What Everyone Must Know About Climate Sensitivity with James Hansen

In this Climate Chat episode, renowned climate scientist James Hansen returns to Climate Chat to discuss Climate Sensitivity. Climate sensitivity relates to how much the Earth will warm in response "forcings" such as human-caused greenhouse gas (GHG) emissions. Such warming includes the direct warming in response to the forcing plus warming due to feedbacks to the direct warming. The total short-term "equilibrium" warming will play out over 100~1000 years. James Hansen and his co-authors (including Climate Chat host Leon Simmons) have said that the "Equilibrium Climate Sensitivity" (ECS) is 4.5ºC for a doubling of CO2, while the IPCC's best estimate is 3ºC. This difference is a "BFD" according to Dr. Hansen and, therefore, the climate sensitivity controversy is a major issue that will impact all humans (and other living things). Dr. James Hansen, formerly Director of the NASA Goddard Institute for Space Studies, is an Adjunct Professor at Columbia University’s Earth Institute, where he directs the Program on Climate Science, Awareness and Solutions. He was trained in physics and astronomy in the space science program of Dr. James Van Allen at the University of Iowa. His early research on the clouds of Venus helped identify their composition as sulfuric acid. Since the late 1970s, he has focused his research on Earth’s climate, especially human-made climate change. Dr. Hansen is best known for his testimony on climate change to congressional committees in the 1980s that helped raise broad awareness of the global warming issue. He was elected to the National Academy of Sciences in 1995 and was designated by Time Magazine in 2006 as one of the 100 most influential people on Earth. He has received numerous awards including the Carl-Gustaf Rossby and Roger Revelle Research Medals, the Sophie Prize and the Blue Planet Prize. Dr. Hansen is recognized for speaking truth to power, for identifying ineffectual policies as greenwash, and for outlining actions that the public must take to protect the future of young people and other life on our planet. Dr. Hansen's paper: "Global Warming in the Pipeline": https://academic.oup.com/oocc/article/3/1/kgad008/7335889 Dr. Hansen's paper: "Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?": https://www.tandfonline.com/doi/full/10.1080/00139157.2025.2434494 Dr. Hansen's recent communications on climate sensitivity: "Seeing the Forest for the Trees": https://www.columbia.edu/~jeh1/mailings/2025/ForestTrees.06August2025.pdf Dan Miller & James Hansen's white paper on Carbon Fee & Dividend: https://csas.earth.columbia.edu/sites/csas.earth.columbia.edu/files/content/Fee-and-Dividend-Miller-Hansen-20191110-1.pdf Dr. Hansen's webpage: https://www.columbia.edu/~jeh1/ Donate to the Columbia Climate Science, Awareness and Solutions Program: https://csas.earth.columbia.edu/giving Dr. Hansen's previous Climate Chat interview: Climate Change: What to Know/What to Do with Climate Scientist James Hansen https://youtube.com/live/8Ag3UVSrlhE Program on cloud feedbacks: Global Warming➔Fewer Clouds➔More Warming! with George Tselioudis: https://youtube.com/live/suFZb2ViHoA Climate Chat "Accelerated Warming" Playlist that includes many programs related to Dr. Hansen's work: https://www.youtube.com/playlist?list=PLep9tQgCqjCoEDD7jLXMFZnPL58eLQmZK Please Like and Subscribe! Dan Miller has been giving talks around the world on the need for urgent climate action for over 20 years. He is co-founder of The Roda Group, a climate-tech venture capital group, and he serves as an advisor for other climate funds and climate-focused non-profit groups. He started Climate Chat in 2021 on Clubhouse and added YouTube in 2023. Follow Dan Miller on X/Twitter: @danmiller999 Follow Dan Miller on Bluesky: @danmiller999.bsky.social and @climatechat.bsky.social Leon Simons is a climate scientist based in the Netherlands. He was a co-author with James Hansen on the 2023 "Global Warming in the Pipeline" paper that discussed the recent acceleration in global warming. Leon's work has focused recently on changes in aerosol emissions from ships and the impact of those changes on climate change. You can access Leon's research writings here: https://www.researchgate.net/profile/Leon-Simons Follow Leon on X/Twitter: @LeonSimons8 and Bluesky: ‪@leonsimons.bsky.social‬ Follow Leon on Patreon: https://www.patreon.com/c/LeonSimons/home Follow Stacey Randecker on X/Twitter: @DrivingMzStacey and Bluesky: @bikingmzstacey.bsky.social Follow Eli Rabani on X/Twitter: @radsci and Bluesky: @radsci.bsky.social You can leave a question in the YouTube Live chat. Please begin your question with "Question:" to make it easy for our mods to spot it. For more Climate Chat episodes, see our YouTube home page: https://www.youtube.com/@climatechat For recent YouTube Live Climate Chat programs: https://www.youtube.com/@climatechat/streams