Pumping Up the Sun

The Sun (2): This is the second of a series on the Sun in the Earth-Sun climate system. The first entry, which focused on some of the basics, is linked here and at the end.

A couple of years ago I was on a review panel for NASA’s Living with a Star Program. What struck me about the proposals was what I will call the “challenges” of understanding the solar variability on the Earth’s climate. There are a number of well known ways the Sun varies: solar flares, a 27 day oscillation associated with the rotation of the Sun, and the most well known solar sunspot cycle. There are a number of ways that the Sun impacts the Earth. Some of these are through changes in the energy that is supplied to the Earth. Other changes are associated with the chemical composition of the atmosphere, for example, ozone production and destruction. A basic fact is that higher in the atmosphere the more direct is the impact of solar variability. This is a good and essential characteristic for surface dwellers; the atmosphere protects the surface from all sorts of unpleasant types of radiative energy.

In the lower atmosphere the density, the mass per unit volume, of the air is much higher than in the upper atmosphere. Because of this high density it takes more force to move air in the lower atmosphere than in the upper atmosphere. Therefore, there is not a direct way for a “storm” in the upper atmosphere to propagate down to the surface. This challenge of identifying mechanisms that can connect the upper and lower atmosphere is formidable.

One of the papers that I use in my climate change class is by Judith Lean, and entitled Living with a Variable Sun, which appeared in Physics Today in 2005. (Here’s a first on this blog. A link to a talk by Judith on YouTube.) This is a concise and clear discussion of the Sun-Earth system; it’s a good paper.

Like the Earth’s atmosphere the Sun is a dynamic place, with all sorts of fluid motions and electrical discharges. Some of this variability is made visible by sunspots on the surface of the Sun. Sunspots have, in fact, been observed for 2000 years (See Yau and Zhentao). (This has not been a systematic and quantitative record of observations.) At the time of the maximum number of sunspots there is an increase of solar energy of about 0.1%. This change is the result of two compensating solar processes. There is a reduction of solar energy of about 1 watt/meter^2 because of the darkening of the surface. There is an increase of solar energy of about 2 watts/meter^2 because of an increase of faculae above the surface of the Sun. (Lean’s paper, Figure 3, is a nice summary of it all.)

Looking a both modern and past records, there are a set of climate signals on Earth that are definitively correlated with solar variability. These include signals of surface temperature and precipitation. One of the systematic errors of climate models is that the signals that are predicted at the Earth’s surface in the climate models are smaller than the signals that are observed in the observations. The observations are that the temperature at the Earth’s surface increase about 0.1 degree Celsius with a 1 watt/meter^2 of the solar energy. This is small. Still, the fact that the climate models underestimate this change, often stated to be by a factor of 5, is an uncertainty that demands attention.

It was stated above that it has been difficult to define heuristic mechanisms that link solar variability in the upper atmosphere to signals at the surface. The bias between observations and models suggest the need for a some mechanism that amplifies the solar signal. On long time scales changes in greenhouse gases and ice are likely sources of amplification. On shorter times scales, the search for mechanisms is generally associated with modes of atmospheric variability like the Northern Atlantic Oscillation. So the question is posed does the state of the atmosphere impact the size of the response of variability associated with the Sun?

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Blogs on the Sun.

The Sun (1)

Blogs on radiative balance

Absorbing
Reflections
Ice Water
Clouds Cool and Warm
Aerosols Cool and Warm

Here is an update of one of the iconic figures of climate change.



This figure shows how solar (visible) and terrestrial (infrared) radiation flows through the atmosphere. This is an updated figure provided by Kevin Trenberth and will appear in the Bulletin of the American Meteorological Society in the article “Earth’s global energy budget,” by Kevin E. Trenberth, John T. Fasullo and Jeffrey Kiehl.