by Judith Curry
“The domain of the Sun-climate relationship. . . has in recent years been marred by unwelcome politics and finance as climate change skeptics have seized on hypothetical solar effects as an excuse not to act on warming. up of man” – Lockwood (2012)
“We consider the Sun/climate debate to be one of those issues where the IPCC ‘consensus’ statements were reached early on through quelling dissenting opinions on science. ” – Connolly et al. (In 2021)
The impact of solar variations on climate is uncertain and the subject of considerable debate. However, you will not infer from the IPCC assessment reports that there is considerable debate or uncertainty surrounding this.
The Sun goes through cycles of about 11 years (Schwabe Cycles) in which solar activity goes up and down. In Earth’s atmosphere, the difference in Total Solar Irradiance (TSI, measured in Watts per square meter W/m2) between the 11-year peak and the minimum is small, on the order of 0.1% of the total TSI, or about 1 W/m2. A multifold increase in TSI will cause global warming (all other factors being equal); similarly, a multilevel drop in TSI would cause a global cooling. Researchers have speculated that more diverse and prolonged changes in solar activity could be a key driver of climate change.
Exactly how the TSI has changed over time is a difficult issue to deal with. Since 1978, we have had TSI measurements directly from the satellite. However, the interpretation of any multi-decadal trends in the TSI requires comparison of observations from overlapping satellites. Significant uncertainty exists in TSI composites between 1978 and 1992. This is largely due to the delay of the ACRIM2 solar satellite mission because of the 1986 Challenger space shuttle disaster. (ACRIM2 was finally launched in late 1991). This delay prevented this record from overlapping with the ACRIM1 record that ended in July 1989. The ACRIM gap prevents direct cross-calibration between the high quality ACRIM1 and ACRIM2 TSI records. [link]
This rather complex issue of cross-calibration of two satellite records has profound implications. There are a number of rival composite TSI datasets, which disagree on whether TSI increased or decreased between 1986-1996. Furthermore, TSI satellite recordings are used to calibrate proxy models, so that past solar variations can be inferred from sunspots and cosmological isotope measurements. Velasco Herrera et al. As a result, in 2015, some datasets for past TSI values (since 1750) have low variability, implying a very low impact of solar variations on mean surface temperatures. global, while a dataset with high TSI variability can explain 50-98% of the temperature variation since pre-industrial times.
The IPCC AR5 has approved low-variability solar renewables, undisputed on this controversial issue. AR5 concludes that the best estimate of the radiant force due to the TSI change over the period 1750–2011 is 0.05 W/m2 (average confidence). For reference, the pressure from greenhouse gases in the atmosphere over the same period is 2.29 W/m2. The IPCC AR5 message is therefore that changes in solar activity are almost insignificant compared to the man-made changes to force climate change.
IPCC AR6 acknowledges a much larger range of estimates of changes in TSI over the past few centuries, states that the TSI between the Maunder Minimum (1645–1715) and the second half of the 20th century increased by 0.7–2.7 W/m2, a range that includes both low and high variability TSI datasets. However, the mandatory dataset recommended for CMIP6 climate model simulations used in AR6 has an average of two datasets with low volatility (Matthes et al. 2017).
The uncertainty and debate surrounding solar variations and their impact on climate is the subject of a Climate, a remarkable blogospheric experiment. The ClimateDialogue is the result of a request by the Dutch parliament to facilitate scientific discussions among climate experts representing a wide range of perspectives on the topic. NS Dialogue on Solar Variations (2014) includes five distinguished scientists with well-publicized records on the subject. One participant conforming to IPCC AR5, argued that solar variations are only a minor factor in Earth’s climate. Two participants debated the larger and even dominant role of the Sun, and the other two highlighted uncertainties in our current understanding.
More recently, a review article was published in the journal Research in Astronomy and Astrophysics by Connolly et al. (In 2021). The paper has 23 co-authors with different points of view, but they have reached a consensus not to apply the IPCC consensus approach. Instead, the paper highlights where dissenting scientific opinions exist as well as identifies where scientific consensus exists. The authors found that the Sun/climate debate was an issue that the IPCC consensus statements were soon reached through by quashing dissenting scientific opinions.
Is directly related to the forecast of 21NS century climate is whether we can expect a significant change in solar activity. On multistage time scales, proxy reconstructions of solar activity show irregular phases of unusually high or low solar activity, respectively known as Grand Solar Minima and Maxima (Usoskin et al. , 2014). A major solar maximum occurs when several solar cycles exhibit greater-than-average activity over decades or centuries.
Solar activity reached an unusually high level in the second half of the twentieth century, although there is disagreement among reconstructions as to whether this peak peaked in the 1950s or continued into the 1990s. It is estimated that about 20 great peaks have occurred in the past 11 millennia (Usoskin et al. 2007), averaging once every 500 years. In the past 11 millennia, there have been 11 major solar minima, with periods between them ranging from a hundred years to several thousand years. The most recent major minimum was the Maunder Minimum, during 1645-1715. [link]
There are several reasons to expect lower solar activity in 21NS century, compared to 20NS century. The recently completed 24 solar cycle is the smallest sunspot cycle in 100 years and the third in a trend of descending sunspot cycles. Solar physicists expect period 25 to be even smaller than period 24. Furthermore, a large maximum is more likely to be followed by a larger minimum than another large maximum (Inceoglu) et al., 2016). Empirical projections imply a new solar minimum starting in 2002–2004 and ending in 2063–2075 (Velasco Herrera et al. 2015) It is estimated that there is an 8% chance The Sun falls into the Great Minimum during the next 40 years (Barnard et al. 2011). However, the depth and length of the solar active phase is as low as 21 .NS century is largely uncertain.
If the Sun falls to its minimum in the middle of the year 21NS century of the magnitude of the Maunder Minimum, how much cooling can we expect? Estimates from climate models and other analytical models show that the cooling is small, ranging from 0.09 to 0.3oC (Fuel 2010). These models assume that the solar–climate interaction is limited to the forced TSI.
However, there is growing evidence that other aspects of solar variability amplify the TSI stress or are independent of the TSI stress, known as the solar indirect effect. Candidate processes include: altering solar ultraviolet rays; precipitation particle energy; atmospheric electric field effect on cloud cover; cloud changes produced by solar-modulated galactic cosmic rays; relatively large changes in the magnetic field; and the power of solar victory. The indirect effects of the sun can be classified as ‘known unknowns.’ Although these indirect effects are not included in CMIP6 21NS century forecast, we can make some inferences based on recent publications. Recent research shows that indirect effects of the sun can amplify anomalies in solar insulation by up to 3-7. Shaviv (2008), Scafetta (2013) Svensmark (2019). If such a gain factor is included, the surface temperature is reduced by up to 1oC (or even more) from Maunder Minimum probable.
So, plausible scenarios for a solar-driven global temperature change in 21NS century? These three scenarios cover quite a bit of scope:
- CMIP6 reference scenario: approx -0.1oC (Mathes et.al 2017)
- Intermediate: -0.3oC, which corresponds to a high Maunder minimum estimate without amplification effects (Fuelner 2010), or a weaker minimum with amplification effects
- Height: -0.6oC, a low solar scenario (not a Maunder Minimum) with amplification by indirect effects of the sun Solheim
Observations over the next 20 to 30 years will reveal a lot about the role of the Sun in climate.
The IPCC acknowledges considerable uncertainty in TSI changes over the past centuries, stating that the TSI between the Maunder Minimum (1645–1715) and the second half of the 20th century has increased by 0.7–2.7 W/m2, a range that includes both low and high variability TSI datasets. However, the mandatory dataset recommended for CMIP6 climate model simulations used in AR6 has an average of two datasets with low volatility (Matthes et al. 2017).
The impact of such large uncertainty in the TSI on the equilibrium climate sensitivity and distribution of 20NS century warming is ignored by the IPCC. If data sets with high variability are accurate, this has important implications for estimates of climate sensitivity to CO2 and the distribution of 20NS century warm. This problem cannot continue to be swept under the rug. Other authors do not overlook this. Here are three recent publications to discuss:
Scafetta: Checking CMIP6GCM simulations against surface temperature records from 1980-1990 to 2010-2020 [link]
Connolly et al: How much has the sun influenced temperature trends in the Northern Hemisphere? An ongoing debate [link]
Girma Orssengo: Determining the relationship between the sun and climate using experimental mathematical models for climate data sets. [link]