Planetary Watch: Predictions of a Cooling Sunby Bill Chameides | November 24th, 2008
posted by Erica Rowell (Editor)
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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?
filed under: faculty, global warming, Planetary Watch
and: greenhouse gases, ice age, solar variation, Sun, sunspots