The Sun vs Forcings and Feedbacks?

Question

So there is something I don't understand about those who do not believe anthropogenic global warming is a reality. They often attribute the warming to the Sun which, as I hope we all know, is recognized by both sides as the main energy source and warming mechanism of the planet. However saying all climate change is specifically due to the Sun is a very simplistic way of looking at it. They then go on to argue that the climate is much to in-depth for us to understand completely and any estimates on future temperature variations and such are just guesswork. They often go on to state that the planet will begin cooling soon due to, what they believe, is the current phase of the Milankovitch cycle. To the 'skeptics' how much of this do you agree with and how much of it do you not agree with? Do you have any data to back up your beliefs? To the 'realists', have you noticed this also? What percentage of change do you attribute to each forcing and feedback?

And another question: Recently it has been shown that the thermosphere is decreasing in size as usually occurs during times of decreased solar input. What effect would this have on temperature variations of the lower tropopause? Adding onto the original question, I have seen 'skeptics' argue that this is the reason for the decrease in temperatures of the upper atmosphere from the mesosphere on up yet state that the Sun is the reason for the recent warming trend. How do both sides feel about this argument?

Answer 1

Total Solar Irradiance

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The ‘it’s the Sun’ argument has become very jaded and I’m surprised that anyone is still using it. We measure the amount of energy from the Sun (total solar irradiance or TSI) as the energy in Watts per square metre per annum on a plane perpendicular to the Earth’s surface at the upper edge of the atmosphere. There are three TSI composites that use the data from the NIMBUS, VIRGO, SORCE, UARS and SOHO satellites, they are…



• Frohlich and Lean - Physikalisch Meteorologisches Observatorium Davos (PMOD)

• Wilson - Active Cavity Radiometer Irradiance Monitor (ACRIM)

• Dewitte et al - Institute Royal Meteorologique Belgique (IRMB)



Here are some graphs showing the PMOD, ACRIM and IRMB composites http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant. ACRIM and PMOD show TSI to be ≈1366W/m²/yr, IRMB has a very slightly higher value closer to 1367W/m²/yr. All composites have a full cycle (wavelength) variability of ≈0.8 W/m²/yr.



In short – the amount of energy received from the Sun varies by less than one thousandth from the peak of a sunspot cycle to the trough of a sunspot cycle.



Yesterday you asked a question about the mathematics of climate sensitivity (CS) and equilibrium climate sensitivity (ECS) (the question is now deleted) and I used figures that illustrated that ECS = 3.7W/m²/yr from a 1750 baseline figure of 278ppmv of CO2 against the 2008 value of 384ppmv. The 3.7W/m²/yr is therefore equal to a rise of 106ppmv of CO2. TSI variability at the edge of the atmosphere is ≈0.8 W/m²/yr of which 0.25 of this ends up as reradiated infrared radiation from Earth, or 0.2W/m²/yr. 3.7 ÷ 0.2 = 18.5, 106 ÷ 18.5 = 5.7. The difference between sunspot cycle maxima and minima is ≈0.8 W/m²/yr which is ≈ 5.7ppmv of CO2 (or 4.4ppmv annual CO2e).



With GHG’s increasing by 2.4ppmv per year (as CO2e), the TSI variation across an entire solar cycle is equivalent to just 2 years of GHG emissions.



Can changes in TSI account for the recent warming (or cooling according to some)? Not even close.



Links to further reading…



• Solar Variability and Planetary Climates - http://books.google.co.uk/books?hl=en&lr=&id=RO0XGT9WwjoC&oi=fnd&pg=PA4&dq=Solar+Variability+and+Planetary+Climates&ots=y5fkyNqdPL&sig=YUGkmOWzn61i5fZFoxROb9m67a4#v=onepage&q&f=false



• Solar Change and Climate: An Update in the Light of the Current Exceptional Solar Minimum

http://www.eiscat.rl.ac.uk/Members/mike/publications/pdfs/2010/261_Lockwood_RSPA2010.pdf



• A very recent publication – Solar Influences On Climate

http://www.eawag.ch/forschung/surf/publikationen/2010/2010_gray.pdf





Milankovitch Cycles

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Using these cycles as an explanation for recent climatic changes is ever more bizarre than blaming it on sunspots.



There are several cycles, the shortest cycle is one of aspidal precession and has a periodicity of approx 24,000 years, the effect on the climate is minimal. The most significant cycle in terms of climatic variability is that of eccentricity – the circularity of Earth’s orbit around the Sun. Each cycle takes 100,000 years and is so momentous that it causes the coming and going of ice ages.



This 100,000 year cycle is typified by approx 90,000 years of gradual cooling and 10,000 years of rapid warming during which time the AGT varies by 7°C. At the peak of this most dramatic of cycles the AGT increases by 1°C for every 1,400 years. Currently the planet is warming 26 times as fast as any of the Milankovitch cycles could allow for.



Further, the eccentricity phase is currently a cooling one. The last warming event occasioned the demise of the last ice-age approx 10,000 years ago and since then eccentricity has dictated cooling of approx 0.00008°C per year.



Can changes in the Milankovitch Cycles account for the recent warming (or cooling according to some)? Not even close.





Thermosphere / Tropopause etc

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The stratosphere is cooling and quite significantly so, the current temperature anomaly is in the order of –0.6°C, this is comparable to the warming anomaly at lower levels of the atmosphere.



Human activities are producing significant amounts of greenhouse gases and these are building up in the atmosphere. As levels of these gases increase so too does the amount of heat they retain. The type of heat being trapped is outgoing thermal radiation – the sort that is radiated outwards from the surface of the Earth and everything on it.



As more heat becomes trapped at lower levels there is a corresponding drop in the amount of heat reaching the stratosphere and thus it cools down.



Even greater cooling has been observed in the upper layers of Earth’s atmosphere. Above the stratosphere is the mesosphere and here the temperature has fallen by about 8°C, by the time you get to the thermosphere (320km to 350km altitude), the temp has fallen by approx 15°C. At these heights there is no ozone and all of the cooling can be attributed to the reduction on thermal radiation reaching this high into the atmosphere.



Regarding the point you made about the troposphere expanding. The boundary between the troposphere and the stratosphere is the tropopause, the location of which is defined as the point where the lapse rate changes from positive to negative (cooling as you go up through the troposphere, equilibrium at the tropopause, warming as you continue up through the stratosphere).



With the troposphere warming it should be expanding. However, with the stratosphere and higher layers cooling they are also contracting and in doing so their density is increasing (the sky is literally falling in). As the lower warming and upper cooling are more or less balanced then it would appear that the increased weight of the upper layers of the atmosphere should be applying as much downward force as the troposphere is applying upward force; in this respect the system would appear balanced.



However, the volume of atmosphere that is contracting is greater than the volume of atmosphere that is expanding and so the additional downward force is weaker than the additional upward force, the overall result of which is that the tropopause is rising and the troposphere is expanding; “by several hundred meters since 1979” according to Science http://www.sciencemag.org/cgi/content/abstract/301/5632/479



If you have a subscription to Science Magazine there’s a good description of “Global Change in the Upper Atmosphere” http://www.sciencemag.org/cgi/content/summary/314/5803/1253 Another good source of info is “Stratospheric Temperature Trends: Observations and Model Simulations” from the NOAA (free) http://www.gfdl.noaa.gov/bibliography/related_files/vr0101.pdf



It should be noted that for purposes of brevity I’ve omitted the role of ozone, stratospheric UV energy imbalance, polar stratospheric clouds, nitric acid trihydrate and chlorine activation. You’ll find that explained in an earlier answer of mine https://answersrip.com/question/index?qid=20100615185906AAQ3Td4



EDIT: Oops, should have stated that he annual GHG increase of 2.4ppmv pa is measured as CO2e.

Answer 2

>>>To the 'realists'...



Yes, I've noticed this as well. I find it odd that a single explanation is given from the same people who assert that the climate is too complex to (do what?) draw a single explanation.



To answer your second question, I don't know enough about each variable to attribute percentages. I don't think that I'm unjustified though, considering that solar output has not increased in the past several decades, in saying that the forcing due to solar output has been more or less zero for the time period. Perhaps negative, at least recently due to the prolonged minimum. Comparing greenhouse gases and feedback v. forcing, I can only give a qualitative statement of what seems to be the case: CO2 is the main forcer; water vapor, methane, and more CO2 seem to be the feedbacks. Quantitatively I'll defer to people with more expertise than myself, such as Trevor. Then you also have the drop in surface albedo due to melting sea ice, which is another feedback mechanism.



As to the thermosphere decreasing in size and the effects on temperatures lower in the troposphere, that's a good question and I do not know the answer to it. However, the connection to the upper atmosphere is specious since the decline in upper atmosphere temperatures has been going on before the last minimum (which had the very drastic effect on the thermosphere).

Answer 3

Well first off I'll link to my "it's the Sun" rebuttal, because it addresses some aspects of your question.

http://www.skepticalscience.com/solar-activity-sunspots-global-warming-advanced.htm



The stratosphere is tricky because its temperature is complicated by ozone depletion and recovery. Nonetheless, there is a very strong long-term cooling trend, about twice as rapid as the tropospheric warming, in fact. But on top of that, the higher layers of the atmosphere (which aren't impacted by ozone) are also cooling as expected from an enhanced greenhouse effect.

http://www.skepticalscience.com/its-not-us-advanced.htm



Yes, I have noticed 'skeptics' contradicting themselves by claiming that we don't know enough about the Sun or the climate in general to understand what's causing global warming, yet they seem very certain that it will soon reverse itself and we don't need to worry about it. As we all know, denial is rife with these sorts of contradictions.



As for the changes in the thermosphere, my understanding is that while it may have significant impacts on some local weather, it will not have a significant impact on global temperatures.



The argument that this change due to the extended solar minimum has caused recent cooling is ludicrous on many levels. First off, 2009 was tied for the second-hottest year on record, and 2010 will likely tie for the hottest on record. Where's the cooling?? Secondly, even during this extended solar minimum, the change in solar activity was very minor. If the climate is so sensitive that such a small change in solar activity can cause a significant global temperature change, then why exactly is it insensitive to CO2 changes, as deniers also claim? And why wasn't the early 20th century warming larger, in response to the much larger change in solar activity at the time?



The bottom line is that the Sun isn't causing the current warming. Deniers really desperately want to be able to blame the Sun because it's a convenient scapegoat as "the source of all warmth on Earth". But it just ain't so.