“Greenhouse gases” may grab the headlines, but other factors are known to play important roles in climate change as well. Key among them are the earth’s irregular orbit, precession (a sort of “wobbliness” in space), and interaction with a changing sun.
Concerning the latter, one researcher from NASA’s Goddard Institute for Space Studies and Columbia University’s Earth Institute has found evidence that “solar radiation has been increasing since the late 19th century.” By piecing together overlapping sets of satellite data collected over the past 24 years, Richard Willson detected “a significant positive trend”—that is, a noticeable increase—in recent sun output or Total Solar Irradiance. The actual change was small—only 0.05 percent per decade. However, such a trend, “if sustained over many decades, . . . could cause significant climate change”—effects that Willson concludes “would have provided a significant component of the global warming the Intergovernmental Panel on Climate Change reports to have occurred over the past 100 years.”
Long-term records of earth’s climate are found in many places. One of the most important is the ice sheet that covers Antarctica. Data from ice borings to a depth of more than 3,000 meters have allowed climatologists to reconstruct conditions back more than 400,000 years. Most significant are four ice ages, where average temperatures fell at least 8 degrees Celsius or 14 degrees Fahrenheit, and five short interglacial warm periods. (Human civilization has arisen in the most recent warm period, which has spanned an unusually long 10,000 years.) Each interglacial period is associated with rising carbon dioxide and methane levels. According to Vostok (Antarctica) ice core investigator J.R. Petit, current concentrations of these gases are “unprecedented during the past 420,000 years.” Previous rises were followed by glacial expansion and global cooling, a surprisingly counterintuitive result.
The glacial-interglacial cycle is also keyed to the effect of the earth’s eccentric path around the sun. “Understanding how the climate system responds to this initial orbital forcing is still an important issue,” writes Petit.
However, because scientists have as yet been unable to get an exact fix on all of these changes (such transitions are seen over 1,000-or-more-year jumps), many hesitate to extrapolate to present-day, short-term climate prediction.