*3.2. Seasonal Trend in Climate and Vegetation*

The changes in LAI and GPP are underlain not only by changes in annual climate but also by seasonal climate changes. Seasonal changes in climate (Figure 5) are associated with increased LAI and GPP (Figure 6), particularly for the temperature-limited region (note that we present seasonal analyses for the Northern Hemisphere only, to avoid contrasting seasonal patterns in the Southern Hemisphere and because the Northern Hemisphere accounts for 68% of total land area).

Boreal summers (June, July, and August: JJA) in temperature-limited regions become warmer, wetter, and brighter (Figure 5a,d,g); conditions that, at least in current Earth system models, lead to a summertime spike in LAI and GPP increases (Figure 6m,p). The precipitation increase is largest in the boreal fall (September, October, and November: SON); warming is at least 4 ◦C in all months, and increases to nearly 10 ◦C in winter. The JJA increase in solar radiation approaches 10 W/m2, but is offset by winter decreases (Figure 5g), leading to no net change in the annual mean (Figure 3g).

**Figure 5.** Easing of seasonal constraints to vegetation growth. Simulated changes in monthly climate and vegetation in the three climate-limiting regions: temperature, (**a**–**c**); precipitation, (**d**–**f**); and radiation, (**g**–**i**). Bars show climatological values (2006–2015), lines show ensemble mean monthly changes from Earth system models of CMIP5. Circles indicate that the trend in the 2006–2099 ensemble mean is significant at the 95% level from a Mann–Kendall trend test, while plus signs show that the Wilcoxon signed rank test is significant at the 95%. Shading is used to highlight changes in the limiting factor for each of the three regions (e.g., blue shading highlights temperature changes in the temperature-limited region).

**Figure 6.** Easing of seasonal constraints to vegetation growth. Same was Figure 5, except for transpiration, **a**–**c**; GPP, **d**–**f**; and LAI, **g**–**i**.
