the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jan 2024
Global Cropland Expansion Enhances Cropping Potential and Reduce its Inequality among Countries
Abstract. Global cropland expansion has been recognized as a key driver of food security. However, cropland expansion induced alterations in biophysical properties of the Earth's surface and greenhouse gas emissions may potentially impact the Earth's climate system. These changes could, in turn, affect cropland productivity and the potential distribution of croplands, although the underlying mechanisms remain relatively underexplored. In this study, a global climate model was employed to quantify the impact of global cropland expansion on cropping potential, utilizing observed and derived cropland expansion data. Our findings reveal that since 10000 BC, a 28 % increase in cropland expansion has led to a 1.2 % enhancement in global cropping potential, owing to more favorable precipitation and temperature conditions. This suggests that global cropland expansion yields dual benefits to crop production. However, in regions with low growth rates of cropping potential, cropland expansion proves to be an inefficient method for augmenting local crop potential yield. As croplands continue to expand worldwide, the capacity to support populations in different regions is altered, thereby reducing cropping potential inequality among nations.
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RC1: 'Comment on esd-2023-47', Anonymous Referee #1, 03 Feb 2024
Summary:
This study investigated the influence of cropland expansion from 10000 BC to present day on the cropping potential. Firstly, AMIP-type simulations based on CESM1.2.1 were conducted to assess the influence of cropland expansion on climate (i.e., temperature and precipitation), by prescribing different cropland conditions. The GAEZ model was then applied to examine the influence of temperature and precipitation on cropping potential. They found that cropland expansion from 10000 BC to 2015 has led to a global enhancement of cropping potential, and the dominant climatic factors differ between regions. In addition, the cropland pressure (combining cropping potential and total population) between rich and poor regions has decreased.
The logic is clear and the paper is generally clearly written. My main concern is about the significance of the impact, as only very limited locations haven shown significant changes in cropping potential in response to cropland expansion. So the conclusions might be overstated. In addition, as only one model is used, the model-dependency of the results needs to be acknowledged. Please see detailed comments below.
- In the AGCM simulations with different cropland conditions, how are the external forcings given (e.g., at the 1850 or 2015 level)? Will the use of different external forcing levels (e.g., at the 1850 or 2015 level) affects the climate response? This is related to the nonlinear responses of climate to land use / land cover change and external forcings such as greenhouse gases and aerosols. At least some discussions on this issue would be helpful.
- L130-145: The forms of Equations (1) and (2) do not seem correct.
- Fig. 3a: The inset blue bars (global mean cropping potential) show no change from 10000BC to 2015. Please check if there is an error. In addition, as the map shows, only very limited locations show significant change in cropping potential (indicated by solid black dots) from 10000BC to 2015. This is also seen from Fig. 4. So I wonder if the conclusion that “a 28% increase in cropland expansion has led to a 1.2% enhancement in global cropping potential” (L24) is overstated. The low significance needs to be clarified.
- L275-278 and Fig. 4: It is indicated that cropping potential growth rate is greater in high latitudes than in low latitudes. I wonder if this statement is robust. This can be confirmed by investigating the relationship between cropping potential growth rate and latitude.
- Fig. 4: The inset plot about the cropland pressure index between “rich” and “poor” regions needs further clarification (e.g., what the x-axis is).
- The analyses are based on only one model, which weakens the conclusion overall. This limitation needs to be acknowledged. In addition, while the climatological mean bias in temperature and precipitation has been corrected, a detailed model evaluation including the spatial distributions and PDFs of temperature and precipitation is needed, to provide basic information about the reliability of the results.
Citation: https://doi.org/10.5194/esd-2023-47-RC1 -
AC1: 'Reply on RC1', Xiaoxuan Liu, 25 Mar 2024
Thank you for your thorough review of our manuscript and for providing insightful comments. We have embraced all of your suggestions and have made the appropriate revisions to improve the quality of our work. The details of the reply could be found in the attachment.
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RC2: 'Comment on esd-2023-47', Anonymous Referee #2, 11 Feb 2024
In this study, the authors aim to understand the role of cropland-expansion on global cropping potential. This is done by utilizing an Earth system model to evaluate the impact of cropland expansion on climate, using land-use data for 10,000 BC, 1850, 1990, and 2015. The authors then use the simulated climatological variables as an input to a cropping potential model and assess changes in cropping potential.
While I believe this is an important and interesting study, I have some major concerns regarding the interpretation of results and addressing the research questions posed by the study.
Major Comments:
C1: The authors motivate the study by describing that the underlying mechanisms behind the effect of cropland expansion on cropping potential remain unexplored (line 23). However, there is very little discussion about the underlying mechanisms. The simulated results by the earth system model are directly used as a forcing to cropping potential model and the assessment about the change in cropping potential is done. I would suggest to add relevant discussion about following points.
C1.1: The tropics and sub-tropics seem to show a clear tendency where wet regions get warmer and dry regions get cooler as a result of cropland expansion. Some discussion should be added for why this could be the case?
C1.2: The warming in tropics (e.g India) can-not be attributed to changes in latent heat flux alone (line 200). Figure S16 clearly shows a decrease in solar radiative heating which primarily reflects decrease in cloud cover. The effect of deforestation on cloud-cover have already been emphasized (Duveiller et al 2021) and cloud-radiative effects have been shown to significantly affect surface temperatures (Ghausi et al., 2023).
C1.3: About warming of northern Eurasia: Changes in warm-air temperature advection mostly results in increased downwelling longwave radiation (Rld) as result of increase in atmospheric heat storage. Therefore, these are confounding effects and not really the independent of each other (See also Tian et al., 2022; Tian et al., 2023). This point should be made clear.
C1.4: Over Europe, the sensible heat flux has reduced substantially. Do authors have an explanation for that?
C1.5: Line 208: There is no discussion about the factors that cause the cooling trend over these regions.
C1.6: Other biophysical factors that can mediate changes in temperatures in response to changing vegetation type should be discussed (Kleidon et al., 1998; Lee et al., 2011; Chen et al., 2020).
C2: Line 223: The cropping potential is described using a cumulative value. Is it the cropland pressure index discussed in lines (170). Does it have a dimension? There should be some information provided to interpret its values.
C3: Figure 3 b,c: How are changes in cropping potential attributed to individual variables? To what extent are the 5 variables (describing temperatures) correlated?
Minor:
M1: There seems to be some typesetting error in equations 1 and 2.
M2: The legend size in figure 3b and 3c should be increased.
References:
Ghausi, S. A., Tian, Y., Zehe, E., & Kleidon, A. (2023). Radiative controls by clouds and thermodynamics shape surface temperatures and turbulent fluxes over land. Proceedings of the National Academy of Sciences, 120(29), e2220400120.
Duveiller, G., Filipponi, F., Ceglar, A. et al. Revealing the widespread potential of forests to increase low level cloud cover. Nat Commun 12, 4337. https://doi.org/10.1038/s41467-021-24551-5 (2021).
Tian, Y., Ghausi, S. A., Zhang, Y., Zhang, M., Xie, D., Cao, Y., ... & Kleidon, A. (2023). Radiation as the dominant cause of high-temperature extremes on the eastern Tibetan Plateau. Environmental Research Letters, 18(7), 074007.
Tian, Y., Zhong, D., Ghausi, S. A., Wang, G., & Kleidon, A. (2023). Understanding variations in downwelling longwave radiation using Brutsaert's equation. Earth System Dynamics, 14(6), 1363-1374.
Kleidon, A., & Heimann, M. (1998). Optimised rooting depth and its impacts on the simulated climate of an atmospheric general circulation model. Geophysical Research Letters, 25(3), 345-348.
Lee, X., Goulden, M., Hollinger, D. et al. Observed increase in local cooling effect of deforestation at higher latitudes. Nature 479, 384–387. https://doi.org/10.1038/nature10588 (2011).
Chen, C., Li, D., Li, Y., Piao, S., Wang, X., Huang, M., Gentine, P., Nemani, R.R. and Myneni, R.B., Biophysical impacts of Earth greening largely controlled by aerodynamic resistance. Science advances, 6(47), p.eabb1981 (2020).
Citation: https://doi.org/10.5194/esd-2023-47-RC2 - AC2: 'Reply on RC2', Xiaoxuan Liu, 25 Mar 2024
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