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Review of The Biophysical Effects of Carbon Fertilization of the Terrestrial Biosphere

The paper examines climate system responses to the biophysical effects of carbon fertilization using a subset of the C4MIP experiments from CMIP6. The paper advances understanding by employing a surface energy balance decomposition, which allows the author to quantify the individual contributions of biosphere-induced climate system changes on surface temperature responses. The paper is well written, and the analyses are appropriate. I have several suggestions that I hope the author will address in the next draft.

General Comments:

1. The experiments here quantify the carbon fertilization effect by subtracting a Preindustrial climate from the end of the 1pctCO2 climate. However, you can also quantify the carbon fertilization effect by subtracting the end of the 1pctCO2-rad climate from the end of the 1pctCO2 climate. Calculating the CO2 fertilization effect using this method provides insight into the influence of the carbon fertilization effect within the context of a warmer (higher atmospheric CO2 concentration) climate. I suggest the author examine whether the climate system responses shown in the current manuscript are consistent with the climate system responses using this alternative method. They do not need to replicate all of the analyses, but should at least focus on replicating the results in the Main Figures 1-3. This will help to understand how robust the changes are to the background climate state, and may lead to new insights that inform the previous analyses.
2. There has been a considerable amount of work dedicated to the analysis of climate responses in these C4MIP-type experiments. The author should expand upon their description of these works. Indeed, much of the results presented here are supported by previous investigations. Some possible papers to describe (among many others):
   • Swann, A. L. S., F. M. Hoffman, C. D. Koven, and J. T. Randerson, 2016: Plant responses to increasing CO2reduce estimates of climate impacts on drought severity. Proc. Natl. Acad. Sci. USA, 113, 10 019–10 024, https://doi.org/10.1073/pnas.1604581113.
   • Skinner, C. B., C. J. Poulsen, and J. S. Mankin, 2018: Amplification of heat extremes by plant CO2physiological forcing. Nat. Commun., 9, 1094, https://doi.org/10.1038/s41467-018-03472-w.
   • Lemordant, L., P. Gentine, A. S. Swann, B. I. Cook, and J. Scheff, 2018: Critical impact of vegetation physiology on the continental hydrologic cycle in response to increasing CO2. Proc. Natl. Acad. Sci. USA, 115, 4093–4098, https://doi.org/10.1073/pnas.1720712115.
   • Zarakas, C. M., A. L. S. Swann, M. M. Laguë, K. C. Armour, and J. T. Randerson, 2020: Plant Physiology Increases the Magnitude and Spread of the Transient Climate Response to CO2 in CMIP6 Earth System Models. J. Climate,  33, 8561–8578,  https://doi.org/10.1175/JCLI-D-20-0078.1.
3. The paper largely groups responses into the tropics vs extratropics. This is mostly appropriate given the distinct responses between those two latitude bands. However, there are some very interesting regional changes that the author should discuss. Namely, the zonally anomalous changes in the Southwest U.S. and parts of Western and Central Asia (e.g., increases in latent heating (from transpiration); reduced downwelling shortwave). These water-limited regions behave differently than energy-limited regions.
4. Line 320-321: The author mentions that in areas with reduced transpiration, evaporation increases to satisfy the evaporative demand of the atmosphere. This may be partially true, but the primary reason for the increase in evaporation is likely because of increased canopy interception (from greater leaf area) and subsequent evaporation from the canopy.
5. Lines 385-388: The large contribution of the surface albedo change to warming in semi-arid regions is likely because these are water-limited areas that see large percentage increases in LAI. These are the regions we expect to see the greatest parentage increases in photosynthesis/fertilization
   • Donohue, R. J., Roderick, M. L., McVicar, T. R. & Farquhar, G. D. Impact of CO2 fertilization on maximum foliage cover across the globe’s warm, arid environments. Geophys. Res. Lett.40, 3031–3035 (2013).
6. Lines 390-392: You mention that the surface albedo effect is largest in the high latitudes due to the presence of snow and ice. Are the vegetation changes occurring in the presence of snow and ice? Aren’t the vegetation responses to CO2 fertilization largely confined to the warmer season?
7. Can you speculate as to why GISS-E2-1-G simulates a decrease in LAI in response to CO2 fertilization? Is there something specific to the treatment of photosynthesis?

Technical Corrections:

1. Line 230-231: “Similar but less significant statements…”. Reword this. It is unclear what less significant means here.