LONG-TERM ATMOSPHERIC MEASUREMENTS HAVE REVEALED A 50% INCREASE IN THE AMPLITUDE OF THE CO2 SEASONAL CYCLE IN NORTHERN HIGH LATITUDES SINCE THE 1960S. THIS INCREASE IS MUCH LARGER THAN THE INCREASE OF THE CO2 SEASONAL AMPLITUDE IN LOWER LATITUDES. THE CAUSE FOR SUCH A DRAMATIC CHANGE HAS BEEN A HUGE PUZZLE FOR THE SCIENCE COMMUNITY. A NUMBER OF STUDIES HAVE PRESENTED CONTRASTING VIEWS ON POTENTIAL CAUSES FOR THIS INCREASE INCLUDING AN EARLIER ONSET OF VEGETATION GROWTH AND ENHANCED RESPIRATION RELATED TO CLIMATE WARMING IN NORTHERN HIGH LATITUDES AS WELL AS AN INFLUENCE ASSOCIATED WITH INCREASED AGRICULTURE PRODUCTION IN MID-LATITUDES. TO QUANTIFY THE CONTRIBUTION OF VARIOUS PROCESSES THAT MIGHT BE RESPONSIBLE FOR THIS DRAMATIC CHANGE IN NORTHERN HIGH LATITUDES WE WILL LEVERAGE VARIOUS NEW DATA PRODUCTS FROM THE NASA S ARCTIC-BOREAL VULNERABILITY EXPERIMENT (ABOVE) NEW SATELLITES (OCO-2 SMAP TROPOMI) AND RECENT ADVANCES IN HIGH RESOLUTION REGIONAL MODELING TECHNIQUES. FIRST WE WILL DEVELOP A HIGH-RESOLUTION REGIONAL INVERSE MODELING SYSTEM CARBONTRACKER-ABOVE THAT BUILDS UPON THE NASA/ CMS FUNDED CARBONTRACKER-LAGRANGE EFFORT. CARBONTRACKER-ABOVE WILL INTEGRATE A VARIETY OF TRACE GAS MEASUREMENTS (AIRBORNE IN SITU AIRBORNE AND SATELLITE REMOTE SENSING AND GROUND-BASED IN SITU MEASUREMENTS) AND SATELLITE-BASED GEOSPATIAL LAND PRODUCTS (E.G. SOLAR INDUCED FLUORESCENCE (SIF) SOIL MOISTURE AND VEGETATION INDICES) TO DERIVE SURFACE FLUXES AT A 0.1O RESOLUTION FOR 2015 2019 WITH WELL QUANTIFIED ERRORS. UNLIKE THE TRADITIONAL BAYESIAN INVERSE MODELING APPROACH WE WILL MAXIMIZE OBSERVATIONAL INFORMATION OBTAINED FROM SATELLITES BY CONDUCTING GEOSTATISTICAL INVERSIONS THAT WILL NOT ONLY BE CONSTRAINED BY CO2 AND CARBONYL SULFIDE (COS) OBSERVATIONS BUT ALSO BE INFORMED BY SATELLITE LAND REMOTE SENSING PRODUCTS. THE SELECTED SATELLITE LAND PRODUCTS ALONG WITH IN SITU AND REMOTE SENSING ATMOSPHERIC TRACE GAS OBSERVATIONS WILL BE USED TO INFORM INVERSIONS OF CO2 AND COS WHICH WILL PROVIDE SEPARATE ESTIMATES OF NET ECOSYSTEM EXCHANGE (NEE) AND GROSS PRIMARY PRODUCTION (GPP) OVER THE ABOVE DOMAIN FOR 2015 2019. SECOND WE WILL EXTRAPOLATE MEASUREMENT-CONSTRAINED NEE AND GPP FROM THE ABOVE DOMAIN TO THE PAN-ARCTIC FOR THE LAST TWO DECADES USING THE HIGH-RESOLUTION SATELLITE-DATA-DRIVEN POLAR VEGETATION PHOTOSYNTHESIS AND RESPIRATION MODEL (POLAR-VPRM). THE CURRENT POLAR-VPRM WAS ONLY CALIBRATED BY FLUXES MEASURED FROM 4 EDDY COVARIANCE TOWERS. WE WILL USE OUR NEE AND GPP FLUXES THAT ARE INFORMED BY A COMPREHENSIVE SET OF ATMOSPHERIC TRACE GAS OBSERVATIONS AND SATELLITE REMOTE SENSING LAND PRODUCTS TO SEPARATELY TUNE PARAMETERIZATIONS OF GPP AND RESPIRATION IN THE POLAR-VPRM. THE IMPROVED POLAR-VPRM WILL BE USED TO DERIVE GPP AND RESPIRATION OVER THE ARCTIC FOR THE LAST TWO DECADES INFORMED BY THE LONG-TERM SATELLITE RECORDS OF EVI AND SIF. THIRD WE WILL INTEGRATE GPP AND RESPIRATION FLUXES ESTIMATED FOR THE PAN-ARCTIC AND AVAILABLE MID-LATITUDE FLUX PRODUCTS INTO THE GLOBAL TRANSPORT MODEL CARBONTRACKER TO SEPARATELY INVESTIGATE THE IMPACT OF GPP RESPIRATION AND TRANSPORT ON THE TREND OF CO2 SEASONAL AMPLITUDE IN NORTHERN HIGH LATITUDES OVER THE LAST TWO DECADES. THE PROPOSED ANALYSES WILL ELUCIDATE PAST CHANGES OF CARBON FLUXES FROM REGIONAL- (THE ABOVE DOMAIN) TO CONTINENTAL- (THE PAN- ARCTIC) AND THEN TO HEMISPHERIC SCALES AND FURTHER QUANTIFY CONTRIBUTIONS OF VARIOUS PROCESSES ON A RANGE OF SCALES TO A DRAMATIC CHANGE OBSERVED IN THE ATMOSPHERE IN NORTHERN HIGH LATITUDES. OUR RESULTS WILL FURTHER PROVIDE INSIGHTS INTO POSSIBLE FUTURE CHANGES OF NET CARBON FLUXES IN THE ARCTIC-BOREAL TERRESTRIAL ECOSYSTEMS IN RESPONSE TO CLIMATE CHANGE.