Keywords:Orbiting Carbon Observatory-2, Greenhouse gases Observing SATellite, Carbon dioxide, Solar induced chlorophyll fluorescence
The first two years of the OCO-2 mission were dominated by the record-setting 2015–2016 El Niño. OCO-2 measurements were combined with GOSAT observations and analyzed to yield high spatial resolution estimates of the column-averaged dry air mole fraction, XCO2, These products clearly resolved reductions in tropical ocean outgassing from net increases in CO2 emissions from tropical forests. These XCO2 estimates were combined with OCO-2 and GOSAT solar-induced chlorophyll fluorescence (SIF) measurements to trace the origin of the forest CO2 emissions to the impacts of drought, temperature stress, and fires. Observations acquired between 2017-2019 documented the atmospheric CO2 response following this El Niño when, unexpectedly, its XCO2 and SIF measurements showed that the natural carbon cycle never fully recovered to the state anticipated by most earlier carbon cycle measurements and models. In particular, tropical forests, once thought to be significant CO2 sinks, continued to be persistent net sources of CO2 throughout this period. Understanding these results and their implications for the response of the natural carbon cycle to climate change requires longer continuous records of CO2 and SIF.