The paper by Palmer et al. compiles 101 papers (although the abstract says 100 plus 50?) from the western US to summarize millennial-scale (EH, MH, and LH), spatial patterns of climatic, ecologic, pyrogenic, and oceanic changes. Overall, I found the paper interesting and applaud its titanic effort. Summarizing data is not easy. The authors do a good job covering the literature with very few exceptions (see below and attached PDF). The methods are sound, the criteria for inclusion reasonable, and the final spatial and temporal interpretations within the bounds of the available science. Note: as a summary paper, I trust that the interpretations by the authors follow those by the primary authors, so I did not double the cited literature.

I provide suggestions/edits/comments below in order of the text, not importance.

1) line 48: under-sampled because of a lack of temperature-sensitive terrestrial records...I think this qualifier should be mentioned. Most WNA records are more water sensitive.

2) line 57: summer insolation was at a Holocene maximum...winter insolation was at a Holocene minimum.

3) add Kaufmann et al., 2020, and Routson et al., 2019.

4) up-date with Swain et al., 2018; Goss et al., 2020 - specific to W US and CA.

5) line 85: need to mention Wise's dipole work since it is the dominant feature of interannual hydroclimate in the W US...also, Dettinger and Cayan...also worth adding something about atmospheric rivers in this section since they are THE source of major hydrologic change - feast or famine W US climate.

6) line 125 - not sure if this Kirby paper actually deals with plant or animal communities? Maybe Kirby et al. (2018) show a strong coupling between hydroclimate and vegetation over 32,000 years at Lake Elsinore? Or, Dingemans et al. (2014)?

7) line 141: because of diverse age control issues between and within the 100 comparison sites, you might qualify this first question to reflect millennial-scale patterns since less than millennial is unlikely without significant age control assumptions across sites?

8) Figure 1: I would prefer a labeling scheme for each site so that the reader can go back and forth from the table to the figure to find the sites...e.g., 1, 2, 3, etc.

9) Figure 1: draw the boundaries as defined by your spatial criteria...PNW, SW, etc...

10) line 163: I think there should be an age control criterion...what is the minimum number of Holocene ages required to make millennial-scale statements??

11) line 163: are you updating the age models? In many of these types of summary papers, the age models are outdated and likely obsolete. Most review papers begin with fresh age models to make sure that papers published 20 years ago are correctly compared to papers published yesterday.

12) line 210: Add Leidelmeijer et al. (2021) - Barley Lake early Holocene.

13) line 230-234: in the SW (west of AZ), a wetter early Holocene was a product of more intense winter ppt associated with low winter insolation...the monsoon plays little if any significant role in the annual hydrologic budget west of AZ...see Kirby et al. (2005, 2007, 2012) and (Bird et al., 2010). It is reasonable that the monsoon boost may have "helped" maintain lakes where playas exist today, but without the increase in early Holocene winter ppt caused by lower winter insolation and its likely impact of storm tracks, the SW (west of AZ) would have been dry.

14) Figure 2: add numbers so that the reader can cross-reference sites to the table.

15) line 240: Leidelmeijer et al., 2021 agrees with a dry early Holocene from Nor Cal.

16) line 266: see earlier comments...the monsoon provided a hydrologic buffer, but it cannot explain the general increase in moisture...winter ppt must be the answer because the climate of the SW (west of AZ) is unimodal and dominated by winter ppt. All the monsoonal rain in the “world [hyperbole]” could not make the SW (west of AZ) wet without ample winter ppt...enhanced by lower winter insolation and its likely modulation of winter season storm tracks over the SW during the early Holocene.

17) line 306: add Barber, Donald C., A. Dyke, Claude Hillaire-Marcel, Anne E. Jennings, John T. Andrews, Maclean W. Kerwin, Guy Bilodeau et al. "Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes." Nature 400, no. 6742 (1999): 344-348.

18) line 415 and elsewhere: I think you are overplaying the significance of the monsoon on the CA annual hydrologic budget. CA is characterized by a unimodal hydroclimate in terms of what matters for its annual hydrologic budget...and what matters is simply winter ppt amount and to a lesser extent, summer evaporation.

19) line 417: most of CA receives no significant monsoonal ppt ever...except Mojave and east (see Hereford, Richard, Robert H. Webb, and Claire I. Longpre. Precipitation history of the Mojave Desert region, 1893-2001. No. 117-03. 2004.).

20) Section 3.2.5 and other human sections: check out...Grenda, Donn R., and Alex V. Benitez. Continuity and change: 8,500 years of lacustrine adaptation on the shores of Lake Elsinore. Statistical Research, 1997.

21) line 628: the Late Holocene Dry Period is reserved for the published LHDP by Mensing et al. 2013...I think you misinterpreted Lund and Platzman's data and LHDP age range...at Zaca Lake, ALL 3 papers show an LHDP period between 2500 and 2000 cal yr BP. The MCA is also present as well as the LIA...but nothing comes close in duration or magnitude as the LHDP.

22) section 3.5: add Crawford, Jeffrey N., Scott A. Mensing, Frank K. Lake, and Susan RH Zimmerman. "Late Holocene fire and vegetation reconstruction from the western Klamath Mountains, California, USA: A multi-disciplinary approach for examining potential human land-use impacts." The Holocene 25, no. 8 (2015): 1341-1357.

23) line 838: really should consistently point out that the early Holocene was characterized by both higher summer insolation and lower winter insolation...BOTH played a role in the millennial-scale Holocene changes you discuss in this paper.

24) line 862: AND, age control issues!!!, proxy sensitivity issues, and differences in the proxies used from site to site.

Before I get to my review, I need to offer my sincerest apologies to the authors for taking so long to review this paper. A series of unfortunate errors on my part led to this paper falling through the cracks for a timely review. The underlying science of the manuscript didn’t warrant such a slow response, and I owe the authors a significant apology for this error.

The manuscript prepared by Palmer et al. is a literature review that incorporates data from 50-100 published studies of marine & terrestrial paleoclimate records from across the US West Coast and western US. The paper is arranged according to early, middle, and late Holocene time intervals, with subdivisions for each time period devoted to regional synthesis, terrestrial climate (including fire reconstructions), marine conditions (mostly SST & upwelling intensity), paleoecology (largely pollen-based as well as some limited consideration of marine diatom & foram fauna), human-environment interactions (e.g., archaeology), and/or specific climate events (e.g., Little Ice Age, 8.2 ka event, European colonization, etc.). Of particular significance is the inclusion of a series of maps that correspond to these different Holocene time intervals and climate interpretations of the underlying reviewed studies.

While the subject matter is of great interest to the field of paleoclimatology generally, and the US West Coast specifically, I take exception to this work on the basis of 4 reasons:

• Many of the subsections listed above are superficial treatments of the subject matter, particularly the archaeology subsections. In several cases, these subsections are based entirely on only 1-2 studies. Some of the human-environment interaction sections are so short, I wondered why the authors even considered writing them (e.g., Sect. 3.1.6, 3.2.5). Why the focus on the Channel Islands? There are thousands of archaeological sites in the western US, and if you really used only the search term “archaeology” as the basis for inclusion in your review, then there should be a LOT more information contained in your review! My suggestion is to either drop the archaeology sections since they are pretty tangential to the main climate thrust of the manuscript, or else improve the thoroughness of the archaeological review sections.

• There is no consideration by the authors of the importance of age control regarding any of the records considered in this synthesis. I know this isn’t the most fun subject to deal with, but you can’t just ignore it. For example, several marine sediment records mentioned (e.g., Gardner et al., 1988; Barron and Bukry, 2007; Barron et al., 2017; McGann, 2015) either contain only 1 or 2 dates to pin down the entire Holocene, or are based on benthic forams that have huge reservoir corrections. I’d argue this fact may be a key reason to describe the lack of synchronicity in adjacent climate records that is mentioned in Line 856, or at least as important as the impacts of local vs regional “factors”.

• In the Methods section, you also highlight that you will “prioritize records with high temporal resolution, continuous records…” [Line 204] = you should state objectively what this high-resolution data threshold is. Also, archaeological midden piles are not continuous records, which again brings into question why the authors opted to discuss these papers in the context of this review.

• This manuscript requires significant re-structuring. The research questions (hypotheses) are not introduced until Line 141, which is far too late in the introduction section. I read Sections 1.1 & 1.2 and got confused as to where this paper was going, as it rambled and lost focus until the hypotheses were introduced. Both of these sections can be culled by 50%. There is also no Discussion section that explicitly addresses the research questions using the results of the review, particularly Questions #2 and #3 (e.g., Lines 142-143).

Minor issues

• There are many grammatical & style issues to address throughout the manuscript, particularly in the introductory paragraphs. Because I recommended you cull 50% of this section, I’m not going to go through that section in detail. However, the authors do need to pay attention to these issues in the rest of the manuscript. For instance, small typos such as in Line 198 […(Figs.s 2,3)…] or Line 335 [add a comma after “of northern California”] require very detailed attention to catch, which the authors clearly need to do.
• The very first sentence of the manuscript’s abstract begins with a prepositional phrase, which is considered bad form in scientific writing, so please re-write. The occasional prepositional phrase is okay, but generally you should avoid using them.
• The authors are inconsistent in their use of capitalization of directions to describe the western United States and the Northeast Pacific Ocean. There are specific rules for how to apply directional adjectives, check out https://editorsmanual.com/articles/capitalizing-directions/ for examples.
• For the initial identification of potential studies using key words, which database(s) were used? Many bibliographical databases have known shortcomings (such as exclusion of key research papers that are older than a decade or two), so it is worth reporting this detail and defending its selection.
• Line 340: What is total carbon? Do you mean total ORGANIC carbon, or carbonate-bound carbon? “Total carbon” is kind of a meaningless proxy, if that is indeed what you are reporting, so please clarify.
• Fig 4b = Y-axis label is wrong, as there are 2 different proxies plotted (opal + sedimentary d¹⁵N). Also, what is the color coding supposed to mean on all of these similar figures (e.g., Figs 4, 5)?
• Appendix A = you have duplicated Columns 1 & 2, please clean it up.

In conclusion, I urge the authors to address these issues and re-submit the manuscript. I don't think any of these complaints are deadly to the manuscript, but some of them will require some time and careful effort to address.  I hope the authors choose to pursue these modifications,  as a Holocene-focused paleoclimatological synthesis of the US West Coast is of great interest to many scientists.