A critical unknown is whether shifting plant phenology is altering tundra carbon cycling and wildlife habitats. Projections of climate feedbacks from high-latitude ecosystems remain uncertain as we do not yet know if carbon losses from warming soils will be offset by increases in tundra productivity. Tundra plant responses to warming could be key for understanding the fate of wildlife populations in a rapidly changing Arctic. Forty years of satellite and field observations have revealed widespread changes in the tundra’s surface that protects large stocks of frozen carbon below. Field studies indicate that plants are coming into leaf earlier in spring, bare ground is becoming vegetated, and plants are now growing taller. While there is scientific consensus that climate change is reshaping Arctic ecosystems, great uncertainty persists about what the greening observed from space means in terms of change on-the-ground. The TundraTime project will answer the fundamental research questions of whether climate warming is leading to longer periods of plant growth and increases in plant productivity in the Arctic. We will test specific hypotheses of whether tundra ecosystems are experiencing: A) increases in productivity, B) shifts in phenology and C) asynchrony of above- and below-ground plant growth. To explore these questions, we will integrate high-resolution drone and time-lapse camera imagery with satellite and in-situ data from 12 focal Arctic research sites. Our findings will inform biome-wide projections of tundra vegetation change and global-scale predictions of climate feedbacks to unprecedented rates of warming. If tundra plant productivity is responding directly to the warmer and longer Arctic growing seasons then tundra productivity will trap more carbon in tundra ecosystems and restructure wildlife habitats. However, if instead tundra plant growing seasons are shifting earlier, then projections of increases in tundra vegetation with warming may be overestimates and earlier timing of key forage could alter migratory behaviour and ultimately wildlife populations. And, if the above- and below-ground responses of tundra plants are asynchronous, plant growth in the now extended snow-free autumns could instead be occurring below ground, which would overturn how satellite data and Earth-system models estimate plant productivity and carbon storage in warming tundra ecosystems. The TundraTime project will test the drivers of Arctic greening by resolving the uncertainty around what role shifting plant phenology plays in the increased tundra productivity with warming. This research will bridge critical scale gaps to resolve the uncertainty between satellite and in-situ observations of changes in the timing of plant growth with accelerating climate warming.