Feb 26 2008

Its the Sun. Again?

Published under Climate Change, Radiation

sun_mercury_transit_inset.jpgThe sun crowd is back at it again. I guess because weve had a cold January and were currently at a minimum in the solar cycle that must mean the two are related. Thats bullocks of course. A few months ago it was the sun that was causing the warming. The sun has approximately 11 year cycles, so it cant be both. So has the sun caused the recent warming, or the recent cooling? Neither or both? This post attempts to find out the answer.

Ive previously shown that the effective temperature of a planet is equal to

effective temperature

where Ap is the albedo, Ip is the irradiance from a sun intercepted by the planet, and σ is the Stefan–Boltzmann constant. Since we have instruments that measure Ip, we can figure out Tp.

According the Gregs TSI Page, the total solar irradiance since 1980 has been somewhere between 1360 and 1375 W/m2. However, if you look at that image, the measurements from the different satellites dont match up. This is because making an absolute measurement of TSI is difficult to do. When aligned to a common scale, it looks much better.

The measurements have been scaled to the TIM instrument. However, other scalings will have little difference than what is shown below.


At around the year 1980, the maximum TSI was 1362 W/m2. Since 1980, the TSI has fallen to about 1360 W/m2. Using the equation above, we can calculate the change in effective temperature due to this change in irradiance. In 1980, the effective temperature would be 254.62 Kelvin. In 2007, the effective temperature would be 254.53 Kelvin. This is assuming an albedo of 0.3. Thus, if the sun were the main driver of temperature changes since 1980, we would expect a decrease in temperatures.

That obviously hasnt happened. By all accounts, the temperature since 1980 have risen. For instance, the group at UAH have shown that temperature have risen by an average of 0.14 degrees C per decade. Other global temperature trends are similar to UAH. I quote theirs because its usually the lowest, and its the easiest to find on the web. Ill happily add other trends if provided.

Have I cherry-picked the dates to use as endpoints in the above analysis? Absolutely. But I chose them to show a worst case scenario. In actuality, the variations due to the solar cycle are less than the computed values above. This is most clearly seen when looking at the global average temperature graphs. If the sun is the main cause, we would expect to see a clearly defined 11 year signal. We dont.


Image courtesy Anthony Watts at Watts Up With That.

In January 2008, there was a sharp drop in global temperatures or around 0.6C reported by at least 4 different groups. Using the equation above, we can see what change in irradiance must be required for this to be caused by solar variations. Im going to estimate that the TSI for December 2007 to be 1360.5 W/m2. This equates to an effective temperature of 254.55K.

What irradiance would be needed to cause the temperature to decrease by 0.6K? This can be computed using the same equation above. I come up with a value of 1347.65 W/m2. When looking at the TSI plot above, we can see this is off the bottom of the chart by almost 10 W/m2. This is almost 5 times more than the variability from each solar cycle. Surely the recent cold cannot be caused by the sun.

Others seem concerned that solar cycle 24 should have started already, and that the lack of current sunspots seems to indicate that were doomed to another ice age. Im not an astrophysicist, so the inner working of the sun are a mystery to me. However, even a cursory glace at the butterfly diagram shows that we arent overdue yet.

An interesting side note: the above inset image is of a transit of Mercury. The TSI page has total solar irradiance values from a transit of Venus, which show that there was a decrease of about 1 W/m2 during the event. This is about 1/2 the value of the solar cycle.

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  • One Response to Its the Sun. Again?

    1. Bob Northon 27 Feb 2008 at 8:33 am

      Atmoz - Thanks for the post, but think there might be a typo in your calculated effective temperatures for 1980 (254.53K) and 2007 (254.62K) since in the text you say 2007 should be lower than 1980 (which makes sense given the slightly lower TSI). Also, is there supposed to be a lag time on the response to changing TSI as there is with CO2? I would assume there is some short-term lag just based on the lag we see on a seasonal basis.


      [Reply: Oops. Thanks for the catch. I reversed the years.

      Regarding lag time, I would suspect there would be since the oceans have such a large heat capacity. If the sun were a major source of climate change within the last few decades, I would still expect to see an ~11 year cycle in the temperature data - possibly lagged from from the TSI data. But we dont see this in the temperatures.]

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