The authors did a set of sensitivity tests using the CMIP6 LULCC forcing (i.e. LUH2) and a bookkeeping model (BLUE) and evaluated the relative importance of different aspects on the uncertainties of LULCC fluxes. It is a very comprehensive analysis with careful simulation design, precise description of different LULCC terms and results. Although the results are only from one model, I believe it documents the details in LULCC carbon flux simulations and answered some common concerns on the uncertainties like the impacts of LULCC, simulation starting time, shifting cultivation and wood harvest. It also gives an important implication that historical LULCC uncertainty is negligible for the LULCC fluxes in the future scenarios. I read the manuscript very carefully and didn’t find any major problem. I therefore recommend this work for publication with small modifications.

Some small concerns:

The title “…based on CMIP6 forcing” is too broad. The work used only the LULCC forcing (i.e. LUH2). May rephrase and make it more concentrating.

If the authors still want to emphasize “CMIP6” and take this study as a reference analysis to compare with ESMs. It will be helpful if they can provide a short summary of how ESMs considered such aspects in the LULCC estimates (i.e. LULCC, simulation starting time, shifting cultivation and wood harvest).

L313-314: “Uncertainty …” I read this sentence several times but still didn’t understand.

L409-410: Did BLUE take bioenergy crops as regular food crops? To me, bioenergy crops are quite different from food crops. What are the possible consequences of taking both as the same?

My last point is kind of to echo the merits of this study that I mentioned earlier. The manuscript is written rigorously with a lot of details and supported materials, but it may be too technical for readers who are not very familiar with the bookkeeping models and LULCC carbon flux estimation. I noticed the authors tried to balance it by adding a summary paragraph at the end of each section which is very thoughtful. There still might be some room to improve by e.g. moving some detailed description of figures to supporting information.

This manuscript describes a study looking at the contribution of several sensitivities underlying the net LULCC flux by assessing their relative importance using a bookkeeping model (BLUE) based on a LULCC dataset (i.e., LUH2). They compared the impacts of LULCC uncertainty, the starting time of the bookkeeping model simulation, net area transitions versus gross area transitions and neglecting wood harvest on estimates of the net LULCC flux. They also revealed how historical LULCC uncertainty affect the net LULCC fluxes in the future scenarios. This study is very interesting and the results could provide some insight into the sensitivities of net LULCC. Basically, in my opinion some minor issues should be addressed and improved before the manuscript can be published.

1. The article title “Net land-use change carbon flux estimates and sensitivities…based on CMIP6 forcing” is a little misleading. The CMIP6 forcing refers to the LUH2 dataset in the manuscript, so I suggest changing the title to make it more accurate.
2. I suggest that Table 2 and 3 can be combined into one table to make it easier to understand the setup of each sensitivity experiment.
3. Some expressions in the manuscript are difficult to understand, so I suggest the author read through the whole text and rephrase some sentences. For example, L184, “It should be noted that even for the nine main experiments differences between the area evolution in BLUE compared to the LUH2 dataset occur.”
4. L256-258, panel a-> Fig. 3a; panel b-> Fig. 3b
5. L313-314, “Uncertainty of wood harvest also explains why REG850 and LO850 produce the same amount of harvest emissions until 1700 (Fig. 2)”. I cannot get the information from the Fig. 2, please confirm if this is about harvest emissions.
6. In the introduction section, the author introduced example studies of Hurtt et al. (2011) and Gasser et al. (2020) in detail, which is good, but I suggest the author add some words about the shortcomings of previous research and the improvements of this work in comparison with the previous studies.