The UK Met Office describes the Anthropogenic Global Warming (AGW) hypothesis as follows: “It is now clear that man-made greenhouse gases are causing climate change. The rate of change began as significant, has becoming alarming and is simply unsustainable in the long term” and the greenhouse effect is depicted on their website thus:
Solar rays hit the earth and heat up the surface (as shown on the left). The earth’s surface emits infrared radiation back in to space thereby cooling the planet (depicted by two of the red arrows in the right hand picture). Greenhouse gases in the troposphere trap some of the infrared rays reflecting heat back down to the surface. The AGW theory suggests that increased CO2 concentration in the atmosphere, caused by humans, is raising global temperatures.
For the theory to hold true, the observable rate of temperature increase would be higher in the troposphere than at the earth’s surface. The rate of temperature increase would be most noticeable in the tropics because that is where the surface would be radiating the most heat.
Yet the observations, from radiosonde (weather balloons) have consistently shown that not to be the case:
The left hand picture (courtesy of Dr David Evans' Missing Signature) is the climate model prediction of warming in the mid troposphere due to greenhouse gases from 1958 to 1999. The computer models predict most warming occurs at the mid troposphere at the Equator. The right hand picture shows actual temperatures measured over the same period by radiosonde (weather balloon). Actual balloon measurements show no increase in the rate of warming in the mid troposphere at the Equator, ie. no evidence of hot-spots in the troposphere and what is more none of the scientific papers supporting the AGW theory have claimed to have found such evidence.
In short the AGW theory is not borne out by the evidence. Had the AGW hypothesis been subject to the proper scientific method, the failure to substantiate this fundamental premise (of increased warming in the troposphere over the Equator) would have rendered the man-made global warming theory invalid.