An emergent constraint on the thermal sensitivity of photosynthesis and greenness in the high latitude northern forests

Despite the general consensus that the warming over the high latitudes northern forests (HLNF)3 has led to enhanced photosynthetic activity and contributed to the greening trend1,2, isolating the4 impact of temperature increase on photosynthesis and greenness has been difficult due to the5 concurring influence of the CO2 fertilization effect3. Here, using an ensemble of simulations from6 biogeochemical models that have contributed to the Trends in Net Land Atmosphere Carbon7 Exchange project (TRENDY), we identify an emergent relationship between the simulation of the8 climate-driven temporal changes in both gross primary productivity (GPP) and greenness (Leaf9 Area Index, LAI) and the model’s spatial sensitivity of these quantities to growing-season (GS)10 temperature. Combined with spatially-resolved observations of LAI and GPP, we estimate that11 GS-LAI and GS-GPP increase by 17.0 ± 2.4% and 24.0 ± 3.0% per degree of warming, respectively.12 The observationally-derived sensitivities of LAI and GPP to temperature are about 40% and 71%13 higher, respectively, than the mean of the ensemble of simulations from TRENDY, primarily due14 to the model underestimation of the sensitivity of light use efficiency to temperature. We15 estimate that the regional mean GS-GPP increased 28.2 ± 5.1% between 1983-1986 and 2013-16 2016, much larger than the 5.8 ± 1.4 % increase from the CO2 fertilization effect implied by Wenzel17 et al.4 This suggests that warming, not CO2 fertilization, is primarily responsible for the observed18 dramatic changes in the HLNF biosphere over the last century.