A Solar-Cycle Climate Trigger

Most climatologists now agree that Earth is getting warmer — records suggest surface temperatures have climbed about 1.3°F (0.7°C during the last century. But there's less agreement on whether this rise has been triggered by an increase in atmospheric greenhouse gases, a change in the Sun's output, or both.

Solar-constant plot
Spacecraft measurements since the 1970s document the slight change in the solar constant during the Sun's most recent activity cycles. Click on the image for a larger view
Careful spacecraft measurements reveal that Sun's total energy output does fluctuate in step with its 11-year-long activity cycle, but it's a tiny blip: just 0.2 watt per square meter, compared to the solar "constant" of 1,366 W/m² reaching the top of Earth's atmosphere. Part of the problem has been trying to understand how a variation of just 0.01% could possibly affect our climate. (Yea, yea, I know all about a butterfly flapping its wings — but that's chaos theory, not direct cause and effect.)

Now a team of climate modelers led by Gerald Meehl (National Center for Atmospheric Research) believe they've found the key to how small solar variations can be amplified enough to alter our climate. In Science for August 28th, they argue that it's a consequence of two factors acting together.

First, the "top down" effect: When the Sun waxes during solar maximum, its ultraviolet output varies by as much as a few percent. This extra energy gets absorbed by the stratosphere, which warms up and creates more ozone (O&sub3;). The warmer stratosphere, primarily over the tropics, alters the circulation lower down in the troposphere and, in particular, more rainfall at latitudes farther from the equator.

Sunlight warms the Pacific
Sunlight beams down on the Pacific Ocean and the Andes in this image snapped by astronauts aboard the Space Shuttle Discovery in 1990.
NASA
Second, the "bottom up" effect: When stronger sunlight passes through relatively cloud-free tropical regions, the ocean surface warms up. It's slight, only about 0.2°F, but enough to cause more water to evaporate. This extra moisture gets carried by trade winds poleward to the Intertropical Convergence Zones, where it rains like crazy.

Both of these factors have been known for some time, but neither seems capable of driving climatic change. The warmer oceans, for example, would seem to require a cyclic change of 0.5 W/m². And while you can see how the two effects might be linked, curiously no single atmospheric model had incorporated both of them — until Meehl's team came along. They actually needed three computer models to draw out the connection. One analyzed interactions between sea-surface temperatures and the lower atmosphere, while a second simulated the stratosphere's ozone response.

It was the third model, which combined features of the other two for the first time, that revealed how the tropical Pacific's response during solar maxima mimicked the actual climatic observations. "The Sun, the stratosphere, and the oceans are connected," Meehl comments in an NCAR press release, "in ways that can influence events such as winter rainfall in North America."

Meehl and his team will have to wait a while to see how their predictions pan out during coming solar maximum, and even then it's likely to be a weak peak. The Sun appears hopelessly stuck in its deep, lasting minimum. According to Texas amateur Tom Fleming, who heads a small but dedicated group of solar observers, the Sun has been completely spotless for 22 weeks so far this year.

13 thoughts on “A Solar-Cycle Climate Trigger

  1. Phil

    Interesting. This research shows that very tiny changes in system input may have unexpectedly far outsized effects. Climate, like many other natural systems, seems to have many destabilizing positive feedback loops and few stabilizing negative feedback loops. I wonder if the anti-climate change crowd would care about this? Nah. They’d just say it’s not worth controlling human inputs into climate, since the natural ones vary so much.

  2. Robert

    As a research meteorologist as well as someone who cares about our environment, it is well to consider that the heat carried in the oceans is many times that carried in the air. In other words, tiny solar variations aside, the oceans are the 900 lb gorilla of the climate system. Unfortunately, the interactions of the oceans with the atmosphere are not well understood. What we do know is that Tropical sea surface temperatures change on multi-decadal scales that have little or nothing to do with anthropogenic CO2 emissions.

    That being said, there are many reasons why we shouldn’t burn all the coal & oil we can get. Pollution is one, the destruction of West Virginia (visible on Google Earth) is another. Who really wants to keep giving the arabs all that $$ anyway?
    I really believe that the solar variations are too small to account for the observed warming, but they might explain 10 – 20% of the change. The rest is divided between ocean heat content and greenhouse gas emission, but no one yet knows how much to ascribe to either of these processes.

  3. Giesterfarher

    Now the researchers need to add a third model to the mix. The one that shows the correlation between solar sunspots, cosmic radiation and cloud formation.

  4. George Chandler

    I hate to be a nitpicker but it looks to me like the peak to peak variation in solar irradiance in the chart you published is a little over 1 watt per square meter, or .07%, whereas in the text you say the variation is about 0.2 watt per square meter, or .01%. Are you getting the value in the text from another source? Later you say a .5 watt per square meter change would be enough to drive the bottom up effect – if so maybe you’ve got it.

  5. Nathaniel Sailor

    I belive humans are to blame for global warming. But the sun producing more energy can amplify those effects. Even more reason to get off of fossil fuels.

  6. Anders

    Nathaniel Sailor’s posting, “Sun adding to global warming,” states that he “believe(s) humans are to blame for global warming,” but presents no evidence to support his belief. Clearly with him, global warming is religion and not science. He must be a disciple of the messiah Al Gore. The S&T article supports the opposite point of view, that climate change (“global warming” to the true believers) is a natural consequence of natural processes, and that the climate models still require further modification before we can safely rely on them. It is far too early to sacrifice our future economic development to the beliefs of the brain-washed.

  7. Anders

    Nathaniel Sailor’s posting, “Sun adding to global warming,” states that he “believe(s) humans are to blame for global warming,” but presents no evidence to support his belief. Clearly with him, global warming is religion and not science. He must be a disciple of the messiah Al Gore. The S&T article supports the opposite point of view, that climate change (“global warming” to the true believers) is a natural consequence of natural processes, and that the climate models still require further modification before we can safely rely on them. It is far too early to sacrifice our future economic development to the beliefs of the brain-washed.

  8. WV Geologist

    This might be a bit tangent to the discussion, but…
    As a Coal Geologist in West Virginia, I usually do not sate my stance in AGW and I will not today. Science has lead to my beliefs but my career might be seen as biasing those beliefs.
    But another issue was touched on above in a post stating that Fossil Fuels cause “the destruction of West Virginia (visible on Google Earth).” While it is true that current mining is visible on Google earth, I assure you that land surface mined 10 years ago is not; it has been reclaimed. As with AGW theory, you shouldn’t believe everything spewed out by the media on the subject of Mountain Top Removal. There are many Scientist (Geologist, Engineers, Chemist…) that work every day to minimize the environmental impact of the large earth moving mining operations responsible for 50% of America’s energy. While Google earth may show 2-3 current mining operations and the so called “Destruction of Mother Earth” they fail to show the hundreds of reclaimed surface mines. You wouldn’t know what you were looking seen on Google earth anyways.
    Sorry about the rant but this is a very hot topic here in WV. I am just trying to keep misinformation from spreading.

  9. Robert Hunter

    re: web article “A Solar-Cycle Climate Trigger”

    Please check your decimal places in the 2nd paragraph (two times; 0.1% vs 0.01% and 2 watts per m2 vs 0.2 watts/m2).

    best regards,

    Robert

  10. Stan Kerns

    I had a friend who was head of Yale’s museum while still in his 20’s–he found a fossil hominid where nobody though there should be any–and it ruined his career–I hope Meehl fares better–The thought that a less than 3% increase in CO2 total production could effect much of anything is ludicrous–especially in light of the documented climate swings in the past few hundred thousand years–not to mention Mars is growing warmer.

  11. mikeemmert

    Just comparing the Milankovich cycles (which can be calculated to several decimal points) to the actual change they produce should make us wary of creating artificial climate change. It may be that extra carbon dioxide also changes climate more than you might think.

    I disagree that addressing climate change will neccessarily produce economic disruption. Quite the opposite; building all those wind & nuke plants will generate economic activity which will be positive. It’s “new money”. I sometimes think that climate change deniers represent “old money”.

    People are ready, it’ll sell. They didn’t expect “Cash for Clunkers” to work, but it’s effect was more than expeceted.

  12. mikeemmert

    Just comparing the Milankovich cycles (which can be calculated to several decimal points) to the actual change they produce should make us wary of creating artificial climate change. It may be that extra carbon dioxide also changes climate more than you might think.

    I disagree that addressing climate change will neccessarily produce economic disruption. Quite the opposite; building all those wind & nuke plants will generate economic activity which will be positive. It’s “new money”. I sometimes think that climate change deniers represent “old money”.

    People are ready, it’ll sell. They didn’t expect “Cash for Clunkers” to work, but it’s effect was more than expeceted.

COMMENT