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

Sunspots and Climate

It is bizarre that anyone is claiming that we should be basing climate models on sunspots at all. What is important is how much energy is incident on the planet, and how the overall energy balance changes. Sunspots may be a proxy for solar output, but we have direct measurements in the modern era, specifically satellite-based measures of total solar irradiance (TSI).

Nonetheless the thing persists.

In April 2008, an article appeared in The Australian alleging there had only been 3 sunspots since January and hence we were headed for an ice age. This article was rebutted by Melbourne academic David Karoly. On 15 May, a letter appeared in The Australian, claiming that Karoly had confused sunspot number (a calculated value) with number of sunspots. It's possible Karoly did, though he was correcting the previous article's manifestly wrong claim that there had only been 3 sunspots in total over a period when there had been many more.

This latest letter claims:

One visible sunspot is represented by a sunspot number of 11. Karoly writes that “the average number of sunspots a day last January was 3.4, followed by 2.1 in February and 9.3 in March’’. In fact, those numbers show months with many days of zero observed sunspots - a very quiet period on the Sun that has now extended longer than expected. Whether that affects the weather cannot be so easily disregarded as does Professor Karoly.

There does seem to be an interesting correlation between the length of sunspot cycles and climate change going back over centuries, compared with the 150 years that is all the CO2 warmers can work with.

There are several problems here.

For a start, it is incorrect to say that a sunspot equates to the number 11 in sunspot numbers. Let's see what the actual definition of sunspot numbers is:

An observer computes a daily sunspot number by multiplying the number of groups he sees by ten and then adding this product to his total count of individual spots... each daily international number is computed as a weighted average of measurements made from a network of cooperating observatories.

That is a nitpick of a nitpick so I will not dwell on it. The substance of the matter is his claim that sunspot cycles are a much better indicator of climate trends than modern models. Aside from the issue of more direct measures of solar output, the last sentence is bizarrely off target. Why should it be better to have centuries-long data of a phenomenon that did not exist centuries ago?

Anthropogenic carbon dioxide emissions began in earnest in the 18th century; by the close of the 19th century, they were less than 10% of current levels.

Saying that sunspot counts stood us in good stead for centuries so we should disregard other measures is like saying horse riding techniques stood us in good stead for centuries, so someone learning to drive a car should go to a riding instructor. In any case, the "centuries" claim is a tad inaccurate. The modern method of counting sunspots was invented in 1848.

Then there's the question of whether sunspot numbers actually correlate to the modern temperature trend. The IPCC has always stated in its reports that climate is a function of natural and anthropogenic influences, and even shows them separated out (here's a paper that does this nicely – see Figure 2(d) in particular, reproduced here). We would expect that solar output could be a major influence on climate before CO₂ emissions became a major factor. But in recent years, this is not the case. In fact the trend in solar output is the opposite to that required to explain the warming trend in the late 20th to early 21st century.

If anyone asks nicely, I may even add in a few correlations.

But in the meantime, here is where you can find the data:

• sunspot numbers
  
• NASA temperatures
  
• Hadley temperatures