Cold Comfort: What Might a Cool Sun Mean for the Future?

by Bill Chameides | March 17th, 2010
posted by Erica Rowell (Editor)

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The Sun's output is not a constant but a variable and this affects climate. But the question is how much? (NASA)
The Sun's output is not a constant but a variable and this affects climate. But the question is how much? (NASA)

Could a cool sun stop global warming in the 21st century?

Obviously a major player in climate is the Sun. Turn up its output and we get hotter. Turn it down and we get cooler.

These scenarios are not just hypothetical: the so-called solar constant or total solar irradiance (TSI), the amount of radiation from the Sun impinging on the Earth, is not, despite its name, a constant. Both direct measurements of TSI in the modern era as well as estimates of TSI in the past using proxies (such as sunspot number and radioisotopes produced by cosmic rays) show that the Sun is a variable star. The variations are small, on the order of a few tenths of a percent, but they are large enough to cause climate variations here on the Earth.

The most obvious and best studied of the Sun’s variations are those that occur over a roughly 11-year period referred to as the sunspot cycle. Over this cycle, TSI varies in a sinusoidal or wave-like pattern with a maximum and a minimum. The total variation from maximum to minimum is typically about 0.1 percent, and our best estimate is that this variation leads to an oscillation in global temperatures of approximately 0.2 degrees Celsius or 0.36 degrees Fahrenheit (source).

Longer Periods of Cool

The Sun can make some more extended excursions. For example, over the past 1,000 years, the Sun experienced five grand minima when its output remained relatively low for a while:

  • Oort Minimum (1040-1080)
  • Wolf Minimum (1280-1350)
  • Spörer Minimum (1450-1550)
  • Maunder Minimum (1645-1715)
  • Dalton Minimum (1790-1830)

Grand minima as measured by Carbon-14 isotopes, a proxy for solar activity.

Of these minima, the Maunder Minimum is probably the most notorious. Its appearance overlaps with part of the Little Ice Age, fueling speculation that the Sun was at least in part the cause of the cold climes our forebears had to endure during that time. (Large volcanic eruptions, which which have coincided with the minima, also likely contributed to the mini freezes.)

The Rebirth of (Extended) Cool?

Could a similar minimum be in the works for us in the near future? Some solar scientists say it could be (e.g., here [pdf] and here).

The Sun has just gone through a very strong and extended minimum in its 11-year cycle and is making a relatively weak recovery from that minimum. Could that be the beginning of a trend?

As it turns out, for much of the 20th century the Sun appears to have been in a so-called grand maximum with relatively high output on average relative. (This grand maximum is estimated to have made a small contribution, roughly 10 percent or less, to the warming of the 20th century.) This has led some solar prognosticators to predict that the Sun’s current grand maximum will soon end and the Sun will enter a new grand minimum.

Aha! — some people say — a new grand minimum will spell the end of global warming and bring about a global cooling and so instead of worrying about rising temperatures, we should be preparing for a new ice age. Brrr.

Paper: Warnings of Coming Ice Age Exaggerated

A new paper in Geophysical Research Letters suggests that warnings of the coming ice age are a bit exaggerated. Using data from past grand minima, authors Georg Feulner and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research find that the amount of cooling from the decrease in solar output during such grand minima is an order of magnitude smaller than the amount of warming that will occur from the increases in carbon dioxide (CO2) that will occur in the 21st century if we continue on a business-as-usual course.

The estimated cooling from past grand minima (of about 0.3–0.5 degrees Celsius) is swamped by the temperature increase of about 3.7 to 4.5 degrees Celsius from greenhouse gas-warming predicted by the climate models used.

I find these results interesting and pretty convincing, but by no means conclusive. The authors depend upon proxies to estimate TSI and temperature during previous grand minima — they could be wrong. And there is no guarantee that a potential 21st-century grand minimum (assuming that it has one) will not be a lot more intense than the ones that have occurred in the recent past. By the same token there is no guarantee there will be a grand minimum at all in this century.

If I were a bookmaker, I would place the odds on the Sun saving us from global warming to be very, very long. The findings of Feulner and Rahmstorf make those odds that much longer. I wonder if the folks who insist on a business-as-usual CO2 emissions path fully realize what a long shot they’re betting on.

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