by Andy May, Watts Up With That:
We define “warming” as a positive rate of temperature change over time. According to the main hypothesis, warming since 1951 has been due almost exclusively to the increase in GHGs (greenhouse gases), of which CO2 is the most important one. The IPCC does not find anything else that has contributed to the observed warming.
According to the IPCC at least 77%, but more probably 120%, and up to 200% of the observed warming, has been caused by GHGs.
The rate of CO2 change (the atmospheric increase in CO2 every year) has been increasing almost linearly since 1959 and is currently ~2.4 ppm/year.
If the IPCC hypothesis was correct, the warming rate should increase (accelerate) if CO2 is increasing rapidly. The warming rate can only decrease (decelerate) if CO2 is increasing more slowly and can only turn into cooling (negative rate) if CO2 is decreasing.
But the hypothesis doesn’t fit the observations. The HadCRUT 4 rate of temperature change (°C/year) is no longer increasing. In fact, it stopped increasing ~1994 and has been decreasing since. Global warming has been decelerating for over 20 years despite CO2 levels increasing at the same rapid rate.