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.
Figure 1. IPCC attribution of warming. AR5 SPM.
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.
Figure 2. Mauna Loa rate of increase in CO2 (ppm/year). Thin line, 12-month increase. Thick line, gaussian smoothing. Red line, 2nd order polynomial least-squares fit to the yearly increase.
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.
Figure 3. HadCRUT 4 rate of temperature change (°C/year). Thin line, 12-month rate of change. Thick line, gaussian smoothing. Red line, 2nd order polynomial least-squares fit to the yearly increase.
Since 2017, the rate of temperature change has become negative.
Figure 4. Zoom of the HadCRUT 4 rate of temperature change (°C/year). The best fit polynomial (black line) shows the long-term evolution in the rate of temperature change.
The global warming deceleration since 1994, and cooling since 2017 are incompatible with the hypothesis that the increase in CO2 is driving global warming. Other factors must be more important than CO2.