Boreal carbon loss due to poleward shift in low-carbon ecosystems

Climate change can be thought of in terms of geographical shifts in climate properties. Tracking the geographical movement of analogous climate conditions between historical and future climate model simulations, and calculating the impact of such shifts on vegetation carbon storage, suggests that boreal forests will lose carbon as low-carbon ecosystems shift in. Climate change can be thought of in terms of geographical shifts in climate properties. Examples include assessments of shifts in habitat distributions1, of the movement needed to maintain constant temperature or precipitation2, and of the emergence and disappearance of climate zones3. Here I track the movement of analogue climates within climate models. From the model simulations, I define a set of vectors that link a historical reference climate for each location to the location in a changed climate whose seasonal temperature and precipitation cycles best match the reference climate. I use these vectors to calculate the change in vegetation carbon storage with climate change due to ecosystems following climate analogues. Comparing the derived carbon content change to direct carbon projections by coupled carbon–climate models reveals two regions of divergence. In the tropical forests, vector projections are fundamentally uncertain because of a lack of close climatic analogues. In the southern boreal forest, carbon losses are projected in the vector perspective because low-carbon ecosystems shift polewards. However, the majority of carbon–climate models—typically without explicit simulation of the disturbance and mortality processes behind such shifts—instead project vegetation carbon gains throughout the boreal region. Southern boreal carbon loss as a result of ecosystem shift is likely to offset carbon gains from northern boreal forest expansion.