Anyone who doubts that the Sun has a profound impact on climate just hasn’t been paying attention. A mere change in the sun angle from January to July means the difference between cold winters and hot summers in New Jersey, where I live. If the Earth were traveling through the darkness of space alone, we wouldn’t have a climate to begin with; the planet would be frozen solid. One way or another, the Sun is the ultimate source of most of the energy on Earth.



That being the case, critics of the idea that global warming is caused by greenhouse gases have suggested that the real culprit is a changing Sun. If they’re right, the clamor to reduce emissions would be a waste of time and resources. And while climate scientists are pretty sure the problem really is greenhouse gases, they can’t just ignore alternate explanations.



Scientists have looked hard at how the Sun might be playing a part in the current episode of climate change.

To be sure, the greenhouse-gas story doesn’t just come out of nowhere. Scientists have had definitive laboratory proof since the 1800s that gases like CO2 trap heat. They know that greenhouse gases are implicated in episodes of climate change going back millions of years. They know that carbon dioxide from the burning of fossil fuels has been building up in the atmosphere since the Industrial Revolution began. They’ve seen the temperature rising, glaciers melting, weather patterns changing. They don’t know all the details, but the story hangs together extremely well.

But just because a scientific story hangs together doesn’t mean you aren’t missing something. The late physicist Richard Feynman once said: “The first principle [in science] is that you must not fool yourself, and you are the easiest person to fool.” A convincing line of reasoning, whether it’s part of a scientific theory or an episode of “Law & Order,” could fool you into overlooking the real villain.

So scientists have looked hard at how the Sun might be playing a part, and how big a part, in the current episode of climate change. There are basically two possibilities. The first and most straightforward is that the Sun has been putting out more energy lately than it used to. The second is that the Sun is tinkering with our atmosphere in more subtle ways — a more complicated proposition.

As to the first idea, there’s no question that the Sun does change brightness. In its youth, for example, billions of years ago, the Sun put out only about 70 percent as much energy as it does now — it’s only thanks to the greenhouse gases thought to have blanketed the planet at that time that all the Earth’s water didn’t turn to solid ice.

For the past couple of billion years, the Sun hasn’t varied anywhere near that much, but its energy output has inched up and down. By one widely accepted calculation, a brightening Sun might account for up to 20 percent of the warming we saw over the 20th century. Most of that, however, was earlier in the century, when greenhouse-gas levels were still relatively low and changes in the Sun had relatively more impact.

Since the 1970s, there has been no upward trend in the sun’s brightness.

Since the 1970s, satellites have been monitoring the Sun’s brightness — technically known as total solar irradiance, or TSI — and with unprecedented precision, using sensitive light detectors that can gaze at the Sun from high above the clouds, moisture, dust, and atmospheric turbulence that interfere with ground-based observations. Over that time, they’ve measured the Sun getting regularly brighter and dimmer as sunspots — essentially, magnetic storms — wax and wane on their normal 11-year cycle. (It’s brighter when there are more sunspots — a seeming paradox since sunspots are dark, but the rest of the surface brightens enough to make up for it.) The total change upward and downward is about a tenth of a percent — enough to change temperatures by a fraction of a degree either way. But overall, there’s been no upward trend over that time in TSI, even as temperatures on Earth have continued to climb.

So the Sun isn’t causing global warming. If the sunspot cycle changes significantly, however, the Sun’s brightness could change as well. Some solar physicists think that may be starting to happen. The most recent sunspot minimum was deeper (meaning even fewer sunspots) and longer lasting than average, and the buildup to the next sunspot maximum has been sluggish. Back in the late 1600s and early 1700s, sunspots laid low for about 80 years, in what scientists call the Maunder Minimum. Temperatures were also low: the sunspot lull coincided with a period known as the Little Ice Age. None of this supports the idea that the Sun is responsible for much of global warming, but it has led to speculation by some experts, including astronomers at the U.S. National Solar Observatory, that we could be in for a stretch of cooling that could counteract rising temperatures, at least for a while.

There are two problems with this scenario, however. First, the Little Ice Age was under way long before the Maunder Minimum kicked in, so sunspots can only have been part of that story. Second, the Minumum would probably have only led to a few tenths of a degree C of cooling in any case, according to Penn State climatologist Michael Mann. Beyond that, says Leon Golub, a solar physicist at the Harvard-Smithsonian Center for Astrophysics, “I’ve been hearing predictions like this for the last three solar cycles, and they’ve always been wrong. We do understand the Sun better now,” he continues, “but we thought we understood it in the past. Our record on this is not very good.

Some scientists see a correlation between sunspot activity and the formation of low-level clouds.

The final nail in the coffin of the solar-brightness explanation for global warming is that the upper atmosphere has actually cooled in recent decades. If the Sun were heating up, the upper atmosphere would feel it, too; if something is keeping heat from escaping — such as greenhouse gases — that would tend to heat the lower atmosphere preferentially, which is exactly what’s happening.

But the Sun could also affect the Earth in a more indirect way. That’s the basis of the second possible Sun-climate connection, and because it’s more subtle than a simple change in brightness, it’s proving more difficult to shoot down. When sunspots are at a maximum, the Sun is also crackling with powerful magnetic fields. The fields send extra bursts of subatomic particles out into space, and when they reach Earth, our own magnetic field begins to crackle with energy too. That’s bad for telecommunications, but it helps keep out cosmic rays that speed across the galaxy.

When the Sun calms down and the Earth’s magnetic field relaxes in response, more cosmic rays get through — and according to Henrik Svensmark and his colleagues at Denmark’s National Space Institute, this turns air molecules into electrically charged ions that spur the formation of low-level clouds, which tend to reflect sunlight and cool the planet. Conversely, a decrease in cosmic rays penetrating the Earth’s magnetic field could lead to the formation of fewer clouds, potentially warming the planet.

The idea is far from outlandish on a theoretical level, and lab experiments at the European Organization for Nuclear Research near Geneva have shown that this can actually happen. Moreover, Svensmark and several collaborators have claimed to see a correlation between the sunspot cycle and cloud cover — more clouds when the Sun is quiet, fewer when it’s acting up.

Scientists would have to throw out pretty much everything they’ve learned in order to switch to a new explanation.

Yet when Terry Sloan and Arnold Wolfendale, physicists at Lancaster University and Durham University, respectively, in the UK, looked at the data, the correlation was there, but the clouds seemed to be changing much more dramatically than the changes in cosmic rays. A direct connection seemed, says Sloan, “a bit farfetched.” Then the scientists looked at whether cloud formation was triggered by nuclear radiation, which can also ionize air molecules. But neither the massive radiation release during the Chernobyl accident nor the spikes in radiation during open-air nuclear testing led to any extra cloud cover. Neither did a blast of charged particles from the Sun in 1980, so powerful it knocked out power to half of Canada. “There was lots of air ionization then,” says Sloan, but no increase in cloudiness.

More problematic still, Sloan, Wolfendale, and Anatoly Erlykin at Durham looked at radioactive isotopes in ice cores that are clues to cosmic-ray bombardments. If cosmic rays were behind global warming, there should be a long-term downward trend in addition to the ups and downs caused by the solar cycle.

Sure enough there was such a downward trend starting in 1900 or so. But it stopped in 1950, a couple of decades before temperatures really took off.

Other scientists have looked not at the statistics of clouds and cosmic rays, but at how ionized air could lead to cloud formation — and that, too, may be a problem for Svensmark’s theory. The ions do attract water molecules, but it’s not clear they pull in enough to make water droplets big enough to make clouds. In fairness, though, says Jeffrey Pierce, an atmospheric scientist at Dalhousie University in Halifax, Nova Scotia, “I don’t feel that I’m in any position to say that cosmic rays can’t be making contributions.”

Even Martin Enghoff, Svensmark’s collaborator at the National Space University, doesn’t claim that they’ve proven the case for a connection between cosmic rays and climate. “There is a clear mechanism [by which cosmic rays can trigger cloud formation],” he says, “but it’s still under investigation whether it is strong enough to explain the observed correlations [between cloud cover and solar activity].” Enghoff points to some studies that suggest a rise in cosmic rays beyond 1950, but even so, he isn’t ready to blame the observed warming of the past several decades on a decrease in cosmic rays penetrating the Earth’s magnetic field.

In order to emerge as a major player, however, the changing-sun theory would not only have to become much stronger than it is today, but the greenhouse-gas explanation would simultaneously have to become a lot weaker. That’s hard to imagine. There are so many different lines of evidence — air samples from the deep past, trapped in ancient ice; modern measurements of changing atmospheric chemistry, projections of warming temperatures that have been validated over recent decades; and much, much more — that scientists would have to throw out pretty much everything they’ve learned about climate in order to switch to a new explanation.

That’s not impossible, and climate scientists must — and do — step back every so often and ask themselves, a la Richard Feynman, if they mightn’t be fooling themselves. But given the consistent strengthening of the greenhouse-gas hypothesis over the past couple of decades, that would be a pretty tall order.



Correction, August 3, 2011: An earlier version of this article incorrectly stated the potential impact of cosmic ray activity on the Earth’s climate. The article should have stated that a decrease in the number of cosmic rays penetrating the Earth’s magnetic field could lead to the formation of fewer low-level clouds, which could warm the planet.