Background/Question/Methods Warmer air temperatures and changes in summer and winter precipitation are major drivers of changes in the Arctic System, as is the more recently appreciated role of grazers in these northern landscapes. For the past 15 years, we have been conducting field experiments in northern Alaska, northern Ellesmere Island, northwest Greenland and west-central Greenland aimed at answering the question: how will high and low arctic tundra respond to changes in winter and summer conditions as they effect changes in CO2 exchange, along with soil and plant traits? Our studies include analyses of winter and growing season processes, allowing us to articulate the linkages between seasons and the role of winter in controlling summer processes. Our studies have used ITEX chambers and IR lamps as means of warming and snow fences to increase snow depth. Results/Conclusions Our findings in northwest Greenland indicate that a doubling of snow depth increases over-winter rates of CO2 efflux and increases summer NEE, GEP and leaf N. Results in northwest Greenland and on Ellesmere Island indicate that changes in CO2 exchange with warming depended strongly upon ecosystem type. In northwest Greenland, for instance, strong responses were observed in a fen, while weak responses were observed in a polar semi-desert. Our work in west-central Greenland has highlighted the importance of herbivores as mediators of ecosystem responses to warming. Grazing by muskoxen and caribou eliminated large increases in GEP and NEE with warming that were observed in the absence of herbivores In northern Alaska, where our snow fence experiment been maintained for 15 years, the soil active layer is increasing and old C is being released to the atmosphere. Our program has led to four major conclusions. First, changes in the winter climate are important drivers of growing season processes and annual C budgets. Second, the magnitude of response to climate change will depend upon ecosystem type. Third, arctic herbivores may alter the trajectory of ecosystem responses and feedbacks to climate change. Fourth, some ecosystem responses to climate change may be cumulative and, therefore, poorly captured by short-term field experiments.