Arguments
Has the sun been cooling or warming in recent decades?
The skeptic argument...
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).
What the science says...
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 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."
ACRIM vs PMOD
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 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 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 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 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 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 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.
BTW, you wrote Challenge instead of Challenger.
http://science.nasa.gov/.../maunderminimum.jpg
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.
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
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.
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.
Thank you,
Chris Shaker
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.
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.
http://solarscience.msfc.nasa.gov/predict.shtml
http://www.skyandtelescope.com/news/123844859.html