THEGREENGROK    Planetary Watch

Planetary Watch: Predictions of a Cooling Sun

by Bill Chameides | November 24th, 2008
posted by Erica Rowell (Editor)

Permalink | 8 comments


How can we know the Sun isn’t behind current warming? Measurements of solar output over the last few decades show no net change. But will the Sun save us from future global warming?

If you’re up on the latest climate “news,” you’re familiar with a rising if small chorus claiming there’s no need to worry about global warming –- the sun is on its way to the rescue. Does a new paper add fuel to the fire –- or is that fuel to the cool?

There are a number of factors that influence our climate, greenhouse gases being one. But you don’t have to be a climate scientist to know that the Sun is another important one. When the Sun heats up, sending more energy our way, things get warmer here on Earth. And when the Sun cools, things get colder. In fact, a relatively cool Sun probably contributed to the Little Ice Age, a period from roughly the 1200s to the mid-1800s with particularly harsh conditions in the northern hemisphere.

And the Sun does go hot and cold. As I explained in an earlier post, there is something called the solar or sunspot cycle in which the sunspot number and solar output increase and wane irregularly over an approximately 11-year period. There are also 22-year and 90-year cycles, and on top of these are irregular excursions known as grand maxima and grand minima –- or periods of extreme high- and low-level solar activity.

With all of these cycles and excursions, how can we be sure that the current warming is not due to the Sun instead of greenhouse gases? The answer lies in the evidence: careful measurements of the Sun’s output over the past few decades show no net change over that period. So the Sun cannot be responsible for at least the most recent part of the temperature record.

But all this leads to another question: if the Sun’s output can change, how will it change in the future? Will it, for example, start to cool, offsetting or perhaps even canceling the greenhouse warming?

Key to answering those questions is being able to predict the comings and goings of the grand solar maxima and minima. For reasons that are not understood, every so often the Sun enters periods of extreme activity (grand maximum), in which solar energy is high, and extreme quiescence (grand minimum), in which solar energy is low. The Maunder Minimum in the late 1600s, a period with very few sunspots, is an extreme example of a grand minima. The fact that the Little Ice Age spans the Maunder Minimum lends some credence to the hypothesis that the Sun was a factor in causing the Little Ice Age.

Right now we are in the midst of a grand solar maximum, which began some 80 years ago. Could this grand maximum end soon? And could it lead to a grand minimum? Are we headed for global cooling instead of global warming?

These are important questions but almost impossible to answer definitively because our understanding of the physical processes that lead to grand maxima and grand minima is insufficient to formulate a predictive model.

Enter J. A. Abreu from the Swiss Federal Institute of Aquatic Science and Technology and his colleagues. These authors carried out a statistical analysis of data that could serve as a proxy for solar variability over the past ~10,000 years (the abundance in ice cores of an isotope of the element Beryllium that is produced by cosmic rays). Their analysis implies that the longest time a grand maximum has lasted over the period studied was about 100 years; most grand maxima lasted between 15 and 35 years. On this basis, they conclude that it is highly likely that the current grand solar maximum will end within the next three decades.

I find their study fascinating, but it is important to keep it in perspective.

  • Abreu and company’s conclusions are based on a statistical model and not on any mechanistic understanding of the processes that cause grand solar minima and maxima. Because statistics do not prove a cause-and-effect relationship, one must be very cautious when using statistics to predict the future.
  • The results are potentially good news. If they are correct (and as long as we are not headed for another Maunder Minimum), it probably means that the Sun’s output is going to modulate the warming from greenhouse gases and give us more time to get our emissions of greenhouse gases under control.
  • However, by the same token, we must not allow Abreu’s work to make us complacent about the task at hand. A cooling Sun, like the great deux ex machina of Greek plays, may come to our climate rescue –- there is no way to be certain this won’t happen. But, by the same token, no one is in a position to know for certain that it will. I am not willing to bet the well-being of future generations on the prospect of a cooling Sun, are you?


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  1. Hank Hill
    Apr 25, 2009

    “The world has been chilling sharply for about twenty years. If present trends continue, the world will be about four degrees colder for the global mean temperature in 1990, but eleven degrees colder in the year 2000. This is about twice what it would take to put us into an ice age.” • Kenneth Watt, Ecologist 1970 He was a little off” title=”Predictions

  2. len zentz
    Nov 27, 2008

    ye have a limited view of our past. the earth has been generally cooling for the past 65 million years. yes there have been oscillations up and down but the overall trend is down. the argument of climate change can only be reasonably attempted if we agree on a set of parameters for discussion. what historical earth/solar data is valid and applicable to our futures? what period in the future are we most interested in? what role should humans play in climate change if we know the climate is changing (anthropogenic or not) adversely for life on earth? earth’s temperatures vary by 10-20 degrees C, so what temperature are we to define as optimum? is our goal to maintain a plus minus 2 degrees C (if so we have no chance anyway)? my gut tells me that glaciers will be much more catastrophic than warming in the next 20,000 years. just consider the last 800,000 years of glacial and interglacial periods. thanks, good discussion, len” title=”climate change

    • erica
      Dec 1, 2008

      From DR. CHAMEIDES – Len – Thank you for the lesson on climate. You didn’t get it quite right, but close. The Paleocene-Eocene thermal maximum occurred about 55 million years ago. Following the thermal maximum, global temperatures did fall. But for the past 2 – 3 million years, temperatures have oscillated between ice ages and warm periods, but with very little net change. And those oscillations occur on time scales of tens of thousands of years. What is the relevance of climate changes that occur on time scales of tens of thousands of years to the problem of global warming in the next few decades? If you think it is of relevance, then I guess you don’t own any shorts or an air conditioner. Why bother having those things when we all know another ice age is on the way in 10,000 years or so?” title=”10,000 years or so

  3. Pete
    Nov 25, 2008

    This from the last paragraph of the paper: “If it turns out that there is a precipitate decline in solar magnetic activity, it may be expected to have a slight cooling effect on the earth’s atmosphere; this will, however, be insignificant compared with the global warming by greenhouse gases.” ” title=”And the authors themselves probably wouldn’t bet…

    • erica
      Nov 26, 2008

      From DR. CHAMEIDES – Pete – Yep, even the scientists who predict a diminished sun don’t think it will save us from global warming from greenhouse gases.” title=”Yep

  4. S2
    Nov 24, 2008

    It’s an interesting study, and it basically agrees with the work done by Sami Solanki at the Max Plank Institute (e.g. Solanki gives an 8% chance of the current maximum lasting for another 50 years, and just 1% for it lasting until the end of the century. As I understand it, if we were to drop into another Maunder-type minimum then global temperatures would only fall by between 0.25 and 0.5 degrees. This would take us back to about where we were 15 to 30 years ago (though there could be regional variations). And as minima don’t last any longer than maxima, it’s quite likely that we would warm again within a few decades (but with additional anthropogenic warming having taken place in the meantime). So no, I wouldn’t bet “the well-being of future generations” on a cooling sun, even if I knew for certain that it was cooling. ” title=”There are similar studies

    • erica
      Nov 25, 2008

      S2, Thanks for providing some more insight into the issue. – DR. CHAMEIDES” title=”Thanks

      • S2
        Nov 25, 2008

        I’m putting a talk together at the moment on how our climate is affected by astronomical influences, so it’s something I’ve been looking into quite keenly for the last year or so. Sorry about the broken link – I guess I shouldn’t have tried using html tags. ” title=”You’re very welcome

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