It’s a blinding hot ball in our daily sky; without it there’d be no plants, no bugs, no death metal bands.
So naturally the Sun is the first place scientists looked to explain modern global warming.
They haven’t found it there. Why do they rule out the Sun as the major cause of climate change over the past seven or so decades?
Because the energy Earth receives from the Sun hasn’t changed much at all over the past few decades. In fact, since about 1960 this energy has been on a slight downward trend, while Earth keeps getting warmer.
Judith Lean, a scientist at the Naval Research Laboratory in Washington, D.C. who studies how the Sun influences Earth’s climate, calls this the most compelling evidence for the conclusion that the Sun is not the cause of Earth’s warming.
“In the past four decades at least — we’ve measured the Sun’s brightness since 1978, using very precise space-based instruments”– Lean explains. “And the overall trend has been downwards by a few tenths of a percent.”
Yet at the same time, the global surface temperature has increased by about 1.1⁰F (0.6⁰C). If the Sun’s irradiance were the dominant force driving changes to our climate, the planet should be experiencing a slight cooling.
Judith Lean (Clip 1): Sun’s brightness measured since 1968 and overall trend down, while global surface temperature of Earth has increased.
The Sun’s Cycles
The Sun puts out an enormous amount of energy – in a millionth of a second the Sun emits as much energy as human civilization uses in a year.
By the time the Sun’s light reaches the top of Earth’s atmosphere, about 1,365 Joules of energy pass through a square-meter every second. Scientists call this the “total solar irradiance” (TSI) or “solar constant,” though it’s actually not a constant but varies with short- and long-term solar cycles by a couple of watts per square-meter (see figure). But this variation, especially over a decade or two, is slight.
After accounting for the Earth’s sphericity and rotation, one-fourth of this is, on average, incident on a unit area of the atmosphere. About 30 percent of this is reflected away by clouds, aerosols, the atmosphere, and the surface, and about 20 percent is absorbed by the atmosphere. A net 168 watts per square-meter is absorbed by the surface.
If the Sun’s output increased or decreased, the amount of energy reaching Earth’s surface would change accordingly. And, all else being equal, any change in the amount of sunlight received at Earth’s surface would lead to a change in the temperature of the surface.
But – and this is crucial – the average temperature of Earth’s surface changes only slowly as the Sun’s output changes – about 0.2⁰F (0.1⁰C) for every additional watt the Sun delivers. One more solar watt would be akin to a 100-watt light bulb hanging over a small house.
Over the Sun’s approximately 11-year cycle, its intensity typically varies by up to two watts per square-meter, from the cycle’s minimum to its maximum. The number of sunspots also varies, from zero to as many as 150. What this means is that “we can detect an increase of ~ 0.1⁰C from the minimum to maximum of a solar cycle,” Lean explains.
Seasons of the Sun
Judith Lean (Clip 2): Reliable records of Sun’s brightness and of Earth’s surface temperature and other factors that affect climate.
Using proxy records from ice cores – such as the accumulation of the radioactive isotope beryllium-10, which varies inversely with solar activity — scientists have been able to deduce changes in the Sun’s intensity going back many millennia.
These records are consistent with the regular counts of sunspots on the Sun by dedicated astronomers going back to 1600 A.D. They reveal a low level of solar intensity during the Maunder Minimum, a period with no sunspots from 1650 A.D. to about 1710 A.D. and a solar intensity as much as 3 watts per square-meter lower than currently (0.2% – see figure).
Another period of weak solar activity was the Dalton Minimum, from 1790 A.D. to 1820 A.D. After it, the Sun’s output increased over the decades, to the Modern Maximum around 1950. Since then, the Sun’s intensity has been on a slight downward trend.
“These associations suggest that changes in the Sun’s energy likely contributed 10 percent to 15 percent or less of the warming since 1900,” says Lean. “So our current understanding of how the Sun’s brightness changes, and its association with solar activity from both direct observations and models, all but precludes the Sun from being responsible for the planet’s warming in the past century or so.”
A Cooler Future?
From a historical perspective, the last several decades have been an active one for the Sun. Since the absolute Modern Maximum around 1958, successive solar cycle peaks have each had a slightly lower irradiance.
Though the most recent peak in irradiance early this year had essentially the same value as the 2000 peak, its number of sunspots peaked at about 150, half of recent cycles. The connection between sunspot activity and irradiance isn’t fully understood, but this prolonged solar minimum has led to speculation that the Sun may be entering a long-term cooler phase, perhaps something again like the Maunder Minimum.
In a 2010 study, space physicist Michael Lockwood used past solar variations to forecast the chances of a return to Maunder Minimum-like conditions in the next 40 years, which he put at 8 percent. Since then he and colleagues raised those chances to 15 to 20 percent, noting that decline in solar activity is faster than at any time in the last 9,300 years covered by solar records.
Would such a decrease be enough to halt global warming, or even reverse it and bring solar cooling?
Authors of several studies on exactly this question say no. Writing in Nature Communications in 2013, Gerald Meehl of the National Center for Atmospheric Research (NCAR) and colleagues found that a Maunder Minimum-like decline from 2020 to 2070 would reduce human-caused global warming by several tenths of a degree, at a time when warming from pre-Industrial times will likely be in the 2.5⁰F to 5⁰F (1.5⁰C to 3⁰C) range.
And, once solar activity returns to more normal values, the slight cooling would disappear and the climate would warm back to where it would have been without the solar cooling.
A Rainbow of Colors
Only some of the electromagnetic radiation we receive from the Sun is visible to our eyes. The white we can see is actually composed of a rainbow of colors, and ultraviolet radiation we can’t see – its wavelengths are too short – can cause sunburn or even cancers. (Fortunately for us, the most dangerous wavelengths are blocked by the atmosphere.) The Sun also delivers a small amount of light on the other side of the visible spectrum, called infrared radiation, but its energy is less than one percent of the Sun’s total.
Even if the total energy from the Sun stays the same, changes in the amounts at each wavelength can have effects on climate. The ocean absorbs ultraviolet light – wavelengths shorter than that of visible light – better than it does visible light delivered to Earth. And changes in ultraviolet irradiance have been linked to changes in surface pressure – changes that resemble those brought by the more familiar phenomena of the Arctic Oscillation and the North Atlantic Oscillation (NAO). In particular, the latter governs the positioning and strength of storm tracks into Europe.
There inevitably is some uncertainty about what ultraviolet variations accompany a change in the Sun’s overall irradiance; but satellite measurements taken during the last half of the last solar cycle, which ended in June 2008, showed ultraviolet variability was significantly larger than thought earlier.
Future ultraviolet variability could lead to noticeable regional climate impacts, according to a recent study also in Nature Communications. By resembling changes brought by the NAO, these changes could bring enhanced cooling over northern Europe and the eastern United States that could be a “significant fraction” of future warming – perhaps around two degrees Fahrenheit.
That’s enough of a change to keep things interesting, but not enough to allay concerns about future warming and where the climate is heading.