Solar Radiation and Climate Experiment (SORCE)

The Sun and Global Warming

Of the many trends that appear to cause fluctuations in the Sun’s energy, those that last decades to centuries are the most likely to have a measurable impact on the Earth’s climate in the foreseeable future. Many researchers believe the steady rise in sunspots and faculae since the late seventeenth century may be responsible for as much as half of the 0.6 degrees of global warming over the last 110 years (IPCC, 2001). Since pre-industrial times, it’s thought that the Sun has given rise to a global heating similar to that caused by the increase of carbon dioxide in the atmosphere. If the past is any indication of things to come, solar cycles may play a role in future global warming.

Though complex feedbacks between different components of the climate system (clouds, ice, oceans, etc.) make detailed climate predictions difficult and highly uncertain, most scientists predict the release of greenhouse gases from the burning of fossil fuels will continue to block a larger and larger percentage of outgoing thermal radiation emanating from the Earth. According to the 2001 report of the Intergovernmental Panel on Climate Change (IPCC), the resulting imbalance between incoming solar radiation and outgoing thermal radiation will likely cause the Earth to heat up over the next century, possibly melting polar ice caps, causing sea levels to rise, creating violent global weather patterns, and increasing vegetation density (IPCC, 2001).

How the Earth’s climate reacts, however, depends on more factors than just greenhouse gases. For instance, some scientists expect that low-level stratocumulus clouds may decrease. Both changes would add to the heating, since an increase in cirrus would trap more infrared, and a decrease of stratocumulous would reflect less sunlight. Such cloud cover changes would intensify global warming. In contrast, an increase of sulfate aerosols created by pollution would likely reflect more sunlight and perhaps also make clouds more


 reflective, thereby countering global warming especially near pollution sources.

Thick, puffy stratocumulus clouds (left) reflect sunlight and cool the Earth’s surface. However, thin cirrus clouds (right) allow most visible light to pass right through them, while blocking thermal radiation, so they warm the Earth. Because of this, how clouds respond to changes in solar energy output is a crucial aspect of the Sun’s influence on climate. (Photographs courtesy Dr. Robert Houze, University of Washington Cloud Atlas)

Sunspot cycles may sway global warming either way. If long-term cycles in solar radiation reverse course and the Sun’s spots and faculae begin to disappear over the next century, then the Sun could partially counter global warming. On the other hand, if the average number of spots rises, the Sun could serve to warm our planet even more. As to the shorter-term 11-year cycles, they may dampen or amplify the affects of global warming on a year-to-year basis.

The Sun’s affect on global warming can mostly be attributed to variations in the near-infrared and visible wavelengths of solar radiation. As previously stated, these types of radiation are absorbed by the lower atmosphere, the oceans, and the land. UV radiation, on the other hand, interacts strongly with the ozone layer and the upper atmosphere. Though UV solar radiation makes up a much smaller portion of the TSI than infrared or visible radiation, UV solar radiation tends to change much more dramatically over the course of solar cycles.

The impacts of undulating UV solar radiation may be substantial. Since UV radiation creates ozone in the stratosphere, the oscillation in UV levels can affect the size of the ozone hole. Absorption of UV radiation by the ozone also heats up the stratosphere. Many scientists suspect that changes in stratospheric temperatures may alter weather patterns in the troposphere. Finally, an increase in the amount of UV radiation could impact human health, increasing the incidence of skin cancer, cataracts, and other Sun-exposure-related maladies (please see Ultraviolet Radiation: How it Affects Life on Earth for more details).