Nir Shaviv: The missing link between exploding stars, clouds and climate on Earth

Figure 4: The correlation between the linearly detrended sea level measured using satellite altimetry (blue dots) and a model fit which includes just two components: The sun and el Niño southern oscillation. The excellent fit implies that the two components are by far the dominant source of sea level change on short time scales

Background:

It has long been known that solar variations appear to have a large effect on climate. This was already suggested by William Herschel over 200 years ago. Over the past several decades, more empirical evidence have unequivocally demonstrated the existence of such a link, as exemplified in the examples in the box below.

The fact that the ocean sea level changes with solar activity (see Box 1 above) clearly demonstrates that there is a link between solar activity climate, but it can be used to quantify the solar climate link and show that it is very large. In fact, this “calorimetric” measurement of the solar radiative forcing is about 1 to 1.5 W/m² over the solar cycle, compared with the 0.1-0.2 W/m² change expected from just changes in the solar irradiance. This means that a mechanism amplifying solar activity should be operating—the sun has a much larger effect on climate than can be naively expected from just changes in the solar output.

Over the years, a couple of mechanisms were suggested to explain the large solar climate link. However, one particular mechanism has accumulated a significant amount of evidence in its support. The mechanism is that of solar wind modulation of the cosmic rays, which govern the amount of atmospheric ionization, and which in turn affect the formation of cloud condensation nuclei and therefore how much light do the clouds reflect back to space, as we now explain.

Cosmic Rays are high energy particles originating from supernova remnants. These particles diffuse through the Milky Way. When they reach the solar system they can diffuse into the inner parts (where Earth is) but lose some energy along the way as they interact with the solar wind. Here on Earth they are responsible for most of the ionization in the Troposphere (the lower 10-20 km of the atmosphere where most of the “weather” takes place). We now know that this ionization plays a role in the formation of cloud condensation nuclei (CCNs). The latter are small (typically 50nm or larger) aerosols upon which water vapor can condense when saturation (i.e., 100% humidity) is reached in the atmosphere. Since the properties of clouds, such as their lifetime and reflectivity, depends on the number of CCNs, changing the CCNs formation rate will impact Earth’s energy balance.

The full link is therefore as follows: A more active sun implies a lower CR flux reaching Earth and with it, lower ionization. This in turn implies that fewer cloud condensation nuclei are produced such that the clouds that later form live shorter lives and are less white, thereby allowing more solar radiation to pass through and warm our planet.

Read the full article here