IPCC scientists assume that human CO2 emissions will continue to accumulate in the atmosphere, remaining anywhere from hundreds to thousands of years.
This assumption is a cornerstone of the AGW hypothesis. The cumulative CO2 growth causes global warming that accelerates (they hypothesize) to a condition of "runaway" temperature increases via positive feedbacks, leading to catastrophic "tipping point" climate change.
To simplify, the CO2-centric AGW hypothesis, and climate models, assume that every additional emission molecule of atmospheric CO2 will accelerate the global warming, to the point of no return. Thus, each new tonne (metric) of CO2 will boost the acceleration via a theoretical positive feedback amplification.
But does the empirical evidence actually indicate that is indeed what is taking place?
Using a combination of the NOAA annual global temperature dataset and two sources of global CO2 emissions from fossil fuels, it can be determined how each new tonne of CO2 emissions is "accelerating" temperatures, or not.
This article's chart provides the answer. As can be observed, each new CO2 emission molecule added to the climate has a smaller and small impact, the opposite of the AGW hypothesis. In 1941, the degree increase per tonne hit a peak. Since then, the impact of each tonne has decreased, significantly - currently it stands at +0.00000000000021°C/tonne.
The AGW hypothesis does not account for this ever smaller impact of CO2. Possibly this is the reason for the "consensus" unexpected global warming 'hiatus', which the IPCC scientists are still at a loss to explain.
If this tiny impact stays constant over the next 30 years, and the growth rate of CO2 emissions over the last 15 years remains the same for the next 30 years (another trillion tonnes emitted), the potential increase of global temperatures will barely be +0.2°C (two-tenths of a degree) by year 2044. And if each tonne's impact continues to shrink, as the evidence suggests, so will the temperature increase shrink.
Now, adding to this miserably low warming influence of CO2 is the recent admission by establishment climate science that natural climatic forces have a powerful say in the trend of global temperatures, regardless of human CO2 emissions. As the Nature science journal indicates, currently, and for the near future, a natural PDO cooling phase may dominate.
More on the above 'C3' chart. Specifically, it plots a ratio of 30-year NOAA temperature changes to the cumulative amount of CO2 tonnes emitted up to that point. For example, the 1941 ratio has a numerator of +0.59°C (30-year annual temperature change) and a denominator of 165 billion CO2 tonnes (the cumulative amount emitted from 1880 through 1941). This ratio calculation is made for each year, starting with 1910 (30 years after 1880).
The ratio allows for depicting visually the influence of all those previous CO2 emissions on moving 30-year climate periods. The chart's additional green and light blue curves simply provide a smoothed sense of direction of the fossil fuel emission influence.
Summary: The observed shrinking of CO2's influence on global warming does not bode well for the future longevity of the AGW hypothesis. Per the well known and documented CO2 physics, this outcome should not be a surprise. It's just another case of 'those stubborn facts' in science.