Merely four years ago worries were afoot that the Sun had fallen 'asleep' and we might well be in store for another "Maunder Minimum" - a period of roughly 70 years beginning ca. 1645, which ushered in what's been called a "little ice age ". The sunspot counts reduced to zero and with them continental Europe experienced the lowest summer time temperatures in recorded history, and which adversely affected the crop growing season.
In 2009 such fears of a new little ice age were re-awakened, on account of 286 total spotless days accumulated in 2008, and 128 by the time of the 40th Meeting of the Solar Physics Division of the American Astronomical Society. This was held in Boulder, Colo. on June 12. However, as the meeting opened and specific papers were presented, some of the fears abated. For example, Frank Hill in his lecture 'A Helioseismic Comparison of the Solar Minima Preceding Cycles 23 and 24', noted the current cycle (24) was not yet exceptional - certainly in terms of spotless days aggregated. He pointed to 311 spotless days in 1913, and 287 spotless days for 1901. However, he also noted the lowest solar wind pressure since cycle 15, as well as the lowest irradiance in the modern (space observation) era.
But by early 2010, activity had begun to pick up and since then we beheld the Sun firing up not only large solar flares but coronal mass ejections - although the general trend of activity was still lower than for a typical cycle, see e.g.
Where the cycle in question is at the right side of the graph. Note the total sunspot numbers generally lie below 100.
If something was disrupting the meriodonal flow and also the solar dynamo - then this pattern would be likely to recur for the next cycle, and hence we'd see the torsional oscillation flow initiated at much higher heliographic latitudes, maybe even 45 degrees or more. One explanation put forward is that the meriodonal flow just below the solar surface is slowing, or perhaps almost ceased. This would delay formation of sunspots at the proper solar latitudes even when high sunspot counts ought to be expected (from standard Babcock-Leighton solar dynamo theory.)
The graphic presented shows one modeling of the solar tachocline subjected to shear, which may account for the pushing up effect on the torsional oscillation flow and slowing of the meriodonal flow.
Now flash forward five years - to yesterday in fact- when it was reported in none other than the esteemed Financial Times, that scientists had found evidence for the Sun "cooling off" and have even forecast a "Grand Minimum" similar to the Maunder Minimum. In fact, the FT cites "1 in 5 solar physicists" as predicting cooler conditions over the next 50 years could rival those seen in Europe during the Maunder Minimum.
But climate change-global warming deniers would do best not to rush in and embrace these findings as a way to pooh-pooh global warming or its import. As I noted above, this behavior was effectively predicted 6 years ago at that SPD meeting when it was noted that changes in the meriodonal flow and also the solar dynamo - "would be likely to recur for the next cycle." Which is precisely the cycle now being forecast as likely displaying this "grand" Maunder minimum.
For one thing, the total net change in global temperature is forecast to be on the order of - 0.1C (or -0.1K). That computes to roughly -0.18F. Six years ago, in his remarkable paper, 'Solar Irradiance: Recent Results and Future Research Plans', Thomas N. Woods of the University of Colorado dealt with the solar irradiance aspect as it pertains to the Cycle 24, and in particular measurements made at the time.
Woods began by noting the assorted recent periods wherein irradiance measurably varied, including: the Medieval maximum, the Sporer minimum (1400s), the Maunder minimum (1600s), and the Dalton minimum (1800s). He noted with some emphasis that there is no single uniform value to characterize a time interval or period, since the irradiance itself can vary hugely on small or local scales. For example, solar flares can propel irradiance increases 50 times over normal and thereby briefly affect the radiance.
On average though, with such violent inputs smoothed out, the Earth's temperature changes by about +0.07 K (kelvin) over a solar cycle. Compare this to the 1.6 K change (current est. increase) arising from global warming over the past 100 years mostly traced to human use of fossil fuels. Thus, the greenhouse component is nearly 23 times greater.
Even if the solar forcing on climate is enhanced by positive feedbacks the amplification is usually no more than a factor 2. So that 0.07 K increases become 0.14 K increases. The human component is still more important by a factor 11.4, a point made by Woods when he emphasized that the recent results support the hypothesis that anthropogenic greenhouse gases are the primary contributor. This despite all the politicos, think tanks and yahoos who keep blabbering that climate change arises from "natural cycles" - meaning the Sun is responsible.
Now, let's consider this projected -0.1K cooling in context. What does it really mean given the preceding? It means at most 0.1K lower temperatures relative to the AGW (anthropocentric greenhouse warming) -forecast increments. In other words, if AGW now forecasts a 2.6K - 3.5 K increase by 2100 that now becomes 2.5K - 3.4K. If humans somehow get their acts together and get serious about controlling carbon inputs maybe we can still hold the line at a 2.0K increase by 2011 - which would then be adjusted to a 1.9K increase. But let's face it, this isn't too likely given humans haven't shown much pluck even with the climate "hangman's noose" staring them in the bloody face.
Of course, regional differences owing to a number of other factors could render the cooling changes greater. For example, some climatologists are forecasting northern Europe could experience cooling from -0.4C to -0.8 C. To put this in context again, if a 2.6C increase is projected by 2050 that means for northern Europe the net warming increment could be tamped down to 2.6C - 0.8C = 1.8C which might make the climate barely more tolerable - say without the specter of the 11,000 heat deaths reported in 2003.
But more sober solar scientists aren't prepared to offer much solace, such as Sarah Ineson of the UK Met Office. She's quoted in the FT as saying:
"What the study shows is the Sun isn't going to save us from global warming".
Indeed. Since most solar physicists concur that the solar meriodonal flow - for which a slowing incepts cooling - isn't likely to remain constant over multiple cycles. Thus, for the next 22-year (polarity) cycle, the flow could speed up, with the result that we'd observe many more sunspots emerging at lower (typical) heliographic latitudes with much higher irradiance. Then you'd have the worst possible combination of high solar irradiance and much higher CO2 concentration- which in 35 years, at the rate of 2 ppm/yr would be nearly 400 ppm + 70 ppm = 470 ppm or within the threshold for the runaway greenhouse effect.
To offer another perspective, during solar cycle 20 – over which I also conducted investigations on solar flares and their effects- the then Solar Max satellite used an active cavity radiometer (ACRIM) to measure temperature increases arising from higher activity – especially as generated by more convection at the periphery of large spots. The differential measured was something on the order of +0.1C at the Sun! Since the radiant energy must now transit 150 million kilometers, and its intensity falls off as the inverse square, one can see this would translate into negligible increases at Earth. In a similar vein, a -0.1C cooling would translate into negligible reductions for the global mean.
What about longer period increases (or decreases) in solar luminosity associated with its possibly being a variable star – as opposed to sporadic sunspot outbursts?
The maximal magnitude of inherent solar -induced climate variability was probably first highlighted by Sabatino Sofia et al in their paper Solar Constant: Constraints on Possible Variations Derived from Solar Diameter Measurements, in Science, Vol. 204, 1306, 1979. Their estimate was a solar change in irradiance of roughly 0.1 % averaged over each solar cycle. (Irradiance is a measure of the energy per square meter received from the Sun).
Thus – if the solar irradiance effect at Earth (solar constant) is normally about 1360 watts/m^2, this would imply an increase (or decrease) of roughly +1.36 W/m^2.. The problem is that there is not enough hard observational evidence to support this for any time in the past century – when global warming spiked to serious levels. Some like Sofia have argued that even if it had occurred, it would only engender a temperature change of perhaps plus or minus one-fourth of one degree, or significantly less than what has been documented.
The bottom line, as Dr. Ineson put it, is we should not look to the cooling Sun for our salvation.
I could not agree more!