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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Has the sun been cooling or warming in recent decades?

What the science says...

Various independent measurements of solar activity all confirm the sun has shown a slight cooling trend since 1978.

Climate Myth...

The sun is getting hotter

There is no single continuous satellite measurement of Total Solar Irradiance (TSI). Instead, the data is composited from various satellite measurements. The two most cited composites are PMOD and ACRIM. According to Nicola Scafetta, ACRIM more faithfully reproduces the observations whereas PMOD assumes the published TSI satellite data are wrong and need additional corrections. In particular, PMOD alters the data from the Nimbus7/ERB record from 1989 to 1991. Nimbus7/ERB data during such a short period show a clear upward trend while PMOD during the same period is almost constant. The alteration of the Nimbus7/ERB data is responsible for the different shape between the ACRIM and PMOD TSI composites (Shining More Light on the Solar Factor).

The ACRIM composite shows a slight increase in TSI - the PMOD composite shows a slight decrease. Regardless of which dataset you use, the trend is so slight, solar variations can at most have contributed only a fraction of the current global warming. Scafetta & West 2006 uses the ACRIM composite and finds 50% of warming since 1900 is due to solar variations. However, the warming from solar influence occured primarily in the early 20th century when the sun showed significant warming. As for the global warming trend that began around 1975, Scafetta concludes "since 1975 global warming has occurred much faster than could be reasonably expected from the sun alone."


While the argument over ACRIM vs PMOD has minimal bearing on the global warming debate, determining the more accurate TSI reconstruction is a significant piece in the climate puzzle. The major difference between the two composites is the handling of data between 1989 and 1991. There is a 2 year gap between ACRIM-I and ACRIM-II (tragically due to the Challenger space shuttle explosion). To fill the gap, both composites use the HF data but in dramatically different ways.

Figure 2: PMOD TSI composite (top) versus the ACRIM TSI composite (bottom). Coloured lines give the daily values with the black solid lines giving the 81 day mean.

PMOD applies corrections to the HF data, which has many sudden jumps due to changes in the orientation of the spacecraft and to switch-offs. Figure 2 demonstrates how the HF corrections are responsible for virtually all of the difference between the long-term drifts of the composites.

Figure 3: The difference between the ACRIM and PMOD composites. The grey line gives the daily values, the black line the 81 day running mean. The step in the ACRIM gap during 1989 is clearly seen and is about half the amplitude of the solar cycle variation.

Independent tests of the PMOD and ACRIM composites

So which composite correctly handled the HF data? Does TSI dramatically increase during the HF period as ACRIM supposes and the raw HF data indicates? Or did PMOD get their calibrations right when they adjusted the data to show slight solar cooling over the ACRIM gap? There are a number of independent measurements that can confirm the trend in solar activity over this period.

  • Lee et al. 1995 compares the ERBS satellite data with the Nimbus HF data and found the HF data drifted significantly over the period of the ACRIM gap while the ERBS data shows a slight cooling.
  • Krivova & Solanki 2003 compares TSI to UV levels. UV levels fluctuate more than TSI - a trend would be more visible. As UV correlates closely with TSI, Krivova concludes PMOD is more accurate and there has been little secular trend in TSI over the past few decades.
  • A reconstruction of TSI using sunspot numbers (Krivova 2007) found the minimum of cycle 23 was lower than the minimum of cycle 22, in contrast to the ACRIM composite.
  • Zurich sunspot counts during the ACRIM gap show a slight downward trend consistent with the PMOD recalibrated data.
  • Ground based measurements of solar magnetograms (Wenzler et al. 2006) show higher correlation with PMOD than with ACRIM. More on magnetograms...

Scafetta & Willson and the SATIRE model

In March 2009, one study claimed the ACRIM composite was independently confirmed by the SATIRE model (Scafetta & Willson 2009). This is a model of TSI created by Krivova and Solanki. In response, Krivova and Solanki published ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996? (Krivova et al. 2009).

There are several versions of the SATIRE model, each developed from different data and optimised for different time scales. For periods after 1974, they calculate TSI values based on daily measurements of solar magnetograms. For longer periods going back centuries, they used sunspot numbers to reconstruct TSI. When parsing sunspot data, averages over several months must be used. Therefore, the sunspot model is significantly less accurate than the magnetogram model on short time scales.

Scafetta & Willson 2009 used the sunspot model in their analysis. By design, the sunspot model is suitable for decadal to centennial scales but significantly less accurate on time scales of months. The more appropriate model is based on daily measurements of solar magnetograms. Therefore, Krivova and Solanki take the next logical step and analyse the TSI results from the magnetogram model over the ACRIM gap. What they found was TSI does not increase over this period. Thus the SATIRE model is independent confirmation that the PMOD composite is the more accurate representation of solar activity.

To put things into perspective, the ACRIM vs PMOD debate is essentially arguing over whether the sun is showing a slight upwards trend or a slight downwards trend or if there's even a trend at all. This only underscores the sharp breakdown in correlation between sun and climate since temperatures started rising in the mid 1970's.

Last updated on 9 August 2010 by John Cook. View Archives

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Further reading

  • PMOD vs ACRIM by Tamino (July 2007) crunches the numbers of Willson's ACRIM reconstruction to show even using ACRIM shows a minimal solar influence on global warming in recent decades. The post features a vigorous discussion with Richard Willson heavily involved and links to the original PMOD and ACRIM data.
  • PMOD vs ACRIM (part 2) by Tamino (July 2007) compares the ACRIM and PMOD TSI reconstructions, explaining how sunspot numbers and magnetograms confirm PMOD's reconstruction. Note: the sunspot and TSI graphs come from this page.
  • PMOD (the Physikalisch-Meterologisches Observatorium Davos) detail the procedures used to reconstruct TSI in Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present.


Comments 1 to 16:

  1. The section on independent tests is really good showing multiple measurements and proxies for TSI, with solid agreement. Are you aware of any measurements over the 1990-92 period using radio wavelengths (eg 10.7cm) that would also serve as a calibration? BTW, you wrote Challenge instead of Challenger.
    Response: Fixed the Challenger typo, thanks for the tip.
  2. No reason to overthink this: It's not rocket science (or big $$$$ research obscuring its findings with acronymic codes to confuse common sense) to see that the latter half of the 20th century was a period of increased solar activity unmatched since recording of which began. The peaks may have decreased somewhat in the last 20 years BUT were still far above the average of the 400 year period- making the expectation that this 20 year period should be observing a cooling effect from solar influence patently absurd. To be exact: "Various independent measurements of solar activity all confirm the sun has shown a slight cooling trend since 1978" Is misleading. It is bad science to be sure. While one could say that solar activity has decreased since 1978 it is completely erroneous to assume that it has decreased in comparison to the 400 years of recorded observation- let alone believe that since 1978 we could calculate that a cooling effect on earth climate should be the result.
  3. batvette, as i'm sure anyone will admit, temperature has gone up from the lows during the Maunder Minimum. This post is on the recent trend, where you have slight decrease in solar output but rising temperatures. More on the Maunder Minimum in this recent post
  4. batvette, in addition to what Riccardo replied, you should read Climate time lag.
  5. Climate lag is precisely the reason I would argue it is fallacious logic to assume there should be a pronounced cooling trend anytime soon after the peak around 1950. Indeed, your own link cites 25 to 50 years. Since the peak was nearly right at 1950 and it was of unprecedented in amplitude over the 400 year period since accurate count began, it should be prudent calculate the residual effect, or lag, should have a duration of 50 years per your own reference, not 30. Even if one accepts a 30 year lag the 4 cycles following it still saw the second, third and fifth most active cycles in the 400 years so should be expected to have continued the warming trend- not cooling. It is furter notable that if you look carefully at that chart you see the bottom swings of each cycle actually became more active than they had been around the 1950 peak, so observations would be skewed as well because the (low) cooling years which countered the unpredented high would not have been as much as had been seen before. To expect any significant cooling after 1980 you'd have to show a cycle or even two which had significantly lower activity than the 400 year average, not merely compare it to an unprecedented high that just occurred and whose residual effects would affect the assumed cooling cycle.
    Response: Note that climate lag doesn't mean there is a delayed response to a change in forcing. Climate responds immediately to forcings. The climate lag refers to the time it takes for the climate to reach equilibrium.

    If the sun cooled, the effect on climate would be immediate, a drop in temperature - but it would take years to decades for the climate to eventually reach equilibrium at a stable lower temperature.

    We are not observing this. There is no negative energy imbalance gradually reducing to equilibrium. Instead, we're observing a positive energy imbalance that is gradually increasing over time. We're moving further and further away from equilibrium. The increasing CO2 forcing is causing an increasing positive energy imbalance.
  6. batvette, the severities of the up parts of the cycles are irrelevant, because the down parts of the cycles cancel them. More precisely, the degree to which the up and down parts of the cycles do *not* cancel each other is reflected in irradiance statistics that filter by 11 years by one or another method. Those filtered statistics are the ones that show either flat or slightly down trend since 1950.
  7. A believer in CO2 was telling me that the sun's output is now 30% higher than it was when life evolved on the earth. Is that true? Thank you, Chris Shaker
  8. cjshaker Yes, that is essentially correct, the sun was about 30% dimmer when the Earth first formed 4.6 billion years ago. This is well known. However that process of stellar evolution is so slow it has no effect on the climate on timescales of thousands of years. The only real relevance of this to the AGW is that it explains how a snowball Earth could ocurr with high levels of atmospheric CO2, but that is a subject for a different thread. Note the climate has been much warmer in the past, even with the dimmer Sun, the most feasible explanation is the higher levels of greenhouse gasses in the atmosphere at the time. This is knwn as the Faint Young Sun Paradox. I note that you have been very active today posting a large number of papers for discussion on a wide range of topics. Some constructive criticism: this gives the impression that you are not really interested in the answers, because very few people would be able to hold a worthwhile discussion on so many topics simultaneously. I would advise in future that you stick to a small number of topics at any one time so that you can have an in-depth discussion that science demands.
  9. A good research link to the sun: Dr. Leif Svalgarrd's Research page His current research shows that the TSI has been constant for well over 100 years. Lot's to digest on the link above.
  10. The maximum of solar cycle 24 is proving to be a fiasco. The cycle 25 can't even occur.
  11. That would be undeserved but welcome break for mankind. Let's hope we can make the most of it before the sun returns to normal activity.
  12. True, even a few tenth of a degree would buy us some time. Undeserved but welcome.
  13. Got a question: have you heard of this one:


    I'm sure it's rubbish; the premise is that Judith Lean, the lone solar physicist on the IPCC, had complete control over solar radiation readings. From what you've written above, this seems like tripe, but I'm not so familiar with the field to be sure.

    Your comment?


    [RH] Hot linked URL that was breaking page formatting

  14. cstanyon69 @13, the chapter in question has just one out of 45 sections dealing with solar forcing.  That section reads as follows:

    "2.7.1 Solar Variability
    The estimates of long-term solar irradiance changes used in the TAR (e.g., Hoyt and Schatten, 1993; Lean et al., 1995) have been revised downwards, based on new studies indicating that bright solar faculae likely contributed a smaller irradiance increase since the Maunder Minimum than was originally suggested by the range of brightness in Sun-like stars (Hall and Lockwood, 2004; M. Wang et al., 2005). However, empirical results since the TAR have strengthened the evidence for solar forcing of climate change by identifying detectable tropospheric changes associated with solar variability, including during the solar cycle (Section 9.2; van Loon and Shea, 2000; Douglass and Clader, 2002; Gleisner and Thejll, 2003; Haigh, 2003; Stott et al., 2003; White et al., 2003; Coughlin and Tung, 2004; Labitzke, 2004; Crooks and Gray, 2005). The most likely mechanism is considered to be some combination of direct forcing by changes in total solar irradiance, and indirect effects of ultraviolet (UV) radiation on the stratosphere. Least certain, and under ongoing debate as discussed in the TAR, are indirect effects induced by galactic cosmic rays (e.g., Marsh and Svensmark, 2000a,b; Kristjánsson et al., 2002; Sun and Bradley, 2002)."

    Your linked blog post claims that Judith Lean was the only solar physicist among the lead authors of the chapter.  That may well be true.  There were in fact 15 Coordinating Lead authors or lead authors to the chapter.  If membership in that group was coordinated based on relevant expertise by section, we would expect just 1 in 45 (or 1/3 rd of a lead author) to be solar physicists.  Given the nature of the topics discussed, that means solar physicists are over represented among lead authors.

    Of course, the blog is carefull to not point out that lead authors are not the only authors.  In fact, in addition to the 15 Coordinating Lead Authors and lead authors, there are 37 Contributing Authors.  Given an assumption of proportionality, we would therefore expect approximately 2/3rds of a Contributing Author to be a Solar Physicist.  In fact, there is at least one in the form of S. K. Solanki (and may be others that I do not recognize).  Apparently the existence of at least one other solar physicist was not considered worthy of mention by the author of the blog.

    The author of the blog also claims the section was based primarilly on just one paper, of which Lean was a co-author.  In fact 16 papers are cited, of which only two have Lean as co-authors (Lean et al, 1995; Wang et al, 2005).  For both of these, they are cited in conjunction with another paper of which Lean was not an author to make the point being made - and the first of these is cited because it was previously cited in Assessement Report 3.  None of Solanki's papers are cited in that section.

    It should be noted that 7 other papers with Lean as a coauthor, and two with Solanki as a coauthor are also included in refferences, but if cited, are cited in other sections of the chapter.

    Not content with misrepresenting or concealing the basic facts of the case, the paper also attempts to claim the sun is responsible for recent warming by trotting out the original graph from Friis-Christensen (1991), which has been resoundlingly rebutted by later work, as explained here.  It also includes some slanderous personal communications that attempt to rebut the PMOD composite by ad hominen, but I'll not adress those.

  15. The sun's heat is increased, but it is not the main factor for the increase in temperature. Even though the sun is burning, burning and spreading the sun, and the sun is expanding, it is hotter.

    The world is being heated by the increase in greenhouse gases caused by the burning of fossil fuels and transportation or transportation. Why global warming does not depend on the green house effect.In spite of this, greenhouse gases have increased and global warming is due to the destruction of the ozone layer.

  16. Chanut, the ozone hole does not cause global warming. Why did you say that?

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