the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Nov 2024
The Colors of Proxy Noise
Abstract. Uncertainty in paleoclimate time series is inherent to the complex biological and physical processes involved in forming and archiving them in the environment for centuries or longer. The timescale-dependency of this uncertainty is often referred to as "noise" of a particular color based on similarities between the power spectrum of a timeseries and the electromagnetic spectrum of light. For example, "white noise" equally affects all timescales, where "red noise" dominates only on long timescales, similar to longwave red light. In paleoclimate research, the frequency characteristics of proxy noise are often assumed based on first principles rather than estimated directly, which risks either inflating or underestimating error at particular frequencies. Here, we synthesize several studies that use a common method to estimate the spectrum of error in ice core, coral, and tree-ring data. We conceptualize how time-scale dependent noise in proxy time series is created through the archive formation and data processing. Our results suggest that the colors of proxy noise are archive- specific, with white noise dominating in depositional archives such as ice-cores and marine sediment cores, while red noise is likely more common in biological archives such as tree rings and corals. Our aim is to clarify these concepts and provide tools for assigning noise terms in proxy system models, data assimilations, and other experiments.
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RC1: 'Comment on cp-2024-73', Anonymous Referee #1, 08 Dec 2024
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Summary: The manuscript discusses the spectral characteristics of non-climatic variability in climate proxies, so-called 'proxy noise'. The manuscript formalizes much of the empirical knowledge about the effect of noise on climate reconstructions based on tree-rings, corals and ice-cores
Recommendation: The information presented in the manuscript is not completely new. It is scattered among the different proxy communities and usually impolitely assumed when reconstructing uncertainty bounds in climate reconstructions. Nevertheless, I found the manuscript interesting and worth publishing, as it presents more formal characterizations and definitions of noise and discusses the effects that, in general, proxy retrieval and post-processing methods have on the final proxy time series. I think this can be a nice contribution to the different proxy communities.
The manuscript does not touch upon one important source of 'noise', namely dating uncertainty, which can be substantial for some proxies and negligible for others (e.g. tree-rings). One appendix briefly indicates that it lies outside the scope of the study, but I think this should be mentioned in the main text and possibly also briefly discussed.
My recommendation is, therefore, that the manuscript needs a few minor revisions. The author may want to consider my suggestions below.
1) Dating uncertain is not considered in this manuscript, but many readers would precisely expect to read what the effects of this uncertainty could be. Could the authors include a brief discussion, perhaps without formal treatment, pointing to a follow-up study?
2) 'whereas the removal of variance constitutes error, but not ‘noise’ per se'
I struggled to understand this sentence. I could get the meaning in the end, but it needs a clearer phrasing.
3) ' represented with a positive slope value (β>0)'
I think it is clear here that the authors are referring to the function of power density as a function of frequency spectral_power(f). However, some readers may also think in terms of period instead of frequency. The function spectral_density(T) as a function of period is not just a variable substitution since there is an additional factor:
P(f)df = P'(T)dT
Since dT and df are not linearly related, there is an additional multiplicative factor that depends on T as well. Perhaps the authors may want to include a warning for those readers. This would also include Figure 2 and its caption.
4) 'Alternatively, estimation of noise spectra can be done with relying solely on proxies by'
by relying
5) 'on ice sheets as wind redistributes snow causing blue noise in noise in annual layer thickness records from ice-cores #
blue noise in noise sounds harder to understand than it should
Citation: https://doi.org/10.5194/cp-2024-73-RC1 -
RC2: 'Comment on cp-2024-73', Anonymous Referee #2, 22 Dec 2024
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Review of the manuscript "The Colors of Proxy Noise" by Mara McPartland and colleagues submitted for publication in Climate of the Past
General:
The authors present set out to conceptualize the colors of proxy noise for different proxy archives. For this they use results that are already published or soon will be, in conjunction with already published data sets. They conclude that their models can be used by the community to account for the range of specific biological and physical processes influencing the proxy system.
In general the manuscript is prepared in a very superficial and simplistic style. The abstract does not present any specific hypotheses or questions that are addressed in the body of the manuscript., i.e. why their study is important and which are the specific new results found ?
The main part also does not include any proper scientific setup with a clear description of methods and a concise description of results, justifying the added value of the manuscript. Therefore the content related to new results is in my opinion not enough to be published in a manuscript in CP, when large parts of the conceptually ideas are already published by other studies in the recent decade (c.f. Smerdon, 2012, Evans et al, 2013, Dee et al., 2017). What is also unclear to me is why the appendix is 30% of the overall length of the (very short) manuscript and does not form a regular method section??
I suggest to reject the manuscript in its present form and the authors should completely re-think their setup and present substantially new results in a revised version.
Specific:
Abstract/Title:
The Title is very unspecific. Authors should be more specific. Actually it is more promised than the article really holds. The abstract also does not contain substantially new results compared to studies cited mentioned or used in the manuscript.
Introduction:
The introduction is a summary of results achieved so far in the context of proxy forward and proxy system modelling. No clear question is formulated. As much as i can understand the study just contains a summary of results presented already elsewhere (c.f. „We show results from three studies that have applied this approach to ice core (Münch & Laepple 2018), tree ring (McPartland et al., 2024), and coral data (Dolman et al., in prep).“ on page 3.
In addition, also the information contained in Text boxes and Fig 1 and 2 is published elsewhere and does not present any new result.
The colors of proxy noise
The (supposedly) main chapter also contains a loose collection of (qualitative) information that is presented without any context. No methods section is presented, nor a concrete formulation of an hypothesis that should be addressed in the study. Again, the concept authors motivate is already published in detail in former studies.
Implications/Conclusions
The conclusions contain a vague summary without any concrete answer to a previously formulated question, without any context to studies and literature published so far in the field of research.
Citation: https://doi.org/10.5194/cp-2024-73-RC2
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