THE AMPLITUDE OF ATMOSPHERIC SEASONAL CO2 CYCLES IN HIGH LATITUDE ENVIRONMENTS HAS INCREASED BY 30-50% OVER THE LAST 50 YEARS. ROUGHLY TWO-THIRDS OF THIS INCREASE IS ATTRIBUTED TO GREATER SEASONAL CO2 EXCHANGE IN ARCTIC TUNDRA AND BOREAL FORESTS. SEVERAL PLAUSIBLE MECHANISMS HAVE BEEN HIGHLIGHTED BUT THE OVERALL CONTRIBUTIONS REMAIN UNKNOWN DESPITE THE IMPLICATIONS FOR ARCTIC AND BOREAL CARBON CYCLING AND CLIMATE FEEDBACKS. FROM A BROAD PERSPECTIVE THE INCREASING AMPLITUDE CAN ONLY BE DUE TO EITHER INCREASED NET CO2 UPTAKE IN THE GROWING SEASON AND/OR INCREASED EFFLUX IN THE DORMANT SEASON. A NUMBER OF STUDIES HAVE IDENTIFIED THIS BUT THE CONTRIBUTION FROM EACH PROCESS REMAINS UNCERTAIN. HOWEVER EVEN A COMPLETE UNDERSTANDING AT THIS LEVEL IS NOT SUFFICIENT FOR DIAGNOSING SYSTEM CHANGES AND PREDICTIVE MODELING AS A VARIETY OF ECOSYSTEM PROCESSES AND RESPONSES TO GLOBAL CHANGE COULD BE RESPONSIBLE. THE PROPOSED PROJECT WORKS TO FILL THIS KNOWLEDGE GAP BY PRESENTING BOTTOM-UP ESTIMATES OF ECOSYSTEM DRIVERS LEADING TO THIS AMPLITUDE TREND. ESTIMATES WILL BE EVALUATED IN THE CONTEXT OF MULTIPLE TOP-DOWN CONSTRAINTS. BECAUSE ANY ONE RESEARCH TOOL IS INHERENTLY LIMITED THE PROJECT WILL UTILIZE SEVERAL TOOLS AND APPROACHES TO ADDRESS MULTIPLE LINES OF INQUIRY WITHIN A HYPOTHESIS-DRIVEN FRAMEWORK. THESE INCLUDE (I) LARGE-SCALE AND LONG-TERM REMOTE SENSING DATA SETS (II) BENCHMARKING DEVELOPMENT AND EXPERIMENTS WITH A LAND SURFACE MODEL (III) MULTI-MODEL INTERCOMPARISONS (IV) ATMOSPHERIC TRANSPORT MODELING AND (V) META-ANALYSIS OF IN SITU FLUX OBSERVATIONS. RESEARCH TASKS ARE LINKED THROUGH A SET OF COMPLEMENTARY AND TRACTABLE HYPOTHESES. OUR PRIMARY GOAL WILL BE TO ATTRIBUTE THE CAUSES OF THE OBSERVED INCREASED CO2 EXCHANGE IN HIGH LATITUDES. THIS WILL IMPROVE OUR COLLECTIVE UNDERSTANDING AND ABILITY TO PROJECT HIGH-LATITUDE CARBON DYNAMICS. HOWEVER OUR PROPOSED RESEARCH WILL ALSO ADVANCE SEVERAL RELATED TOPICS OF BROAD INTEREST TO THE COMMUNITY. THESE INCLUDE HOW VEGETATION PROPERTIES HAVE CHANGED IN TUNDRA AND BOREAL FORESTS AND HOW MUCH CHANGE STEMS FROM DISTURBANCE; HOW CLIMATE AND OTHER FACTORS HAVE ENHANCED PRODUCTIVITY; AND HOW INCREASED TEMPERATURE SUBSTRATE QUANTITY VEGETATION COVER AND SNOW DYNAMICS HAVE INFLUENCED DORMANT SEASON RESPIRATION. OUR PROJECT ADDRESSES SEVERAL HIGH-LEVEL RESEARCH QUESTIONS AND PRIORITIES FOR DOE NASA AND THE US CARBON CYCLE SCIENCE PROGRAM AS THEY RELATE TO ELEMENT 3.1.2 OF THE CARBON CYCLE SCIENCE SOLICITATION. AMONG OTHERS THESE INCLUDE (I) HOW DO NATURAL PROCESSES AND HUMAN ACTIONS AFFECT THE CARBON ON LAND AND IN THE ATMOSPHERE? (II) HOW ARE ECOSYSTEMS IMPACTED BY INCREASING GREENHOUSE GAS CONCENTRATIONS AND THE ASSOCIATED CHANGES IN CLIMATE? (III) WHAT IS THE DIRECTION AND STRENGTH OF ARCTIC/BOREAL CLIMATE FEEDBACKS? AND (IV) HOW CAN WE BEST REPRESENT MECHANISTIC CARBON FEEDBACKS IN EARTH SYSTEM MODELS? BECAUSE OF THE STRONG LINKS WITH PROCESS-LEVEL UNDERSTANDING EARTH SYSTEM MODELS AND CLIMATE FEEDBACKS WE ARE RECOMMENDING OUR PROPOSAL BE REVIEWED BY DOE. HOWEVER THERE ARE STRONG TIES TO NASA PRIORITIES AND WE PROPOSE TO INCORPORATE EXISTING AND NOVEL REMOTE-SENSING BASED DATA SETS INTO SEVERAL PROJECT COMPONENTS; THUS CO-REVIEW WITH NASA WOULD BE APPROPRIATE.