This statement includes the assumption that 100% of the warming observed in the last 140 years,
can be attributed to CO2, which is clearly incorrect. The IPCC themselves mostly discuss warning since 1950.
Such attribution has to include the idea, that it is made by subtracting out the assumed other known contributors.
TSI (Total Solar Intensity) the sunlight striking the top of the atmosphere, increased until 1958, and has been declining
since then, in cycles. TSI is not the whole story however, because solar insolation, the amount of sunlight
that actually reaches the ground, showed massive decreases between 1950 and 1985, followed by substantial brightening
from 1985. Wild 2012.
Link to ENLIGHTENING GLOBAL DIMMING AND BRIGHTENING
"
Recent observational evidence suggests that sub-stantial changes in surface solar radiation (SSR; also known as global radiation) indeed occur over time and may profoundly affect our environments. "
The numbers attached to the brightening phase, could account for a substantial portion of the observed warming since 1980.
Mind you, the dimming and brightening were still human caused, just not related to CO2 emissions.