Monday, 7 July 2008

Nothing Like the Sun?

A popular affectation among climate inactivists is to insist that all climate change is driven by the sun. Let's look first at figures for total solar irradiance or TSI, a value giving the energy from the sun incident on the earth. This number has been available from satellite data since 1978. It has been estimated in the past by indirect means ("proxies") but let's stick with the modern data the accuracy of which is easier to determine. Also, there is a long-term record of suns spots but in the period when satellite TSI data is available, they are also an approximation – so let's stay with the more accurate data.

How does that data compare with the temperature trend over the same period? The HadCRUT3 data from the UK's Hadley centre is one of several that are widely used. This one is sufficient to illustrate the point. Look at the two graphs closely, and what do you see? Probably not that much: they look pretty different.

In both graphs, the points joined by the broken line are the actual measurements, averaged over each year, and the points joined by the unbroken line are the 5-year mean, which smooths out short-term irregularities. I also use these 5-year means (for each year, found by averaging the two years before and after plus the current year) for comparing the two trends, to eliminate short-term variations.

Note also the trend lines. In each, the number before the "x" is effectively the trend per year. The "R2" value is a measure of how well the trend line represents the main graph (in this case, the 5-year mean). An R2 near 1 means there is a strong relationship; near 0 means the trend line is a poor approximation to the original data. Note that the temperature trend is going up at 1.83 degrees per century, with R2 = 0.92, a very strong relationship, well outside chance. The sun trend, on the other hand, is -0.74 degrees per century, a slow downward trend, with R2 = 0.0143. If you plotted a series of random numbers, you would get a "trend" about this strong.

In other words, the temperature trend is strongly up; the sun is down, but not convincingly.

So the trend lines are different; how come some people then are saying that the sun is the best explanation of climate variation? Let's look at another measure: correlation. Correlation tells you how well two different data sets track each other. A strong positive correlation, near 1, tells you they are following the same trend. A strong negative correlation, near -1 tells you they are heading the opposite way. Anything closer to 0 tells you they are (relatively, depending how close to 0) unrelated.

Let's look then at the correlation of the temperature trend for the given years with the TSI data. It comes out as 0.37 – a reasonably strong positive correlation. What is interesting though is to look at the correlation since 1990, when the warming trend has strengthened. The correlation is -0.11 – negative but no longer a strong relationship. Look at the correlation for the period 1978-1989, and it's 0.17. Positive, but not very strong.

How could this have happened?

Answer: the sun does indeed drive climate, but something superimposed on that trend from 1990-2007 disguises that relationship. In other words, a good explanation of what's going on is that the sun is responsible for the irregular dips and peaks in the temperature graph, but when you try to make the sun explain an upward trend, it doesn't work anymore.

For some reason, at this point, I'm reminded of a line from one of Shakespeare's sonnets:
My mistress's eyes are nothing like the sun