ACCORDING to the international panel on climate change (IPCC) any direct temperature rise from increasing carbon dioxide levels is greatly amplified by positive feedback from water vapour. As the theory goes, rising carbon dioxide levels from human activity causes some temperature rise which causes more water to evaporate. Because water vapour is the dominant greenhouse gas, the additional water vapour absorbs even more energy, so global temperatures rise even, more causing still more water to evaporate and so on in an amplifying spiral. In this way the roughly half degree direct impact from doubling carbon dioxide is claimed to be amplified to three degrees or more.
An interesting theory, but now consider the following scenario;
We know the earth rotates about an axis tilted about 23 degrees relative to the sun. This is what causes the seasons and what sets the Tropics of Cancer and Capricorn. Imagine a location on the Tropic of Capricorn (23 degrees south) – say Mackay in Queensland. In summer the sun is directly overhead – average solar input of around 310 watts/sq meter. In winter the sun is at maximum elevation 44 degrees – average solar input of around 220 watts/sq meter. That is a difference summer to winter of about 90 watts/sq meter which, according to Stefan’s law, without any feedbacks would give a temperature difference summer to winter of about 16 degrees. The amount of positive or amplifying feedback claimed by the IPCC would inflate that about 6 times to more than 90 degrees C, extinguishing all life in Mackay.
Yet according to the long term records from the Bureau of Meteorology, the average summer maximum is 30.3C and the winter maximum 22C, a difference of less than 8 C. That is not only far less than 90 degrees but is far less than even the 16 degrees predicted in the absence of any feedback at all. How is that possible?
Maybe the thermal mass of the environment averages out much of the summer winter difference? Unlikely, consider the significant temperature change between day and night. If the temperature can change significantly in a few hours it could certainly change profoundly over 6 months. Also, if the above were the reason one would also expect to see it similarly averaged out in other places. Yet if we compare Mackay with say Alice Springs, both are at very similar latitudes yet again according to the Bureau of Meteorology long term records the summer maximum in Alice Springs is 36.4 and the winter maximum 19.7C a difference very close to the predicted 16C and double that of Mackay. Interestingly, not only is the summer winter difference greater at Alice Springs but the annual average is also greater 28C versus 26C and this is despite the fact that Alice Springs is at an elevation of 550metres which should make it 3.5 degrees cooler.
What does stand out at Mackay relative to Alice Springs is that the humidity is much higher in summer than it is in winter. Higher humidity means more water vapour content in the air and a higher summer temperature increases the water vapour content even further. But isn’t this going the wrong way? According to the IPCC, all this extra water vapour should massively increase the retained heat, making the temperature difference between summer and winter much larger not smaller. It should also raise the average temperature compared to Alice Springs not reduce it.
Well maybe the green house effect acts very slowly taking a long time to become apparent. Certainly IPCC has suggested exactly that with claimed time constants in decades. Our senses can give us some insight into that claim. Step outside any winters evening and you can immediately tell if the sky is cloudy or clear. I am sure most of you will agree from personal experience that it will be much colder when the sky is clear and a thermometer will confirm it. This comes about because clouds act exactly like greenhouse gasses, trapping energy radiated from the surface. Indeed clouds form a significant part of the overall greenhouse effect. Of course, clouds, unlike green house gases, also have a second effect and that is to reflect incoming solar energy away from Earth (the albedo effect). Again anyone who has been outside when a cloud comes between them and the sun can testify that the drop in radiant energy is immediately detectable. What our simple experiments are showing is that green house effects are clearly detectable at a very local level over time scales much less than a day, rather than only over 10’s of years.
So how is it possible that the summer/winter temperature variation at Mackay is so much less than the change in solar input would suggest? The reason is that water vapour generates lots of feedback effects and many of them are very strongly negative (feedback that reduces the impact of changes in energy input). Evaporating water takes a very large amount of energy. As more water evaporates in the summer it absorbs the necessary energy from the environment reducing the temperature rise. Greater humidity also leads to more condensation and thus clouds which reflect a larger fraction of the incoming solar energy back out to space and away from Earth’s surface. It also takes energy to lift the water vapour several kilometres into the atmosphere against earth’s gravity to the altitude where it condenses to form clouds and falls back as rain (after all that is where the energy for hydroelectricity comes from). The temperature data clearly shows that these negative feedback mechanisms strongly dominate over the slight increase in retained energy with increasing water vapour concentration.
The IPCC temperature rise claims are based on an assumption of strong net positive feedback in our climate system yet natural systems virtually all exhibit strong negative feedback around an equilibrium point. Negative feedback is the opposite of positive feedback. It acts to oppose any disturbance acting on a system and seeks to maintain the current equilibrium. In short it is a stabilising factor whereas positive feedback is a destabilising factor. Long term stability of any natural system almost guarantees that there is strong negative or stabilising feedback in operation. The climate, while showing periodic variations, has been stable enough for life to form and flourish for millions of years despite significant changes in forcing over the millennia and this makes it virtually certain that strong negative feedback is in operation. The fact that IPCC claim a large degree of positive feedback in our climate system suggests there may be a flaw in their theory.
If, as I believe, the net feedback from water vapour is negative rather than positive then the actual temperature rise by 2070 even with a doubling of carbon dioxide is likely to be less than the half a degree predicted in the absence of feedback, probably no more than 0.2 to 0.3 degrees.
by Michael Hammer
Michael Hammer graduated with a Bachelor of Engineering Science and Master of Engineering Science from Melbourne University. Since 1976 he has been working in the field of spectroscopy with the last 25 years devoted to full time research for a large multinational spectroscopy company.