The solar forecast calls for sluggish times ahead, according to scientists in Boulder, Colo.—which could have a cooling effect on Earth. A better understanding of solar processes and their climatic impacts will be key to understanding how far such quiescence could go to counteract global warming caused by pollution from fossil fuels.

There's little doubt the sun's been in a funk ever since solar cycle 23 wound down in 2007. (Astronomers began to number the solar activity cycles in 1755.) The ramp-up to the peak of the next cycle is taking longer than expected and has been feeble, with precious few sunspots.

Normally, at the apex of each 11-year solar cycle—for cycle 24 it should occur around 2013—the sun's surface becomes especially frenetic, erupting into a climax of sunspots and flares. The outbursts can bombard the planets with gusts of highly charged solar wind, which triggers effects on Earth ranging from spectacular aurorae to interference with communications networks and even—in the worst cases—disruptive surges in power grids.

As the sun pushes toward the height of cycle 24, we'll still see a rise in solar activity, predicts Tom Woods, a solar physicist at the University of Colorado at Boulder's Laboratory for Atmospheric and Space Physics (LASP), but "I think you can say with pretty fair confidence we're going to have a low maximum."

Predictions of global cooling became popular among climate change contrarians during the unexpected pause between solar cycles 23 and 24 and haven't completely subsided. But the degree to which a quiet sun could chill Earth, especially in the face of pollution that's warming the planet, is uncertain. Solar researchers are increasingly eager to quantify the sun's role, given the global importance of the question.

An Earth-orbiting satellite called Glory—a project of NASA and LASP—will tackle two pieces of the puzzle after it launches next week.

Pollution may be stealing the sun's power
Woods said sluggish solar cycles tend to come in groups of three or more—and when they persist, the chilling effect on Earth becomes noticeable. The Maunder Minimum of the late 17th century is an oft-cited example of a time when a quiet sun—as documented by a paucity of sunspots—coincided with lower temperatures on Earth and, in particular, bitterly cold winters in Europe.

The Dalton Minimum was another period of low solar activity from about 1790 to 1830, when better weather records were kept; rivers in Europe stayed frozen throughout winter—a condition that is unheard of today. In general, a solar minimum is thought to bring a cooler climate to Europe and North America as well as a warm Greenland.

But Earth might not respond the same way it did during the Dalton Minimum. That's because there's a new variable that is part of the equation these days: the air pollution that has accompanied the Industrial Age.

The Gleissberg Minimum at the end of the 19th century may foreshadow the future of sun-induced cooling. The sun was quiet, but "not a lot of people have recognized that as a colder period of time for the United States," Woods said. "We're starting to see other effects from greenhouse gas burning. It was masked to some degree."

Judith Lean, senior scientist for Sun–Earth System Research in the Space Science Division of the U.S. Naval Research Laboratory, says greenhouse gases influence climate change much more than the sun can: Although solar variations may change Earth's temperature by 0.1 to 0.2 degree Celsius globally over a couple of centuries—more in specific regions—human-induced global warming is projected to raise temperatures up to three or four degrees C globally in that same time frame.

Eyes on the sun
Still, the bulk of Earth's energy comes from the sun—and so understanding its role in climate is key.

Glory is NASA's most recent attempt to quantify the sun's role. The 1.9-meter-long, 525-kilogram satellite will launch shortly after 2 A.M. local time on February 23 from Vandenberg Air Force Base in California. It will be lofted into a polar orbit with a mission set for at least three years, where it will monitor from above both the total solar energy reaching the atmosphere as well as the airborne particles that variably absorb and reflect that energy.

Aerosols include salt, mineral dust, soot and smoke, and come from a variety of sources, such as vehicle exhaust, campfires, volcanoes and even desert winds and sea spray. NASA climate expert and Glory science team member James Hansen has said the range of uncertainty associated with the climate impact of aerosols is three or four times that of greenhouse gases.

Raytheon's Aerosol Polarimetry Sensor, an instrument mounted on the Earth-facing side of the spacecraft, will observe the movement of aerosols through the atmosphere over time, especially on a seasonal scale. And Glory's sun-facing side will sport the Total Irradiance Monitor, which will measure the intensity of solar radiation at the atmosphere's upper reaches.

The measurements will continue a 32-year climate data record from a dozen NASA, National Oceanic and Atmospheric Administration (NOAA) and European instruments. Greg Kopp, a researcher also at LASP, is principal investigator for the irradiance monitor. He says existing data has already helped researchers understand variations on the scale of the sun's 11-year activity cycles, but in order to capture longer trends observations must continue.

Four solar irradiance instruments are already in space, including VIRGO (for Variability of Solar Irradiance and Gravity Oscillations), launched in 1995 on board the sun-orbiting SOHO spacecraft, and the SORCE satellite, sent into Earth orbit in 2003 carrying three irradiance monitors. Three of those instruments, however, have long exceeded their designed mission lifetimes, and are deteriorating. The European PICARD microsatellite mission, launched in 2010, and NASA's Glory mission are the new solar vanguard.

Kopp says, to fully understand the sun–climate connection, "we would like very accurate measurements of the amount of radiant energy coming in to the Earth, the total solar irradiance, and the amount leaving it. Any imbalance between the two is available to heat or cool the planet, causing climate change."

The outgoing measurements are much more difficult to acquire, he says, although NASA's CLARREO Decadal Survey mission, with a planned launch in 2018, is one effort in that direction.