Here Comes the Sun

The Sun (1): Some time ago I promised to write some blogs about the Sun and solar variability in climate change. Recently, there has been a lot of discussion about how the future variability of the Sun is accounted for in climate change predictions. This is the first in a series of blogs about the Sun.

First, however, I want to go back to fundamentals. Early on in this blog I did a series on the basics of climate science. The links to those blogs are given at the end, and here is a link of a collection of these blogs that I have put together. Briefly, the source of energy for the Earth and its climate is the Sun. Most of the energy of the Sun comes to the Earth as visible radiation, but there is energy in other parts of the radiative spectrum, for example, infrared and ultraviolet. The radiative energy that makes it to the Earth is either absorbed at the Earth or it is reflected back to space.

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



Figure 1: 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.

Most of us were probably taught that the energy that came from the Sun was “constant.” From grade school through graduate school I was taught that the solar constant was a number that was something like 1366 Watts/meter^2. There were various measurements that gave a “range” of this constant.

As our education got more sophisticated, we were ultimately taught that the solar constant was not, in fact, constant. One, we knew that the Sun’s output varied, with the most widely known source of variability correlated with the sunspot cycle. Two, in as much as the solar constant was the amount of radiative energy that fell upon the Earth, the various asymmetries and wobbles in the Earth’s orbit around the Sun caused some variability. Still, given the known (or hoped for?) stability of the Earth’s climate, the output from the Sun was commonly viewed as “constant.”

To climate scientists, solar physicists, and astrophysicists the Sun has never been constant. Here is a paper from the astrophysical community that has been cited many times in the climate community. (It happens to be by my big brother. Newman and Rood, “Implication of Solar Evolution for the Earth’s Early Atmosphere”, Science, 1977) This paper looks at very long time scales, over the lifetime of the Earth and the lifetime of the Sun (billions of years). Over this length of time, the energy coming from the Sun has increased by about 25%. This paper talks about the consistency between the observations of the climate of the Earth in the presence of this increasing energy from the Sun. Briefly, the surface of the Earth is warmer than it “should” be, and it is the fact that there is an atmosphere with greenhouse house gases that keeps the Earth warm (This is the part called “back radiation” on the right hand side of the figure.). Long ago, when the Sun was faint and young, the composition of the atmosphere had to be different, to contain more greenhouse gases, in order for the Earth’s temperature to be as high as it is known to have been.

The warmth and habitability of the Earth comes from the Sun, but the climate of the Earth is largely determined by the concentration of greenhouse gases in the atmosphere and ice on the surface of the Earth. In the absence of greenhouse gases, Earth would be frozen and white (super reflector!). Because of the large impact that greenhouse gases have on the Earth’s climate, the variability that comes from the Sun is buffered. Again, looking at the figure, a 1% change in the “incoming solar radiation” would be about 3.5 Watts/meter^2. Under the assumption that a 1% change would partition itself throughout the system in a proportional way, the 3.5 Watts/meter^2 change at the top of the atmosphere would translate to about 1.65 Watts/meter^2 at the Earth’s surface. This would then be portioned throughout the atmosphere, land, and ocean. The change from the Sun stands in stark contrast to the 333 Watts/meter^2 of “back radiation” that comes from the greenhouse effect of the atmosphere and clouds. From this sort of estimation, often called scaling arguments, if the Sun varies on the order of 1-2 %, then the impact on Earth’s surface is expected to be small. Variability from the Sun is spread throughout the Earth, and the 1% is, effectively, diluted.

That said: there are many signals that are observed in the Earth’s climate that are correlated with variability of solar energy. Some of these correlations are likely to be coincidence, but some of the correlations are quite compelling. (Remember correlation does not lead to a conclusion of cause and effect.) In order to establish the cause and effect a physical mechanism needs to be established. Often, in solar-terrestrial physics for the physical causality to be established between the Sun and temperature and precipitation at the surface of the Earth requires something that “amplifies” a relatively weak solar signal. The likely mechanisms to amplify a solar signal reside in, again, ice and greenhouse gases. More next time.

r

Blogs on radiative balance

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