Beyond the Wild Frontier: The Sun-Climate Link

By Sallie Baliunas, Ph.D., and Willie Soon, Ph.D.
Harvard-Smithsonian Center for Astrophysics

Editor’s Note: With this issue, we introduce “Cutting Edge,” a monthly feature that reports on the latest developments in ongoing climate change research.

It is well known that the computer simulations of the earth’s climate lack knowledge of some physical processes of climate change. With this in mind, we embark on an expedition—a journey to the largely unknown frontiers of climate change science.

As dawn breaks on this new WCR department, our first installment devotes itself to the role of the mercurial sun in climate change.

First, some history. In 1801, astronomer Sir William Herschel speculated:

“I am now much inclined to believe that openings [i.e., sunspots] with great shallows, ridges, nodules, and corrugations, instead of small indentations, may lead us to expect a copious emission of heat, and therefore mild seasons.”

Conversely, Herschel thought that eras with few sunspots would lead to “spare emission of heat” and “severe seasons.”

What does this have to do with the price of wheat in 18th-century Europe? Quite a lot, actually.

Herschel was systematically exploring the wild frontiers of the sun-earth climate link. But he lacked the modern instrumentation needed to explore his hypothesis. With no temperature measurements to turn to, he used the only evidence available to test his notion.

He figured that a severe season (one lacking in sunspots) would drive up the price of wheat. Reconstructing past climate conditions and scanning historic wheat prices, he found his link. During five lengthy periods during which sunspots were scant, wheat was indeed more expensive.

Herschel took his ideas to the farthest reaches of available knowledge, but he understood the limitations of his study.

When the great astronomer, best known for discovering the planet Uranus, presented his ideas to the Royal Society, they laughed him out of the room. This brave pioneer would have benefited from knowing what we know today about the sun:

• The number of sunspots increases and decreases in a roughly 11-year cycle (discovered by Heinreich Schwabe, 1843).

• From about 1640 to 1720, the number of sunspots and the 11-year cycle were greatly suppressed (that period is called the Maunder Minimum, after Edward Maunder, who popularized this observation in 1894).

• Sunspots are cooler than the surrounding surface of the sun, as George Ellery Hale discovered in 1908. Hale also found that sunspots are areas of intense magnetic fields; therefore, the number of sunspots is related to the strength of solar magnetism.

• By the 1980s, NASA satellites had measured changes in the total solar irradiance (the sun’s output) occurring in step with the sunspot cycle.

This recent discovery is significant because it shows how variations in the sun’s output to the earth might affect global temperature.

Satellites began measuring solar output in 1979 and revealed that the amount of variation—0.1 percent over a sunspot cycle—is small. All in all, the variation just seems too tiny and over too short a time scale to push the global temperature around very much.

This supposition has led some to conclude that solar-irradiance changes (which translate to roughly 0.3 watts per square meter at the surface of the earth through each sunspot cycle) are insignificant compared with the effect of increased greenhouse gases (which totals 2.4 watts per square meter, or eight times as much as the solar changes). At first squint, the solar influence seems forgettable.

But here’s the problem with dismissing the sun as an agent of any significant climate change on time scales of decades to centuries: The signature of solar variability shows up too well!

For example, Karen Labitzke found a positive correlation between the solar cycle and winter temperatures in the upper atmosphere over the North Pole. Closer to the surface of the earth, there is a correlation between the length of the sunspot cycle and the Northern Hemisphere’s temperature history over the last 250 years (Figure 1).

Figure 1.  Length of the 22-year solar magnetic cycle (closely related to the 11-year sunspot cycle) and reconstructed temperature history for the land areas of the Northern Hemisphere are highly correlated over the last 250 years.

Now for some armchair exploration: What if the sun’s irradiance changes by several tenths of a percent on time scales of decades to centuries?

If we plug this into a climate model, as the authors did, with colleague E. Posmentier, it produces plausible temperature changes. Consider: For a change of total solar output of up to 0.4 percent and timed to variations given by the observed sunspot record, the earth’s actual temperature changes can be reasonably explained. Neither experimental nor theoretical evidence disallows such a solar change. Indeed, hot off the presses at Science is R.C. Willson’s report of an observed difference of irradiance over the course of the last sunspot cycle that would amount to about 0.4 percent over a century!

But a change of solar irradiance of 0.4 percent over say, a century, is still only 1 watt per square meter, or about half that of the human-enhanced greenhouse effect. With an expected doubling of greenhouse gases in the next 100 years, surely the sun’s influence will diminish further.

Or will it? The question on the wild frontier is whether the climate responds to other solar influences. In other words, is it the total solar-irradiance change that affects our climate? Or do individual components of the solar spectrum play important roles?

For example, solar ultraviolet radiation may change the chemistry in the bottom 50 miles of the atmosphere. Visible wavelength irradiance changes may affect the lower atmosphere and sea surface. Both portions of the solar-irradiance spectrum may combine to influence the jet stream and the trade winds.

Then, too, the sun’s surface magnetism and the solar wind change the galactic cosmic rays hitting the earth’s own magnetic field, affecting the electrical and chemical properties of the upper atmosphere. Even cloud characteristics and coverage may change.

The observed sunlike signatures in the climate system leave us with the feeling that perhaps the old assumption of “equivalence” between the sun’s energy input to the earth and the added energy due to increased greenhouse gases is false. The facts are these: The sun’s rainbow comes in many colors; the sun emits energetic particles; and both are variable in time, space, and frequency. Obviously the different components of the earth’s atmosphere and surface respond to different aspects of the sun’s diverse energy outflows.

Incidentally, knowledge of mechanisms of sun-caused climate change other than total irradiance would rebound through the issue of detecting the fingerprint of human activity on climate. But detecting whether the fingerprint is ours or Mother Nature’s requires knowing all the relevant factors and considering them simultaneously in a model. Once such a model is verified, then and only then can fingerprints for each suspect be identified.

But the case of the sun remains unsolved, so the mechanisms of climate change are not fully known. And the models are unverified. So conclusive fingerprinting isn’t possible.

Almost 200 years ago, Herschel made a very modern speculation on the influence of the sunspots on the climate of the earth. It remains to be seen what the mechanism(s) of solar change and its (their) climate response are.

But studying sunlight is essential to creating the best climate simulations possible. As Herschel himself put it:

A constant observation of the sun with this view, and a proper information respecting the general mildness or severity of the seasons, in all parts of the world, may bring this theory to perfection or refute it if it be not well founded.

References:

Herschel, W. (1801). Observations tending to investigate the nature of the sun in order to find the causes or symptoms of its variable emission of light and heat. Philosophical Transactions, 91, 265.

K. Labitzke (1987). Sunspots, the QBO, and the stratospheric temperature in the North Polar region. Geophysical Research Letters, 14, 535.

Soon, W., et al. (1996). Inference of solar irradiance variability from Terrestrial Temperature Changes, 1880–1993: an astrophysical application of the sun-climate connection. Astrophysical Journal, 472, 891.