I below post, for the public record, my anonymous review of a new paper published in Journal of Geophysical Research.
I rank the paper: “Good” because the paper’s methods seem solid. Yet, depth with regard to examining causal factors is missing. Further, the paper’s main point, as it seems, that recent warming is not without precedent, may already be obsolete because 2010 was such an extreme melt year AND that more warming in Greenland is likely simply for Greenland to be in sync with the northern hemisphere. The paper thus, in the very least, requires a revision that includes consideration of 2010 data. Yet, consideration of causal factors of cooling and warming and treatment of the Box et al. (2009) prediction, which for 2008-2010 has been accurate, would give the paper the depth consistent with JGR’s standard.
As is, the only depth of the paper is the statistical modeling, that is, the regressions to reconstruct melt area and comparison of the recent warming versus past warm episodes. There is theory to explain warming and cooling episodes in Greenland. Yet, the paper does include this important dimension. Therefore, to increase the depth or impact of the work, the paper should elaborate causal factors that explain the ups and downs in the reconstruction.
The paper may already be obsolete without considering the extreme melting in 2010. I would therefore not recommend accepting the paper without a revision that included 2010. the numerous statements throughout the paper, like that in line: 19 “We find that the recent period of high melt extent is similar in magnitude but, thus far, shorter in duration, than a period of high melt lasting from the early 1920s through the early 1960s.”
One thing different about the recent warming versus the 1920s warming is that Greenland climate continues to lag the northern hemisphere pattern… The work should therefore reflect on the prediction made in Box et al. (2009) that: simply to be in sync with the northern hemisphere pattern, Greenland climate must warm (after year 2007) by 1.0 – 1.5 C. In the years after 2007, that is, 2008-2010, this prediction has held true. And that still more warming should happen in Greenland in the coming few years is more likely than not. A major volcanic eruption, of course, see relevant literature, would cool Greenland’s climate for 1-3 years.
The pre-1840 results should be abandoned because is cannot or at least it has not been demonstrated that there sufficient sampling to compare with the subsequent complete series.
Title: A less ambiguous time frame should be included in the title than: “A Reconstruction of Annual Greenland Ice Melt Extent Going Back to 1784” is needed…Something like: “A Reconstruction of Annual Greenland Ice Melt Extent 1784-2009”. Why? If the paper is published, some years down the line, the title would become ambiguous.
line 12 “three decades” instead of “several decades”
line 52: the following statement seems not accurate: “Such a comprehensive, annually resolved reconstruction has not previously been undertaken, and will better place current observations of melt extent in a longer-term historical perspective.” Box et al. (2009) modelle an annually resolved temperature reconstruction for the Greenland ice sheet.
line 103 define “closely match” quantitatively.
line 124 define “quite similar” quantitatively.
line 140: Does this relationship account for sub-monthly melt frequency? “Our Greenland melt reconstruction therefore focuses on the relationship between monthly average temperatures” I suspect a reduced sensitivity to melt intensity for 2 reasons: 1.) summer variability is minimal; 2.) a summer average of e.g. 0 C still includes periods above melting.
line 162: explain “the direct measure of JJA temperature subsumes the summer NAO influence.”
line 166 “winter conditions act to pre-condition summer ice melt through a snow/albedo response” certainly because of thermal erosion of heat content. “snow/albedo response” is vague and does not mention important heat content issue.
line 195: suggest “strong warming trend” instead of “strong positive trend”
line 195: “~1979-2009” instead of “The last ~30 years”
line 211: By the same token as the arguments that the recent warming is not statistically unprecedented, the following statement need be substantiated using probabilities: “several sustained periods can be identified when a greater and/or more prolonged”
line 221-223: a good point: “It is worth noting that the satellite observations of Greenland‘s total ice melt, which begin in the late 1970s, start during a time that is characterized by the lowest sustained extent of melt during the past century (Figure 2).”
line 248: remove “much”, overstatement