Guest commentary by Georg Feulner
During a meeting of the Solar Physics Division of the American Astronomical Society, solar physicists have just announced a prediction that the Sun might enter an extended period of low activity (a ‘grand minimum’) similar to the Maunder Minimum in the 17th century. In this post I will explore the background of this announcement and discuss implications for Earth’s climate.
It has been known for a long time that solar activity shows a very regular pattern. Every 11 years the Sun is particularly active, and numerous dark sunspots are visible on its surface. These maxima of solar activity are separated by times of low activity when only few (if any) sunspots appear.
Figure 1: The Sun in visible light during an activity maximum (left) and during the last (and rather extraordinary) 11-year minimum during which it appeared spotless most of the time. Source: NASA Earth Observatory/SOHO.
One could think that the Sun emits less light during a solar maximum because of the many dark spots. In fact it is the other way round, since active regions around the sunspots emit more radiation than is “lost” in the cooler sunspot areas. This effect can be best seen in ultraviolet images of the Sun.
Figure 2: The Sun in ultraviolet light during a maximum (left) and a minimum (right). Source: NASA Earth Observatory/SOHO.
An analysis of historic sunspot observations shows that the 11-year solar activity cycle was interrupted during the late 17th century.
This period of time, during which the Sun appeared without sunspots most of the time, was called the Maunder Minimum by Jack Eddy in his famous Science paper. (Alliteratively named after Edward Maunder, although it was actually first discovered by Gustav Spörer.)
Figure 3: Observations of the number of sunspots over the last four centuries. Source: Wikimedia Commons/Global Warming Art.
The Maunder Minimum falls within the climatically cooler period of the “Little Ice Age”, during which temperatures were particularly low over continents in the Northern hemisphere (especially in winter). It has long been suspected that the low solar activity during the Maunder Minimum was one of the causes of the Little Ice Age, although other factors like a small drop in greenhouse gas concentrations around 1600 and strong volcanic eruptions during that time likely played a role as well.
Solar physicists do not yet understand how an extended solar-activity low like the Maunder Minimum arises. Yet there is recent observational evidence for an unusual behavior of the Sun during the current cycle 24, including a missing zonal wind flow within the Sun, decreasing magnetic field strength of sunspots and lower activity around the poles of the Sun. These observations prompted Frank Hill and colleagues to suggest that the Sun might enter a new Maunder-like minimum after the current 11-year cycle ends (i.e. after 2020 or so).
It remains to be seen whether this prognosis turns out to be true (there have been some doubts expressed), but since grand minima of solar activity did occur in the past, it is certainly interesting to explore what effects such a minimum might have on 21st century climate if it did occur. This is precisely the question Stefan Rahmstorf and I investigated in a study published last year (see also our press release. (Earlier estimates for the size of this effect can be found here and here.) In our study we find that a new Maunder Minimum would lead to a cooling of 0.3°C in the year 2100 at most – relative to an expected anthropogenic warming of around 4°C. (The amount of warming in the 21st century depends on assumptions about future emissions, of course).
Figure 4: Rise of global temperature (relative to 1961-1990) until the year 2100 for two different emission scenarios (A1B, red, and A2, magenta). The dashed lines show the slightly reduced warming in case a Maunder-like solar minimum should occur during the 21st century. Source: PIK.
According to these results, a 21st-century Maunder Minimum would only slightly diminish future warming. Moreover, it would be only a temporary effect since all known grand solar minima have only lasted for a few decades. Critics of this result might argue that the solar forcing in these experiments is only based on the estimated change in total irradiance, which might be an underestimate, or that does not include potential indirect amplifying effects (via an ozone response to UV changes, or galactic cosmic rays affecting clouds). However, our model reproduces the historic Maunder minimum with these estimates of solar irradiance. Furthermore, even if one multiplied the solar effects by a huge factor of 5 (which is unrealistic), no absolute cooling would take place (the temperatures would be temporarily cooler than the base scenario, but the trends would still be warming).
It is clear that if a grand minimum were to happen it would be a tremendously exciting opportunity for solar physicists, however it is unlikely to be very exciting for anyone else.
Update 23 June: Here is a nice tongue-in-cheek video on the media response to this story.
The Alley quote come from this:
http://www.usgcrp.gov/usgcrp/seminars/980217DD.html
“Abrupt Climate Changes Revisited: How Serious and How Likely?”
USGCRP Seminar, 23 February 1998
Note the date.
Jonty:
There is no evidence for this. The temperature changes associated with solar minima, or for that matter prolonged solar inactivity are very small compared to what you get with a doubling or tripling of CO2 in the atmosphere. There’s some interesting dynamical feedbacks and regional responses (see section 4 of the paper I linked to in post #22), but we have evidence for what happened during the Maunder Minimum, and the cooling trend was relatively modest, and probably depended on other things as well (like volcanic activity).
As I said in my post #1, the “business-as-usual” expectations for the global temperature by 2100 (or beyond) if we continue to emit carbon are something like an order of magnitude greater than anything the sun has done over the last 10,000 years, so even if you believe in these amplification factors or if clouds enhance the sun-induced cooling by a factor of two or three, you are still left with a big warming signal. Though, as I also said, there’s probably some interesting work to be done with short-term prediction when the effects of CO2 are comparable to both the decline in sunlight and natural variability.
Your father also wants the argument to go one way: if clouds operated as a positive feedback to decreased solar (further causing cooling) then they could just as well act as a positive feedback to the elevated greenhouse gases (further enhancing the warming). Clouds are a very uncertain subject in general though, but most estimates having them as a neutral to positive feedback. It’s not settled, but a huge cooling trend in the near future seems well beyond the constraints we have.
@50
Yes that is the source. Alley was doing the bulk of his paleo work in the 90s. He is now monitoring change in glaciers and ice sheets but I believe he still considers his earlier LIA data quite valid. Got a link that supports your statement that he no longer believes the NH LIA was -(1-2C)? Perhaps I will ask him.
@52 Chris says “The temperature changes associated with solar minima, or for that matter prolonged solar inactivity are very small compared to what you get with a doubling or tripling of CO2 in the atmosphere.”
There is simply no scientific support for this statement Chris. When solar activity falls far below the sunspot threshold, far below any time that we have measured TSI with satellites we are in uncharted territory. We do not know what effect this will have on climate because we have never been there. Gavin suggests Maunder was mild at only -.3C but Richard Alley, historical records, glacier movement, migration patterns, clothing trends, crop records, ice cores suggest a much deeper NH dip in temps (1-2C). It important to get this right so we have a better idea of what to expect over the next 70 years or more. If Gavin is right, AGW will continue with a modest reduction. If Alley (and historical record) is right, a deep solar minima may neutralize AGW for the 21st century only to emerge when solar activity increases again. To consider this possibility is just good clean science at work.
@50 Yes that is the source. Alley was doing the bulk of his paleo work in the 90s. Now he is mostly focused on changes and movement of the cryosphere but I believe he still feels his LIA data is valid. Got a link to suggest he changed his mind?
@52 Chris says: “The temperature changes associated with solar minima, or for that matter prolonged solar inactivity are very small compared to what you get with a doubling or tripling of CO2 in the atmosphere.”
Sorry Chris but there is no evidence to support your statement either. If solar activity drops off far below the threshold to produce sunspots for a prolonged period we are in uncharted territory with no instrumental precedent. TSI estimates are completely unknown and we don’t really know what the climate response will be. If Gavin is right AGW will continue with only mild attenuation for a while. If Richard Alley was correct in his assessment of NH temps during the LIA, the quiet sun may play a stronger climate forcing role than GHGs for most of this century. Interesting times!
[Response: From Richard Alley’s Earth: The Operator’s Manual (2011), Chapter 9 (The Moving Finger Writes), p. 115-116 (section on “Solar Stuff”): The small variations in the sun’s output during sunspot cycles have caused small variations in Earth’s temperature. The cold of the Little Ice Age of a few centuries ago seems to have involved both a little extra sun-blocking effect from explosive volcanic eruptions, and a small reduction in the sun’s output….There is a small but interesting literature looking for amplifiers that might allow tiny changes in the sun to cause larger changes in climate…However, the few tenths of a degree from such influences are very small compared to the possible warming if we burn most of the fossil fuels”. Any further discussion of Richard’s views will be considered O-T. -mike]
This is what’s been quoted in the last 3 weeks, suggesting a possible end to sunspot cycles for a while:
http://www.boulder.swri.edu/~deforest/SPD-sunspot-release/SPD_solar_cycle_release.txt
Same press release from SRO:
http://www.nso.edu/press/SolarActivityDrop.html
One news story based on that:
http://earthsky.org/space/major-drop-in-solar-activity-ahead-scientists-say
Might as well quote the bit that’s being left out of the blog science copypastes:
______________________
In response to news inquiries and stories, Dr. Frank Hill issued a follow-up statement:
“We are NOT predicting a mini-ice age. We are predicting the behavior of the solar cycle. In my opinion, it is a huge leap from that to an abrupt global cooling, since the connections between solar activity and climate are still very poorly understood. My understanding is that current calculations suggest only a 0.3 degree C decrease from a Maunder-like minimum, too small for an ice age. It is unfortunate that the global warming/cooling studies have become so politically polarizing.”
—————-
http://www.nso.edu/press/SolarActivityDrop.html
@53
Thank you for your response Mike. I am aware of Alley’s views on the possible causes of the LIA. It is the classic textbook answer that is most common. I am certain that volcanic activity played a role in brief cold periods throughout the LIA just as Pinatubo did in the early 90s. Your response does fail to answer the question asked though:
Does Richard Alley still believe that the LIA was characterized by NH temps falling 1-2C over an extended period as he did in 1998 while doing his paleo studies, or has he invalidated his own data and accepted the more consensus view (Gavin et al) that it was a far milder event (approx -.3) as stated in the op?
As I am sure you are aware the NH climate implications of a difference between (-.3) vs (-2C) are huge. Both scientists are bright guys with strong credentials. It would be useful to get clarity on what really happened during Maunder as we face a possible sequel in the near future.
To Jonty, his “extreme global cooling” father, ivp0, and anyone else who might seriously believe that the next few decades will bring strong cooling:
Off and on for years, I have been trying to make a large, public bet about the next few decades with someone who disagrees strenuously with mainstream climate science. Alas for my retirement fund, whenever I get a hint of a worthy opponent, they melt away in a flurry of anonymity, coy disclaimers, and general commitment-phobia.
Since most of us on both “sides” agree that we are effectively betting our grandchildren’s futures anyway, it seems a small thing to ask.
If you are serious, let’s find a suitable forum to work out the details.
Please put up or shut up.
@57
No need to get nasty there Ric. I am just a curious guy asking questions from the climate experts here. I have made no such cooling predictions (feel free to re-read my posts) and really just want to know if I need to buy sunshades or long johns for the next 50 years. I do hate to ignore history though. So sorry if my questions have somehow ruffled your feathers.
> ivp0 says:
> 8 Jul 2011 at 10:03 AM
> your response does fail to answer the question asked though:
> Does Richard Alley still believe …
You’re copypasting a faux-clever trick question widely circulated a year ago. You’re rebunking.
Look it up, like this: http://www.google.com/search?q=%22Richard+Alley%22+%22little+ice+age%22+temperature+change
The first hit debunks the question you’re pretending to ask:
“Richard Alley’s name has been thrown around a bit by bloggers asserting that ice-core records from Greenland show that carbon dioxide has scant, if any, influence on climate…. I sent a query to Dr. Alley about such interpretations of his work and the ice-core record and he sent a reply, the heart of which is pasted below…..”
http://dotearth.blogs.nytimes.com/2010/02/08/richard-alley-on-old-ice-climate-and-co2/
Read. Think for yourself.
ivp0:
Look, the NH is not the world, and as has been stated repeatedly the LIA wasn’t caused only by a drop in TSI anyway. The 0.3C figure is for average global temps, not the continental masses of the NH.
It’s not enough to offset the global 4C warming we expect to see by 2100, and in the NH would serve to diminish warming but not start another LIA. That’s the point. Global warming of (say) 3.7C vs. 4C means NH continental warming of 10C+/- a bit in either case.
@59
Sorry Hank,
I guess I am not that clever as I did not find the answer to my question in your links. I do admit to being somewhat of a fan of Dr. Alley. His observations, teaching style, and curious mind are quite refreshing. I also believe that the earth is warming and manmade CO2 certainly plays a key role in this. As always, the devil is in the details and that often means asking the hard questions. I apologize if this process somehow offends you.
*eyes rolling*
Right. Just a noble seeker after truth. Mm-hmm.
I didn’t say anything “nasty” (your word, ivp0).
I notice any candidates to take a serious bet have melted away, as I predicted. Seems to be a good correlation with arctic sea ice there, maybe a subject for research. Jonty’s father, oh where are you?
An awful lot of disagreement with mainstream climate science, so many mentions of decades of cooling, none of them serious.
Chris:
Thanks for the info/ammunition – Will put it to pa (will probably end up agreeing to disagree as usual)