Claim: It's the sun that is causing global warming.

Why this claim is wrong: A large number of studies all show that the sun, whether by straightforward luminosity, or by UV flux, or by modulating cosmic rays, cannot account for more than a small fraction (if that) of global warming in the past 60 years. The sun's total luminosity does fluctuate slightly with the solar cycle, as magnetic field energy thermalizes and emerges as luminosity. However, it is weak, only 0.1% peak to trough. Since 1957, solar cycle maxima have been slightly decreasing in strength, and the same for solar energy flux. And therefore too, all known variants of solar forcing. Meanwhile, this period of time shows the most rapid and accelerating global temperature rises, in lockstep with the rising and accelerating GHG emissions due to human activities. Here's a good article with graphs and summaries of many published studies on the solar influence on climate. What about the claim that somehow, through unknown physics, solar forcings must be higher than the pure irradiance effect, because look at how severe the "Little Ice Age" was, when the solar cycle virtually disappeared for several cycles? The Little Ice Age appears to have been a coincidence of several verified causes beyond the solar activity effect, including volcanic activity, reforestation pulling CO2 out of the atmosphere after the Black Death. The Columbian Exchange (European diseases introduced to the New World) alone is estimated to have caused global human population to drop 20% during the 1500's and 1600's for example.

Solar activity and hence solar luminosity, in the form of sunspot number, showing a decline since the mid 1950's, while global temperatures accelerate upward.

Solar luminosity translated into global surface temperature forcing, in yellow. A slight increase in the first half, and slight decline in the second half, of the 20th century. Temperatures show halt in mid century due to strong aerosol pollution before the Clean Air Act passed in early 1970's. From Rahmstorff.

Back in 2007, Lockwood and Froelich published a careful study of the solar impact on climate and found that contrary to the denialists, the solar luminosity, when smoothed over the solar cycle effect, shows a consistent decrease over the time. We've had good satellite data on total solar luminosity for about 40 years, preciselyl when temperatures have shown a consistent sharp increase. A short video with good visuals on the solar cycle and solar/climate connection, and the bogus claims of right-wing news that the sun is taking us into a new Ice Age (!)

The denialists will have you believe that global warming is a scam designed to pump up research dollars to the climate scientists. They provide no evidence, nor even any believable rationale, for this claim, only slander. By their logic, who would have more incentive than the Stanford Solar Center (where I worked for a time as a PhD student) to implicate the sun in global warming? Yet, see what they have to say here.


Denialists also claim that other planets are brightening and therefore global warming must be due to the sun. While Neptune is indeed getting brighter - it's been well shown for over a decade that this is a seasonal effect . From this source I quote: "Neptune's nearly constant brightness at low latitudes gives us confidence that what we are seeing is indeed seasonal change, as those changes would be minimal near the equator and most evident at high latitudes where the seasons tend to be more pronounced." (Remember too, that Neptune's seasons last 164 times longer than ours, the warming half of the Neptunian "year" lasts 82 of our years). Denialists will also claim that Mars is warming, based on a 3 year dataset, showing it must be the sun. Climate scientists have debunked the Mars myth as well. Also see here. Mars' "year" is ~2 of our years, has no oceans to buffer temperature, has global dust storms which greatly affect the albedo of the polar cap, and a very elliptical orbit which means solar distance is a major factor in Mars' annual climate, whereas it is not for Earth, which has a nearly circular orbit. The other planets for which global warming is might be claimed are all outer planets with seasonal cycles that last one to many decades - much longer than the one year for the Earth. The planet data is sparse, and consistent with being seasonal effects, and not due to secular changes in the sun's energy. In fact, you can see that several measures of solar luminosity, including high precision satellite data, show only the solar cycle effect, and that the secular trend in luminosity is actually slightly down, as graphs shown here demonstrate.

Some good graphics for the solar myth are in this short video

Update 2015

There has been some legitimate ambiguity about how much solar luminosity may have contributed to Earth warming in the early 20th century, and the suspicion that solar luminosity has been increasing in a secular way since at least the 1600's when records began. The sunspot numbers (Wolf sunspot number) have supported the idea that until 1957, there was increasing solar luminosity then. But getting reliable sunspot numbers is tough because the telescopes of the 17th and 18th century were more primitive than those of today. While this effect has been calibrated out, the reliability of the calibration has some uncertainty. Today we have a much better understanding of how to group sunspots (their magnetic polarity), which was unavailable for the early data, but which can be calibrated during more recent times by comparing the classic Wolf number to the new grouping. The result (Svaalsgard 2012) is that there was a substantial over-estimate of the number of sunspot groups prior to 1855, and the area of spots also was not properly measured. The new calibration of consistent Wolf sunspot number now going back 250 years, shows no overall trend in solar luminosity, although sunspot cycle-to-cycle variations are still there, and increasing solar activity remains in the early 20th century, so some solar effect likely is in the global temperature rise of that time.

Clette and Svaalgard (2015) have extended the Svaalgard (2012) work back for the entire sunspot historical record, and this new calibration shows virtually no long term trend. The Maunder Minimum remains, as do the unusual minima at ~1800, at ~1900, and the declining numbers during the late 20th and early 21st century also remain in the data (the Gleissberg cycle).

The erroneous earlier Wolf number on top, and the re-calibrated Wolf number on the bottom. Note the secular trend disappears.


In Short: Solar luminosity (after smoothing out the 11 yr solar cycle) shows no increase (in fact, a continuing decrease) for the past ~60 years, precisely when climate heating rates are accelerating dramatically, and solar output due to even dramatic shifts in solar magnetic activity, is small compared to the magnitude of global warming and CO2 forcing. This data has been unambiguously clear since the 1991 solar cycle peaked and we finally had two solar maxima of satellite data on solar luminosity, which is when I stopped giving any possible credibility in my astronomy lectures to the solar/climate argument as contributing measurably to global warming. Climate denialists continue to use it for as long as they have an uninformed audience with whom they can get away with it.


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