This week Jiahua Guo will be presenting at ClimaTea on Stevens (2015) "Rethinking the Lower Boundary on Aerosol Forcing". Bjorn Stevens will be visiting Harvard next week, and discussing this paper is a chance for us to better understand the work before his talk on Thursday. Jiahua's thoughts on the paper are: A better estimate of radiative forcing is critical to understand climate change. However, a huge uncertainty of the negative radiative forcing by aerosol arises not only from a poor understanding of aerosol-cloud-radiation interaction, but also from the coarse resolution of Earth system models that can hardly resolve clouds. In this paper, a simple statistical model has been proposed to refine the uncertainty range of the negative radiative forcing from anthropogenic aerosols. The lower bound (-1.0W/m2) is much less negative than what is commonly believed. In addition, individual terms are estimated through a bottom-up approach utilizing observations, comprehensive modeling, and physical understanding, which provides more confidence for the narrow range of uncertainty.