Urban Land Expansion and Spatiotemporal Dynamics of Urban Green Spaces in Africa

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Overall, the manuscript “Urban land expansion and spatiotemporal dynamics of urban green spaces in Africa” presents a timely and comprehensive analysis of urban expansion on the African continent. The study employs remote sensing datasets (Global Urban Boundary and GLC_FCS30) to quantify changes in urban extent, impervious surfaces, and urban green spaces over two decades. The work is highly relevant given the rapid urbanization occurring in Africa and its implications for environmental sustainability and urban planning. The study covers a broad spatial scale and analyzes over 8000 urban boundaries, thereby capturing the dynamics across diverse urban settings. This extensive coverage is commendable. The use of both urban expansion metrics (OEA, CAGR) and land cover change indicators (UGS and IS proportions) provides a robust framework for assessing urban dynamics. The temporal segmentation (2000–2010 and 2010–2020) highlights an important shift in urban development patterns, with implications for green infrastructure planning. The manuscript is well-organized, and the integration of regional and country-level analyses enhances the overall clarity and depth of the study.

Major Concerns:

1. While the paper provides equations and indicators used for analysis, some methodological details require further elaboration. For instance, more information on the accuracy and limitations of the underlying datasets would help readers assess the reliability of the results.
2. The study is heavily reliant on medium-resolution global datasets. It would be useful to discuss the potential implications of spatial resolution on the estimation of urban boundaries and land cover fractions, especially in areas with heterogeneous urban landscapes.
3. While the paper effectively documents spatial and temporal patterns, it would benefit from a deeper discussion on the socioeconomic drivers behind these changes. Integrating qualitative or secondary socioeconomic data might help explain some of the observed patterns.

Minor Concerns:

1. Some figures (e.g., Figures 3–5) could be enhanced with clearer legends and annotations to facilitate interpretation. In addition, ensuring consistency in units and labels would improve clarity.
2. The discussion would benefit from a more detailed comparison of the results with findings from similar studies in other regions, thereby situating the findings within the broader urban sustainability literature.
3. While the manuscript is generally well-written, a careful proofreading for minor errors and sentence structure would enhance readability. Such as km2 in Abstract, Error! Reference source not found in line 223.

In conclusion, the manuscript makes an important contribution by providing a continent-wide analysis of urban expansion and green space dynamics in Africa. Addressing the methodological and contextual issues noted above will strengthen the paper and broaden its appeal to both academic and policy-oriented audiences.

Author Response

Major Concerns:

1. While the paper provides equations and indicators used for analysis, some methodological details require further elaboration. For instance, more information on the accuracy and limitations of the underlying datasets would help readers assess the reliability of the results.

Response 1: The methodology part revised to improve clarity and we added the limitations and future research part at the end of the discussions.

1. The study is heavily reliant on medium-resolution global datasets. It would be useful to discuss the potential implications of spatial resolution on the estimation of urban boundaries and land cover fractions, especially in areas with heterogeneous urban landscapes.

Response 2: This was indicated as a potential limitation that could be improved in future works.

1. While the paper effectively documents spatial and temporal patterns, it would benefit from a deeper discussion on the socioeconomic drivers behind these changes. Integrating qualitative or secondary socioeconomic data might help explain some of the observed patterns.

Response 3: We added the part to evaluate the correlation between population growth and GDP growth with urban expansion. However more in-depth analysis of socioeconomic drivers is still needed.

Minor Concerns:

1. Some figures (e.g., Figures 3–5) could be enhanced with clearer legends and annotations to facilitate interpretation. In addition, ensuring consistency in units and labels would improve clarity.

Response 1:  All figures were revised as suggested

1. The discussion would benefit from a more detailed comparison of the results with findings from similar studies in other regions, thereby situating the findings within the broader urban sustainability literature.

Response 2: Some comparisons with existing literature were included in the discussions

1. While the manuscript is generally well-written, a careful proofreading for minor errors and sentence structure would enhance readability. Such as km2 in Abstract, Error! Reference source not found in line 223.

Response 3: These errors were rectified

Reviewer 2 Report

Comments and Suggestions for Authors

I carefully reviewed this article. The author explored the spatiotemporal dynamics of urban expansion and its urban green spaces (UGS) and impervious surfaces (IS) in Africa from 2000 to 2020. The article focuses on the scale and rate of urban expansion in Africa, as well as the changing trends of urban green spaces and impervious surfaces, and explores the impact of urbanization on urban environmental quality. The article has a complete logical structure, but the following issues need to be addressed before publication:

Major issues:

1. The research results describe the scale and rate of urban expansion, but lack a detailed analysis of the causes of urban expansion. For example, why did some regions accelerate urban expansion during 2010-2020, while others did not? It is recommended to increase in-depth analysis of the causes of urban expansion, including factors such as population growth, economic development, and policy intervention.
2. The description of the study area is detailed, but the discussion section lacks a specific analysis of the differences in urban expansion in different regions of Africa. The characteristics and differences of urban expansion in different regions of Africa (such as East Africa, West Africa, etc.) should be emphasized. In addition, the article mentions the climate diversity in Africa, but lacks a discussion on how these climate characteristics affect urban expansion and green space changes.

Specific issues:

1. The article mentions the importance of urban green space, but lacks a description of the current status of urban green space in Africa. It is recommended to add the current status and challenges of urban green space in Africa to enhance the pertinence of the background.
2. The background section details the environmental impact of urban expansion and the importance of green space, but it is obviously insufficient and should include aspects such as climate change, land use, and human activities. It is recommended to refer to the following literature to improve the comprehensiveness of the introduction. — Past and future impacts of land-use changes on ecosystem services in Austria；Multi-Scenario Simulation of Land Use Change and Ecosystem Service Value Based on the Markov–FLUS Model in Ezhou City, China；Vulnerability assessment of hydrological ecosystem services under future climate and land use change dynamics；Assessing the relative contributions, combined effects and multiscale uncertainty of future land use and climate change on water-related ecosystem services in Southwest China using a novel integrated modelling framework；Effects of urban land change on ecosystem service values ​​in the Bolgoda Wetland, Sri Lanka
3. The results section describes in detail the scale and rate of urban expansion in Africa, but lacks a detailed analysis of the causes of urban expansion. It is recommended to increase the discussion of the causes of urban expansion, such as population growth, economic development, etc. For example, the authors mentioned a significant increase in urban expansion during the period 2010-2020, but lack an analysis of the specific reasons for the acceleration of urban expansion during this period.
4. The results section describes in detail the changes in green space and impervious surfaces, but lacks a detailed analysis of the causes of these changes. There should be more discussion on the reasons for changes in green space and impervious surfaces, such as policy intervention, urban planning, etc.
5. The discussion section analyzes the reasons for the dynamic changes in green space and impervious surfaces, but lacks discussion on specific policy recommendations. What are the specific recommendations on how to promote the increase of urban green space through policy intervention?

Author Response

Major issues:

1. The research results describe the scale and rate of urban expansion, but lack a detailed analysis of the causes of urban expansion. For example, why did some regions accelerate urban expansion during 2010-2020, while others did not? It is recommended to increase in-depth analysis of the causes of urban expansion, including factors such as population growth, economic development, and policy intervention.

 

Response 1: We added the part to evaluate the correlation between population growth and GDP growth with urban expansion. However, more in-depth analysis of socioeconomic drivers is still needed.

2. 
   The description of the study area is detailed, but the discussion section lacks a specific analysis of the differences in urban expansion in different regions of Africa. The characteristics and differences of urban expansion in different regions of Africa (such as East Africa, West Africa, etc.) should be emphasized. In addition, the article mentions the climate diversity in Africa, but lacks a discussion on how these climate characteristics affect urban expansion and green space changes.

 

Response 2: The regional differences of urban expansion were analyzed with correlation to population growth

Specific issues:

1. The article mentions the importance of urban green space, but lacks a description of the current status of urban green space in Africa. It is recommended to add the current status and challenges of urban green space in Africa to enhance the pertinence of the background.

 

Response 1. The background of the study was revised to include information about urban expansion and UGS dynamics in Africa.

2. 
   The background section details the environmental impact of urban expansion and the importance of green space, but it is obviously insufficient and should include aspects such as climate change, land use, and human activities. It is recommended to refer to the following literature to improve the comprehensiveness of the introduction. — Past and future impacts of land-use changes on ecosystem services in Austria；Multi-Scenario Simulation of Land Use Change and Ecosystem Service Value Based on the Markov–FLUS Model in Ezhou City, China；Vulnerability assessment of hydrological ecosystem services under future climate and land use change dynamics；Assessing the relative contributions, combined effects and multiscale uncertainty of future land use and climate change on water-related ecosystem services in Southwest China using a novel integrated modelling framework；Effects of urban land change on ecosystem service values ​​in the Bolgoda Wetland, Sri Lanka

 

Response 2: we have referred to existing literature as suggested to strengthen the arguments presented in the background of our study.

3. 
   The results section describes in detail the scale and rate of urban expansion in Africa, but lacks a detailed analysis of the causes of urban expansion. It is recommended to increase the discussion of the causes of urban expansion, such as population growth, economic development, etc. For example, the authors mentioned a significant increase in urban expansion during the period 2010-2020, but lack an analysis of the specific reasons for the acceleration of urban expansion during this period.

 

Response 3: The discussion part was revised to include more about the reasons of the observed trends and patterns

4. 
   The results section describes in detail the changes in green space and impervious surfaces, but lacks a detailed analysis of the causes of these changes. There should be more discussion on the reasons for changes in green space and impervious surfaces, such as policy intervention, urban planning, etc.

 

Response 4:  Some reasons for the observed changes in green space and impervious surfaces were included in the discussion section.

5. 
   The discussion section analyzes the reasons for the dynamic changes in green space and impervious surfaces, but lacks discussion on specific policy recommendations. What are the specific recommendations on how to promote the increase of urban green space through policy intervention?

 

Response 5: General recommendations were mentioned in our conclusion.

Reviewer 3 Report

Comments and Suggestions for Authors

MS review:

Urban land expansion and spatiotemporal dynamics of urban green spaces in Africa

Jean Pierre Muhoza, Weiqi Zhou

 

The authors conducted a comprehensive analysis of urban expansion, green space, and impervious surface dynamics in Africa from 2000 to 2020 using global medium-resolution datasets. A dramatic increase in urban areas of 307% (or 207?) was observed, mostly due to the emergence of small cities. The authors found that the first decade of the study period was characterized by a decline in green space and a corresponding expansion of impervious surfaces, indicating a negative correlation between urbanization and environmental quality. However, a noticeable shift occurred in the 2010s, with a 20% increase in urban green space and a decline in impervious surfaces.

The article is interesting and relevant.

Easy to understand.

The numbers analyzed and the results obtained are clear and logical.

Perhaps some statistical studies would be useful.

Some calculations may need to be adjusted. For example, according to the data provided, the area of ​​cities increased by 307%, from 28,461.7 km2 in 2000 to 87,337.2 km2 in 2020. However, a simple mathematical verification of this thesis shows that the area increased by 3.07 times, which is 207%, not 307%: if the parameter increased by 2 times, then it is 100%, and by 3 times, then by 200%. Interestingly, when calculating the change in the UGS area from 12101.5 to 39353.6 km2, a similar error was not repeated.

Author Response

Comment 1: Some calculations may need to be adjusted. For example, according to the data provided, the area of ​​cities increased by 307%, from 28,461.7 km2 in 2000 to 87,337.2 km2 in 2020. However, a simple mathematical verification of this thesis shows that the area increased by 3.07 times, which is 207%, not 307%: if the parameter increased by 2 times, then it is 100%, and by 3 times, then by 200%. Interestingly, when calculating the change in the UGS area from 12101.5 to 39353.6 km2, a similar error was not repeated.

Response 1: You are right there was an error, it is 207% instead of 307%. However, for the calculation of the change in the UGS area, I double checked and found no error.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Good job.  Accept!