Even though the myth that the current warming trend is due to changes in the sun has been debunked numerous times, climate change skeptics continue to insist that the sun is responsible. Scientists at the recent AAAS meeting again tried to correct the misinformation coming out of the skeptic tank.
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.
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 yearsand, 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.