The sun and global warming: can it be just a coincidence that the Earth's climate appears to be warming as the Sun is getting more active? Clearly there is more to global warming than greenhouse gases.

As any homeowner in the northern latitudes knows, there are two ways to keep warmer in the winter. The first is to increase the amount of insulation in one's home. The other is to increase the amount of output from the home's heat source, the furnace. Of the two, increasing the amount of heat generated by the home's furnace causes the most noticeable increase in temperatures.

The same may be true of the globe. So far, global-warming research has focused on the role played by the increase of insulating greenhouse gases. But what if the Sun, the globe's heat source, is actually growing more intense? If so, then any warming trend is probably the result of a rise in solar intensity.

This is, in fact, what some researchers and scientists are beginning to believe is the major source of global warming. In 1999, a team of Dutch and Russian scientists published a paper in the scholarly journal Quaternary Science Reviews pointing out that solar activity coincided in the past with climate changes. The team examined the relative levels of a certain carbon isotope that is more commonly created when the Sun is quiet and solar radiation is at a minimum. They found that substantial increases in the carbon isotope coincided with global cooling events at about 850 B.C. and 1600 A.D. The latter date corresponds to the so-called Little Ice Age.

According to the researchers, "It is well documented that periods of decreased solar activity ... often coincide with climatic change. The best-known example is the Maunder Minimum (1645-1715), a solar event that is coinciding with one of the coldest phases of the Little Ice Age.... According to Lean et al. (1992) the sun during the Maunder Minimum was 0.25% less bright than it was during the solar minimum of 1985-1986. Climate model experiments indicate that such a decrease in solar irradiance is capable of causing a global cooling of about 0.5[degrees]C [0.9[degrees]F]."

As a result of their research, the Dutch and Russian scientists reached the conclusion that the climate is very susceptible to changes in solar radiation. "Accepting the idea of solar forcing of Holocene and Glacial climatic shifts has major implications for our view of present and future climate," they wrote. "It implies that the climate system is far more sensitive to small variations in solar activity than generally believed. For instance, it could mean that the global temperature fluctuations during the last decades are partly, or completely, explained by small changes in solar radiation."

In fact, the Sun has been more active in recent years than in many centuries previous, an intriguing correspondence with the perceived rise in temperatures over the last century. In 2003, another team of European researchers led by Ilya Usoskin of the University of Oulu in Finland published a paper in the peer-reviewed journal Physical Review Letters documenting the increase in solar activity. "The most striking feature of the complete SN [sunspot number] profile is the uniqueness of the steep rise of sunspot activity during the first half of the 20th century. Never during the 11 centuries prior to that was the Sun nearly as active." They pointed out that periods of high solar activity corresponded with periods of warmth on Earth and that periods of low solar activity likewise correspond with periods of wet, cool weather. The researchers concluded that the "current high level of solar activity may also have an impact on the terrestrial climate. We note a general similarity between our long-term SN reconstruction and different reconstructions of temperature: both SN and temperature show a slow decreasing trend just prior to 1900, followed by a steep rise."

Can it be just a coincidence that the climate appears to be warming as the Sun is getting more active? Clearly there is more to global warming than greenhouse gases.