01 Aug 2011: Analysis

Probing the Role of the Sun
In an Era of Global Warming

Some skeptics have suggested the real culprit behind rising temperatures is increased solar activity. But a wide variety of data and experiments still provide no solid evidence to refute the scientific consensus that greenhouse gas emissions are the major reason the planet is heating up.

by michael d. lemonick

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.

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
Scientists have looked hard at how the Sun might be playing a part in the current episode of climate change.
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, 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
Since the 1970s, there has been no upward trend in the sun’s brightness.
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.

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

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


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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.

POSTED ON 01 Aug 2011 IN Business & Innovation Climate Climate Energy Policy & Politics Science & Technology Asia 


Really? Even though NASA has admitted that its satellites have detected more heat leaving the Earth than arriving?

Posted by William Hoy on 01 Aug 2011

How strong is the warming impact of rising man-generated CO2 without including any theoretical positive feedback as compared to the fluctuations in the Sun Spot Activity and Solar Energy Output?

Isn't the entire concern over man generated CO2 the result of models saying that the man generated CO2 could be the straw that breaks the camel's back?

Posted by Dialla Ingalis on 01 Aug 2011

You can't be serious. No solid evidence that the sun is effecting the climate.

Can you produce just one piece of solid evidence (not failed computer models) that supports the hypotheses that the small temperature increase from CO2 would result in more water vapour leading to positive feedback (more warming) rather than negative feedback (cooling from increased cloud cover)?

This is what it's all about (which most people don't realise). No-one disputes that an increase in CO2 could result in a small degree of direct warming - that's not the issue.

Posted by R James on 02 Aug 2011

Michael, just to note that in its earlier days the Earth was indeed frozen, and more than once ("snowball Earth").

Nice try, William.

Posted by Steve Bloom on 02 Aug 2011

Don't worry, it will be cooled by wind turbines!

Dr.A.jagadeesh Nellore(AP),India

Posted by Dr.A.Jagadeesh on 02 Aug 2011

There is that word again. "Consensus"

I might point out that is difficult to provide hard evidence against anything unless there is hard evidence for something. It may exist but I can't find any.

Posted by pesadia on 02 Aug 2011

A very poor article (especially when it relies on a quote by Mann!). A bit of straw man mixed with appeals to authority along with some ad hom! And this from a "science writer"!

Getting nervous about the outcome of the CLOUD experiment Micheal, after all, all you have is non empirical models!?

Posted by Pete H on 02 Aug 2011

Gases in a glass jar can trap heat, not in an open system. Lets just throw away the laws of thermodynamics and make it easier for mann et al.

Posted by vibes on 02 Aug 2011

1. No organization expressly created to defend a theory is nonpartisan! His job is to dismiss alternative ideas. Does his effort amount to anything more than pseudo-scientific nonsense?

2. The correlation between solar activity and temperature is far better than the correlation between CO2 concentration and temperature. Just because there was no good explanation for this observation is no excuse to dismiss the importance of our Sun.

With Henrik Svensmark theory of cosmic rays, a possible explanation has now been provided. While evidence for the domination of the greenhouse theory remains weak, observational support for Svensmark 's theory grows steadily.
Highlights: Good correlation between solar activity - magnetic flux/solar wind and cosmic ray concentration in the lower atmosphere; good correlation between cosmic ray concentration and lower clould coverage; inverse correlation between cloud coverage and temperature. Now it has been demonstrated in a laboratory that high energy particles, such as cosmic rays, can produce aerosols, the precursor to clould formation.

For those who are interested in Svensmark's theory, can form their own opinion by watching a 52 min tape on You Tube - Henrik Svensmark: the cloud mystery.

Posted by peter bartner on 02 Aug 2011

Here's a full-bore crock:

"projections of warming temperatures that have been validated over recent decades"

What 'validation'? Doomsday cries that didn't pan out? Secretly 'adjusted' temperature and sea-level records? Editorial rejection of critical papers?

Perhaps you mean Hansen's 1980 shrieking that 2010 sea level would be 5 feet higher, and world temperatures 5 deg higher? Down the memory hole!

Those so-called 'projections' are nothing but fantasies embodied in jiggered computer programs 'modeling' a fictitious flat earth with constant average sun, scribbled by government-bribed pseudo-scientist flunkies who'd be unemployed in the real world.

The so-called 'scientific consensus' was totally bought and paid for by the printing presses of a bankrupt socialist government.
Posted by Bill the Disbeliever on 02 Aug 2011

I have very little faith in your abilities to communicate climate science when you cannot even get Svenmark's theory correct.

Svenmark says that the cosmic ray induced cloud formation results in cooling by shielding the sun's rays, whereas you imply warming because you say that the clouds are heat trapping.

As a science writer and especially as one who teaches other's the trade, you should be a little more objective and apply the same rules of probability to both sides of the argument. Correlation between Co2 and temperature can only be shown over a twenty year period. Correlation between temperature and the sun's activity goes back to at least the medieval warm period, though perhaps if you are a disciple of Mann, you may not even accept the existence of the medieval warming period.

I reiterate your own advice. Remember Feynman.

Posted by Colin Porter on 03 Aug 2011

Good point, Steve Bloom. Although there's some dispute over whether the "Snowball Earth" episode literally froze all the water on the planet, you're quite right that without greenhouse gases, Earth would far, far colder than it is.

I won't address all the other comments-- especially those delivered with sarcasm or a sneer, because they're not worth responding to. But Peter Bartner's point 1, above, strikes me as being inaccurate. No organization was created to "defend a theory." If he's talking about the IPCC, it was created to assemble and synthesize research on the topic of climate change. Big difference, that.

For those who suggest that the cosmic ray hypothesis has overturned the greenhouse-gas theory simply by virtue of not being ruled out, even Svensmark's collaborator, quoted in my piece, doesn't make that assertion. Why the commenters here think they know more than he does escapes me completely.

Posted by Mike Lemonick on 03 Aug 2011

Colin Porter, another reader pointed out the error, but in his case, in a gentlemanly way, and
I've corrected it.

The assertion that temperature and CO2 can only be correlated for the past 20 years is demonstrably false, and has been refuted so many times it's a staple of FAQ lists.

I'm not a "disciple of Mann," because I don't consider these to be religious questions we're discussing.

As for Feynman, his statement shouldn't be interpreted to mean "everything scientists have concluded about the world is automatically wrong."

To Bill the Disbeliever, I can only say that if you start with a false premise, you can prove anything you like. Even assuming you're accurately relaying Hansen's views at the time, false premise here is that any single prediction by a single scientist or model represents the overall consensus. That's what the IPCC was formed to do: take all of the various projections, figure out what the overall consensus was, and report it. The temperature increases seen so far are perfectly consistent with the non-doomsday scenarios climate scientists were projecting in the late 80's, as reported in my TIME cover story on the topic in 1987.

Posted by Mike Lemonick on 03 Aug 2011


I'm not sure why I would be nervous about the CLOUD experiments, or nervous about any scientific result (unless it proved that skin cancer is caused primarily by wearing cotton shirts or something along those lines). I'm not emotionally wedded to any particular scientific result.

Professionally, I'd love to be the one to break a story convincingly refuting the greenhouse gas explanation for climate change, if that should happen.

I do find the phrase "all you have is non-empirical models" kind of odd, though. First, I neither have nor have models or experiments of any kind; I just report about them.

Second, it was proven empirically that carbon dioxide absorbs infrared radiation way back in the 1800's, and that CO2 is increasing in the atmosphere no later than the 1950's. So I'm not quite sure what you're getting at.

Posted by Mike Lemonick on 04 Aug 2011

The article demonstrates the beauty of science, a self-correcting search for the truth that demands skepticism about announced possibilities for explanations of some phenomena and the high standards which must be met by un-self-serving skeptics. Even after all that, the currently accepted "truth" is only seen as un-disproven.

The bickering in the commentary is mostly unhelpful in establishing what we can use as momentary additions or subtractions to what we think we know. If we are to establish collective objectivity, then attacking another's speculations or purported facts is counter-productive.
Posted by Burton Mac-Holmes on 04 Aug 2011

Too bad to see the commentary here heading for the gutter. Used to follow the climate discussion at Accuweather but then it got way too polarized and nasty. In the end this is about science which is a process that is designed from the get-go to take on criticism and self-correct, not opinions, self interested or otherwise, that are designed from the get-go to support belief systems and ideologies despite the evidence. Tobacco companies learned how to do it decades ago and today's climate discussions have the same familiar smell.

Posted by Adam on 06 Aug 2011

To R James:

This is just one example of one of a few studies that are starting to show that co2 emissions are causing less low-altitude cloud cover, increasing global warming.

Posted by Bob on 06 Aug 2011

"the greenhouse-gas explanation would simultaneously have to become a lot weaker. "

Evidence that this is the case is actually coming out from Murray Salby. Go to Judith Curry's blog and listen to the podcast to decide for yourself.

"Salby’s argument is that the usual evidence given for the rise in CO2 being man-made is mistaken....He suggests that its warmth which tends to produce more CO2, rather than vice versa – which, incidentally is the story of the past recoveries from ice ages."


Posted by David McMahon on 08 Aug 2011

I think the missing element in the models of climate change has to do with Gravity. - The Milankovitch Cycles are always looked at [on both sides of the argument] only in the perspective of sun light intensity. No one looks at the fluctuations in Earths gravity due to the Sun or the other Planets in our system. [Nor that of periodic Comets passing by.] - We know gravity has a HUGE effect on Earths environment by the simple fact that the tides follow our relatively small moon. We also know that Sea Levels vary slightly due to the density of the nearby land masses. Stands to reason that the Sun's Gravity as felt by the Earth is going to vary in sync with the Milankovitch Cycles just as the Sun Light intensity does. That could have a huge affect Ocean Currents and Tides. - No one is looking at that and we only -really- began looking looking at Earths Gravity fluctuations in the last 10 years. - Thanks for your time. - SJJ

Posted by PCBONEZ on 05 Sep 2011

Regarding the hypothesis that warming drives carbon dioxide levels: In all of the graphs I have looked at, which trend - carbon dioxide levels or temperatures - leads the other is inconsistent. This suggests that it is a feedback loop; change the balance on one, and the other follows.

In our times, there is a simple enough explanation for why higher temperatures would lead to more carbon dioxide: Much of the excess carbon dioxide generated since the Industrial Revolution has been dissolving into the oceans forming carbonic acid. As temperatures rise, solubility decreases.

Posted by Daniel Gilsdorf on 13 Sep 2011

Comments have been closed on this feature.
michael d. lemonickABOUT THE AUTHOR
Michael D. Lemonick is the senior writer at Climate Central, a nonpartisan organization whose mission is to communicate climate science to the public. Prior to joining Climate Central, he was a senior writer at Time magazine, where he covered science and the environment for more than 20 years. He has also written four books on astronomical topics and has taught science journalism at Princeton University for the past decade. In other articles for Yale Environment 360, Lemonick has written about the effect of clouds on climate change and how satellite technology is used to track melting ice.



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