A 450-ppm equivalent CO[2] target by 2050 is an often-proposed goal under a future global emissions agreement, but there is considerable high side risk in global-warming models due to cloud formation, feedbacks in dissolved organic carbon from peat bogs in polar regions, and unaccounted solar dimming by particulates. The 450-ppm figure is predicated on absorption of some CO[2] by oceans, but increasing acidification may dictate that CO[2]produced during the next 50 years may have to be reduced further to preserve species diversity in the oceans. By consideration of industry capability, a 400-ppm target is shown to be affordable and achievable by renewables using 2/3 direct solar electricity by 2050 with technology currently being proven, together with aggressive energy efficiency. Sequestration of liquid CO[2] under pressure in enormous continually excavated caverns at 2-km depth is technically much less certain and later in development. (Contains 5 graphs.)