In his recent article on ‘the greenhouse effect’ Dr Roy Spencer presents his ‘Alabama Two Step’ argument which contends that the basic facts of radiative physics as presented by the IPCC are correct, but the devil in the detail, feedbacks, means that the warming effect of extra co2 in the atmosphere is strongly limited by Earth’s internal climate system. For Roy, the question of co2 driven warming is not a matter of whether it exists, but a question of how strong it is.
I think this is correct as far as it goes, but misses out much of the real situation regarding the question of how energy flow in the Earth’s climate system is organised, and what causes what. Roy talks about ‘the greenhouse effect’ as if atmospheric radiation and convection are the only important factors. My contention is that he is only discussing the radiative greenhouse effect, and that this misses a large part of the totality of the ‘greenhouse effect’.
There is an old joke about the recent rediscovery of an ancient Gaelic book: ‘Gaelic Dancing part two: Arms’, and it applies to Roy’s Alabama two step as well. Atmospheric radiation and atmospheric convection are probably less than half of what really causes the Earth’s surface to be much warmer on average than the Moon’s.
So what kind of arm-waving argument am I going to offer in support of this assertion?
One aspect indicating that we need to take a fresh look at the greenhouse effect is the question of its magnitude. Phd Scientists Ned Nikolov and Karl Zeller report that recent MSU measurements of the Moon’s surface temperature made by an instrument carried aboard the Diviner spacecraft show that the Moon’s surface temperature is much colder than previously thought. In fact it is empirically determined to be in the region of 161K. This figure agrees well with the theoretical calculations made in Nikolov and Zeller’s paper ‘Reply to comments on the UTC part 1‘, which arrives at a figure of 155K. This possible error of around 6K is a lot less than the error arrived at using the Stefan-Boltzmann law in the way it has been traditionally employed to estimate the average surface temperature of airless celestial bodies. The same discrepancy has been noted on Mercury, and after a bit of prompting from the Talkshop, Wikipedia has removed the mis-estimate generated by the mis-application of the S-B law from its page on the planet Mercury.
However, the Moon’s average surface temperature lies at one end of the limits of Holder’s Inequality, due to the poor conductivity of heat by the Moon’s surface. A theoretically perfect black body would instantaneously spread the energy arriving at its surface evenly over its entire surface area. If the Moon’s surface were like that, it would have an average surface temperature equal to that predicted by the other limit of Holder’s inequality, at around 255K.
This 255K figure is of course familiar to us already. It is the notional temperature of the Earth without greenhouse gases used by the IPCC to claim that the radiative greenhouse effect is what is responsible for the difference between that figure and the balmy average 288K we enjoy where we live on the ground. So what is it about Earth’s surface that means that it is at the upper end of the range determined by applying Holder’s inequality to the grey body temperature of a bare, Moonlike planet earth? The answer is of course that Earth has oceans and an atmosphere, which spread the heat around the planet and retain a lot of heat on the night-time side of the planet.
The oceans have a massively higher heat capacity than the atmosphere, around 2.5m of ocean has the same heat capacity as the entire atmosphere above it. It is also buffered from the coldness of space by that atmosphere, as it spreads the solar energy it receives from the equatorial tropical region where most of the year-round insolation arrives, to higher latitudes where much of the re-radiation to space via the atmosphere occurs. The average temperature of the bulk of the ocean is around 275K or about 2C, and its surface at an average 17C or 290K. This vast body of heat retaining fluid is therefore maintaining most of the enhanced temperature of the Earth’s surface. But how does it get to be that warm?
The standard theory says that the atmospheric greenhouse effect is responsible, but there are a couple of simple observations which give us cause to doubt that. The first concerns cause and effect. Since the advent of satellite observations of lower tropospheric temperature in 1979, we have been able to make accurate comparisons of air and sea surface temperature. What we find is that changes in sea surface temperature precede the consequent changes in air temperature by an several months. The ocean surface temperature is apparently driving air temperature, not the other way round.
The lag of air temperature in red behind sea surface temperature in green is clear from this plot:
The second concerns thermodynamics. The average sea surface temperature is around 3C warmer than the near surface air temperature. Heat cannot pass from cooler to hotter according to the second law of thermodynamics. Conduction isn’t going to do the job. Convection is upwards not downwards. Longwave radiation cannot penetrate the surface of the ocean beyond its own wavelength. There are some limited circumstances when the air temperature exceeds that of the ocean, but in terms of heating the ocean bulk, this is about as effective as trying to heat an open air swimming pool with a hairdryer.
But if the atmospheric radiative greenhouse effect can’t heat the ocean fast enough to keep it at its present temperature, how does the ocean get to be warmer than the limit of Holder’s inequality for Earth of 255K?
I think the answer is as follows. In order to get rid of energy as quickly as it absorbs it and reach equilibrium, the temperature of the ocean has to be such that it can radiate, evaporate, convect and conduct energy upwards fast enough to match the rate at which it arrives from the Sun. Incoming solar shortwave radiation can easily penetrate the ocean to a depth of several hundred feet and thermalise its upper layer, because like the atmosphere (though not as much so) water is largely transparent to short wave radiation. But once thermalised, the ocean has to lose heat from a much lower temperature than the source of the original energy which heated it. This means that the warmed water has to emit longwave radiation. But longwave radiation has a hard time travelling in water, because water is nearly opaque to it. So radiated energy has a very tortuous and convoluted series of absorbance and re-emittance events on its way back up to the surface. There are of course other ways the ocean can lose heat, but these are also restricted in various ways. Water conducts heat slowly. It convects heat fairly quickly to the surface, but once there, the heat has to get into the atmosphere. That mostly happens via evaporation and the now liberated radiation.
But the rate of evaporation is limited by surface pressure. It is well known that water boils at a lot less than 100C at high altitude, where air pressure is lower. Down at sea level, the weight of the atmosphere on the surface is much higher, and water has to reach a higher temperature in order to evaporate or boil. The ocean therefore has its own greenhouse effect, or ‘hot water bottle effect’ Stephen Wilde colourfully coined it sometime in 2008. But this is not simply a radiative greenhouse effect, ‘trapping’ or ‘delaying’ the longwave radiation emitted within itself. It is also a pressure effect, whereby the weight of the atmosphere (not its composition) limits the rate at which the most important heat loss mechanism, evaporation, can occur.
These are the missing arms which gives the greenhouse theory legs.
Now there have been several spurious arguments bandied around in some noisy and poorly managed threads concerning these issues around the ability of ‘back radiation’ to heat the ocean or slow its rate of cooling, and the question of whether pressure induced by gravity acting on atmospheric mass can affect the Earth’s surface temperature. I hope that this short exposition will help cut through some of the confusion engendered by the tossing in of red herrings such as demands that any alternative to the radiative greenhouse theory must be able to work in a GHG-free atmosphere etc. This is nonsense. GHG’s exist in our atmosphere and do a fine job of radiating heat to space from altitude. If my hypothesis is correct, whatever warming effect they might have is a secondary issue to that primary role, because most of the reason the surface of our planet is as warm as it is, is due to the effect of atmospheric mass limiting the rate at which the ocean can lose energy. This is evidenced by cause and effect and thermodynamic consideration of simple observations as outlined above.
So obfuscatory arguments about imaginary planets will not be featuring in the comments section of this post, because we are dealing with the real situation here. Likewise, spurious arguments about heat from back radiation striking the surface of the ocean and being ‘mixed down’ into the ocean will also be given short shrift, because mixing occurs when wind blows. When wind blows, the rate of evaporation rockets upwards, removing heat from the ocean far more quickly than wafting its surface with back radiation is going to entrain heat into it. Arguments by assertion unaccompanied by supportive reasoning will be headed for the bit bucket whether submitted by any turtle, magic or not, or anyone else who thinks that argument by assertion is an acceptable form of scientific debate.
I will be happy to be proved wrong however, by anyone who has a sound, logical argument to offer. It may well be that the result of discussion will be ‘case not proven’. This is ok by me, because I don’t think the case for the radiative greenhouse effect being the sole cause of the ‘greenhouse effect’ is proven either. Note well however that I’m not disputing the basic physics of radiative gases, or that some of the outgoing longwave radiation from the surface is absorbed and re-emitted by gases in the atmosphere with radiative properties (mostly water vapour – around 1% of the atmosphere, and co2 around 0.04% of the atmosphere). Nor am I disputing the fact that around half of this absorbed radiation is re-emitted back towards the surface, or even that it might slow down the cooling of the air near the surface. I do have doubts about the extent it can measurably directly affect the oceans rate of cooling, but even allowing that it might, the effect will be smaller in comparison to the pressure effect I have outlined.
Perhaps the best we can hope for until someone comes up with a fiendishly clever empirical experiment to decide the issue is that it will be accepted that the biggest heaviest heat retaining fluid on the planet and the surface pressure it is subject to might just have something to do with its elevated surface temperature.