The Sun and Climate
by Judith Lean and David Rind
Of the many objects in the universe, only two are essential for life as we know it: the Earth itself, and the Sun: the star around which it circles, year after year. Burning steadily in stable, middle age, the Sun--now about five billion years old--provides an unfailing source of light and energy. The Sun's heat is so intense that at a distance of 93 million miles it warms the surface of the otherwise cold and lifeless Earth some 250°Centigrade, to -18°C (0°Fahrenheit). Thus warmed, the solid Earth releases a portion of its heat in the form of infrared radiation, which is trapped by atmospheric greenhouse gases, further raising the surface temperature to a more comfortable 15°C (59°F). In this way, the Sun's radiation and the Earth's blanket of greenhouse gases sustain the mean global temperature at a level supportive of life. Sunlight also powers photosynthesis, and provides energy for the atmospheric and oceanic circulations that profoundly affect all living things.
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In global average, increases in greenhouse gas concentrations or in solar radiation bring warmer surface temperatures since they add energy to the climate system. In contrast, increased industrial and volcanic aerosols restrict the penetration of solar radiation to the Earth's surface and lead to surface cooling. A drop in the concentration of ozone in the lower stratosphere should also produce a net cooling at the surface. Changes in albedo that increase the planet's reflectivity will lead to cooling, and those that make it less reflective and more absorbing, to a temperature rise.
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The Earth receives most of its direct heat from the visible and near- infrared spectrum of sunlight, retaining about 70 percent of what pours down on its day-lit hemisphere. The rest is returned, by reflection, back into the cold of space. A part of what the ground and oceans and lower atmosphere absorb also leaks outward through the atmosphere in the form of infrared radiation. The remaining fraction--trapped in part by greenhouse gases--sustains the habitable environment to which we are accustomed. Any variation in total radiation from the Sun will force an accompanying change in mean-surface temperature.
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Isotopes of chemical elements are distinguished by different atomic weights. The most valuable for what they tell of the Sun are carbon of atomic weight 14 (14C, or "radiocarbon"), found in tree-rings, and beryllium of weight 10 (10Be) that is naturally sequestered in polar ice deposits. Well-dated records of both of these indirect indicators of solar activity reach back many thousands of years and exhibit cyclic variations of about 2300, 210 and 88 years, as well as 11 years, all of which are ascribed to the Sun.
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The sensitivity of climate to solar radiation changes, as defined earlier, is not well known. A conservative estimate is that a 0.1 percent change in solar total radiation will bring about a temperature response of 0.06 to 0.2°C, providing the change persists long enough for the climate system to adjust. This could take ten to 100 years.
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Solar radiation received at the Earth can vary by means that are unrelated to any changes on the Sun itself. The best studied of these are very long-term changes in the Earth's orbit around the Sun, which alter the distribution of sunlight both geographically and seasonally. They are now believed to trigger the coming and going of the major Ice Ages. As such, they may provide a powerful demonstration of the impacts of changes in solar radiation on the climate system.
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CONCLUSIONS
That the Sun plays a critical part in the Earth's climate system is indisputable; moreover, both the Sun and the climate change continually, over all time scales. Yet the physical connections that might link the variations seen on one with the variability presently occurring in the other are but poorly known. One and a half decades of continuous monitoring of direct solar radiation have provided long-needed information, but this short period of time is but a wink of an eye in the life of the Sun, and a woefully inadequate sample of the full range of its possible behavior.
We now know--thanks to recent spaceborne monitoring--that sunlight received at the Earth follows the drum beat of the eleven-year sunspot cycle, with both the total and short wavelength emissions varying in phase with solar activity. At the peaks of recent eleven-year cycles the total energy from the Sun increased by about 0.1 percent, and the UV portion by several percent or more. In years of the cycle when sunspots are few--as is presently the case--both total and UV radiation are diminished.
We also know from isotopic archives of solar activity that the Sun exhibits greater variability on time scales that exceed the eleven-year cycle. Detecting and confirming larger-amplitude, longer-period cycles in solar radiation--if they indeed exist--will require reliable continuous solar monitoring, well into the next century. NASA's Office of Mission to Planet Earth and the European Space Agency are presently conducting these measurements on, respectively, the Upper Atmosphere Research Satellite and the Solar Heliospheric Observatory. Future solar radiation monitoring is planned for NASA's Earth Observing System and the NOAA/DoD National Polar- Orbiting Operational Environmental Satellite System, although in the latter case not at UV wavelengths. Redundancy afforded by multiple space-based instruments is critical for separating solar and instrumental effects in these difficult radiometric measurements, and to ensure overlap so that the long- term database is not broken. Supporting observations from the ground are also necessary in order to interpret the causes of measured variations. Present lack of access to space, the overall low priority of solar influences in global change research, and continued reductions of Earth environmental monitoring threaten the accomplishment of this task.
Climate model simulations indicate that changes in solar radiation a few times larger than those confirmed in the eleven-year cycle, and persisting over multi-decadal time scales, would directly affect the surface temperature. Since such models cannot account for the climate system's apparent sensitivity to small perturbations in solar energy apparently brought about by the very long term changes in the Earth's orbit about the Sun, they may also underestimate climate sensitivity to energy output fluctuations caused by solar activity, even during the eleven-year Schwabe cycle. Nor do these simulations yet include potential effects of changes in the solar spectrum, including the more variable UV.
The unsolved puzzle of how subtle variations driven by Earth-orbit changes can affect the climate suggests a closer look at feedback processes, including other pathways than direct solar radiative forcing. These should include known changes in solar energy inputs across the entire spectrum and possible connections that might amplify small changes in radiation, not only at the Earth's surface but from the top to the bottom of the atmosphere. Such studies of solar perturbations can serve the broader cause as diagnostic probes of the atmosphere and climate system.
Ambiguities regarding projected greenhouse warming call in much the same way for clearer information regarding the role of the Sun, as a possibly important contributor to the current warming trend. Climate simulations using only greenhouse gas changes predict a warming that exceeds the 0.5°C that is documented in the instrumental record of the past 140 years. To reconcile the difference between the observed and the predicted values, either the models are wrong or other, natural and anthropogenic forcings must be properly factored in.
If variations in the output of the Sun are indeed limited to the tenth of a percent that is recorded in direct measurements, future solar changes will likely have but a small effect, one way or the other, on the surface warming of a few degrees that is expected to result from doubled concentrations of greenhouse gases. If we include reasoned deductions from what we know of the Sun and climate in the past, we must allow that solar changes could potentially alter the anticipated effects of carbon dioxide and other greenhouse gases on the surface temperature of the Earth.
In this case the Sun could make a difference of about 0.5°C in the surface temperatures now projected by consensus climate models for doubled concentrations of CO2. A fortuitous future cooling of this amount, due to the Sun, would not fully compensate for the effects of increases in greenhouse gases, which are projected to warm the Earth by 1 to 3°. Furthermore, the overall, long-term level of solar activity would have to fall steadily and systematically over the next few hundred years from the current, high sunspot numbers that have characterized the greater part of the 20th century. Were that to happen, the principal impacts would be to cloud for a time the unambiguous detection of enhanced greenhouse warming and to soften, by at most about a half, its projected impact on the temperature of the planet. The rapid warming since 1970 is several times larger than that expected from any known or suspected effects of the Sun, and may already indicate the growing influence of atmospheric greenhouse gases on the Earth's climate.
