An impressive cycle for its unimpressiveness.
You will recall that the Sun goes through an 11-year cycle of activity. At the high point of its activity a relatively large number of spots appear on its surface (see here and here for the cause of the spots) and at the low point relatively few appear. The cycle from one low to another is about 11 years.
The current cycle -- called cycle 24 -- began in 2008-2009 (see here and here) and even as it started, there were signs indicating it could be weaker than what we've seen in a long, long time (though the predictions were mixed). As cycle 23 wound down in 2007, the sunspot number plummeted to extreme lows. Then cycle 24 took a relatively long time to get underway, and once it did it, it showed relatively paltry numbers of sunspots. Solar scientists whose business is to study and predict the intensity of solar cycles began to revise their estimates of the current cycle's strength significantly downward.
So now it's 2013, about four years since cycle 24 began, and it continues to impress by its unimpressive numbers. For example this from NASA's website: "The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906." Scientists now predict that cycle 24 will peak at about half the intensity of cycle 23.
The Climate Connection
Some have speculated that cycle 24's weakness may not be an isolated anomaly but the start of a new regime in the life of our Sun. The Sun has lots of cycles in addition to the 11-year cycle in which we see ups and downs in solar activity. Over the past ~100 years the Sun has been in what is referred to as a grand maximum with relatively high numbers of sunspots. Could this new weak cycle be a harbinger of a change, marking a change from a grand maximum to a grand minimum?
This is relevant to the climate because the amount of energy that emanates from the Sun varies with sunspot number, more at high activity and less at low activity. The variation is not a lot -- from minimum to maximum in a given cycle the Sun's luminosity varies by about 0.1 percent or less. (See here and here [pdf].) The best estimates are that such variations over the course of a cycle have an effect on global temperatures, but since these variations go up and down, the net effect is negligible.
But what if we are now headed from a grand maximum to a grand minimum; that is, what if we're in for a long-term change in the amount of energy coming from the Sun? Some have speculated that such a change would put a halt to global warming and even harbinger a period of cooling. There have been a few articles that have looked at this question (see here, here and here -- this last article from NASA is less conclusive) and generally concluded that even with a new grand minimum, a cooling in global temperatures is not in the cards (see here and here).
A Slow-down at Best, but No Guarantees
Now a new paper by Gerald Meehl of the National Center for Atmospheric Research and co-authors reporting in the journal Geophysical Research Letters have come to a similar conclusion.
They ran a series of climate-model simulations assuming reduction of solar irradiance by 0.25 percent from 2020 to 2070 and rising emissions of greenhouse gases. (They used a reduction of 0.25 percent based on estimates of solar irradiance during the so-called Maunder Minimum, a period of extremely low solar activity that began in the mid-1600s and lasted into the early 1700s.)
The authors' conclusion: the imposed 50-year decrease in solar irradiance does depress temperatures somewhat, but it does not halt, let alone reverse the warming trend. And once the assumed grand minimum ends, the "warming catches" up to what would have occurred without the depressed solar activity. Meehl and co-authors write: "Thus, a future grand solar minimum could slow down but not stop global warming."
You know what? I'd gladly take a little bit of "slow down" on climate change. In my book, a new grand minimum would be grand.