WHAT CLIMATE SCIENTISTS KNOW FOR SURE PART- 1: WHEN DID AGW HUMAN CAUSED CLIMATE CHANGE START? Callendar 1938: It started in 1900 and warmed steadily from 1900 to 1938 with the warming driven by rising CO2 which in turn is attributable to fossil fuel emissions. Hansen 1988: It started in 1950 because in the 30-year period 1950-1980 there is a strong measurable warming rate with 99% probability for human cause. IPCC 2001: It started in 1750 when the Industrial Revolution kicked in and atmospheric CO2 began to rise. IPCC 2015: It started in 1850 by when sufficient fossil fuel carbon had entered the atmosphere for a measurable response of temperature to CO2. NASA 2020 [LINK] It started in 1950 because from then the relationship between CO2 and temperature we see in the climate models closely matches the observational data. Climate Scientist Peter Cox 2018: It started in the 1970s because it is since then that we see a measurable responsiveness of surface temperature to atmospheric CO2 concentration according to the theory of the greenhouse effect of CO2. PART-2: WHAT SCIENTISTS KNEW FOR SURE IN 2001: Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 5C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet. WHAT SCIENTISTS KNEW FOR SURE IN 2007 Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 4C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet. WHAT SCIENTISTS KNEW FOR SURE IN 2013 Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 3C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet. WHAT SCIENTISTS KNEW FOR SURE IN 2015 Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 2C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet. WHAT SCIENTISTS KNEW FOR SURE IN 2018 Fossil fuel emissions is causing atmospheric CO2 concentration to go up and that in turn is causing global mean temperature to go up. If we don’t take climate action to reduce and eliminate emissions, the temperature will continue to go up and when it warms 1.5C above pre-industrial, warming will become irreversible – meaning that it will no longer be possible to attenuate warming with climate action and that will make it impossible for us to save the planet. WHAT SCIENTISTS KNOW FOR SURE IN 2020: Insights from Earth system model initial-condition large ensembles and future prospects, C. Deser, F. Lehner, K. B. Rodgers, T. Ault, T. L. Delworth, P. N. DiNezio, A. Fiore, C. Frankignoul, J. C. Fyfe, D. E. Horton, J. E. Kay, R. Knutti, N. S. Lovenduski, J. Marotzke, K. A. McKinnon, S. Minobe, J. Randerson, J. A. Screen, I. R. Simpson & M. Ting: Nature Climate Change (2020). Abstract: Internal variability in the climate system confounds assessment of human-induced climate change and imposes irreducible limits on the accuracy of climate change projections, especially at regional and decadal scales. A new collection of initial-condition large ensembles (LEs) generated with seven Earth system models under historical and future radiative forcing scenarios provides new insights into uncertainties due to internal variability versus model differences. These data enhance the assessment of climate change risks, including extreme events, and offer a powerful testbed for new methodologies aimed at separating forced signals from internal variability in the observational record. Opportunities and challenges confronting the design and dissemination of future LEs, including increased spatial resolution and model complexity alongside emerging Earth system applications, are discussed.