WARMER SPRINGS AND DRIER SUMMERS ARE EXPECTED TO INCREASE THE CO2 UPTAKE OF TERRESTRIAL ECOSYSTEMS BY LENGTHENING THE GROWING SEASON BUT REDUCE SUMMER UPTAKE THROUGH WATER STRESS. ONE OF THE KEY CHALLENGES IN CARBON CYCLE SCIENCE IS DETERMINING THE BALANCE OF THESE PROCESSES HOW THEY ARE COUPLED THROUGH CARBON-WATER CYCLE INTERACTIONS AND THE IMPACT ON ATMOSPHERIC CO2 CONCENTRATIONS. THE CARBON UPTAKE RESPONSE TO SEASONAL WARMING AND DRYING IS UNCERTAIN BUT APPEARS TO BE INCREASINGLY DEPENDENT ON AVAILABILITY OF LIQUID WATER IN SOILS WHICH IS LIMITED PRIMARILY BY WATER SCARCITY IN WARM TEMPERATE CLIMATES AND FREEZING TEMPERATURES IN COLD NORTHERN CLIMATES AND MODULATED BY DROUGHT AND THAWING/FREEZING PROCESSES RESPECTIVELY. THEIR INTERACTIVE EFFECTS WITH CO2 PHOTOSYNTHETIC GAIN (GPP) AND RESPIRATION LOSS (ER) SHOWS HIGH SEASONAL SPATIAL AND BIOME DEPENDENCE. HERE WE PROPOSE TO EXPLOIT AN EXPANDING SET OF MULTI-SCALE CARBON AND WATER CYCLE OBSERVATIONS WITH STATE-OF-THE-ART LAND AND ATMOSPHERE MODELS TO INVESTIGATE THE RESPONSE OF SEASONAL PHENOLOGY CO2 EXCHANGE AND ATMOSPHERIC CO2 TO SPRING WARMING SOIL THAW AND LIQUID WATER AVAILABILITY SINCE 2002. WE WILL USE A COUPLED HYDROSPHERE-BIOSPHERE-ATMOSPHERE ANALYSIS FRAMEWORK FOCUSING BROADLY ON NORTH AMERICA (NA) AND SPECIFICALLY ON CLIMATE SENSITIVE SEMI-ARID AND ARCTIC-BOREAL ECOSYSTEMS. WE SEEK TO ANSWER THE FOLLOWING CRITICAL SCIENCE QUESTION: HOW HAS TERRESTRIAL CARBON UPTAKE IN PARTICULAR GPP AND ER EVOLVED IN THE PAST DECADE AND WHAT HAS BEEN THE IMPACT ON ATMOSPHERIC CO2 CONCENTRATIONS?"