Interactive comment on “ Pathway dependence of ecosystem responses in China to 1 . 5 ◦ C global warming ”

This vegetation model-based quantification of various components, including rising CO2, O3 pollution, and warming, influencing carbon sequestration across terrestrial ecosystems in China is not less than being complete. Moreover, there are many places that are quite interesting to me and would appeal to the broad communities around ACP. For example, to supplement with diffuse radiation the CMIP5 data the authors compiled empirical relationships between total and diffuse radiation and identified the best one therein to derive the diffuse radiation. What’s also interesting is that the authors drew a conclusion that the allowable carbon budget is higher than expected to achieve the 1.5 deg C goal under a stabilized pathway.


50
The past decade has seen record-breaking warming largely related to anthropogenic 51 greenhouse gas emissions (Mann et al., 2017). This warming trend presents a challenge 52 to achieve the temperature control target of 1.5°C above the pre-industrial (PI) level set 53 by the 2015 Paris climate agreement. Many studies have shown that a conservative 54 warming such as 1.5°C is necessary to limit climatic extremes (Nangombe et al., 2018), 55 avoid heat-related mortality (Mitchell et al., 2018), reduce economic loss (Burke et al.,56 2018), and alleviate ecosystem risks (Warszawski et al., 2013) compared to stronger 57 anthropogenic warming. To achieve this target, each country must aim to control its 58 greenhouse gas emissions. A full understanding of regional ecosystem response to the 59 changing climate and environmental stress is essential to reduce uncertainties in 60 allowable carbon budget estimates at 1. 5°C (Mengis et al., 2018). China is covered with 61 a wide range of terrestrial biomes (Fang et al., 2012). While China's ecosystem 62 response to possible future climate has been explored (Wu et al., 2009;He et al., 63 2017;Dai et al., 2016), impacts on the regional carbon budget of differing pathways to 64 the 1.5°C target are not known. 5 of GMT relative to PI period  from two scenarios are examined (Fig. S1a). 110 The low emission scenario RCP2.6 yields an equilibrium ∆GMT of 1.85°C by 2100. 111 We remove 8 climate models predicting stabilized ∆GMT higher than 1.85°C by the 112 end of century. The 7 remaining models yield an ensemble warming close to 1.5°C

113
(1.49°C for 2050-2070, Fig. S1b). Meanwhile, ∆GMT in the high emission scenario 114 RCP8.5 grows fast and realizes a transient 1.5°C warming around the year 2021-2041. 115 Daily meteorology from 7 selected models (Table S1)  Here V d is the original daily variables and " # is the scaled value. S w is the 2-  The original CMIP5 archive does not provide diffuse component of shortwave radiation.

152
Here, we use empirical relations between total and diffuse radiation from 11 studies to 153 calculate hourly diffuse radiation (Table S4). The diffuse fraction k d in all equations 154 depends on clearness index k t , which is defined as the ratio between global solar 155 radiation I t and extra-terrestrial solar radiation I 0 (Ghosh et al., 2017): Here I sc = 1367 W m -2 is solar constant, N is Julian day of the year, and φ is solar zenith.

159
The empirical equations are evaluated using hourly total and diffuse radiation from  We apply the YIBs model (Yue and Unger, 2015; (Spitters, 1986). Sunlit leaves can receive both direct and diffuse radiation, while 183 shading leaves receive only the diffuse component   (Spitters, 1986). Simulated GPP responses to direct and diffuse radiation show good 210 agreement with observations at 24 global flux tower sites from FLUXNET network 211 (Yue and Unger, 2018). In general, diffuse radiation is more efficient to enhance canopy 212 photosynthesis compared to the same level of direct radiation.

215
We perform two main groups of simulations, one for RCP2.6 and the other for RCP8.5.

216
For each group, 7 sub-groups are designed with varied climatic or CO 2 forcings (  The main focus of this study is to quantify how the differences of anthropogenic 244 emissions, including both CO 2 and air pollution which are usually associated, will cause 245 different responses in land carbon budget to the same global warming target. Especially, 246 the role of air pollution on land carbon cycle has always been ignored. The assumptions 247 of land use can be quite uncertain among future pathways (Stehfest et al., 2019), and 248 these assumptions are not necessarily associated with CO 2 and air pollution emissions.

339
The changes in carbon fluxes follow the variations in atmospheric composition and 340 climate ( Fig. 6 and Figs. S8-S11). By the global warming of 1.5°C, a dominant fraction 341 of GPP enhancement in China is attributed to CO 2 fertilization (Fig. 6a). For the RCP2.6 342 scenario, CO 2 alone contributes 0.83 Pg C yr -1 (77%) to ∆GPP, with the highest 343 enhancement of 0.8 g C m -2 day -1 over the southeast coast (Fig. S8a). For RCP8.5, CO 2 344 fertilization increases GPP by 0.95 Pg C yr -1 , even higher than the total ∆GPP of 0.82

347
The 12 ppm differences in CO 2 concentrations lead to a change of 0.12 Pg C yr -1 (1.7%) 348 13 in GPP. This sensitivity of GPP to CO 2 , 0.14% ppm -1 , falls within the range of 0.05-349 0.21% ppm −1 as predicted by 10 terrestrial models (Piao et al., 2013) and that of 0.01-350 0.32% ppm −1 as observed from multiple free-air CO 2 enrichment (FACE) sites 351 (Ainsworth and Long, 2005). The higher ∆GPP in RCP2.6 instead yields a weakened 352 NEE (more positive) due to the CO 2 effects (Fig. 6b). The stabilization of CO 2 353 concentrations in this scenario (Fig. 3a)  the high emission pathway (Fig. 3b) Changes in meteorology account for the rest of the perturbations in the carbon fluxes.

383
At the global warming of 1.5°C, temperature in China increases by 0.90°C for RCP2.6 384 and 0.91°C for RCP8.5 (Figs. S12a-S12b) compared to present-day climate. The spatial 385 pattern of these changes is very similar without significant differences (Fig. S12c), and S9e). Precipitation increases by 0.14 mm day -1 (4.6%) over eastern China in 397 RCP2.6 but decreases by 0.03 mm day -1 (1.2%) in RCP8.5 (Figs. S12d-S12e), leading 398 to higher soil moisture in eastern China for RCP2.6 (Figs. S13d-S13e For the RCP2.6 scenario, the net effect of climate change causes an increase of 0.15 Pg 403 C yr -1 in GPP with a range from -0.54 to 0.62 Pg C yr -1 (Fig. 6a). Such large variability 404 in ∆GPP is related to the uncertainties in meteorology from different climate models.

405
For RCP8.5, climate-induced GPP change is only 0.04 Pg C yr -1 with a range from -0.6 406 to 0.26 Pg C yr -1 . The discrepancy of ∆GPP for the two pathways is mainly caused by 407 the different radiation impacts, which enhance GPP by 0.2 Pg C yr -1 in RCP2.6 but only 0.11 Pg C yr -1 in RCP8.5 (Fig. 6a). Photosynthetically active radiation (PAR) is higher 409 by 2.8 W m -2 in RCP2.6 than in RCP8.5 (Fig. 3c). The distinct changes in radiation are 410 related to aerosol radiative effects, because global analyses also show radiation 411 enhancement in regions (e.g., U.S. and Europe) with aerosol removal (Fig. S14). The 412 lower AOD in RCP2.6 helps increase solar insolation at surface by reducing light 413 extinction (Yu et al., 2006), and promote precipitation with weaker aerosol semi-direct 414 and indirect effects (Lohmann and Feichter, 2005). Although lower aerosols in RCP2.6 415 slightly decrease diffuse radiation (Fig. 3d), which is more efficient in increasing 416 photosynthesis (Mercado et al., 2009;Yue and Unger, 2018), the overall enhancement 417 in total radiation helps boost GPP. Climate-induced ΔNEE is -0.02 Pg C yr -1

435
The slow warming increases the allowable cumulative anthropogenic carbon emissions.  The YIBs simulations do not consider nitrogen cycle and its limitation on carbon uptake.

467
Inter-model comparisons show that models without nutrient constraints tend to 468 overestimate GPP responses to CO 2 fertilization (Smith et al., 2016). As a result, the 469 difference of CO 2 contributions in RCP scenarios would be smaller than predicted (Fig.   470 6a), suggesting that GPP enhancement in RCP2.6 might be even higher than RCP8.5 if 471 nitrogen cycle is included. In contrast, nitrogen deposition in RCP2.6 would be much 472 smaller than that in RCP8.5 due to emission control (Fig. S5), leading to lower nitrogen 473 supply for ecosystem in the former scenario. Consequently, plant photosynthesis is 474 confronted with stronger nutrient limit in RCP2.6 than that in RCP8.5, resulting in Assimilation in Leaves of C-3 Species, Planta, 149, 78-90, 10.1007/Bf00386231, 1980    total shortwave radiation (W m -2 ) and (d) diffuse radiation derived with method M01 (Table S4)    using YIBs vegetation model driven with daily meteorology from 7 CMIP5 models.

766
The O 3 damaging effect is included with predicted ensemble O 3 concentrations from 12 767 ACCMIP models. For each grid, significant changes at p<0.05 are marked with dots.

768
The total changes (Pg C yr -1 ) over China are shown in each panel.