It's not the Sun

The graph above shows annual values and 11-year running means for TSI[1] and GMST anomalies[2] from HadCRUT5. It's very clear that 11-year running means for TSI have varied by very little. The brightest years differ from the dimmest years by only about 0.5 W/m^2, and some of the dimmest years have begun after 2010. However, GMST began to warm around 1910, and after a brief pause from 1940-1970, GMST has increased by an an average of 0.2°C/decade. There is not much evidence for correlation in this graph. If I plot both so that they are similar from 1850 to 1910, they part ways a little after 1910 and then diverge in trend after about 1960.

Of course, you could argue that I've chosen a scale for TSI to exaggerate the differences, though this could only potentially be true from 1910-1960, but we can get past this objection by plotting annual TSI values on the x-axis and annual GMST anomalies on the y-axis. Here it's clear that annual TSI is a terrible predictor of GMST anomalies. I suppose I could make this look a little better if I plotted 11-year means for TSI and GMST, but then we run into autocorrelation issues. 
Furthermore, if warming was caused by solar variability, we would see the stratosphere warming with the troposphere, since sunlight must travel through the stratosphere to reach the troposphere. But this is not what is happening. The stratosphere has been cooling since 1979, since more thermal radiation is trapped in the warming troposphere. Satellite observations since 1979 are incompatible with the hypothesis that warming is caused by the Sun and essentially falsify they hypothesis that solar variability is responsible for any significant fraction of current warming.
And we also have the problem that GMST does not respond to TSI specifically but to the fraction of TSI that is absorbed by the earth. Absorbed solar radiation (ASR) can be calculated as ASR = TSI*(1-α)/4, where α is Earth's albedo. Since Earth's albedo is about 0.3, we can compare trends in variability in solar forcings and CO2 forcings to see the relative influence of ASR and CO2 forcings. In the graph above, I set all three (GMST and CO2 and solar forcings) to a 1850-1900 baseline. These forcings are directly proportional to changes in GMST, so this allows a clear comparison of the relative effects of ASR and CO2. Clearly the temperature response is following the CO2 curve and largely ignoring the very small variability in solar forcings.
If I plot CO2 and GMST anomalies, I get a very strong correlation, with an r^2 of 0.89 for the relationship between GMST and ln(rCO2) in HadCRUT5 since 1850. The correlation is a little less strong for GloSAT, since uncertainties are larger prior to 1850. This is very much in line with what we would expect if CO2 is the primary driver of global warming. Of course correlation itself is not proof of causation, but there are many studies demonstrating causation independently from correlation. We can rule out the possibility that the Sun is driving current warming.

Some of you may object that ASR has increased since 2000 because of decreasing albedo over that time frame, and this has been the result of reductions of cloud cover (and snow/ice cover). However, albedo does not change by itself. As I've shown in other posts, cloud cover changes in response to forcings. It can not be a driver of global warming. We know that decreasing cloud cover is occurring as a response to both warming temperatures and reductions in aerosol pollution. And reductions in aerosol pollution are removing some of the masking effect that aerosols have had on GHG-induced warming. Decreasing snow/ice cover is also a direct result of warming. Neither can be external forcings driving current warming. So the long term trend in decreasing albedo must be seen as positive feedback to warming and reductions in aerosols. It is not and cannot be evidence that current warming is caused by the Sun.


References:

[1] Vaughan R. Pratt, Formal centennial climate, Stanford University. December, 2024. http://clim.stanford.edu/MATLAB/CentennialCC/

[2] Morice, C. P., Kennedy, J. J., Rayner, N. A., Winn, J. P., Hogan, E., Killick, R. E., et al. (2021). An updated assessment of near-surface temperature change from 1850: the HadCRUT5 data set. Journal of Geophysical Research: Atmospheres, 126, e2019JD032361. https://doi.org/10.1029/2019JD032361


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