In the series CO2 – An Insignificant Trace Gas? we concluded (in Part Seven!) with the values of “radiative forcing” as calculated for the current level of CO2 compared to pre-industrial levels.
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.
Earth from Space - pretty but irrelevant..
Accuracy in instrumentation is a big topic but very boring:
- absolute accuracy
- relative accuracy
- repeatability
- 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:
Total Solar Irradiation, as measured by various satellites
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:
Composite TSI from satellite, 1978-2004, Frohlich & Lean
As you can see, their reanalysis of the data concluded that there hasn’t been any trend change during the period of measurement.
Proxies
What can we work out without satellite data – prior to 1978?
The Sun
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:
Reconstruction of changes in TSI, Solanski & Fligge
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:
Second reconstruction of TSI back to 1870, Solanski & Fligge
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:
Reconstructions of TSI back to 1600, IPCC (2001)
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.
Conclusion
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.
References
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)
The Science of Doom 
It is interesting to read how constant the TSI is and then to look at the forcings used in many of the models. The forcings show a strongly rising TSI over time. Presumably, this is to account for “natural” climate varitions before CO2 kicks-in as the main driver. For me, it raises questions about the various models’ accuracy as a forcasting tool.
Chad had a good post looking at this in “Trees for the Forest”
http://treesfortheforest.wordpress.com/2010/01/04/another-brief-look-at-climate-model-solar-forcing/
Tim
You might want to take a closer look at the reconstructions presented in the graphs above.
They don’t show the TSI to be constant in the first half of the 20th century, but rising.
All the models do is plug in the data from the reconstructions, in essence.
Where else would you propose they get their data, if not from the published literature?
Your accusation that they’re in essence making stuff up to account for “natural” variation is wrong. Again, they’re not making it up, they’re taking it from the published literature.
dhogaza,
You are right, I had implied an accusation and I was wrong to do so. I retract it and apologise.
Do you know if more recent model runs have used updated TSI reconstructions or do they use The Hoyt or Lean reconstructions? The older reconstructions had about 14 W/m^2 variance vs the 1 W/m^2 variance in the more recent reconstructions (ie Svaalgard, Premiger & Walton).
I don’t know what reconstruction is considered most accurate currently. Svaalgard’s appears to show a small upward trend in the 1900-1950 timeframe, flat in recent decades.
Be careful you don’t fall into the trap of demonstrating less solar variation and therefore more climate sensitivity to CO2, though 🙂
Tim W:
Are you talking about back to the Maunder Minimum (MM)? Of course even more uncertainty as we go further back before 1870.
From what I was reading, on the MM (early 1700s):
Solanski and Fligge said:
They themselves, with this approach, come up with 4W/m^2 (change since Maunder Minimum).
The IPCC AR4 (2007) said:
Then they list the relevant work, dates and the values (recalculated to radiative forcing – so converting back to TSI..)
Solanski (1998) 4W/m^2
Lean (2000) 2.2W/m^2
Foster (2004) 1.6W/m^2
Wang (2005) 0.6 W/m^2
I don’t really know why I am putting decimal points in. Perhaps “not much” or “not far off 10” might better indicate the uncertainty.
[Interesting read for anyone who wants to learn more – Ch2, AR4 pp188-193., get it from ipcc.ch]
Well, as one who had is mind made up before knowing the facts, I’m trying to learn. I also have some un-learning to do which is why I’m at this site.
That said, I don’t think anyone really knows what the change in TSI was prior to directly measuring it. It seems to be anywhere between huge and insignificant. However, the models seem to have a lot riding on the real answer — and that leaves a lot of room for uncertainty in the projections.
I have to confess that I mis-read the scale when eyeballing a graphical comparison of several reconstructions from 1850 -on. The 14 W/m^2 i cited earlier should have been 4 W/m^2.
(note to self Think — then think again — then post)
Tim W:
I tend to agree about the lack of knowledge prior to 1978, which is one reason why I didn’t spend too much time on it.
However, from 1870s onwards there have been very detailed observation on the sun. There seems less variation in the reconstruction numbers.
From 1700-1870, it’s definitely pick your own number between 0-10 (or maybe greater) !
But it does create an interesting question as to the models. You could take a 0.4W/m^2 TSI number from 1870 to today. Or 2.5 W/m^2 TSI number from 1870 to today.
I would guess that the whole range has been used in various models to see what effect it creates over the last century and a bit.
Remember that greater solar output 1900-1950-ish is only one explanation for the rise in temps – it was also a period of quietness for large-scale volcano eruptions. This *is* a direct observation (“hey, look – kaboom!”).
I don’t see where you get your conclusion, though. Direct forcing due to CO2 absorption of LW IR comes from physics, confirmed by a variety of observations, with no dependence on whatever caused warming 1900-1950. Total sensitivity due to the combination of forcing and feedbacks is quite tightly constrained by the geologic records and paleoclimate reconstructions. If anything, they point to 3C as being more likely a minimum sensitivity value.
If one can’t make things balance out for 1900-1950 for a lower TSI figure such as Svalgaard’s, given what’s known about CO2 forcing and constraints on the sensitivity figure would cause one to focus on the TSI reconstruction with at least as much scrutiny as the CO2 issue. Or to look for something else that was varying at the time.
“Total sensitivity due to the combination of forcing and feedbacks is quite tightly constrained by the geologic records and paleoclimate reconstructions. ”
Its hard to swallow that one without a lot more info. Maybe science will eventually do a post on that.
Speaking of radiative forcing, do you know what the approximate forcing would be for the loss of the summer ice in the Arctic, which seems more and more like a 10-20 years for now thing?
My crude back-of-the-envelope assuming 1.4% of the earth’s area went from an albedo of 0.7 (ice) to 0.08 (open water) under 400W/m^2 irradiance times six months of the year (0.5). I got about 1.7W/m^2, which seems awfully high.
Robert
I think your numbers about albedo change are roughly right -without checking them. As to whether 10-20 years is correct I don’t know. I might get the chance to look at this question again later, but anyhow, take a look at this-
Here is what I wrote about the subject in a different forum.. looking at it from a different point of view..
By refreezing the arctic and northern europe and therefore increasing the albedo of the planet.
If the albedo of the planet increases by 1% that’s equivalent to about 3W/m^2, or a little more than the current impact of CO2.
The current albedo of the northern mid to high latitudes is around 45-55%.
If we took an area that was grassland, soil or forest and covered it with snow or ice it would go from around 20% to 60%-80% albedo.
If we took ocean and covered it with ice it would go from 6% to 60% albedo.
So an extra 7M km^2 of water with ice and an extra 3M km^2 of land with snow/ice would make that change.
That’s about 50% more sea ice in the arctic at the moment and a pretty big increase in northern europe/n.russia ice & snow.
Also, if as a result of the THC shutdown the energy in the climate system is simply “re-arranged” then the tropical/sub-tropical regions will be warmer as a consequence of the northern high latitudes being colder.
As energy is radiated at T^4, this will also result in a higher overall radiation. Back of envelope..
20% of the world’s surface decreases from 0 to -5’C and 20% of the world’s surface increases from 15-20’C then there will also be a 1.1W/m^2 increase in radiation from the earth’s surface (averaged across the whole planet).
“If the albedo of the planet increases by 1% that’s equivalent to about 3W/m^2, or a little more than the current impact of CO2.”
I was hoping I could plug a number like that into a simple calculation based on the long-term summer ice extent (about 1.4% of the earth’s surface). Unfortunately, I realized the average will tend to mislead, since the Arctic gets less solar radiation than average.
My number one reason for thinking I cannot possibly be right is that we have already said goodbye to at least a sixth of the summer ice. That would be a positive feedback of about 0.3W/m^2, which should be big enough to show up on the those nice tables of forcings (like the one here: http://www.columbia.edu/~mhs119/Storms/).
Maybe I’m just the first person on earth to realize we need to start factoring albedo change forcings from the loss of Arctic sea ice. Seems implausible, but I have been eating a lot of fish recently.
Robert:
The reason it’s not in a table of forcings is because it is a feedback. So your quick calc might well be right.
You are correct about the solar radiation adjustment. And it’s not so difficult to do, just a “quick” integration
insolation, I = S cos Z
where Z = zenith angle, S is the solar “constant”, less reflections and absorptions in the atmosphere (ouch, see end)
Then Z is “easily calculated”:
Z = cos-1 (sin L sin D + cos L cos D cos H)
where
L = Latitude (doesn’t change of course)
H = Hour Angle = 15′ x (Time – 12) (Angle of radiation due to time of day. Time is given in solar time as the hour of the day from midnight.)
D = Solar Declination Angle (slow change throughout the year from -23.5′ to +23.5′
Then how about the changes in atmospheric reflection and absorption at that latitude? Generally, a little over 50% of solar radiation makes it to the ground – on average. Is the average ok to use?
Still might be interesting to do the calculation and see what churns out..
Here’s a great way to see the solar insolation – someone who’s done all the hard work for us:
http://pvcdrom.pveducation.org/SUNLIGHT/SHCALC.HTM
Just move the slider around for latitude and time of year and the daily insolation curve is produced!
Thank you! I didn’t realize that, as a feedback, it wouldn’t be included.
Cool site, too.
Leif Svalgaard shows a much flatter trend for TSI in the 20th Century.
Click to access TSI-LEIF.pdf
There is some discussion and justification of it here:
http://climateaudit.org/2007/11/30/svalgaard-solar-theory/
Apologies if you’re already aware of this reconstruction. Enjoying this blog, btw. Plenty of information, very little invective. I hope you keep it going.
Yarmy:
Thanks for the links and the kind words. I hadn’t seen the ClimateAudit presentation and discussion. And I haven’t read any of Svalgaard’s papers but now I will try and take a look.
Climate science is a vast subject – so no need for anyone to apologize even for highlighting the foundational stuff.
In fact, good papers or websites that explain the basics – or a particular point of view well are worth highlighting.
Plenty of people visiting – plus me – who will appreciate it.
[…] solar heating was the direct cause (see Here Comes the Sun) the stratosphere would not be cooling. However, other effects could possibly also cause […]
Dead post?
Hi
I’ve got a technical question, I hope you can spare a minute or two. You say this
“showed an increase of 0.5 W/m2 (converting that to the “radiative forcing” = 0.09 W/m2)”
Can you do the ‘idiots guide’ to converting change in TSI to radiative forcing.
Thanks
nother example from your post.
“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”
Thanks
HR:
For the conversion, check out The Earth’s Energy Budget – Part One
Thanks