“The Green Discount at a Steep Price” is presented by Ramakrishna Nemani, Senior Earth Scientist in the Advanced Supercomputing Division at the NASA Ames Research Center.

Abstract: The last time CO2 concentrations in the atmosphere passed 400 ppm; humans did not walk on Earth. As a consequence of the rapid increases in CO2 , particularly over the past five decades, projected changes in sea level, the frequency of monster storms, floods, droughts, and heat/cold waves may all come sooner than expected – creating a bumpy ride for us humans. What about the plants on Earth that evolved through millions of years and have seen much higher CO2 concentrations? Earth system models predict that climate changes resulting from increases in atmospheric CO2 relax climatic constraints to plant growth such that growing conditions improve globally. Models show, and satellite observations corroborate, that increases in photosynthesis from 1950-2100 are not only supported by direct CO2 fertilization but by three other factors: 1) warming in temperature-limited regions (arctic/boreal), 2) increased water-use efficiency accompanied in some areas by increasing precipitation benefiting water-limited regions (semi-arid regions) and 3) increased insolation through reductions in cloud cover in sunlight-limited regions (equatorial tropics). With ongoing interactions between plants, CO2 and climate, we may continue to get the 25% green discount that we have enjoyed over the past 60 years – only half the emissions stay in the atmosphere each year with the other half sequestered about equally on land and oceans. Even with this discount, we will be reaching dangerous levels of CO2 in the atmosphere, over 550 ppm at the current rate by 2100. The only way to avoid such a scenario is to reverse the steadily increasing trend of our greenhouse gas emissions.