Open Mind

Here Comes the Sun

January 22, 2007 · 9 Comments

For those who don’t deny global warming but deny that it’s man-made, probably the most popular alternative theory to explain it is solar variations.

Its appeal is obvious. Earth’s climate gets it energy from the sun! If the sun gets hotter, then of course the earth will too. This isn’t complex physics involving the selective absorption of infrared radiation by trace gases in the atmosphere — it’s something everybody can understand, and is so obvious you don’t need Al Gore to explain it.

Solar variation is so popular as an “alternative” to global warming, that there is a bewildering array of theories about it. So I’ll address the topic in multiple posts.

The most obvious candidate for a solar influence on climate is the total energy output of the sun; this is called total solar irradiance (TSI). TSI is notoriously hard to pin down; it has been measured by satellites, but only for the last 25 years or so. Nonetheless it can been estimated using a number of evidences (proxies), including “cosmogenic isotope abundances” and the strength of the sunspot cycle. Perhaps the most-used estimate of TSI for the last several centuries is from Lean (2000, Geophysical Research Letters, 27, 2425).

If we look at the graph of global average temperature over the last century and a half, the rise in temperature is rather obvious (data are from the Hadley Centre/Climate Research Unit (HadCRU)):


Likewise, if we look at the graph for TSI over the last century and a half there is a similar marked rise. The squiggly line is the actual estimate of TSI; there is a fluctuation with period about 11 years, due to the roughly 11-year sunspot cycle. The thick line is a smoothed version (using a low-pass filter) to show the overall trend:


There’s certainly some similarity in the change of temperature and the change in TSI. To quote from Al Gore’s film An Inconvenient Truth, “Did they ever fit together?”

One of the problems with this idea is that no, they don’t fit together. We can see this by plotting them on the same graph:


They both show a rise from about 1915 to 1945. But since then, TSI has not shown any appreciable increase, while temperature has risen dramatically. So solar variation (change in TSI) may explain part of the rise in the early 20th century, but cannot explain the rise in the modern global warming era (1975 to the present).

Another problem with using TSI to explain global warming is that the change in TSI just isn’t big enough. The change in average TSI throughout the 20th century is no more than 1.6 watts per square meter (W/m^2). But this does not mean that solar variation is contributing a “climate forcing” of 1.6 W/m^2. That’s because TSI measures the intensity of the sun, but the sun’s energy is spread out over the whole surface of the earth. The earth intercepts solar energy according to its cross-sectional area, but that energy is spread over the earth’s surface area. The surface area of a sphere is exactly four times its cross-sectional area, so the global average solar input to the climate system is one fourth of the solar irradiance. That means that the change in TSI in the 20th century creates a climate forcing of at most 1.6/4 = 0.4 W/m^2.

And not all that energy even gets into the system. A certain fraction of incoming solar energy is reflected right back to space, never entering the climate system at all; this is called the albedo of the earth. Earth’s albedo is about 0.31, so 31% is reflected away, only 69% of incoming solar energy is absorbed. This means that of the 0.4 W/m^2 of “forcing” from 20th-century change in TSI, only 69% — about 0.28 W/m^2 — actually generates a “climate forcing.” The climate forcing due to a doubling of CO2 is much larger, about 4 W/m^2.

The climate sensitivity to forcing is the change in global average temperature caused by a change in climate forcing. A good estimate of sensitivity (including feedback effects) is 2/3 of a degree Celsius for every 1 W/m^2 of forcing. So, the change caused by the 0.28 W/m^2 of climate forcing from 20th century solar variation is about 0.28 x 0.67 = 0.18 deg.C. That’s not nearly enough to explain the observed temperature change in the 20th century.

In fact solar variation is one of the causes of the temperature increase in the early 20th century. Climate scientists (the ones who are warning us about man-made global warming) acknowledge that the sun is an important influence in climate. Denialists want you to believe that they either refuse to do so, are too ignorant to realize it, or underestimate it by a vast amount. In fact, unless we take solar variation into account, computer models aren’t able to match the temperature changes we’ve already observed; solar variation is an integral part of climate science, and is not neglected or underestimated as denialists would have you believe.

But solar variation doesn’t even explain all of the temperature change in the early 20th century, there are other factors at work as well. One of the factors is volcanic activity; there was an unusual lull in volcanic activity during the early 20th century, leading to a less-than-average concentration of “volcanic aerosols” and their cooling effect. Another factor in early 20th-century warming is: greenhouse gases (they weren’t nearly as great back then as they are today, but they were still there, in greater concentration than during pre-industrial times). All these factors conspired to warm the earth in the early part of the 20th century.

The total inadequacy of increased TSI to explain modern global warming has led some to claim that the effect of changes in solar output on climate are amplified far beyond the usual estimates of the effects of feedback mechanisms, and/or that the primary influence of the sun on climate is through mechanisms other than the sun’s total energy output. These are topics for future posts…

Categories: Global Warming

9 responses so far ↓

  • Yelling in the fog // January 22, 2007 at 2:53 pm

    Tamino, interesting post and I think it shows the influence of solar very well (better than I have ever seen it done).

    I will note that as opposed to the paper that you quote (Lean, 2000), another widely quoted one is Wilson, 2003 which shows a slight but significant rise in the solar output. The difference between Wilson and Lean is how they interpret the early satellite records.

    If you use Wilson’s data you get an increase in solar influence during the last 20 years - albeit not a large one. Consequently it was a favourite used by the deniers for a while.

    I confess that I don’t know how things stand now, does anyone have an update on the status of this dispute?

    [Response: Indeed! The work of Willson is about the satellite estimates of TSI, while the Lean data are an historical reconstruction covering the period of sunspot observations. The satellite estimates come from a number of different satellites, and most of the disagreement centers on how to patch their data together. The ACRIM composite (from Willson) shows a small increase in TSI lately, while the PMOD composite shows none. But ... that is the topic of a future post.]

  • kennebecriver // January 22, 2007 at 3:53 pm

    Another excellent, informative post. Thanks.

    As a side note on the “skeptic” front — it seems disingenuous or at least illogical to cite the reality of one known type of climate forcing (TSI) while dismissing the existence of another forcing (GHG); and to acknowledge the impact of natural GHG while dismissing the impact of human-caused GHG. Oh well.

    [Response: that's not true of all solar advocates. For example, Scafetta & West have made some claims about the response to solar variation which in my opinion are not correct, greatly exaggerating the solar influence. However, they do not deny human influence; I think their latest estimate is that solar variation is responsible for 40% of modern global warming, leaving 60% to other factors (including human influence).

    However, it's all too true of many non-scientist solar advocates.]

  • Hans Erren // January 22, 2007 at 4:00 pm

    more on solar proxies here.

    And its not only TSI that determines solar activity. We have seen some really big solar particle storms recently.

  • Andrew Dodds // January 23, 2007 at 1:56 pm

    Hans -

    A solar contribution to temperature is known and accounted for; is there any particular evidence that the sun is primarialy responsable for the recent warming (since 1975)? I find a lot of the stuff on the climateaudit site (and especially your sire) very disjointed..

  • nanny_govt_sucks // January 23, 2007 at 8:25 pm

    A solar contribution to temperature is known and accounted for;

    Andrew, check out the “level of scientific understanding” of solar forcing, according to the UN IPCC:

  • Catherine Morgan // January 24, 2007 at 3:44 am

    Hi “tamino”….just so you know your site is listed in my links under “good blog for global warming”….hope thats o.k. You seem to have a lot of great information here. If you get a chance check out my last comment on my global warming post….I would love to know if you agree.

    But, the reason I am writing is; I heard something today about “geo-thermal energy”, I never heard of it before but apparently it has been around for awhile. I Googled it and found some info….so I now know what it is. Apparently it can be used as a substitute for nuclear and coal energy. Sounds like a good thing….but I was wondering… this something that would help solve some global warming issues or contribute more to it??? Thought you might know???

    [Response: I don't know much about geothermal, but I can tell you that the geothermal contribution to the energy budget of earth's climate system is negligible. So my best guess is that it's a useful form of energy for human consumption, and doesn't contribute to warming the surface or to greenhouse gases. However, as far as I know it's not easy to get everywhere. Places like Iceland, which is situated on the mid-Atlantic ridge, have easy access to it, and I believe Iceland makes good use of geothermal (but I'm not really sure). Readers: anybody know more about this?]

  • Dano // January 24, 2007 at 5:15 am

    Geothermal in itself is benign, but the manufacturing processes to make the tubing and dig the trenches and install are not benign.

    The key is [goober caveat: I've built strawbale before and my next house after the next one will be strawbale] to calculate carbon footprint and efficiency: if you are geothermally heating water to radiant heat a huge concrete slab, that’s carbon positive. If you are using ICFs and polystyrene forms for your foundation and earth-friendly materials (perhaps SIPs), then you might be OK.

    Take away message: 5 minutes with The Google isn’t enough.



  • Andrew Dodds // January 24, 2007 at 8:44 am

    NGS -

    What’s the problem? Quite apart from the picture being based on data that must be getting on for a decade old, the fact that the effect is taken into account and has relevant error bars makes the original point; it’s an effect but smaller than that of GHGs.

    Catherine -

    It depends. There are some highly volcanic areas, such as iceland, where geothermal works very well - where the geothermal gradient is high enough that a shallow borehole gets very hot water. Unfortunately, over most of the planet the borehole would have to be many kilometers deep, which cancels out any benefit.

    These systems also suffer from mineralisation and the fact that the hot rocks cool over time, since rocks are such bad conductors of heat.

  • Victor de Vries // March 16, 2007 at 12:40 pm

    Thanks for your post. You show it clear and simple , but I think too simple.

    I don’t deny man-made effect on global warming but the influence of the sun is rather complex.

    My first point is the irrediance spectrum. Chances in solar irradiance due tot sunspotscycles occurs typical in the higher frequences as blue and UV. The most of the extra sun-energie will be absorpt by stratopheric ozon and cannot be reflected by earth and clouds. There’s a growing evidence for changing stratospheric circulations due to sun-activity. Furthermore should earth-albedo changing during the sunspot cycle. That’s because the albedo of a coloured body depends on the light that shines on it. A blue body reflects higher frequences of the light better than low frequences, that’s why we called that body ‘blue’. A red body reflects the larger wavenumbers better. Hence, a red body, like Mars gets a lower albedo during periodes of higher sun-activity and becomes warmer. This effect should also possible on earth, especially on rocky- and sandy landscapes.

    My second point is that you use the same sensitivity for all forcings. Because the different global contributions of different forcings it seems to me that you can’t use the same sensitivity. Simply, TSI warms the tropics most and GHG’s warms overall, voor CO2 especially on higher latitudes. These forcings will not trigger the same feedback effects.

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