Solar Doubt Doth Growby Bill Chameides | March 23rd, 2011
posted by Erica Rowell (Editor)
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The Sun’s surface is a busy place. Seen here in UV light, a big sunspot group is visible as the bright area near the horizon. Sunspots (or lack thereof) might help explain the Little Ice Age. Or they might not. (NASA/TRACE Project)
Did a quiet Sun cause the Little Ice Age? Maybe not.
Remember the Little Ice Age? Well, no one can actually “remember” the Little Ice Age in the sense of having lived through it. But you know what I mean. The Little Ice Age was that period ranging from roughly 1300 to 1850 when temperatures across much of the Northern Hemisphere were, by modern standards, cold. Emblematic of this period are paintings showing snow-covered European villages and iced-over rivers and lakes. (More on the Little Ice Age here and here.)
Sunspots the Key?
One explanation for all that cold period is the Sun — or more specifically a quiet (as in inactive) Sun. Support for this hypothesis comes from the fact that the Little Ice Age included the so-called Maunder Minimum, a period from about 1645 to 1715 when solar activity, indicated by the number of sunspots seen on the Sun’s surface, was extremely low — almost non-existent in fact.
Sunspots, you will recall, are areas of strong magnetic fields that manifest as dark “spots” on the Sun’s surface and are caused by disruptions in the Sun’s magnetic field. Modern observations show there is a relationship between the Sun’s sunspot number and the amount of energy emitted from the Sun that impinges on the Earth (called the Total Solar Irradiance or TSI). And, so the argument goes, since the sunspot number was so low during the Maunder Minimum, it must have also been a period of low TSI and thus we have an explanation for the Little Ice Age.
So how much lower was the TSI during the Maunder Minimum?
Since the middle of the twentieth century, scientists have tried to answer that question by taking modern observational data (like radio flux and modern TSI) and correlating those numbers with modern sunspot data to derive a relationship that they can run backwards in time to when only sunspot data exists to get at TSI for that earlier period. The result: TSI during the Maunder Minimum was indeed quite low and this dip in TSI could very well explain the cold temperatures experienced during this period [pdf].
A False Conclusion?
Maybe, says C. J. Schrijver of Lockheed Martin Advanced Technology Center in Palo Alto, California, and his co-authors, writing in last week’s edition of Geophysical Research Letters. They make the case that using the observational data from recent decades to derive a relationship between TSI and sunspot number may be problematic especially given that sunspot minima during recent times have never approached the minimum seen during the Maunder Minimum. In other words, is it appropriate to extrapolate data from relatively active periods at the end of the 20th century to the extremely quiescent Maunder Minimum?
In the past this has been an intriguing but moot question, since we didn’t have relevant minima data to answer it. But in 2008 and 2009 that changed when we saw the Sun go through an extreme solar minimum — more extreme than we have seen since 1913 and the first such extreme event since we have had the ability to reliably measure TSI. (See yearly mean sunspot numbers from 1700-2009.) The authors note that “from mid-2008 until 2009/09, the fraction of spot-free days fluctuated around 82%, unprecedented in the age of modern instrumentation.”
Using the 2008/09 sunspot minimum data, Schrijver et al estimate that the decrease in the TSI during the Maunder Minimum may have been significantly smaller than previously estimated. Using the 2008/09 sunspot minimum, the authors estimate that the TSI was about 0.2 – 0.5 watts per square meter smaller than in 1996. Previous estimates ranged from 0.5 to 1.5 watts per square meter. For comparison, the greenhouse warming due to long-lived greenhouse gases is about 2.6 watts per square meter.
Not Just the Sun at Play?
The authors conclude: “If the 2008–2009 solar … activity is indeed similar to the Maunder Minimum level … then it would appear that drivers other than TSI [i.e., the Sun] dominate Earth’s long-term climate change.”
Interesting, but I don’t think I would go quite that far. Just because the Sun may not have played a major role in the Maunder Minimum does not necessarily mean that the Sun is not a driver in all long-term climate change.
What I take away from Schrijver et al’s work is that the climatic forcing (whether it came from the Sun or something else) that led to the Little Ice Age may have been much smaller than we thought. And what that means is that the climate sensitivity — the amount of temperature change for a given climate forcing — is also larger than what’s included in current climate models. And that would mean that the eventual climate response to the rise in carbon dioxide and other greenhouse gases will be larger than now estimated. It’s all a fascinating puzzle that, given the current trajectory of things, we seem hell-bent on doing the experiment so that we can make sure all the puzzle pieces fit.filed under: carbon dioxide, carbon dioxide emissions, climate change, faculty, global warming, Planetary Watch
and: climate, climate models, climate science, global temperatures, greenhouse gas emissions, Little Ice Age, Maunder Minimum, Sun, sunspots, total solar irradiance (TSI)