In the 1980s much of my work addressed the problem of nuclear winter, the climatic effects of nuclear war, demonstrating long-term (several year) effects with a computer model, disproving the dirty snow effect, and discovering observational evidence of surface cooling due to forest fire smoke plumes in the atmosphere. I am now once again doing research in this area, using modern climate models to look at the climatic effects of regional and global nuclear conflicts, funded by the Open Philanthropy Project and the Future of Life Institute.  Our latest work (including PowerPoints and papers) shows that even a "small" regional nuclear conflict could have severe global climatic effects, that there are still enough nuclear weapons in global arsenals to produce nuclear winter, and that the impacts would last for a decade.  This is the most serious environmental threat faced by humans and demands immediate policy attention.  If you are a scientist, you can join me on the Physicists Coalition for Nuclear Threat Reduction to work on policy changes.  Even Fidel Castro was interested.