1. We have good measurements of the output of the sun over the past few decades and they show that total solar irradiance has varied by only 0.07% in that time. For more on this, see the review by Foukal et al. in Nature. Combined with our estimates of climate sensitivity, this observed increase in solar output cannot produce anywhere near the observed increase in temperature over the past few decades.
2. If the climate turns out to be much more sensitive than previously thought, and solar brightening turns out to be responsible for the temperature increase, one has to explain why increases in CO2 and other greenhouse gases are not also driving large temperature increases. Based on our present understanding, it would be difficult for solar brightening to be driving a large warming without greenhouse gases also significantly contributing.

But what about more exotic explanations, like galactic cosmic rays (GCRs)? The idea here is that GCRs stimulate cloud formation. Since clouds are powerful regulators of our climate, variations in GCRs can cause climate variations.

This is an interesting idea for which there is little evidence, for or against. As a result, the scientific community leaves it an open issue. More data will eventually establish whether this is a reasonable explanation.

In the meantime, however, one has to consider greenhouse gases a much more likely culprit. And if it turns out that GCRs are the dominant cause of the recent temperature rise, one still has to explain why greenhouse gases are not affecting our climate.

Program Note: As you probably know, at present there is no way to get an RSS feed for a particular Gristmill author. As a result, I'm going to start posting links to my new Gristmill stories on my old blog. If you get an RSS feed to that site, you'll be alerted whenever a new grist story by me is released.