by Dana Nuccitelli, "Climate Consensus - The 97%," The Guardian, January 19, 2014
Fortunately for us, solar activity is quite stable, and a solar minimum would only have a small effect on global temperatures. Photograph: saesnes
Recent articles in the Danish newspaper Jyllands-Posten (translation available here) and in the Irish Times both ran headlines claiming that another grand solar minimum could potentially trigger an "ice age" or "mini ice age" this century. These articles actually refer to the Little Ice Age (LIA) – a period about 500 to 150 years ago when global surface temperatures were approximately 1 °C colder than they are today. This is quite different from an ice age, which are more like 5m°C colder than today. The LIA was not actually very cold on a global scale.
So, in order to trigger another LIA, a new grand solar minimum would have to cause about 1 °C cooling, plus it would have to offset the continued human-caused global warming of 1 to 5 °C by 2100, depending on how our greenhouse gas emissions change over the next century.
In the Jyllands-Posten article, Henrik Svensmark (the main scientist behind the hypothesis that the sun has a significant indirect impact on global climate via galactic cosmic rays) was a bit more measured, suggesting,
"I can imagine that it will become 0.2 °C colder. I would be surprised if it became 1–2 °C"
So these two articles are suggesting that a grand solar minimum could have a net cooling effect in the ballpark of 1 to 6 °C, depending on how human greenhouse gas emissions change over the next century. Is it plausible that a grand solar minimum could make that happen?
The short answer is, 'No.'
Fortunately, Solar Output is Stable
We're fortunate that the amount of solar radiation reaching the Earth's surface is very stable. Climate contrarians will often ask if we'd prefer if the planet were warming or cooling, suggesting that global warming is a good thing because at least the planet isn't getting colder. This is a false dichotomy - an ideal climate is a stable one.
The relatively stable climate over the past 10,000 years has allowed establishment of human civilization, by making it possible to create large stationary agricultural farms because we could rely on stable weather patterns. During that time, net global surface temperatures changes haven't exceeded 1 °C from the coldest to the hottest climates, though we're now approaching that degree of change, with 1 °C warming since the LIA, 0.8 °C of that over the past century, with much more to come.
What difference would a grand solar minimum make in the amount of solar energy reaching Earth? Two examples are the Maunder Minimum, a period of very low solar activity between 1645 and 1715, and the Dalton Minimum, a period of low (but not as low as the Maunder Minimum) solar activity between 1790 and 1830.
400 years of sunspot observations data. Created by Robert Rohde, via Wikipedia.
Relative to current levels, the Dalton Minimum represents a 0.08% decrease in the amount of solar radiation reaching the Earth's surface, and the Maunder Minimum represents a 0.25% decline. That's how stable solar activity is. That's also why we're playing with fire by increasing the greenhouse effect so much and so quickly. We're threatening the stability of the climate that has been so favorable to our development.
Peer-Reviewed Research Says Global Warming will Continue
There have been several studies in recent years investigating what impact another grand solar minimum would have on global surface temperatures, since solar research suggests it's possible we could be due for another extended solar minimum. Generally these studies will run climate model simulations under a given greenhouse gas emissions scenario with stable solar activity, then run the same scenario with the sun going into a grand minimum, and look at the difference in resulting global surface temperature changes.
Using this approach, Feulner and Rahmstorf (2010) (PDF available here) estimated that another solar minimum equivalent to the Dalton and Maunder Minima would cause 0.09 °C and 0.26 °C cooling, respectively.
The global mean temperature difference is shown for the time period 1900 to 2100 for the IPCC A2 emissions scenario. The red line shows predicted temperature change for the current level of solar activity, the blue line shows predicted temperature change for solar activity at the much lower level of the Maunder Minimum, and the black line shows observed temperatures through 2010. Adapted from Feulner & Rahmstorf (2010) by SkepticalScience.com
Jones et al. (2012) (PDF available here) arrived at a nearly identical result, with cooling from another Dalton or Maunder Minimum at 0.09 °C and 0.26 °C, respectively. Similarly, a new paper by Anet et al. (2013) found that a grand solar minimum will cause no more than 0.3 °C cooling over the 21st century.
Consistent with these previous studies, Meehl et al. (2013) (PDF available here) estimate a Maunder Minimum would cause about 0.26 °C cooling, but as soon as solar activity began to rise again, that cooling would be offset by solar warming. This is a key point, because a grand solar minimum would not be a permanent change. These solar minima last for a few decades, but eventually solar activity rises once again. Thus any cooling caused by a solar minimum would only be temporary.
The cooling effect of a grand solar minimum can also be estimated very easily without the aid of climate models, because the change in the amount of solar radiation reaching the Earth's surface is directly proportional to the temperature change it causes. Performing this calculation yields the same result as the model-based research: approximately 0.3 °C cooling from another Maunder-type grand solar minimum. Click here to see the details behind the calculation.
The Heating of the Deep Oceans
In the Jyllands-Posten article, Svensmark also disputes the data showing the accelerated accumulation of heat in the deep oceans.
"How can the ocean below 700 meters be heated up, without the upper ocean warming up accordingly?"
This is an increasingly common argument made by climate contrarians, and a bit of a strange one. The data are what they are - we've measured the deep ocean warming, including with reliable instruments on Argo buoys for close to a decade now. Even if we couldn't explain how the heat got there, it's there.
5-year averages of ocean heat content 0-700 meters (red) and 0-2000 meters (black), from the National Oceanographic Data Center
But let's address the question anyway - do we expect to have seen some obvious indication of heat being transferred from the shallow to deep ocean layers?
It's certainly not clear that we should. Consider the analogy of a bathtub. Water from the faucet represents heat entering the shallow ocean layer. Water exiting the drain represents heat leaving the shallow oceans and entering the deep oceans. The water level in the bathtub represents the heat in the shallow ocean layer (which is what we measure).
If the amount of water entering the tub from the faucet is the same as the amount of water draining out of the tub, the water level in the tub won't change. Yet the water still flows down the drain. Climate scientist Gavin Schmidt has discussed this point, summarized here.
In short, we wouldn't necessarily see the heat being transferred through the shallow to the deep oceans. However, there has been plenty of warming of the shallow oceans that could have been transferred to the deeper oceans. In our case, the water is flowing into the tub faster than it's draining out - the shallow oceans are warming fast, as the figure above illustrates.
Svensmark Gets Ocean Warming Wrong
Unfortunately Svensmark appears to be unfamiliar with this ocean heating data, saying,
"The thousands of buoys that we have deployed after 2003 to measure the ocean temperature, have not registered any temperature rise."
This is just totally wrong, even if we ignore the rapid warming of the deep oceans (as is clear from a simple examination of the figure above). The ocean heat content data can be downloaded from the National Oceanographic Data Center here. The heating trend since 2003 in the upper 700 meters of oceans is equivalent to nearly 1 Hiroshima atomic bomb detonation per second (plus another 3 per second in the deep oceans). Both the shallow and deep oceans are accumulating a whole lot of heat, with no signs of slowing whatsoever. If anything, the heating of the oceans and the planet as a whole is accelerating.
Human Influence on Climate Change is Bigger than the Sun's
The bottom line is that the sun and the amount of solar radiation reaching Earth are very stable. Even during the Maunder and Dalton grand solar minima, global cooling was relatively small - smaller than the amount of global warming caused by human greenhouse gas emissions over the past century.
A new grand solar minimum would not trigger another LIA; in fact, the maximum 0.3°C cooling would barely make a dent in the human-caused global warming over the next century. While it would be enough to offset to about a decade's worth of human-caused warming, it's also important to bear in mind that any solar cooling would only be temporary, until the end of the solar minimum.
The science is quite clear that the human influence on climate change has become bigger than the sun's. At this point, speculation about another mini ice age is pure fantasy.
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