It's widely known that the ultimate driver of the earth's climate system, the sun, has a variable output. Short-term variations and an 11-year solar cycles have been observed, and hints of longer-term cycles appear in the records. It's tempting to speculate that this variability can account for the rise in temperatures that we've seen over the last 50 years; indeed, editorials in business journals and local papers have done just that. But attendees of last week's American Association for the Advancement of Science meeting were told in no uncertain terms that this speculation was badly misguided.

It took a while to get there, though. The first talk of the session came from Dr. Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics who has explored the possibility of long-term cycles in the sun's output using everything from earth-bound isotope records to observations of other stars similar to the sun. The data she presented suggested a number of possibilities, including a 200 year variability and potentially an even longer 2200-2500 year cycle. Changes in 14C and 10Be fluxes from various locations around the globe's sea floors also suggests a millennial scale variation.

The small magnitude of the changes and large uncertainties in the data, however, leave the evidence for these cycles largely suggestive. And, as Dr. Baliunas noted, without a detailed model of a star's internal dynamo, understanding long term patterns is difficult.

Dr. Judith Lean of the Naval Research Lab took a shorter-term view, focusing on what we know about solar variability based on observations made since the advent of the space age. Lean detailed how solar forcings fit into the larger climate system, along with forcings from volcanic eruptions, aerosols, the El Niño southern oscillation (ENSO), and greenhouse gases.

Given satellite data, Lean started subtracting various forcings and explored what happens in different layers of the atmosphere. For example, the ENSO had a far larger effect than the sun on the lower atmosphere, while the sun's forcings dominated the stratosphere—except after large volcanic eruptions, in which case all bets are off. Lean concluded that, over the past 100 years, changes in the sun's output have driven the temperature up by about 0.1 K, but greenhouse gases have had a seven fold effect compared to the sun—0.7 K.

The first two speakers focused largely on technical issues, leaving the answers to the big question a bit hard to discern. The final speaker, Dr. Casper Ammann from the National Center for Atmospheric Research, changed that. His message, which he repeated a number of times, is that the increase in temperatures since the 1950s isn't due to the sun and, even if the next solar activity cycle doesn't arrive at all, temperatures are likely to continue to rise.

He got there in part by noting that the signature of the 11-year solar cycle is actually really difficult to detect in the climate; it's apparent only if one takes the quasi-biennial oscillation (QBO) into account and modern statistical tools are used. While clearly visible, the amplitude of the effect due to the solar variation is still less than 0.1 K. He also pointed out that increased solar activity should warm the atmosphere uniformly; instead, we see an increase of temperature at the surface and mid-troposphere, but a temperature decrease in anything above that. This scenario is exactly what one would see if greenhouse gases were the cause of the global warming.

Dr. Ammann also discussed some climate modeling results. He showed that modern climate models that take into account natural forcings do a rather good job at predicting past climate data—they're all within the uncertainty of the temperature measurements. These models and reality only diverge in the past 50 years and, when greenhouse gas forcings are added in, the two match closely all the way to the present.

The message of the talks was clear: although the sun's output is variable, those variations have occurred over a defined range for the last 50 years, with no trend once the variation is smoothed out. It's not the sun, and the only way to get the rising temperatures we see is to include greenhouse forcings in the climate models. This wasn't a shock, given that Nobel Intent has covered a couple of peer-reviewed papers saying exactly that, but it appears to be worth repeating, given that newspaper editorialists are apparently unaware of it.

Incidentally, those who might have hoped that an unexpected drop in solar activity could bail us out were in for some disappointment. Ammann calculated what would have to happen to the total solar irradience (TSI) to counteract the greenhouse gas forcings. Apparently, the TSI normally varies around 1365W/m2 give or take 1.4 W/m2. To bail us out, it would have to drop to 1330 W/m2—something that can pretty much be guaranteed not to happen.