Impacts of 1.5 °C and 2 °C Global Warming on Net Primary Productivity and Carbon Balance in China’s Terrestrial Ecosystems

Round 1

Reviewer 1 Report

General comments

 

This is an interesting approach to explaining interactions between carbon balances, such as NPP and NEP, and 1.5 and 2 global warmings. What is done seems to conform to well-known facts. For example, NPP will increase during the 21^st century. However, I am wondering that the slope analysis used in this study is robust enough to characterize the trends of NPP and NEP since, from Fig. 5, NEP decreases from 2030 to 2050 under RCP4.5. Another issue of concern to me is that CO₂ fertilization effects will play a significant role in determining NPP and NEP dynamics. Therefore, only analyzing relationships between temperature and NPP and NEP would not provide comprehensive and robust results. In addition, model validations nor cross-modal comparisons were not provided (just mentioned briefly, “is consistent with the simulation…”). Thus, an uncertainty analysis should be dealt with in the manuscript. Overall, nice work. However, there are several points that should be addressed before publication.

 

Detail comments

 

P2 – L61: Please provide reasons why recent studies projected decreases in net carbon uptake.

 

P3 – L87: Need citations for “Extensive analyses…”

 

P4 – L109: What criteria were used to select the four ESMs data? Although the focus of this study is to analyze NPP and NEP, MPI-ESM-LR and NorESM1-ME do not provide NEP data (Table 1).

 

P4 – L113: CIMP Phase 6 is available now.

 

P4 – L121: The website is not working. Please check the URL.

 

P5 – L134: Where were soil types and soil texture downloaded?

 

P5 – L135: Please provide a URL for Global Land Cover 2000 database.

 

P5 – L143-145: Different periods and different RCPs have different CO₂ and precipitation patterns and trajectories. What are the impacts of CO₂, precipitation, and solar radiation on NPP and NEP?

 

P5 – L150-151: What types of interpolation were used? Was the interpolation method strong enough to accurately represent the spatial distributions of NPP and NEP?

 

Results: Although the different study periods for RCP4.5 and RCP8.5 were used in this study, this study did not consider the influences of CO₂, precipitation, and solar radiation on NPP and NEP. These should be considered to make the conclusions driven from this study robust.

 

P7 – L192-195: Under what scenarios? RCP4.5 or RCP8.5?

 

Fig. 2: The subfigures (a to d) look almost the same. A statistic representation of the results (e.g., pdf, bar plot, or difference between them) may be used to see the differences between them.

 

P9 – L224-226: Please provide reasons why “it is greater in northwestern China than…”

 

P11 – L255-256: Net carbon uptake is also driven by CO₂ increase.

 

Fig. 5: is not used in the text. I was wondering why NEP decreases from 2030 to 2050 under RCP4.5? Is the use of slop to analyze the NEP robust to conclude the results found in this study?

 

P18 – L391 and L401-402: As noted by the authors, this study should consider the effects of CO₂ fertilization on NPP and NEP.

Author Response

General comments

 This is an interesting approach to explaining interactions between carbon balances, such as NPP and NEP, and 1.5 and 2 global warnings. What is done seems to conform to well-known facts. For example, NPP will increase during the 21^st century. However, I am wondering that the slope analysis used in this study is robust enough to characterize the trends of NPP and NEP since, from Fig. 5, NEP decreases from 2030 to 2050 under RCP4.5. Another issue of concern to me is that CO₂ fertilization effects will play a significant role in determining NPP and NEP dynamics. Therefore, only analyzing relationships between temperature and NPP and NEP would not provide comprehensive and robust results. In addition, model validations nor cross-modal comparisons were not provided (just mentioned briefly, “is consistent with the simulation…”). Thus, an uncertainty analysis should be dealt with in the manuscript. Overall, nice work. However, there are several points that should be addressed before publication.

 Detail comments

Dear reviewer and editor,

We appreciate the editor and the reviewers for their thoughtful comments and constructive suggestions, which are very helpful for revising and improving our manuscript. In the revised manuscript, we have carefully addressed all the comments/suggestions and have made substantial edits to the manuscript. Please find detailed responses to the reviewers’ comments as followed.

 P2 – L61: Please provide reasons why recent studies projected decreases in net carbon uptake.

 [Response: Revised as requested. We added a couple of phases to provide the reasons for the projected decrease in net carbon uptake. According to previous studies (Ciais et al., 2013; Hadden and Grelle, 2016; Bispo et al., 2017; Ellsworth et al., 2017; Sihi et al., 2017), many factors contribute to the decrease of net carbon uptake, including enhanced soil organic matter decomposition induced by warming, water deficit, climatic extreme events and land use and land cover change, etc.]

P3 – L87: Need citations for “Extensive analyses…”

[Response: Revised as requested.]

P4 – L109: What criteria were used to select the four ESMs data? Although the focus of this study is to analyze NPP and NEP, MPI-ESM-LR and NorESM1-ME do not provide NEP data (Table 1).

[Response: This study mainly focused on the responses of terrestrial ecosystems to global warming targets, especially on NPP and NEP, so we considered multi-factors when we selected models in CIMIP5. We also considered the vegetation types used in different models, so that they can approximately match with the CEVSA2 model. Based on these criteria, we have chosen four ESMs and averaged to an ensemble with our ecosystem model to analyze the responses of the terrestrial ecosystem. Simulating NEP is very limited by ESMs directly, but it is very important to both climatic and ecological systems. Considering the research and knowledge about 1.5℃ and 2℃ global warming impacts on the ecosystem at regional scales still very shortage, especially in China, so we believe this work could contribute to further understand the responses and risks of terrestrial ecosystems to global warming on regional scales. In the revised manuscript, we described the criteria of the model selection in a more explicit fashion: the availability of monthly NPP and NEP, and approximate classification of vegetation function types.]

P4 – L113: CIMP Phase 6 is available now.

[Response: Our work was conducted when most of CIMP6 data were not available. Because spatial interpolation is very time-assuming, we keep using the results from the CMIP 5 datasets in this study. However, we will continue our study by including the CMIP 6 datasets in the next step..]

P4 – L121: The website is not working. Please check the URL.

[Response: The URL should be updated or moved, we gave another available URL.]

P5 – L134: Where were soil types and soil texture downloaded?

[Response: The map was digitized by our previous studies, which was based on the map of soil texture in China. In the revised manuscript, we added URL of the map.]

P5 – L135: Please provide a URL for Global Land Cover 2000 database.

[Response: we added the URL for the GLC2000 data.]

P5 – L143-145: Different periods and different RCPs have different CO₂ and precipitation patterns and trajectories. What are the impacts of CO₂, precipitation, and solar radiation on NPP and NEP?

[Response: We couldn’t agree with the anonymous reviewer’s opinion more, all of these factors are very important to NPP and NEP in the future, and the impacts of these factors also were used for exploring the driving mechanism of NPP and NEP change. we added the explanation in the section of Discussions and analyzed the impacts of main climatic factors(P21 L400). In this paper, our main concerns were the responses and risks of terrestrial ecosystem to comprehensive effects of climate conditions under different global warming targets, not very much on the impacts of each factor of climate and environment. So we briefly show the change of temperature and precipitation at 1.5℃ and 2℃ global warming and its connection with the change of NPP and NEP, we did not give more detail analysis in these aspects.]

P5 – L150-151: What types of interpolation were used? Was the interpolation method strong enough to accurately represent the spatial distributions of NPP and NEP?

[Response: In this study, we interpolated the dataset of running CEVSA2 model into the latitude-longitude grid at a resolution of 0.1°´0.1° by the ANUSPLIN4.0 software and run the model at 0.1°´0.1° spatial resolution. The results of ESMs we interpolated to 0.1°´0.1° resolution by ArcGIS software. Then, all the results were averaged to an ensemble for the analysis. We acknowledge that the interpolation of NPP and NEP involves uncertainties and have added relevant uncertainty discussion in the revised manuscript; like the effects of climate factors and atmosphere CO2 concentration etc. We also added more explanation in the section of Instruction part in the revised manuscript.]

Results: Although the different study periods for RCP4.5 and RCP8.5 were used in this study, this study did not consider the influences of CO₂, precipitation, and solar radiation on NPP and NEP. These should be considered to make the conclusions driven from this study robust.

[Response: In this paper, we focused on the responses of terrestrial ecosystems to the comprehensive effects of climate with different global warming targets. Considering the importance of these factors to NPP and NEP, we added figures about temperature and precipitation changes with different global warming, and added a explanation of the influences briefly in the part of Discussions.(P21 L400)]

P7 – L192-195: Under what scenarios? RCP4.5 or RCP8.5?

 [Response: We rephrased the sentence.]

Fig. 2: The subfigures (a to d) look almost the same. A statistic representation of the results (e.g., pdf, bar plot, or difference between them) may be used to see the differences between them.

 [Response: We added a column in figure2 to show the differences of the extra 0.5℃ warming, hoping that they would show the difference clearly.]

P9 – L224-226: Please provide reasons why “it is greater in northwestern China than…”

[Response: We added an explanation about the reasons as suggested. Indeed, the absolute value of NPP increase is larger in southern and southeastern China. We used the relative change of NPP to express the sensitivity of plant growth to warming. The greater relative increase of NPP in Qinghai-Tibet Plateau, northwestern and northern China indicates that the regions with lower temperature and water deficit are higher sensitive to warming and change of precipitation. These results agree with previous studies (Nemani et al., 2003; Gu et al., 2015).]

P11 – L255-256: Net carbon uptake is also driven by CO₂ increase.

[Response: We agree with the opinions, we analyzed the influences of increasing CO₂ concentration in Discussions. In the CEVSA2 model, annual CO₂ concentration was used as one of the input environmental data in the transient simulations, so the NPP and NEP results have included the effects of elevated CO₂ concentration.]

Fig. 5: is not used in the text. I was wondering why NEP decreases from 2030 to 2050 under RCP4.5? Is the use of slop to analyze the NEP robust to conclude the results found in this study?

 [Response: In the revised manuscript, we added more interpretations of NEP decrease during the 2℃ warming period in Discussion section. Many studies have demonstrated that warming can stimulate plant growth and carbon uptake. However, increased air temperature also stimulates autotrophic respiration in plants and soil organic matter. The responses of net carbon uptake represent an integrated effect of changes in temperature and water status on photosynthesis and respiration. Some studies show that respiration is more sensitive to rising temperatures than photosynthesis. So, we think the high elevated temperature depresses the carbon uptake capacity by enhanced respiration.]

P18 – L391 and L401-402: As noted by the authors, this study should consider the effects of CO₂ fertilization on NPP and NEP.

 [Response: Thanks for the comment; in the revised manuscript, we analyzed the influences of increasing CO₂ concentration in Discussion section.]

 

Reviewer 2 Report

Li Yu et al. show the impacts of 1.5℃ and 2℃ global warming on terrestrial carbon budget in China. Results are robust and meaningful that additional 0.5℃ warming brings the decline of terrestrial carbon sink. However, I think there are three major concerns which should be solved before re-consider for publishing in Sustainability.

Major concerns
1. Only four models?
Authors mentioned that there are only four models based on the availability of monthly NPP and NEP in RCP4.5 and 8.5 scenarios. Is it really? I cannot believe that many previous studies used other models together such as Shao et al. 2013, Kim et al. 2016, Kim et al. 2017. I think if carbon emission by fire(fFire) do not provide by model, NBP(Net Biospheric Production) is equal to NEP. Can you check the availability of NBP in RCP4.5 and 8.5 scenarios?
Only four models are not enough to get a general feature of CMIP5, so I strongly recommend using more models to conduct your analysis if possible. 

2. ESM diversity
I think you show only Multi-Model Ensemble (MME) result for averaging BNU-ESM, IPSL-CM5A-LR, MPI-ESM-LR, NorESM2-ME and CEVSA2. I am not sure each model shows same result of your main conclusion. All model show declined growth rate of NEP with the additional 0.5℃?

3. The physical reason for declined growth rate
I cannot find the physical reason for the declined growth rate of NEP with the additional 0.5℃ throughout the manuscript. As the authors mentioned in the Introduction part, mean air temperature over Asia will increase more than global warmings such as 1.5℃ and 2℃, but the authors did not show temperature differences in between RCP4.5 and 8.5 which can contribute to the declined growth rate of NEP. Also, I am not sure, but changes in precipitation and other physical factors can contribute to the declined growth rate of NEP (e.g. soil moisture depletion, Kim et al. 2017 Nature Communications). On the other hand, enhanced extreme heat events can bring compound drought event which reduces GPP (Wang et al. 2020). I think the authors should provide at least temperature changes in the used model in this manuscript. 

Minor comments
L18 0.5℃warming -> 0.5℃ warming (Cross the manuscript, I found same  typos such as L31, L33, L99, L153, L160, L182, L183, L184, L193, L199, L202, L203, L223 and captions for Figs. 2,3,4,5,7 and many other lines...)

L56 net primary productivity (NPP) -> Net Primary Productivity (NPP) 
L58 net ecosystem productivity (NEP) -> Net Ecosystem Productivity (NEP)
L65 Is it annual mean temperature?
L86 earth system models -> Earth System Models (ESMs)
L990.5℃warming -> 0.5℃ warming
Table 1 Land surface component -> Land surface model
Table 1 Each land surface model mentioning needs references. 

L191 and L192 Can you calculate and provide 95% significance interval (SI) for annual NPP mean value? Such as 518 (±???) C m^-2 yr^-1
Also, you need to use the minus symbol (−), not short dash (-).
L196 I think "during" should be edited here.

Fig.1 Can you merge Fig1a and b? Y-axises are same, so I think you can merge together and it would be better than check differences between RCP4.5 and 8.5.
Fig. 2 Spatial pattern is important but I cannot check what is difference between a,b,c and d. How about draw one more row and column that differences for NPP2 and NPP1 (also RCP8.5 and RCP4.5)? 
Also, you already show differences in RI_NPP, but it seems just all greenish. Please use another color level for Fig 3 in order to distinguish differences. 
L250 ~ means approximately, not range. Please use – (En dash)
https://www.btb.termiumplus.gc.ca/tpv2guides/guides/pep/index-eng.html?lang=eng&page=punct_4_hyphens_dashes
L356 Land Use and Land Cover (LULC)

References
Kim, J.-S., J.-S. Kug & S.-J. Jeong (2017), Intensification of terrestrial carbon cycle related to El Niño–Southern Oscillation under greenhouse warming, Nature Communications, 8, 1674, doi:10.1038/s41467-017-01831-7
Kim, J.-S., J.-S. Kug, J.-H. Yoon & S.-J. Jeong (2016), Increased atmospheric CO2 growth rate during El Niño driven by reduced terrestrial CO2 capture in the CMIP5 ESMs, Journal of Climate, 29, 8783–8805, doi:10.1175/JCLI-D-14-00672.1
Wang, J. et al. (2020), Anthropogenically-driven increases in the risks of summertime compound hot extremes. Nature Communications, 11, 528, doi:10.1038/s41467-019-14233-8

Author Response

Comments and Suggestions for Authors

Li Yu et al. show the impacts of 1.5℃ and 2℃ global warming on terrestrial carbon budget in China. Results are robust and meaningful that additional 0.5℃ warming brings the decline of terrestrial carbon sink. However, I think there are three major concerns which should be solved before re-consider for publishing in Sustainability.

Dear reviewer and editor,

We appreciate the constructive advice of reviewer and hard work of editor. We took care to incorporate all recommendations and conducted a careful modification and improvement. We think that the comments helped to improve the quality of the manuscript. Please find below our replies (blue italics) to the comments and content added in manuscript by bracket.

 

Major concerns
1. Only four models?
Authors mentioned that there are only four models based on the availability of monthly NPP and NEP in RCP4.5 and 8.5 scenarios. Is it really? I cannot believe that many previous studies used other models together such as Shao et al. 2013, Kim et al. 2016, Kim et al. 2017. I think if carbon emission by fire (fFire) do not provide by model, NBP(Net Biospheric Production) is equal to NEP. Can you check the availability of NBP in RCP4.5 and 8.5 scenarios?
Only four models are not enough to get a general feature of CMIP5, so I strongly recommend using more models to conduct your analysis if possible. It is very useful to evaluate the temporal correlations between global annual mean carbon and climatic variables over land in the historical simulation. Light, T, P

[Response: We are sorry that we did not express clearly. We used monthly NPP and NEP simulating by four ESMs of CIMP5 in this paper, it doesn’t mean there are only four ESMs provided these simulating results, we rephrased it in the text. In this paper our work mainly focused on the responses of terrestrial ecosystem to global warming targets, especially on NPP and NEP, so we considered multi-factors when we selected models in CIMIP5, except the simulating of NPP and NEP, we also considered the vegetation types used in different models, so that they can approximate match with CEVSA2 model. And some ESMs, which shared the same land model, they are similar in predicting NPP and NEP. For these models, we only choose one of it. In addition, based on other people researches, we think the projection of future climate in China by the four ESMs also suggest the overall characteristics of future climate change in China area.]

2. ESM diversity
   I think you show only Multi-Model Ensemble (MME) result for averaging BNU-ESM, IPSL-CM5A-LR, MPI-ESM-LR, NorESM2-ME and CEVSA2. I am not sure each model shows same result of your main conclusion. All model show declined growth rate of NEP with the additional 0.5℃?

[Response: We analyzed the results of each model, and compared the main results for cross-model. The simulations of all models showed the almost consistent trends (as follow figure), so we averaged the results of all models.  Considering the main concerns of this paper and the length of manuscript, we did not detail on this part in our manuscript.]

Figure: the annual NPP of the models used in this manuscript under RCP4.5 and RCP8.5 scenarios during the period of 1970-2060. Red line and blue line represent the averaged NPP of all models under RCP8.5 and RCP4.5, respectively.

3. The physical reason for declined growth rate

I cannot find the physical reason for the declined growth rate of NEP with the additional 0.5℃ throughout the manuscript. As the authors mentioned in the Introduction part, mean air temperature over Asia will increase more than global warmings such as 1.5℃ and 2℃, but the authors did not show temperature differences in between RCP4.5 and 8.5 which can contribute to the declined growth rate of NEP. Also, I am not sure, but changes in precipitation and other physical factors can contribute to the declined growth rate of NEP (e.g. soil moisture depletion, Kim et al. 2017 Nature Communications). On the other hand, enhanced extreme heat events can bring compound drought event which reduces GPP (Wang et al. 2020). I think the authors should provide at least temperature changes in the used model in this manuscript. 

 [Response: We added two figures of temperature and precipitation difference at two global warming targets, and added a paragraph of main reasons about projected decrease in net carbon uptake, also provided the relative references (P21 L400). The response of net carbon uptake represents an integrated effect of changes in temperature and water status on plant growth and respiration. Although the pattern of warming is widespread in all areas, precipitation change varies in spatial. The response of ecosystem to rising temperature and change of precipitation varies because of different environmental background and vegetation type, as well as the response of plant growth and respiration have the different response to warming. So the response of net carbon uptake will show large spatial variations due to all these control factors. In Discussion, we also try our best to explain the decline of growth rate of China ecosystem NEP and the decrease of net carbon capacity in some areas. ]

Minor comments

L18 0.5℃warming -> 0.5℃ warming (Cross the manuscript, I found same  typos such as L31, L33, L99, L153, L160, L182, L183, L184, L193, L199, L202, L203, L223 and captions for Figs. 2,3,4,5,7 and many other lines...)

[Response: Thanks to the anonymous reviewer for pointing out these typos, we corrected them all.]

L56 net primary productivity (NPP) -> Net Primary Productivity (NPP) 

[Response: Revised as requested.]

L58 net ecosystem productivity (NEP) -> Net Ecosystem Productivity (NEP)

[Response: Revised as requested.]

L65 Is it annual mean temperature?

[Response: We added some words to make it clearer.]

L86 earth system models -> Earth System Models (ESMs)

[Response: Revised as requested.]

L990.5℃warming -> 0.5℃ warming

[Response: Revised as requested.]

Table 1 Land surface component -> Land surface model

 [Response: Revised as requested.]

Table 1 Each land surface model mentioning needs references.

 [Response: We add a column about the references of the EMSs.]

L191 and L192 Can you calculate and provide 95% significance interval (SI) for annual NPP mean value? Such as 518 (±???) C m^-2 yr^-1. Also, you need to use the minus symbol (−), not short dash (-).

 [Response: We recalculated annual NPP mean value with SI, and revised the minus symbol. Thanks for that.]

L196 I think "during" should be edited here.

 [Response: Revised as requested.]

Fig.1 Can you merge Fig1a and b? Y-axises are same, so I think you can merge together and it would be better than check differences between RCP4.5 and 8.5.

[Response: We redrew figure 1 and figure 5, hoping that they would show the difference of the two scenarios clearly.(p11, P17)]

Fig. 2 Spatial pattern is important but I cannot check what is difference between a,b,c and d. How about draw one more row and column that differences for NPP2 and NPP1 (also RCP8.5 and RCP4.5)?  Also, you already show differences in RI_NPP, but it seems just all greenish. Please use another color level for Fig 3 in order to distinguish differences. 

[Response: Because the pattern of NPP is basically similar, and the information about NPP also shows in the original figure3, we would like to delete the figure 2, and added the mean NPP for each period under different scenarios. We also added the figure about the change of temperature and precipitation at different global warming and discussion about the impacts of climate factors to NPP and NEP.(P21 L400)]

L250 ~ means approximately, not range. Please use – (En dash) https://www.btb.termiumplus.gc.ca/tpv2guides/guides/pep/index-eng.html?lang=eng&page=punct_4_hyphens_dashes

[Response: Revised as requested. Thanks for information.]

L356 Land Use and Land Cover (LULC)

[Response: Revised as requested.]

References

Kim, J.-S., J.-S. Kug & S.-J. Jeong (2017), Intensification of terrestrial carbon cycle related to El Niño–Southern Oscillation under greenhouse warming, Nature Communications, 8, 1674, doi:10.1038/s41467-017-01831-7

Kim, J.-S., J.-S. Kug, J.-H. Yoon & S.-J. Jeong (2016), Increased atmospheric CO₂ growth rate during El Niño driven by reduced terrestrial CO₂ capture in the CMIP5 ESMs, Journal of Climate, 29, 8783–8805, doi:10.1175/JCLI-D-14-00672.1

Wang, J. et al. (2020), Anthropogenically-driven increases in the risks of summertime compound hot extremes. Nature Communications, 11, 528, doi:10.1038/s41467-019-14233-8

 [Response: Thanks for information.]

 

Reviewer 3 Report

You investigated the results for the NPP and NEP thoroughly. Nevertheless, I miss the discussion of the causes of the differences (temperature and precipitation changes, ....) . As well, I don't understand why the differences in the slopes are as important as the fact, that NPP and NEP are increasing (in most areas). Please explain in the text.

Comments:

Abstract:

line 33: You mention negative effects. What are the negative effects?  Please specify.

Whole Text:

1. very often, there is no space after  °C and the next word- please insert (See e.g. line 199)
2. Results chapter: discuss causes of regional and temporal differences in  NPP and NEP changes: temperature, precipitation,...? What causes the differences  in NPP and NEP between the RCP8.5 and RCP4.5 simulations e.g.  for a temperature rise of 1.5°C?  Perhaps show additional figures  for temperature and precipitation changes. What is the role of the different vegetation distributions in the models?
3. Explain  the importance of  increase rates.
4. Warming: in  which time period?  2°C warming for China or global? Please specify, as you also write: "The Coupled Model Intercomparison Project, Phase 5 (CMIP5) model simulations showed that the mean air temperature over Asia will increase 2.3℃ and 3.0℃,  respectively, in response to 1.5℃ and 2℃ global warming (Shao et al., 2013)."    -->  which warming is meant with 2 ° warming?
5. You also mention: "In China, the mean air temperature has increased by 0.9-1.5°C during 1909-2011" – so warming starts 2011? Please specify in the text.

Line 92: simulatiing à simulating

 Line 96 ff: We combined the simulations of CEVSA2 model and the ESMs –results of EMS were used as boundary conditions for CEVSA2?

Line 121  link does not work- please change and specify, e.g.  which datasets you used –

Table 1: cite each dataset with ID/DOI

Table 1: explain PFT

Line 128 vision or version?

Line 140: years 2020 and 2049 calculated as initial years of 1.5 ° C and 2 ° warming with the combination of 12 GCMS – why didn’t you use the 4 GSMs of your investigation?

Line 146 ff: That is, 2020-2039 and 2040-2059 are represent 1.5℃ 146 and 2℃warming period for RCP4.5 scenarios. -->  Are representing

Line 151: Although the simulated absolute values of NPP and NEP by all models varied 
--> please specify, how much the values varied  (e.g. list all model results for the reference period in a table)

Line 152 So, we averaged  the results of all models in each respective period for 1.5℃ and 2℃ warming.

-->How were the results averaged? The NPP for each year, model and grid point? Pleas specify.

Line 166: To  eliminate the impact of assumed initial state, the model was run repeatedly for dynamic  simulation.

--> A transient simulation for 140 years with 14 days timestep? please specify

Line 186: Departure of annual NEP calculated the differences between mean NEP during different warming period and that during the reference period, to represent the impacts of different  warming on NEP.

--> better: Departure of  mean annual NEP

Line 199:  scale, the increase of NPP is greater in the 2℃than that in the1.5℃warming period  -->  missing space

Figure 1 caption and all other captions: specify, how NPP was averaged (all models, 20 years, all grid points)

Figure 2 and all other figures: only continental China was investigated- don`t show the islands.  What do the white areas mean in the figures mean? Zero NPP ? (should be red, see color bar)

Caption figure 2:   missing space

Discuss results of figure 2: why does the NPP not change in the northern areas? Only in south-eastern parts? Perhaps show additional temperature maps ?

Line 251: On national scale, the total NEP is projected to be increased by 53% at 1.5℃warming, and by 81% at 2℃ warming compared with that in reference period. --> please specify the scenario.

Line 258 ff: For RCP4.5 scenario, both linear trend and mean value of NEP 

--> please add mean values of NEP either by displaying it in figure 5 or with a table

Line 335: Although NEP shows an increasing trend at both two warming targets, the abnormity of NEP displays a substantial differ from that of NPP in spatial and temporal patterns

--> please rephrase

Line 372: three out of the five models --> please  specify which models: the 4 GCMs, that were investigated?

Author Response

Comments and Suggestions for Authors

You investigated the results for the NPP and NEP thoroughly. Nevertheless, I miss the discussion of the causes of the differences (temperature and precipitation changes, ....) . As well, I don't understand why the differences in the slopes are as important as the fact, that NPP and NEP are increasing (in most areas). Please explain in the text.

Dear reviewer and editor,

We appreciate the constructive advice of the editor and reviewers, which are greatly helpful for improving the manuscript. In the revised manuscript, we took care to incorporate all recommendations and conducted careful modification and improvement.

In the section of Data and Methodologies, we added an explanation about the differences in slopes used in this study. The slopes were used to represent the changing trends of NPP and NEP, which smoothed the annual fluctuation and captured the major characteristic of the indicators (NPP and NEP) in the study periods.

In the Discussion section, we added two figures for illustrating differences in changing temperature and precipitation under two warming scenarios (1.5^oC and 2^oC) (P21). We also added a paragraph to explain the reasons for the projected decrease in net carbon uptake, with references cited.

Response of net carbon uptake represents an integrated (combined) effect of changing temperature and water status on plant growth and respiration. Although the pattern of warming is widespread over most of the areas, the precipitation shows substantial variations in temporal and spatial patterns. Ecosystem responses to the rising temperature and changing patterns in precipitation greatly varies due to high spatial heterogeneities of the land surface (such as diverse environmental background and vegetation types), which results in different responses of plant growth and respiration to the warming and changes in water status. These processes largely explain why the responses of net carbon uptake have large spatial variations as influenced by multi-factors. In the Discussion section, we added more text to explain the decline of the growth rate of China ecosystem NEP and the decrease of net carbon capacity in some areas.

Comments:

Abstract:

line 33: You mention negative effects. What are the negative effects?  Please specify.

[Response: We added some negative effects due to the changing trend of NPP and NEP, such as the drop of NPP growth and the reduction of carbon sequestration capacity.]

Whole Text:

1. very often, there is no space after  °C and the next word- please insert (See e.g. line 199)

[Response: Thanks for pointing out these typos, we have corrected them all.]

2. Results chapter: discuss causes of regional and temporal differences in NPP and NEP changes: temperature, precipitation,...? What causes the differences in NPP and NEP between the RCP8.5 and RCP4.5 simulations e.g. for a temperature rise of 1.5°C?  Perhaps show additional figures for temperature and precipitation changes. What is the role of the different vegetation distributions in the models?

[Response: Thanks for the reviewer’s comments. In the revised manuscript, we added the explanation in the section of Discussions and analyzed the impacts of main climatic factors. In this paper, our main concerns were the responses and risks of terrestrial ecosystems to the comprehensive effects of climate conditions under different global warming targets. For separating the effects of each single climatic factor, more simulation experiments are needed and are not the scope of this study. We appreciate the suggestions of the reviewer and will focus this scientific question in our very near future studies(P21).

Therefore, in this study, we briefly show the changes in temperature and precipitation under 1.5℃ and 2℃ global warming scenarios and their connection with the NPP and NEP variations. Regarding the vegetation distribution,  the vegetation map we used to drive the CEVSA2 model represents the contemporary vegetation distribution in China, and we also checked the vegetation types adopted by our selected models in CMIP5, to make sure their vegetation maps approximately match with that of CEVSA2 model. We agree with the reviewer that dynamic vegetation distribution may have an important role in regulating future magnitudes and patterns in NPP and NEP; however, most of the CMIP5 models did not consider dynamic vegetation, making it impossible to conduct relevant investigations in this study at this moment. ]

 Explain the importance of increase rates.

[Response: We added an explanation about the differences in slopes used in this paper. The slopes were used to represent the changing trends of NPP and NEP, which can smooth the annual fluctuations and capture the major characteristic of the indicators (NPP and NEP) in the study periods. The increasing rates (the slopes) indicate how the terrestrial carbon cycling would respond in magnitudes and patterns to climate change and provide important information about tipping points of carbon sources to carbon sinks.]

1. Warming: in which time period?  2°C warming for China or global? Please specify, as you also write: "The Coupled Model Intercomparison Project, Phase 5 (CMIP5) model simulations showed that the mean air temperature over Asia will increase 2.3℃ and 3.0℃,  respectively, in response to 1.5℃ and 2℃ global warming (Shao et al., 2013)."    -->  which warming is meant with 2 ° warming?

[Response: We revised in the text as the reviewer suggested.]

2. You also mention: "In China, the mean air temperature has increased by 0.9-1.5°C during 1909-2011" – so warming starts 2011? Please specify in the text.

[Response: We revised in the text as the reviewer suggested.]

Line 92: simulatiing à simulating

 [Response: Thanks for pointing the typo. Revised.]

 Line 96 ff: We combined the simulations of CEVSA2 model and the ESMs –results of EMS were used as boundary conditions for CEVSA2?

 [Response: Thanks for the reviewer’s question. In the revised manuscript, we added a couple of sentences to explain how we used the results of ESMs and CEVSA2.]

Line 121  link does not work- please change and specify, e.g.  which datasets you used –Table 1: cite each dataset with ID/DOI

 [Response: The URL should be updated or removed, we have updated the URL in the revised manuscript.]

Table 1: explain PFT

 [Response: Revised as requested.]

Line 128 vision or version?

 [Response: Thanks for pointing the typo. Revised.]

Line 140: years 2020 and 2049 calculated as initial years of 1.5 ° C and 2 ° warming with the combination of 12 GCMS – why didn’t you use the 4 GSMs of your investigation?

 [Response: Time horizon that surpasses a warming threshold of 1.5 and 2 varied in different ESMs, and there are different methods to treat global warming intervals. Based on references(Chen et al.,2015), we note that the same time horizon of warming used to assess the impacts of global warming targets, we think it will be benefit for the comparison and integration of research results. So we cited a result by an ensemble of GCMS.]

Line 146 ff: That is, 2020-2039 and 2040-2059 are represent 1.5℃ 146 and 2℃warming period for RCP4.5 scenarios. -->  Are representing

[Response: Revised. Thanks for that]

Line 151: Although the simulated absolute values of NPP and NEP by all models varied --> please specify, how much the values varied  (e.g. list all model results for the reference period in a table)

[Response: We analyzed the results of each model, and compared the main results for cross-model. The simulations of all models showed similar changing trends (see the figure as followed), so we averaged the results of all models.  Considering the main concerns of this paper and the length of the manuscript, we did not include details of this part in our manuscript.]

Figure: the annual NPP of the CMIP5 models used in this manuscript under the RCP4.5 and RCP8.5 scenarios during the period of 1970-2060. Red line and blue line represent the averaged NPP of all models under RCP8.5 and RCP4.5, respectively.

Line 152 So, we averaged the results of all models in each respective period for 1.5℃ and 2℃ warming.

-->How were the results averaged? The NPP for each year, model and grid point? Pleas specify.

 [Response: We rephrased in the text. Generally, we interpolated the results of ESMs to the 0.1º resolution, then averaged all the grid datasets to an ensemble, which used to be analyzed.]

Line 166: To eliminate the impact of assumed initial state, the model was run repeatedly for dynamic simulation.--> A transient simulation for 140 years with 14 days timestep? please specify

 [Response: We rephrased in the text. We run the CEVSA2 model driving by the same transient dataset for three times, and every time the simulation was conducted for 140 years (1961-2099). The time step of dataset is at 10-day and its spatial resolution is 0.1°× 0.1° latitude and longitude.]

Line 186: Departure of annual NEP calculated the differences between mean NEP during different warming period and that during the reference period, to represent the impacts of different warming on NEP.--> better: Departure of  mean annual NEP

[Response: Revised as requested.]

Line 199:  scale, the increase of NPP is greater in the 2℃than that in the1.5℃warming period  -->  missing space

[Response: Revised as requested.]

Figure 1 caption and all other captions: specify, how NPP was averaged (all models, 20 years, all grid points)

[Response: we added the explanation in the text. NPP was averaged by the annual gridded data for all models.]

Figure 2 and all other figures: only continental China was investigated- don`t show the islands. What do the white areas mean in the figures mean? Zero NPP ? (should be red, see color bar)

[Response: Due to map censorship rules of the Ministry of Natural Resources, PRC, all maps published and circulated within mainland China must include these islands. In the revised manuscript, we revised the caption of Figure 1 to explain the white areas.]

Caption figure 2:   missing space

Discuss results of figure 2: why does the NPP not change in the northern areas? Only in south-eastern parts? Perhaps show additional temperature maps?

[Response: Because the pattern of NPP is basically similar to each other, and the information about NPP is also shown in the original figure3, we deleted the figure 2 and added the mean NPP for each period under different scenarios in the revised manuscript. We also added figures (Fig8 and Fig9)about the change of temperature and precipitation under different global warming scenarios and discussed the impacts of climate factors on NPP and NEP.]

Line 251: On national scale, the total NEP is projected to be increased by 53% at 1.5℃warming, and by 81% at 2℃ warming compared with that in reference period. --> please specify the scenario.

[Response: Revised as requested.]

Line 258 ff: For RCP4.5 scenario, both linear trend and mean value of NEP --> please add mean values of NEP either by displaying it in figure 5 or with a table

 [Response: We added the mean value of NEP at two warming targets under two scenarios in section 3.3.]

Line 335: Although NEP shows an increasing trend at both two warming targets, the abnormity of NEP displays a substantial differ from that of NPP in spatial and temporal patterns--> please rephrase

 [Response: Revised as requested.]

Line 372: three out of the five models --> please specify which models: the 4 GCMs, that were investigated?

 [Response: Revised as requested.]

 

 

 

Round 2

Reviewer 1 Report

The authors addressed the concerns raised. 

Author Response

Dear reviewer,

 

We appreciate your constructive advice of our manuscript during the process. We think that the comments improved the quality of the MS greatly.

 

Your sincerely,

 

Mei Huang on behalf of the authors

Reviewer 2 Report

The authors have addressed all of my comments, and now make clear where in the manuscript they have been responsive to the reviews.

Author Response

Dear reviewer,

 

We appreciate your constructive advice of our manuscript during the process. We think that the comments improved the quality of the MS greatly.

 

Thank you again.

 

Your sincerely,

Li Yu and Mei Huang on behalf of the authors

Reviewer 3 Report

Most of the sentences of the manuscript are  correct - as I am not a native speaker  I can't be sure. Nevertheless, there are some sentences which don't have an end (last sentence of the abstract) of where the grammer definitely isn't correct.  Therefore, the paper needs to be read and corrected by someone with a good knowledge of the english language.

Here are all the sentences, of which I think that they are not correct:

Abstract: end of sentence is missing (after i.e., in line 35.)

line 346: Although the pattern of warming was widespread in all areas, precipitation change varies in  spatial --> varies spatially

line 346: Temperature increasing is a dominant reasons for NPP changes at national scale or some regional scales --> The increase of the temperature is the dominant reason for ….

Line 348: The increase of temperature and precipitation is both shows greater in northern areas compared to  in southern areas, and also higher in high-altitude areas compared to low-altitude areas.

-->  The increase of temperature and precipitation is both higher in northern areas compared to southern areas, and also higher in high-altitude areas compared to low-altitude areas.

Line 351: Relative to southeastern China, northwestern China and Qinghai-Tibet Plateau have lower 351 mean annual temperature, and higher sensitive to the global warming -->

Relative to southeastern China, northwestern China and Qinghai-Tibet Plateau have lower mean annual temperatures, and higher sensitivity to the global warming

Line 353: Please rephrase the sentence: The increase of precipitation would alleviate the depress of water deficit on the plant growth in the areas which limited by water shortage

Line 356: Please rephrase the sentence: So, NPP was also projected to a greater  increment accordance reasonably with spatial pattern of temperature and precipitation  increase.

Line 359 Respiration shows higher sensitive -->  higher sensitivity

Line 364: Therefore, the effects of climate change on the terrestrial  ecosystem in different regions are inconsistent

-->you stated before that there are many reasons why the effects can differ- I wouldn’t use the word inconsistent.

Line 433: Our results show clearly that the interaction between global warming and elevated CO2 concentration.

--> Sorry, I don’t understand the sentence. Please rephrase. Perhaps you meant: Our results show clearly the interaction between global warming and elevated CO2 concentration.

Line 453: Furthermore, climate  extremes should be extraordinarily concerned in a warmer world.

-->Sorry, I don’t understand the sentence. Climate extremes can’t be concerned….. Please rephrase.

Line 456: In  China, climate extreme events are projected particularly sensitive to the additional 0.5℃ in the Northwestern, Northeastern, and Tibetan Plateau subregions. --> Please rephrase.

Line 463: adapttive --> adaptive

Line 461: Therefore, if the warming target was set to the 2℃, the adapttive capacities of terrestrial ecosystems to climate change in special regions are expected to enhance to mitigate the potential risks of warming on productivity and carbon sequestration. --> instead of “are expected to enhance”  “should enhance” ?  Please rephrase the sentence

 

Author Response

Dear reviewer,

 

We appreciate your constructive advice and of our manuscript during the process. We think that all comments and advice improved the quality of the MS greatly.

 

In this revised manuscript, we conducted careful modification and improvement. Please find below our replies to the recommendations and content added in manuscript.

Thank you again.

 

Your sincerely,

 

Li Yu and Mei Huang on behalf of the authors