the Little Ice Age. During this time, global temperatures were much lower than that are today. The effects of temperature drops were noticeable: glaciers in the Alps advanced, access to Greenland was largely cut off by ice, and canals in Holland routinely froze solid (see image to right).

During a normal solar cycle, the number of sunspots varies with solar minimum and maximum. In the coldest part of the Little Ice Age (about 1645 to 1715) there was very low sunspot activity observed. Scientists now dub this decrease of solar output as the Maunder Minimum. NASA climate models indicate that low solar activity could have changed atmospheric circulation patterns on Earth, thus affecting global weather. The Maunder Minimum and the Little Ice Age stand out as a prominent example of how change in solar activity can affect global temperature.

Debates and Research Continue

The Sun-Earth system is complex. When compiling climate models and predicting long-term change in global temperatures, researchers have numerous factors to consider. Direct and convincing lines of evidence have established that both greenhouse gas increase and variations in solar output can affect our climate. The difficulty comes when trying to resolve how much each factor influences the overall system. This feature only included two elements (e.g., greenhouse gases and solar variation) that influence what we should expect in future weather patterns. There are many other things to consider when compiling climate models, such as cloud cover, Earth’s orbit (i.e. Milankovitch cycles), feedback mechanisms, and various biogeochemical cycles.

In concluding this series, it is hoped that the reader gained an appreciation of the complexity in understanding our Sun and how it influences our planet. As much as Carl Sagan was correct in dubbing Earth as the pale blue dot, it is important to recognize that our planet does not operate in isolation. Earth is very much dependent upon interactions within the solar system. Solar influence affects and sustains our planet, and it is the main energy source driving so many of the biochemical reactions that we refer to as life. It is clear that if we are to continue to further our understanding of Earth, we must consider it as a larger system and carry on the solar research that has taught us so much already.