|The manuscript is in in better shape than before but it still needs some revisions. Abstract still needs improvements. It doesn’t deliver the primary message clearly and in fact contact few incorrect statements. The overall “loose” coupling of CLM and GCAM appears so “non-scientific”. I am not sure if it is the way it is described or if it actually is, probably the former.|
Abstract. Only on reading the manuscript the second time it was obvious to me what authors meant by “… which can incorporate land-use change as static inputs but do not simulate the policy or economic forces …”. This sentence still doesn’t make sense. How can land-use be “changing” and still “static”. What authors mean here is likely “prescribed”. That land-use change is prescribed makes sense. I still can’t appreciate “but do not simulate the policy or economic forces”. This needs to be better worded.
Abstract. “ … focusing on how CESM climate effects on the carbon cycle can be shared with GCAM”. No, this can’t be your focus since this is an offline study.
Abstract. “ … quantify how carbon flux changes driven by climate (e.g. CO2 fertilization) …”. CO2 fertilization is NOT a climate effect.
Abstract. “ … to manipulate GCAM’s assumptions of equilibrium ecosystem steady state carbon”. Please consider using another word for “manipulate”.
Line 55. “ … different policy choices vis-à-vis LUC and carbon may result in greatly different configurations of the future carbon cycle”. Still a vague sentence.
Lines 64-66. “Such models may incorporate LUC as static inputs, but do not simulate policy options or economic forces, a significant limitation given how strongly humans can perturb the earth system”. See comments above regarding “static” and “simulate policy options or economic forces”.
Line 99. “CLM simulates the cycling and land-atmosphere exchange of energy, water, chemical elements, and trace gases”. CLM simulates fluxes of CO2 and N2O, as far as I know. “land-atmosphere exchange of chemical elements” sounds confusing. Please be explicit here.
Line 122. Market-clearing prices is a term unknown to ESM community.
Line 126. What is a logit mathematical model?
Line 130. “GCAM’s carbon model”. Wouldn’t a more appropriate term be “GCAM’s terrestrial carbon cycle model”.
Line 137 and 140. “When a particular land-use contracts” and “When a land-use expands”. Wouldn’t it be appropriate to tell your reader what kind of land-uses GCAM deals with to provide some context.
Line 168-169. “have a suitable proxy by which to change GCAM’s steady-state carbon assumptions”. You are not changing GCAM’s steady state carbon assumptions (it still assumes that pools are steady state) but rather the magnitude of the pools which GCAM assumes are in steady state. Please also address this subtlety elsewhere in the manuscript.
Line 171. “ … if the carbon stock of a CLM forest changes from one time step …”. Please reword to “ … if the land carbon pool size of a grid cell with forested fraction simulated by CLM changes …”
Line 205. “In simulations S2-S4, we used the same looped climate, …”. Reword to “In transient 1850-2010 S2-S4 simulations, we used the 1901-1920 climate repeatedly …”
Line 206, 207. Replace “factors” by “forcings” and elsewhere too.
Line 237. “… even a perfect proxy variable will be subject to climate and LUC …”. You mean subject to “climate change and variability”. “subject to climate" doesn’t mean anything.
Line 258. “To distinguish LUC from climate signals …”. Doesn’t this actually mean, “To distinguish the effect of LUC, on primary CO2 fluxes and land carbon pools, from that of climate …”
Line 267. “The scalars were then …” What scalars are being referred to here?
Line 300-302. “ … because a few CLM grid cells, located in GCAM’s “Middle East” region, were subject to LUC at the end of CLM’s spinup phase”. This is confusing for me as an ESM person. Typically a spin up is performed when all forcings are fixed. For example, a typical ESM control run spinup is done with land cover fixed at 1850 conditions and concentration of all GHGs at their 1850 values. In the context of this paper the spinup S1 is done with 1901-1920 climate used repeatedly. Table 1 doesn’t say anything about land cover so I presume it would be specified and not changing but the above sentence implies that land cover is changing. If land cover is changing in a spin up simulation then how come it is a spin up.
Equations (1) and (2). Shouldn’t equation (1) be more appropriately written as
CA_i = CA_0 x (NPP_i/NPP_0)
In regards to equation (2), isn’t a more appropriate quantity to describe change in the belowground carbon pool (NPP – HR)_i/(NPP – HR)_0, as opposed to the average of the NPP_i/NPP_0 and HR_i/HR_0 ratios. After all the rate of change of soil carbon is equal to NPP minus HR, assuming NPP equals litterfall rate. In a transient simulation NPP increases faster than HR, so the difference between the two will reflect how soil C pool is changing.
Also, I presume, equations (1) and (2) are applied every 5 years.
Line 336. “The initial change in NPP was able to explain 20-92% of the variance …”. You mean, “The change in NPP over the 2005-2009 period was able …”
Lines 348-353. These sentences are redundant and provide no useful information.
Section 3.3. Wouldn’t a more appropriate title be “Distinguishing the effects from LUC from climate”
Line 360. “ … that as long as fewer than ~25% of the simulation cells were perturbed, the error (between the known climate signal and that inferred by the outlier test) ..” What exactly is this error? What are its units? What are the units of the “climate signal”? I am sorry but I am unable to understand what error is being talked about here.
Line 375. “Climate changes” doesn’t seem right. It’s either “Climatic changes” or “Climate change”.
Line 377,378. “In addition, it guarantees that if climate changes affect the carbon cycle, GCAM’s equilibrium assumptions will change correspondingly for the same vegetation type and spatial location, feeding back into economic decisions about industrial and LUC CO2 emissions (e.g., Le Page et al., 2013), emissions that propagate back to CLM (Di Vittorio et al., 2014)”. Are you sure, this is incorrect. I don’t think LUC CO2 emissions from GCAM propagate back to CLM. It’s the land use information, which is then translated to land cover information that is seen by CLM.
Lines 384, 385. “In addition, the outlier-exclusion step will break down under extreme LUC scenarios, scenarios that can be a useful tool”. I am unable to understand the last part of this sentence – “scenarios that can be a useful tool” – how does this relates to the first part of the sentence.
Line 400. “This work is only one step to a full coupling of an ESM and IAM; the second is described by Di Vittorio et al. (Di Vittorio et al., 2014).” Either don’t mention the second step or if you do want to mention it then please briefly explain what is it.
Figure 5. I am unable to understand Figure 5. Like I mentioned earlier, what exactly is error. What are its units? The caption says “error between the inferred climate change signal and the known (artificial) signal”. But what is “climate change signal”? Caption says x-axis is ‘LUC intensity” and the figure itself says “LUC effect”.