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	It's the sun
	Climate's changed before
	There is no consensus
	Surface temp is unreliable
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	It's cooling
	Ice age predicted in the 70's
	Al Gore got it wrong
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ACRIM vs PMOD - Is the sun getting hotter?

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 Challenge 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...

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.