I keep on saying that the “forcing” effects of CO2 are logarithmic while the cooling response of the planet rises exponentially. I’m not the only one saying this, serious heavy weight skeptics like Lindzen are saying the same thing. So what do these terms really mean? OK, a bit of background and then onto the pictures.
What is often quoted is that CO2 doubling causes an increase in radiance to earth’s surface of 3.7 watts/meter squared, which in turn raises temperatures about 1 degree Celsius. Why the reference to “doubling”? Because we’re talking about light and filtering materials. Consider that you have several pairs of sun glasses, each of which blocks 50% of the light. If you put two pairs in a row, do they block 100%? Of course not. The first pair blocks 50% and the second pair blocks 50% of what is left, which is 25% of the original light. The third pair would only block 12.5% of the original light. CO2 suffers from the same law of diminishing returns. What keeps getting left out of the climate discussion is what happens after the first doubling. The pre-industrial levels (1900 AD or so) of CO2 are commonly quoted at 278 PPM (parts per million) and the current levels are at about 385 ppm. If we look at this graph, it becomes pretty clear that we would have to generate a LOT of CO2 to get much more effect than we are already:
However, to keep the big picture in mind, we have to also remember that as the earth gets warmer, it radiates heat to space. The ideal black body formula to calculate how much heat is being dissipated to space is P=(5.76×10^-8)(T^4) where P is power in watts per square meter and T is temperature in degrees K or Kelvin. To convert from the more common degrees C, just add 273. The “average” temperature (there’s really no such thing) of earth is often quoted as 15 degrees C or 288 K. This graph shows how much additional heat the earth sends into space as it gets just a few degrees warmer:
So how much does CO2 in theory heat the planet? If we use the formula above, we see that increasing the earth’s temperature by just 1 degree, from 288 K to 289 K, results in an increase in earth radiance of 5.5 watts per square meter. This brings up the obvious question. If earth radiance goes up by 5.5 watts, how could it be caused by only a 3.7 watt rise? The climatologists have a variety of explanations for this. In brief, CO2 doesn’t reflect long wave radiance as many people think, it absorbs it. This heats the CO2 up, which causes it to radiate more heat, but the photons it releases can be emitted in any direction. Up, down, sideways… long story short, some escapes to space and some gets sent back to earth, about 3.7 of the 5.5 additional watts. This issue alone is a long complicated discussion, but rather than argue it, let’s just accept the numbers. Doubling CO2 levels from the pre-industrial level of 278 PPM causes an increase of 3.7 watts per meter squared, and that results in a temperature increase of 1 degree C. The various theories then go on to claim that increased temperatures result in increased water vapour, which is itself a greenhouse gas and supposedly adds another 2 degrees C to the warming. We’ll debunk both of those, but let’s put aside the water vapour for the moment and just focus on the CO2.
In order to put the whole thing in perspective, we have to keep in mind two things. The first is that in order to get a second 3.7 watts (after the first doubling) we would have to double CO2 again. So the first doubling would be 278 x 2 = 556 PPM = 3.7 watts. To get to 7.4 watts, we would have to double again to 1,112 PPM. As the earth heats up though, the amount of additional power required to raise the temperature just one more degree also goes up. So, to put everything in perspective, let’s take a look at how much CO2 would be required, without water vapour feedbacks, to directly raise the temperature of the earth from 288 K (15 degrees C) by four degrees. As you look at the graph, just to put things in perspective, consider the two thin lines at the bottom. The green line is what CO2 was at pre-industrial, and the red line is where we are at after a century of burning fossil fuels:
Several things jump out at us. The first is just how ridiculous the idea of a “tipping point” really is. The amount of heat the earth radiates to space just goes up too fast for that, and the amount of CO2 that is required to maintain any temperature increase at all goes up even faster. If we were to double the rate at which CO2 in the atmosphere is increasing in comparison to the last 30 years, it would still take well over a century to get to just two degrees of warming from CO2. If we tripled the rate, it would take almost four centuries to get to three degrees. But what about positive feedback from water vapour?
There are plenty of things wrong with that theory. In principle, the amount of water vapour the atmosphere is capable of holding about doubles for every 10 degree rise in temperature. The theory goes that just a small rise in temperature would increase water vapour which over all has a much larger greenhouse effect than does CO2. Estimates range anywhere from double to quadruple the additional warming. The average quoted most often is 1 degree of warming from CO2 and 2 more from water vapour feedback. Is this reasonable?
If the amount of water vapour in the atmosphere always “maxed out” it might be, but we know that doesn’t happen. Instead, let’s look at what has actually happened. Since the pre-industrial levels of 278 PPM one hundred or so years ago, CO2 levels have gone up about 38%, not even close to doubling. In that time, various estimates based on surface station readings around the world have suggested that the earth has warmed up about 0.6 degrees C. But, we must keep in mind that due to the logarithmic effects of CO2 forcing, and the increased radiance of the earth as it warms, the first 38% has a much larger effect than the next 38%. In fact, if we go back to our graph and look at where we are now, it is easy to see that whatever effects doubling CO2 actually has, almost 70% of that is already happening:
Even if we accepted the notion that positive feedback from water vapour triples the effects of CO2, we clearly are not seeing that in actual earth temperatures. If the rough estimates of CO2 doubling = 3.7 watts per square meter = 1 degree plus 2 more from water vapour were correct, we would have seen a temperature increase over the last century of 2.1 degrees, but we’ve only seen 0.6 degrees. It could be argued that there are natural cooling fluctuations, and the difference between what the earth’s temperature is now, and what it would have been without the extra CO2 would be 2.1 degrees. That also seems far fetched given that the earth has been in a general warming trend for the last 300 years, and the rate of warming over the last century has been about the same as the previous ones.
The more logical explanation is twofold. First, the effects of CO2 and positive feedback from water vapour have been far over estimated. Secondly, even doubling or tripling the amount of CO2 we put into the atmosphere would not appreciably change the warming effects of the CO2 levels we have currently… and then not by much. This isn’t me making numbers up, it is just a matter of extending the IPCC claims and putting them in perspective to show that the worst is already behind us, and is over estimated in any event. Even if the estimates of CO2 warming were correct (which they clearly are not) the fact is the bulk of the damage (if any) has already happened, and the amount of fossil fuels we would have to burn to appreciably change that is completely beyond our production capacity.