Radiative fluxes at the Earth surface are major determinants of ambient climate and provide the energy for a variety of vital climate processes. Variations in these fluxes therefore may play a crucial role in various environmental issues such as global warming, glacier retreat, water availability and carbon budgeting. On a more applied level, changes in the amount of solar radiation reaching the Earth surface may substantially affect factors involved in climate change.
Anthropogenic interference with climate occurs primarily through modification of Radiative fluxes in the climate system.
There is, however, a dawning realization among researchers that even these apparently tiny variations can have a significant effect on terrestrial climate. A new report issued by the National Research Council (NRC), “The Effects of Solar Variability on Earth’s Climate,” lays out some of the surprisingly complex ways that solar activity can make itself felt on our planet.
Understanding the sun-climate connection requires a breadth of expertise in fields such as plasma physics, solar activity, atmospheric chemistry and fluid dynamics, energetic particle physics, and even terrestrial history. No single researcher has the full range of knowledge required to solve the problem. To make progress, the NRC had to assemble dozens of experts from many fields at a single workshop. The report summarizes their combined efforts to frame the problem in a truly multi-disciplinary context.
One of the participants, Greg Kopp of the Laboratory for Atmospheric and Space Physics at the University of Colorado, pointed out that while the variations in luminosity over the 11-year solar cycle amount to only a tenth of a percent of the sun’s total output, such a small fraction is still important. “Even typical short term variations of 0.1% in incident irradiance exceed all other energy sources (such as natural radioactivity in Earth’s core) combined,” he says.
Of particular importance is the sun’s extreme ultraviolet (EUV) radiation, which peaks during the years around solar maximum. Within the relatively narrow band of EUV wavelengths, the sun’s output varies not by a minuscule 0.1%, but by whopping factors of 10 or more. This can strongly affect the chemistry and thermal structure of the upper atmosphere.
Several researchers discussed how changes in the upper atmosphere can trickle down to Earth’s surface. There are many “top-down” pathways for the sun’s influence. For instance, Charles Jackman of the Goddard Space Flight Center described how nitrogen oxides (NOx) created by solar energetic particles and cosmic rays in the stratosphere could reduce ozone levels by a few percent. Because ozone absorbs UV radiation, less ozone means that more UV rays from the sun would reach Earth’s surface.
Indeed, Gerald Meehl of the National Center for Atmospheric Research (NCAR) presented persuasive evidence that solar variability is leaving an imprint on climate, especially in the Pacific. According to the report, when researchers look at sea surface temperature data during sunspot peak years, the tropical Pacific shows a pronounced La Nina-like pattern, with a cooling of almost 1o C in the equatorial eastern Pacific. In addition, “there are signs of enhanced precipitation in the Pacific ITCZ (Inter-Tropical Convergence Zone) and SPCZ (South Pacific Convergence Zone) as well as above-normal sea-level pressure in the mid-latitude North and South Pacific,” correlated with peaks in the sunspot cycle.
The Concept Behind This Linkage
Increasing releases of greenhouse gases into the atmosphere lead to an enhancement of thermal radiation from the atmosphere to the surface, thereby causing global warming. Yet not only thermal radiation undergoes substantial decadal changes at the Earth surface, but also incident solar radiation (SSR) often in line with changes in aerosol emissions.
Land based observations widespread declines in SSR from 1950s to 1980s (also known as ‘global dimming’), a partial recovery (termed as ‘brightening’) since mid-1980s, and indication for an ‘early’ brightening in 1930s and 1940s. No similar extended observational records are available over oceans.
Though, modeling studies, conceptual frameworks and available satellite derived records point to the existence of decadal variations also over oceans. These changes in radiations overall match with decadal variations in observed warming rates, suggesting that solar output variations may effectively modulate greenhouse gas-induced warming. This is how scientists speculate the possible connection between solar output and its impact over factors determining climate change.
Causes of Variation
The decadal variation in SSR as described above cannot be explained by changes in the luminosity of the Sun, as these are at least an order magnitude smaller and uncorrelated. The observed SSR variations therefore have to originate from alterations internal to the climate system that affect the transparency of the atmosphere for solar radiation. This transparency depends on the presence of clouds, aerosols and radiatively active gases.
Radiative energy available at the Earth surface drives a variety of essential climate processes, and any change in its amount has the potential to significantly alter the state of Earth’s climate and environment. Scientific observations suggest mutlidecadal variations in SSR at widespread land based observation sites. It is still debated, however, to what extent the two major modulators of the atmospheric transparency, i.e., aerosol and clouds, contribute to the SSR variations.
In a concluding panel discussion, the researchers identified a number of possible next steps. Foremost among them was the deployment of a radiometric imager. Devices currently used to measure total solar irradiance (TSI) reduce the entire sun to a single number: the total luminosity summed over all latitudes, longitudes, and wavelengths. This integrated value becomes a solitary point in a time series tracking the sun’s output.
A better long-term record of the sun’s irradiance might be encoded in the rocks and sediments of the Moon or Mars. Studying other worlds might hold the key to our own.