My name is Nicola Jones and I write for Yale Environment 360 magazine and the journal Nature. With a background in chemistry and oceanography, I cover the physical sciences, from environmental issues to quantum physics. In my work as a freelance journalist, I’ve contributed to Scientific American, Globe and Mail, and New Scientist, and serve as the science journalist in residence at the University of British Columbia. In my recent Yale Environment 360 story, “How the World Passed a Carbon Threshold and Why It Matters” [http://e360.yale.edu/features/how-the-world-passed-a-carbon-threshold-400ppm-and-why-it-matters], scientists Ralph Keeling and Dana Royer join me to understand what Earth’s climate was like in previous eras of high CO2 levels and portray a sobering picture of where we are headed. Last year marked the first time in several million years that atmospheric concentrations of CO2 passed 400 parts per million. Environmental scientists see this threshold as a clear red line into a danger zone of climate change. But, as humans keep digging up carbon out of the ground and burning it for fuel, what will this mean for our future? My name is Ralph Keeling, and I am the Director of the Scripps CO2 Program, Professor of Geochemistry at UC San Diego, and Principal Investigator for the Atmospheric Oxygen Research Group at Scripps Institution of Oceanography. My research interests include measurements of variations in atmospheric oxygen, recent perturbations to the global carbon cycle, air-sea gas exchange, detection of ocean heat storage and transport using atmospheric gases and Paleoclimate theory. I continue to research the “Keeling Curve,” which was developed my father Charles David Keeling in 1958, at Scripps CO2 Program. My name is Dana Royer and I am a Climatologist and Professor of Earth and Environmental Sciences at Wesleyan University. I explore how fossil plants can be used to reconstruct ancient environments (especially CO2, temperature, and climate sensitivity), and the (paleo-) physiological underpinnings behind these plant-environment relationships. Recent and current projects include the reconstruction of paleo-atmospheric carbon dioxide levels from the stomatal distributions in plant leaves, and the development of mechanistically-grounded proxies for climate and leaf ecology from the size and shape of fossil leaves. I also compile ancient carbon dioxide records and investigate the strength of carbon dioxide-temperature coupling over multi-million-year timescales. We will be answering your questions at 1 pm EST – Ask Us Anything! Thank you everyone for tuning into this dynamic discussion on crossing the carbon threshold. We’ve received many questions during this AMA session, and tried our best to answer as many as possible. We apologize if we didn’t have time to get to your submission. But, please continue this conversation! To stay updated on the latest climate change stories, you can visit our website www.e360.yale.edu or follow us on FB & Twitter (@YaleE360). Cheers, Nicole, Ralph, Dana & Yale Environment 360 staff.