We are. Forcing is climate change outside of natural cycles. What else could cause it? Volcanos, changes to the Earth's orbit, and solar intensity.
Quote:
The correlation statistics indicate highly significant detection of
solar irradiance forcing in the NH series during the ‘Maunder Minimum’ of solar activity from the mid-seventeenth to early eighteenth century which corresponds to an especially cold period. In turn, the steady increase in solar irradiance from the
early nineteenth century through to the mid-twentieth century coincides with the general warming over the period, showing peak correlation during the mid-nineteenth century. The regression against solar irradiance indicates a sensitivity to changes in the ‘solar constant’ of,0.1 KW-1m-2, which is consistent with recent model based
studies42. Greenhouse forcing, on the other hand, shows no sign of significance until a large positive correlation sharply emerges as the moving window slides into the twentieth century. The partial correlation with CO2 indeed dominates over that of solar irradiance for the most recent 200-year interval, as increases in temperature and CO2 simultaneously accelerate through to the end of 1995, while solar irradiance levels off after the mid-twentieth century. It is reasonable to infer that greenhouse-gas forcing is now the dominant external forcing of the climate system. Explosive volcanism exhibit the expected marginally significant negative correlation with temperature during much of 1610–1995 period, most pronounced in the 200-year window centred near 1830 which includes the most explosive volcanic events.
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http://iri.columbia.edu/~goddard/EESC_W4400/CC/mann_etal_1998.pdf
So to say that GHGs are 100% responsible for warming is in fact true because solar intensity has decreased from the mid-century mark and volcanoes serve to cool.