From Gavin Schmidt et al, to appear in JGR:
The attribution of the present-day total greenhouse effect:(highlight mine)
Abstract. The relative contributions of atmospheric long-wave absorbers to the present-day global greenhouse e ffect are among the most misquoted statistics in public discussions of climate change. Much of the interest in these values is however due to an implicit assumption that these contributions are directly relevant for the question of climate sensitivity.... With a straightforward scheme for allocating overlaps, we find that water vapour is the dominant contributor (50% of the effect), followed by clouds (25%) and then CO2 with 20%. All other absorbers play only minor roles. In a doubled CO2 scenario, this allocation is essentially unchanged, even though the magnitude of the total greenhouse e ect is signi cantly larger than the initial radiative forcing, underscoring the importance of feedbacks from water vapor and clouds to climate sensitivity.