The Waxing Sun and Warming Climate

You don’t have to be a climate scientist to know that the Sun is the main driver of climate.

But that doesn’t mean that it must be the main driver of global warming. For that to hold, the Sun’s output would have to have changed by an appropriate magnitude to explain the current increased temperature.

Careful Monitoring Shows Solar Changes Alone Do Not Explain Recent Warming

Over the past two decades or so, scientists have been carefully monitoring the Sun from space, and so we have a pretty good idea of how that oh-so-important orange ball has been changing. The data show pretty convincingly that solar changes alone do not explain most of the warming since the 1980s — warming from greenhouse gases is needed.

But wait a minute, some people say. We know that the climate was considerably colder in the 16th century through the 18th, so much so that it is been dubbed the “Little Ice Age.” Then, in the early to mid-19th century, a slow warming started that largely continues today. Surely, these same people might say, you’re not claiming that that warming was due to greenhouse gases. And they do have a point.

Records of sunspots — those dark areas on the Sun caused in part by intense magnetic fields — indicate that the Little Ice Age was a period of uncommonly low solar activity.

In fact, there was an extended period, known as the Maunder Minimum (MM), in the late 1600s to early 1700s when there was almost a complete absence of sunspots. More than likely, this absence was indicative of a relatively weak Sun that provided a little less warmth to the Earth. And so scientists have concluded that the cold temperatures of the 16th through 18th century were caused by the Sun.

It follows, therefore, that the warming that began in the 18th century was also triggered by the Sun. For one, it is highly unlikely that the onset of warming was caused by greenhouse gases, as the rates of fossil-fuel burning and forest-clearing were still quite low. Secondly, the increase in sunspot numbers in those decades provides strong evidence that the solar output was increasing and driving the warming.

So, you might ask, if the Sun caused the warming that began in the 18th century, doesn’t it follow that today’s warming is also due to a warming sun?

That’s a fair question, but be careful of jumping to wrong conclusions. Just because the Sun has driven warming in the past does not mean it is driving the current warming. In the end it’s a matter of degrees (sorry, no pun intended). The Sun is not the only force that can drive a warming trend. Greenhouse gases can too, and we know that carbon dioxide and other greenhouse gases have increased quite substantially since the industrial revolution. So, which is more important to today’s warming: the Sun or greenhouse gases?

Measuring Warming From Both the Sun and Greenhouse Gases

To answer this riddle, scientists have been working on quantifying the amount of warming the two have provided to the Earth over the centuries. (We refer to the amount of such warming as “radiative forcing” and express it in units of Watts per square meter [W/m2]; i.e., the average amount of heat deposited on one square meter of the Earth’s surface per unit of time.)

(Source: Intergovernmental Panel on Climate Change http://ipcc-wg1.ucar.edu/wg1/FAQ/wg1_faq-2.1.html)

Using records of variations in sunspot number, the Intergovernmental Panel on Climate Change (IPCC) concluded that the forcing from the Sun since the MM was about 0.12 W/m2.

Calculations of total forcing from long-lived greenhouse gases, on the other hand, amount to about 2.6 W/m2; if one takes into account the cumulative effect of both human-caused cooling (e.g., aerosols from air pollution) and warming effects, the forcing drops to about 1.6 W/m2. (See Table 2.12, Chapter 2 here [pdf]).

Last Century's De-Icer Is Not Necessarily the Current One

The conclusion is obvious: while increases in the Sun’s output might have been the de-icer that got us out of the Little Ice Age, that warming has been significantly eclipsed by greenhouse gases that have provided much of the heat for the temperature runup of the 20th century.

Now a new paper by F. Steinhilber of the Swiss Federal Institute of Aquatic Science and Technology and colleagues in Geophysical Research Letters has provided independent and perhaps even more accurate confirmation of that conclusion.

Instead of using sunspot numbers, the authors scoured a composite ice-core record for concentrations of the radioactive element Be-10, which is produced by cosmic rays impinging upon atmospheric nitrogen and oxygen. Their method is based on empirical evidence that when the Sun becomes more active, its magnetic field changes, which in turn affects cosmic rays impinging on the Earth, which in turn affects the abundance of Be-10. By measuring the change in Be-10 present in the ice cores as a function of depth, the authors determined the change in solar output as a function of time dating back some 9,000 years.

The result: the solar output (or irradiance) from the Sun has increased by about 0.9 W/m2 since the MM. Taking into account geometry and the planet’s reflectivity, that translates into a radiative forcing of about 0.16 W/m2. That’s a little larger than the forcing estimated by the IPCC but still way smaller than the forcing by greenhouse gases (the 2.6 W/m2 referred to above).

So yep, the Sun is the big player in the climate sweepstakes. But if the Sun is standing pat, little guys, like those lowly greenhouse gases, can make a significant play.