That value is essentially a top of atmosphere (TOA) increase in longwave radiation. The value from CO2 is 1.7 W/m2. And taking into account all of the increases in trace gases (but not water vapor) the value totals 2.4 W/m2.
Comparing Radiative Forcing
The concept of radiative forcing is a useful one because it allows us to compare different first-order effects on the climate.
The effects aren’t necessarily directly comparable because different sources have different properties – but they do allow a useful first pass or quantitative comparison. When we talk about heating something, a Watt is a Watt regardless of its source.
But if we look closely at the radiative forcing from CO2 and solar radiation – one is longwave and one is shortwave. Shortwave radiation creates stratospheric chemical effects that we won’t get from CO2. Shortwave radiation is distributed unevenly – days and nights, equator and poles – while CO2 radiative forcing is more evenly distributed. So we can’t assume that the final effects of 1 W/m2 increase from the two sources are the same.
But it helps to get some kind of perspective. It’s a starting point.
The Solar “Constant”, now more accurately known as Total Solar Irradiance
TSI has only been directly measured since 1978 when satellites went into orbit around the earth and started measuring lots of useful climate values directly. Until it was measured, solar irradiance was widely believed to be constant.
Prior to 1978 we have to rely on proxies to estimate TSI.
Accuracy in instrumentation is a big topic but very boring:
- absolute accuracy
- relative accuracy
- long term drift
- drift with temperature
These are just a few of the “interesting” factors along with noise performance.
We’ll just note that absolute accuracy – the actual number – isn’t the key parameter of the different instruments. What they are good at measuring accurately is the change. (The differences in the absolute values are up to 7 W/m2, and absolute uncertainty in TSI is estimated at approximately 4 W/m2).
So here we see the different satellite measurements over 30+ years. The absolute results here have not been “recalibrated” to show the same number:
We can see the solar cycles as the 11-year cycle of increase and decrease in TSI.
One item of note is that the change in annual mean TSI from minimum to maximum of these cycles is less than 0.08%, or less than 1.1 W/m2.
In The Earth’s Energy Budget we looked at “comparing apples with oranges” – why we need to convert the TSI or solar “constant” into the absorbed radiation (as some radiation is reflected) averaged over the whole surface area.
This means a 1.1 W/m2 cyclic variation in the solar constant is equivalent to 0.2 W/m2 over the whole earth when we are comparing it with say the radiative forcing from extra CO2 (check out the Energy Budget post if this doesn’t seem right).
How about longer term trends? It seems harder to work out as any underlying change is the same order as instrument uncertainties. One detailed calculation on the minimum in 1996 vs the minimum in 1986 (by R.C. Willson, 1998) showed an increase of 0.5 W/m2 (converting that to the “radiative forcing” = 0.09 W/m2). Another detailed calculation of that same period showed no change.
Here’s a composite from Fröhlich & Lean (2004) – the first graphic is the one of interest here:
As you can see, their reanalysis of the data concluded that there hasn’t been any trend change during the period of measurement.
What can we work out without satellite data – prior to 1978?
The historical values of TSI have to be estimated from other data. Solanski and Fligge (1998) used the observational data on sunspots and faculae (“brightspots”) primarily from the Royal Greenwich Observatory dating to back to 1874. They worked out a good correlation between the TSI values from the modern satellite era with observational data and thereby calculated the historical TSI:
As they note, these kind of reconstructions all rely on the assumption that the measured relationships have remained unchanged over more than a century.
They comment that depending on the reconstructions, TSI averaged over its 11-year cycle has varied by 0.4-0.7W/m2 over the last century.
Then they do another reconstruction which includes changes that take place in the “quiet sun” periods – because the reconstruction above is derived from observations of active regions – in part from data comparing the sun to similar stars.. They comment that this method has more uncertainty, although it should be more complete:
This method generates an increase of 2.5 W/m2 between 1870 and 1996. Which again we have to convert to a radiative forcing of 0.4 W/m2
The IPCC summary (TAR 2001), p.382, provides a few reconstructions for comparison, including the second from Solanski and Fligge:
And then bring some sanity:
Thus knowledge of solar radiative forcing is uncertain, even over the 20th century and certainly over longer periods.
They also describe our level of scientific understanding (of the pre-1978 data) as “very low”.
The AR4 (2007) lowers some of the historical changes in TSI commenting on updated work in this field, but from an introductory perspective the results are not substantially changed.
Second Order Effects
This post is all about the first-order forcing due to solar radiation – how much energy we receive from the sun.
There are other theories which rely on relationships like cloud formation as a result of fluctuations in the sun’s magnetic flux – Svensmart & Friis-Christensen. These would be described as “second-order” effects – or feedback.
These theories are for another day.
First of all, it’s important to establish the basics.
We can see from satellite data that the cyclic changes in Total Solar Irradiance over the last 30 years are small. Any trend changes are small enough that they are hard to separate from instrument errors.
Once we go back further, it’s an “open field”. Choose your proxies and reconstruction methods and wide ranging numbers are possible.
When we compare the known changes (since 1978) in TSI we can directly compare the radiative forcing with the “greenhouse” effect and that is a very useful starting point.
Solar Irradiance since 1874 Revisited, Solanski & Fligge, Geophysical Research Letters (1998)
Total Solar Irradiance Trend During Solar Cycles 21 and 22, R.C.Willson, Science (1997)