Over the weekend, a paper published in the American Geophysical Union's journal Eos attracted a lot of attention, as it suggested that the levels of magnetic activity associated with recent sunspots indicated that the sun might be returning to a state of low activity, similar to that of the Maunder Minimum, which occurred in the late 17th century. That change in solar activity was notable for setting off what's called the Little Ice Age, which plunged Europe into a deep chill. Left undiscussed is what that might mean in a world where greenhouse gas changes are threatening a period of extended high temperatures.
To understand how a significant change in sunspot levels might be felt in the Earth's climate, we'll back up and look at how sunspots relate to solar output, how that output gets felt on Earth, and how it interacts with changing levels of greenhouse gasses. The answer appears to be that it could reverse the climate change that occurred during past century, but would only delay the changes expected by the end of this century.
Sunspots and solar output
At a very basic level, a sunspot is an area of unusually strong magnetic fields on the surface of the sun. They have the effect of preventing hot gasses from the interior of the star from reaching the surface and, as a result, the sunspot itself is relatively cool compared to its surroundings—in effect, it looks dark. At the same time, however, the areas around the sunspot brighten relative to the rest of the sun's surface. Thanks to the nearby bight areas, on balance, the net effect of a sunspot is positive: more light emitted by the sun.
This can be seen by tracking the total solar irradiance over the course of a typical 11-year solar cycle. As the number of sunspots increase to the peak of the cycle, the total solar irradiance becomes more variable but, on average, increases. It's not just visible light that increases; other aspects of solar activity, such as magnetic field strength, mass ejections, and UV emissions, also go up. So, in general, sunspots act as a proxy for how active the sun is.
We have several hundred years of observational data on sunspots, and can track them further back in time by following changes in two radioactive isotopes of carbon and beryllium (more on that later). These records make two things clear: the Earth's climate responds to changes in solar output (no surprise, really), and that the solar output can change independently of the 11-year solar cycle.
These larger-scale changes are most obvious in the case of the Maunder Minimum, a period that ran roughly between 1645 and 1715. Observations of this period suggest that the solar cycle essentially stopped during this time, as very few sunspots were recorded during the entire period. Temperatures responded accordingly; current reconstructions indicate that the global mean temperature dropped by 0.4?C. That may not sound like much, but recent climate reconstructions indicate the effect was more pronounced in the Northern Hemisphere, and felt especially keenly in winter. The net result is that Europeans recorded this period as the Little Ice Age.
For reasons that aren't clear, the solar cycle returned at the end of this period, and temperatures rose accordingly. They rose again slightly at the start of the 20th century—the IPCC estimates that up to 30 percent of last century's warming can be accounted for by this change—before stabilizing at mid-century. The dramatic rise in temperatures in the latter half of the century has largely been ascribed to the rising levels of greenhouse gasses.
Current events
That brings us to the current situation where the last solar cycle drew to a close as expected, leaving the sun with very few sunspots for the past couple of years. What has been surprising is the slow pace at which the next solar cycle is starting. That delay has set the stage for the Eos paper, which suggests that the few sunspots that have been visible are associated with extremely weak changes in solar magnetism. In some models of solar dynamics, this apparently indicates that we are heading for a period similar to the Maunder Minimum, at least as far as the sun is concerned.
It's worth pointing out that this is in no way a sure thing. We certainly don't know enough about the internal dynamics of the sun as we'd like, and even the authors recognize that it's probably too soon to tell, writing, "Other indicators of the solar activity cycle suggest that sunspots must return in earnest within the next year."
Assuming this one model is right and we are inching towards a second Maunder Minimum, can we expect another Little Ice Age, wiping out any impact of the greenhouse gasses that we've pumped into the atmosphere? The answer is "probably not." Nearly any way you calculate it, the effects of the greenhouse gasses that will be in the atmosphere by the end of this century appear likely to dwarf the changes that accompanied the Maunder Minimum.
This is apparent if you express things in terms of changes in the energy flux caused by solar variability, compared to that caused by greenhouse forcings, as we reported in 2008. An alternate way to look at things is to compare the global average temperature changes estimated for the Maunder Minimum, (about 0.3 to 0.4?C) with that which has occurred since the middle of the 20th century, which are about 0.4 to 0.5?C. The last way is to compare that figure with the warming expected by the end of this century, which the IPCC estimates at over two full degrees.
All of these numbers indicate that a return to Maunder-Minimum-like conditions could take us back to the conditions of the first half of last century, which would be a significant change in the climate. That would certainly take a bit of the time pressure off our attempts to limit our use of fossil fuels. But the impact would be less than we'd expect from a doubling of atmospheric carbon dioxide, meaning it would only delay some of the more significant climate changes.
Nothing is ever simple
Of course, in any system with significant unknowns, it's impossible to rule out anything unexpected. So, for example, just as a return to Maunder Minimum conditions would have been largely unexpected, there's also the remote possibility that solar radiance might drop below the level expected based on the loss of sunspots. There is also the chance that one of the associated changes that occur during the solar cycle, like the changes in levels of UV light, might have an unexpected impact on the climate.
Still, it's worth remembering that these are hypotheticals twice removed—potential differences caused by a potential event (changes in solar activity) that we're not even sure is happening at this point.
The last thing that seems to require a comment is the idea, popularized by Henrik Svensmark, that changes in the solar magnetic field associated with sunspots can have an indirect affect on the climate. These changes influence the number of cosmic rays that reach the earth's atmosphere—weaker solar magnetic fields mean that the Earth gets hit by more cosmic rays. These cosmic rays are responsible for the production of the radioactive isotopes of carbon and beryllium mentioned above, which is why we can use them to track sunspot numbers.
Svensmark's idea is that the impact of cosmic rays (registered by 14C and 10Be levels) isn't simply a proxy for solar activity, but is part of a process that influences the climate directly. He proposes that the cosmic rays form ions in the lower atmosphere that seed clouds, which cool the planet by reflecting sunlight back out.
At the moment, however, there is essentially no evidential or mechanistic support for this idea. Under normal atmospheric conditions, the particles that actually seed clouds are significantly larger than the ions generated by cosmic rays, and nobody is aware of a mechanism by which these ions can grow to the appropriate size. Cosmic rays are also more prevalent in the upper atmosphere, where cloud formation tends to have an insulating effect. Meanwhile, some studies have suggested that, at least over the oceans, the lower atmosphere is already saturated with particulates the appropriate size to seed clouds.
Finally, although there did seem to be a sunspot-cloud cover correlation during an earlier solar cycle, the correlation fell apart during the most recent. So, overall, Svensmark's ideas haven't gained much traction within the scientific community, although they will appear if you do Web searches for sunspots and climate.
The takehome
Overall, the Eos paper suggests that current data is consistent with a decline in the sun's magnetic field activity, which could potentially end in a sunspot-free period. We care about this because sunspot numbers act as a proxy for the amount of radiation sent out by the sun, which can have a significant influence on the Earth's climate. But the sun is one of a large number of factors that influence the climate, and the changes in solar radiance caused by sunspots appear likely to have a smaller impact on the climate than that caused by our ever-increasing levels of greenhouse gasses.
Still, even a relatively small effect may buy humanity valuable time in coming to grips with the CO2 we're putting into the atmosphere (at least when it comes to temperatures—ocean acidification is a different problem entirely). According to the paper, we may know whether a new solar minimum is occurring as soon as 2015.
Listing image by NASA's Astronomy Picture of the Day
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