This is the second post in a 4-post series on the connection between the sun, sunspots, and climate.

Sunspots and cosmic rays are not the cause of the recent warming. How do we know? It’s all in the data.

Solar variations can have and have had a huge effect on our climate — that’s obvious. But have they been the cause of climate change over the past 30 years or so, when we have seen an accelerated rate of warming? (I will address the issue of the past ten years of warming or lack thereof in a subsequent post, so please hold your comments on that one.) In the first part of this series, I presented data from space-borne instruments that show that changes in solar output (i.e., total solar irradiance or TSI) were minimal over this period and therefore did not drive the temperature change.

But what about sunspots and cosmic rays?

The Basics

Sunspots are areas of strong magnetic fields on the surface of the Sun. They are caused by disruptions in the Sun’s magnetic field (believed to result from the different rates of rotation in the polar and equatorial regions of the Sun, which create a drag in the magnetic field). Why are they called “sunspots?” Because they appear as small dots on the Sun’s surface. Want to learn more about sunspots? Check out this site.

Sunspots correlate closely with what we call solar activity. They tend to reach a maximum at the same time that the solar output or TSI is at a maximum and vice versa. That is why the 11-year cycle, in which TSI cycles up and down, is called the "sunspot cycle."

A wide variety of parameters related to the Sun vary with the sunspot cycle. One of these is solar wind. The solar wind is a stream of charged, energetic particles emanating from the Sun’s surface and outwards into the solar system. (Read more about solar wind.)

The term cosmic rays is a catchall to describe the energetic particles impinging upon our atmosphere from outer space. They include galactic cosmic rays coming from outside the solar system, solar energetic particles coming to us in the solar wind, and anomalous cosmic rays consisting of particles that are not in the solar wind and not from outside the solar system. (Learn more about cosmic rays.)

The next section has been corrected as per Mark Adams's first comment

Hypothesis: Sunspots, Cosmic Rays, and Climate Are Linked

The hypothesized climate connection is often credited to Danish scientist Henrik Svensmark, who published a paper on the subject in the late 1990s.

The idea goes like this (corrected version--see comments):

1. An increase in solar activity (characterized by an increase in sunspots) results in a strengthened solar wind;
2. A strengthened solar wind allows fewer galactic cosmic rays to penetrate into the solar system and impinge on the Earth’s upper atmosphere;
3. The decrease in galactic cosmic rays raining down on the atmosphere decreases the ionization in the upper atmosphere;
   
4. The decreased ionization in the upper atmosphere suppresses cloud formation in the lower part of the atmosphere;
5. Fewer clouds mean less reflection of sunlight and more absorption of sunlight by the Earth; and
6. Higher temperatures.

Is the Connection Real?

The connection between sunspots, solar activity, and cosmic rays has been verified; it exists, no question. The graphic shown here (plotted by scientists at the National Oceanographic and Atmospheric Administration, or NOAA, who monitor cosmic rays in Colorado) clearly shows the inverse relationship between sunspots and cosmic rays. Both run up and down with the 11-year sunspot cycle, but in opposite directions.

The connection between cosmic rays and cloudiness is more speculative. Some analyses of historical cloudiness data suggest that a correlation between the two does exist, as hypothesized; others do not (see here).

Maybe the Connection Exists, but There Is No Trend

Let's suppose for the sake of argument that there is a connection between cosmic rays and cloudiness, as speculated. Could this explain the temperature increase over the past few decades? Clearly not. Why? Because there has been no net change in sunspot numbers or cosmic rays over this period. Take a look at the graphic above. Cosmic rays cycle up and down with the sunspot cycle, but there is no net change over the cycles covering the period from 1960 to 2005.

So is there a sunspot-cosmic ray-climate connection? Quite possibly.

Could it hold the key to understanding the linkage between solar activity and climate change on long-time scales? Maybe.

Does it explain the warming we have experienced over the past 30 years? No.

And this no is based not on models or theories — it comes from just data.

Next up in this series – has global warming stopped?

Other Posts in Global Warming and Predictions of an Impending Ice Age

Part 1: Total Solar Irradiance

Part 3: Global Warming Since 1998

Part 4: Predicting Future Climate