About the Assessment of Cover Crop Albedo Potential Cooling Effect: Risk of the Darkening Feedback Loop Effects

Round 1

Reviewer 1 Report

The introduction of cover crops over Europe results in a cooling impact due to an increase of soil organic carbon stocks, a decrease in the use of fertilizers, and an increase in surface albedo of the croplands, which can facilitate effective climate mitigation strategies. Based on the remote sensing data, land cover database, meteorological data, national agricultural statistics and ground measurements, this study discusses the maximum greening potential in Europe and the associated indirect surface properties changes. This manuscript is well written. The followings need to be commented or addressed before it is publishable.

General comments

1)      As is shown in Fig.4, there are some areas experienced positive radiative forcing and some areas experienced negative radiative forcing due to CC introduction. The authors should give the possible reasons for those.

2)  The CC introduction affects the temperature not only through changing the surface albedo, but also through other processes, such as evaporation, carbon exchange between the crop and atmospheric. The authors should add a discussion on the difference between your results and others which consider these processes.

specific comments

1）  Add a detailed description of the simulation scenarios

2）  Reference 29 and 30 are duplicated.

Author Response

We are grateful to the reviewer 1 for his time and effort in providing valuable feedback on our manuscript. We greatly appreciate the reviewers' constructive feedback, which has helped to improve the quality and clarity of our study.

Here below are the answers to reviewer 1 for the article entitled “About the assessment of cover crop albedo potential cooling effect: risk of the darkening feedback loop effects” by Gaétan Pique et al

General comments

1)      As is shown in Fig.4, there are some areas experienced positive radiative forcing and some areas experienced negative radiative forcing due to CC introduction. The authors should give the possible reasons for those.

>> You are right, we should have better explained why some areas have negative radiative forcing while others have positive. This behavior is mainly caused by the soil albedo of the area under consideration. The introduction of a cover crop only induces a negative radiative forcing if the albedo of the vegetation is higher than that of the bare soil. In some areas, e.g. in Spain, with bright calcicols, introducing CC would result in a positive radiative forcing due to the initial high bare soil albedo. We have therefore amended the paragraph on Spanish soils to clarify this point (L.334-338).

2)  The CC introduction affects the temperature not only through changing the surface albedo, but also through other processes, such as evaporation, carbon exchange between the crop and atmospheric. The authors should add a discussion on the difference between your results and others which consider these processes.

>> Indeed, in our study we focused only on the radiative effect of CC induced by albedo changes, while this agronomic practice affects the climate in several ways. Now we discuss this at the end of the first paragraph of the discussion. See (L.422-439).

specific comments

1）  Add a detailed description of the simulation scenarios

>> We do not understand this comment, as there is already a section (2.3) in the paper describing the simulation scenarios with a Figure summarizing them. All the scenarios are based on those presented in Carrer et al. (2018), and the modifications made to simulate the additional ones are explained in the text and in Figure 3.

2）  Reference 29 and 30 are duplicated.

>> Corrected, thanks.

Reviewer 2 Report

This work presents an interesting methodology to increase surface albedo in cultivated areas in Europe, as an example of the so called Geoengineering. It also studies the effects of such methodology, such as the darkening of the soil, as a negative side effect. The method used to increase soil albedo could easily be implemented in large crop areas.

All in all, the work deserves to be published in Remote Sensing. However, a few  points need to be addressed before being accepted for publication.

1.- I dont understand how modis product is used. First, the authors use the modis albedo product in combination with data from ECOCLIMAP to calculate αveg and αsoil. Although the authors provide a reference, they should also provide a brief explanation in the manuscript. Secondly, I do not understand why they calculate the actual albedo as a linear combination of αveg and αsoil instead of using the albedo value provided by the modis product.  

2.- Regarding figure 2: the pixels of the image correspond to what sensor or database? What is the spatial resolution of the data?

3.- When calculating the RF in equation (5), should not if be calculated comparing the albedo of the soil and the albedo of the soil covered with CC? Δα in equation (5) compares the albedo of the vegetated + soil with that of the soil covered by the cover crop. Maybe I have not understood properly, but it seems that the Cover crop covers the whole area during the fallow period.

Author Response

We are grateful to the reviewer 2 for his time and effort in providing valuable feedback on our manuscript. We greatly appreciate the reviewers' constructive feedback, which has helped to improve the quality and clarity of our study.

Here below are the answers to reviewer 2 for the article entitled “About the assessment of cover crop albedo potential cooling effect: risk of the darkening feedback loop effects” by Gaétan Pique et al

1.- I dont understand how modis product is used. First, the authors use the modis albedo product in combination with data from ECOCLIMAP to calculate αveg and αsoil. Although the authors provide a reference, they should also provide a brief explanation in the manuscript. Secondly, I do not understand why they calculate the actual albedo as a linear combination of αveg and αsoil instead of using the albedo value provided by the modis product. 

>> As explained in Section 2.1.3, the alb_veg, alb_soil and veg products used in our study are coming from Carrer et al., 2014 and were obtained by combining MODIS products, the ECOCLIMAP classification and a Kalman-Filter method. But in our study, we did not use MODIS products directly. We don't think that we should go further in explaining how the albedo and vegetation indices used in our study were produced, as it risks losing the reader in non-essential information but if you think it's essential information, we'll add it.

Note that the MODIS albedo products can’t be used directly in our study since it corresponds to a total albedo of the surface while we need separate albedo for soil and vegetation, that is why we rebuilt for cropland an albedo as a combination of alb_veg, alb_soil for cropland weighted by veg for cropland. We modified L.140-142 to make it more clear.

2.- Regarding figure 2: the pixels of the image correspond to what sensor or database? What is the spatial resolution of the data?

>> The background of figure 2 corresponds to the fraction of summer and winter crops used in our approach. It comes from the fraction of C3 and C4 crops of the ECOCLIMAP classification and it is explained in section 2.1.2. We have added a recall in the text L.217.

3.- When calculating the RF in equation (5), should not if be calculated comparing the albedo of the soil and the albedo of the soil covered with CC? Δα in equation (5) compares the albedo of the vegetated + soil with that of the soil covered by the cover crop. Maybe I have not understood properly, but it seems that the Cover crop covers the whole area during the fallow period.

>> In fact, the total albedo of a pixel is calculated as a combination of alb_veg, alb_soil weighted by veg. But veg in not necessarily equal to 0 when a CC is introduced (e.g. residual vegetation can be present in the fields during the fallow, e.g. weeds or spontaneous regrowth). Since we want to assess the radiative impact of CC compared to a reference, we also need to estimate Δα relative to the reference, so we need to consider alb_veg and alb_soil. Your point would be correct if veg were equal to 1 (full soil coverage) or 0 as soon as vegetation is present or absent on the field, but this is not always the case.