Temperature and vegetation seasonality diminishment over northern lands

Global temperature is increasing, especially over northern lands (>50 N), owing to positive feedbacks1. As this increase is most pronounced in winter, temperature seasonality (ST)—conventionally defined as the difference between summer and winter temperatures—is diminishing over time2, a phenomenon that is analogous to its equatorward decline at an annual scale. The initiation, termination and performance of vegetation photosynthetic activity are tied to threshold temperatures3. Trends in the timing of these thresholds and cumulative temperatures above them may alter vegetation productivity, or modify vegetation seasonality (SV), over time. The relationship between ST and SV is critically examined here with newly improved ground and satellite data sets. The observed diminishment of ST and SV is equivalent to 4 and 7 (5 and 6) latitudinal shift equatorward during the past 30 years in the Arctic (boreal) region. Analysis of simulations from 17 state-of-the-art climate models4 indicates an additional ST diminishment equivalent to a 20 equatorward shift could occur this century. How SV will change in response to such large projected ST declines and the impact this will have on ecosystem services5 are not well understood. Hence the need for continued monitoring6 of northern lands as their seasonal temperature profiles evolve to resemble those further south. The Arctic (8.16million km2) is defined here as the vegetated area north of 65N, excluding crops and forests, but including the tundra south of 65N. The boreal region (17.86million km2) is defined as the vegetated area between 45N and 65N, excluding crops, tundra, broadleaf forests and grasslands south of the mixed forests, but including needleleaf forests north of 65N (Supplementary Fig. S1). These definitions are a compromise between ecological and climatological conventions. Importantly, they include all non-cultivated vegetation types within these two regions.