Solar variability has a small effect on climate change

The Earth wouldn't have much of a climate if it weren't for the Sun. But it's a different thing entirely to conclude that because of its essential role the Sun contributes significantly to climate change. To alter the climate, the amount of energy sent our way by the Sun would have to vary significantly. And most studies have found that, while the Sun's output does vary, it hasn't seemed to have changed enough to have left a mark on the recent climate record.

But a few studies have suggested that the Sun's influence may be much larger. In fact, the range of estimates differ by an order of magnitude. One of the high-end estimates attempted to infer historic solar activity based on an examination of the details of the Sun that we can currently observe. And that, as its title suggests, "leads to large historical solar forcing."

 A team from the University of Edinburgh decided to figure out if that actually made any sense. So, they compared a climate model's output with reconstructions of the Northern Hemisphere's temperatures for the last 1,200 years (Northern Hemisphere data is much more complete than Southern). Within the climate model, they set both large and small values for the influence of solar activity on the climate.

And the large values simply don't work very well. With a high value for solar influence, nearly three hundred of the 1,000 years of the comparison failed to line up—the model output failed to match the historical record. In contrast, with a low value of solar influence, the number of mismatched years was cut by more than half. 

There was also an extended period at the start of the last millennium where the Northern Hemisphere's temperatures were high (commonly called the Medieval Warm Period), yet the solar activity was relatively low.

Doing a fingerprint analysis, which identifies the climate influences that produce the climate changes we actually measure, researchers showed that volcanoes and greenhouse gasses were the largest influences on the climate over the last 1,000 years, with greenhouse gasses playing a role even before their recent rise due to industrialization. In addition, they find that volcanic eruptions have both a short-term impact on climate (which was known) as well as a longer-term cooling impact.

Clearly, this study is limited by being focused on the Northern Hemisphere, when what we generally care about is the global effect. If solar activity did have a strong global influence, however, there should be periods where at least some of that effect was apparent in the Northern Hemisphere. It's also limited by being focused on a single climate model. The authors confirmed that a second model produced similar results, and they note that the fingerprint analysis depends only on the timing of changes, and not their magnitude. As a result, they "conclude that large solar forcing is inconsistent with reconstructions of climate of the past millennium."

That doesn't mean that the Sun couldn't force changes if its activity shifted more significantly than it has over the last thousand years or so. But that period includes both the Maunder and Dalton minimums, which are periods of exceptionally low activity in the historical record. It also doesn't rule out solar activity driving regional changes that are swamped when averaging across the entire Northern Hemisphere.

Nevertheless, the study is another point against the idea that the Sun's variability has had a significant influence on the historic climate. And, in that, it's consistent with the majority of other results.