For the last 30 years satellite bourne instruments have been measuring the globally averaged temperature of the lower troposphere (ref) These measurements are based on the microwave thermal emission of oxygen. These instruments have been bourne on a series of NOAA polar orbitting satellites which scan the Earth’s surface and are calibrated internally against empty space (Cosmic Microwave background) and an on board blackbody source. The estimated accuracy is 0.2C. Since 2002 the Advanced Microwave Sounding Unit (AMSU-A) flying on NASA’s Aqua satellite has been the main source of data.
There has been a lot of controversy about global surface temperature data because of for example 1) urban heating effect 2) human selection of data 3) various averaging methods. Therefore a purely instrumental satellite measurement is in principal better, but clearly it can only cover a limited time period period. Since 1979 John Cristy and Roy Spencer have published monthly averaged data now covering a 30 year period. This is now giving some really interesting results.
Overall 2010 has come out just a bit below the warmest year to date namely 1998. (+0.41 against + 0.42 above the mean). However what I find really interesting about this data are the following observations.
1) There are clear regular variations on a 3-4 year cycle with an amplitude of around 0.2C. What is the mechanism for this? Is it El Nino/La Nina, solar, volcanic, astronomical or what ?
2) There is evidence of a small underlying linear increase over the 30 year period in temperature which fits a gradient of 0.013C +- 0.002 per year.
From 1979 until today the CO2 levels as measured in Maona Loa have risen from 336.8 ppm to 389.8 ppm – a total rise of 53 ppm. This gives an average yearly increase of 1.77 ppm/year. If we assume that the IPCC is correct and that the only effect on climate is due to anthropogenic increasing greenhouse gases then we can simply use this data to derive a linear relation between global temperatures and CO2 concentration. This works out at :
DeltaT = 7.3*10**-3 * DeltaCO2
This empirical relation predicts that if CO2 levels were to rise by say 300 ppm ( i.e. more than double that of pre industrial values) then the temperature would rise by about 2.2 degrees C. The other interesting conclusion is that it would actually take another 170 years of “business as usual” from today to reach such CO2 concentrations, assuming no curbing of carbon emissions in the meantime! So there really is no emergency and a healthy skepticism of imminent doom is the right approach right now.
My conclusion is that AGH is a real effect but it is way off being the world’s number one urgent problem. I suspect that an over-reaction which forces through drastic cuts in carbon emissions will disrupt the global economy thereby causing far worse damage on developing nations than otherwise. The good news is that basic physics (if we ignore so-called feedbacks) also predicts that the greenhouse effect on temperature is logarithmic with CO2 concentrations. That means that for the temperature to eventually rise by a further 2.2 degrees (4.4 degrees in total) would require a quadrupelling of CO2. Even supposing that man could somehow economically exploit enough new resources to keep burning coal, oil and gas for the next 500 years in order to reach a concentration of 1200 ppm, the rise would still be half that caused by an Ice Age. In reality we would appear to have well over a hundred years of scientific R&D in order to solve our energy needs.
In conclusion Science must prevail and measurements will eventually decide between rival models. I think, right now it is verging on the irresponsible to trumpet that the debate is over, thereby hyping fears to scare the public and educate our children into what are essentially predetermined quasi-political positions which may well turn out to be wrong.