A new paper by Italian climate scientist Nicola Scaffeta published in GeoScience Frontiers has reopened the whole discussion about the sun’s role in climate change. Or perhaps we should say has kept it open despite the alarmists’ efforts to hush the matter up, because Scaffeta begins by discussing the ACRIM/PMOD controversy about which we also made a video. The IPCC insists on using a reconstruction of solar activity over the past 150 years that implies it has changed very little, so the sun can’t explain global warming. But other solar reconstructions show that solar output has increased a lot compared to the early 1800s, which if true would mean the sun may be responsible for much of the warming. Scaffeta presents a statistical analysis that looks at what the alternative solar reconstructions imply about the role of CO2 in global warming. But despite all the chatter about “settled science” he also asks whether we even understand the ways the sun interacts with the climate, and once he allows for indirect solar effects in his model, the role of the sun gets much bigger while that of CO2 gets much smaller.
Nobody disputes that solar output varies over time, and the main measure of its strength is called “Total Solar Irradiance” or TSI. Nor does anybody dispute that the change in energy reaching the atmosphere as TSI fluctuates isn’t enough to cause much warming or cooling directly. But the sun also affects the climate indirectly, for instance by affecting cloud formation. And while there are a few theories about how these indirect effects function, it remains unsettled. So Scaffeta argues that rather than try to settle it from first principles, we could just ask what the data tell us.
He estimates a standard statistical model in which the sun affects the climate only through TSI, and he gets the usual result that greenhouse gases explain roughly all the warming over the past century, with an ECS for CO2 of about 2 degrees C, in line with a few other recent studies. Then he estimates a different model in which the sun is allowed to exert both direct and indirect effects. In this case the sun’s role turns out to be over 4 times larger than is usually assumed and most warming over the past century can thereby be explained by the strengthening of solar output. In this case the ECS for CO2 is about 1.8 degrees C. Then when he uses one of the solar reconstructions overlooked by the IPCC which implies a relatively large increase in irradiance since the 1800s, the sun’s role grows even larger and the ECS of CO2 drops to 1.1 degrees C.
From that Scaffeta concludes that scientists really ought to get to work nailing down what the potential indirect mechanisms of solar influence on the climate might be. Sadly such work is hard to get funded, and even when it’s published the IPCC ignores it. But if Scaffeta’s results are correct, we will see far less warming in the coming years than alarmists have predicted, and at some point we’re going to want to know why. Now would be a good time.