# Constraints to Vegetation Growth Reduced by Region-Specific Changes in Seasonal Climate
## 4 Discussion  
Our analysis suggests fundamental future increases in the amount of vegetation and photosynthesis (LAI and GPP in Figure 1), mainly arising from relaxing climate constraints on vegetation growth. This feedback effect can explain the discrepancy between the models and the observation in the \(\beta\) factor [14]. We argue that the results are consistent with three lines of observational evidence and a considerable body of paleoclimatic evidence of dramatically different vegetation composition during past high-CO\({}_{2}\) periods [41; 42]. Figure 11: Monthly contribution of climate feedback and radiative climate change to vegetation growth in increases in CO\({}_{2}\) by 1%/year to 4 xCO\({}_{2}\) experiment (1 pctCO2) for each climate-limited area. The monthly contribution was calculated for each climate-limited region. The green and purple bars show the contribution of climate feedback and radiative climate change, respectively. In each region, the total of the contribution (CO\({}_{2}\) fertilization, climate feedback, and radiative climate change) was summed up to 100%. First, if our central claim that vegetation physiological process reduces transpiration, reduces cloud cover, and increases radiation is correct, then cloud cover, particularly low-level clouds, which strongly influence the planetary shortwave radiation budget, should decrease. Recent climate modeling studies indeed simulate a decrease in low-level cloudiness due to the vegetation physiological effect [23; 29; 35]. Further, several modeling studies indicate that the rapid adjustments of the troposphere for the combined radiative and physiological effects of increased CO\({}_{2}\) are associated with a decrease in low-level cloud cover over land, but increased boundary layer cloud cover over oceans [29; 43; 44]. We re-analyzed the NDP026 ground observation of cloud cover [26] and show that the modeled processes have indeed occurred over the period 1971 to 2005 using the Mann-Kendall trend test. During this period, cloud cover significantly decreased by a few percent points per decade over much of the land surface, and increased over the ocean (Figure 12). Second, the same models we used for future projections under RCP 8.5 produce simulations of the historical climate and vegetation that are broadly consistent with independent observations (Figure 13). Although historical skill does not guarantee future performance, region-level simulations of climatological temperature, and precipitation are statistically indistinguishable from the Climate Research Unit (CRU) product [25] in all months (Figure 13a,b). Simulated radiation, assessed for the radiation-limited regions of the Northern hemisphere against the CRUNCEP radiation dataset, has climatological differences of up to 15 W/m\({}^{2}\), but a similar seasonal cycle (Figure 13e). The Earth system models also capture seasonal satellite-observed variations in LAI in all three regions (Figure 13b,d,f). Figure 12: Changes in observed (NDP026) and CMIP-simulated cloud cover are consistent with a CO\({}_{2}\)-induced down-regulation of stomatal conductance, resulting in reduced transpiration. Trends from observations are shown as decreasing (red) or increasing (blue) annual average cloud cover for 1971–2005: Red would tend to support our hypothesis of reduced transpiration and cloud cover as a result of stomatal down-regulation. If the CMIP5 trend and the observed trend have the same sign, the corresponding box is hatched. Location and shape of boxes corresponds to the coverage in the observational dataset, the 1971–2005 comparison period represent the overlap between the observational dataset and the historical CMIP5 runs. A change during the mid-1990s in cloud cover observation methodology in the US precluded their use in trend evaluation in NDP026. A chi-squared test between the two data sets rejected the null-hypothesis that they are independent (\(p=0.05\)).