While I seriously doubt whether climate skeptics will thank me for pointing it out, I don’t believe their arguments impress the swing voters in the climate debate as convincingly as they might. With this in mind I’d like to propose a strengthening of the skeptic argument that downward longwave radiation or DLR, popularly called back radiation, cannot be held responsible for warming the surface of the Earth.
On a clear summer day, when the surface temperature is 20 °C (as it is outside my house this morning), the atmosphere is below freezing at all altitudes above 2 km, based on a lapse rate of 10 °C/km. That is, 70% of the air above is colder than a block of ice. When I measure the DLR by pointing my Microtemp MT250 infrared thermometer at the sky it registers 20 degrees below zero, icy indeed and corresponding to 2.534 * 5.67 = 232 W/m2 of DLR. Pointing it at the ground, it registers 20 °C or 2.934 * 5.67 = 418 W/m2 of upwards longwave radiation.
Meteorologists tell us that this block of ice overhead warms the ground. Well known meteorologist Alistair Fraser, who taught meteorology at Penn State from 1978 to 2001 (CV at http://www.ems.psu.edu/~fraser/cv/ ), puts it this way at http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html :
“The surface of the Earth is warmer than it would be in the absence of an atmosphere because it receives energy from two sources: the Sun and the atmosphere. The atmosphere emits radiation for the same reason the Sun does: each has a finite temperature. So, just as one would be warmer by sitting beside two fireplaces than one would have been if one fireplace were extinguished, so, one is warmer by receiving radiation from both the Sun and the atmosphere than one would be if there were no atmosphere. Curiously, the surface of the Earth receives nearly twice as much energy from the atmosphere as it does from the Sun.”
Now if you sit beside a block of ice, does it make any sense whatsoever to say that the block of ice is warming you? Obviously it cools you, and you’re warming the ice, not the other way round.
The counterargument would be that the ice block, as a black body at 0 °C, is radiating 2.734 * 5.67 = 315 W/m2 towards you. But that’s just a number, what does it actually mean as a physical phenomenon? Well, moving the ice further away reduces its cooling effect on you and your warming effect on it, so presumably that number must be reconciled with the evident fact that you’re warming the ice, not vice versa.
Thinking perhaps that Professor Fraser’s remarkable statement would have elicited at least one objection, I searched the web for references to his website. I found a great many praising him for the clarity with which he explained global warming, but not a single objection to his claim that the atmosphere warms the surface.
On that basis it seems safe to assume that the proposition “the atmosphere warms the surface” has been assimilated and accepted by the non-skeptical public as an axiom about global warming.
So what’s going on here? In particular what is the basis for Professor Fraser’s seemingly extraordinary claim that an aerial object significantly colder than a block of ice supplies twice as much energy to the surface as the 5700 K Sun? Could it really be so, or is this just misleading numerological prestidigitation?
While Fraser does not supply a reference at that page, one need not look far to confirm this factor of two. Figure 7 of the famous 1997 paper of Kiehl and Trenberth shows the Earth’s surface receiving 168 W/m2 from the Sun and 324 W/m2 from the atmosphere.
At this point let me digress with a popular skeptic argument about net CO2 fluxes into the atmosphere. Consulting any figure depicting the carbon cycle, http://earthobservatory.nasa.gov/Features/CarbonCycle/ for example, we see that, of the 220 GtC (gigatonnes of carbon) or so being emitted annually into the atmosphere, only about 9 GtC or 4% is of anthropogenic origin. Evidently our contribution is negligible compared to nature’s. What’s the big deal?
The problem with this argument is that the CO2 flux is bi-directional, and it is the net flux that matters. Preindustrially this flux was in balance and CO2 was not under any great pressure to change rapidly in a single century (though over millions of centuries it has certainly drifted greatly).
The problem with that “small” 4% we’re adding is that, unlike nature, we’re not taking it back out! Hence that 4% is accumulating, as we can see from the Keeling curve, which shows atmospheric CO2 rising at about 2.3 ppmv these days. That 2.3 ppmv per year represents roughly half of our 4% contribution; nature has stepped up her own removal program and is busy removing the other half of the 4% as we speak.
Back to Professor Fraser’s claim. Well, what do you know, the exact same fallacy. At the same time as 324 W/m2 is entering the surface from the atmosphere, 390 W/m2 is leaving the surface. The net flux is 66 W/m2 upwards. The atmosphere is not warming the surface, it’s cooling it!
But this should have been obvious when I pointed my IR thermometer at the sky and the ground and observed 232 W/m2 down and 418 W/m2 up, a net flux of 186 W/m2 cooling the surface. On a cloudy day the sky might be 13 °C or 2.864 * 5.67 = 379 W/m2, a mere 39 W/m2 less than the radiation from the 20 °C ground. And at night a cloudy atmosphere may even occasionally warm a slightly cooler Earth. So an annual global average of 66 W/m2 upwards may well be the case, but that’s still cooling.
Experienced meteorology professor Alistair Fraser says the atmosphere warms the surface. I, an amateur hack in this area with no meteorological credentials at all, say the surface warms the atmosphere, and moreover at an average power density of 66 W/m2 and sometimes hitting upwards of 200 W/m2. Who’s to be believed?
Judging from the many favorable references on the web to his website, it is clear that Fraser has persuaded the public of the correctness of his argument. His reasoning is just as plausible as the 4% argument about CO2 made by the other side, and he taught the subject for nearly quarter of a century, so one would have to be firmly wedded to the skeptic position to object to his website.
Those who’ve thought long and hard about whether back radiation makes sense may well have arrived at the conclusion that it doesn’t. Furthermore even if they can’t articulate their conclusion with the force they would like, it may nevertheless seem intuitively clear and irrefutable to them.
What we have here is a failure to communicate. If climate science has any expectation of being able to communicate with the skeptic community, I don’t think shooting down skeptic arguments on the ground that they’re obviously weak is the way to go about it. You’ll get nowhere attacking what skeptics say, you have to address what they’re thinking, which may be based on sounder intuition than is expressed in their arguments.
Which is tough to do if they have a harder time articulating exactly what they see wrong about back radiation than seeing intuitively that the atmosphere cools the surface. To them it is totally unreasonable to view the atmosphere as warming the surface, and they find it very frustrating not being able to explain to the satisfaction of the non-skeptics what they find unreasonable about it.
Hopefully I’ve made skeptical thinking a little clearer here, at least as it bears on the back radiation concept. If this were to help the two sides to focus on their real differences instead of getting off on irrelevant tangents to do with their communication difficulties and incompatible thought processes I’d be thrilled.
Just to clarify my own position, it is clear to me, based solely on the temperature and CO2 records to date, that if CO2 continues on its present course it will reach between 850 and 1000 ppmv by 2100, by which time temperatures worldwide will have increased some 2 °C. The concept of back radiation does not enter into my projections, and I have no idea how I would use it for forecasting climate change. I’ve been unable to persuade myself that back radiation is anything more than a computationally unusable extrapolation from John Tyndall’s insightful experiments in the 1850s.