For those worried about tundra melt and methane outgassing, this study might dampen those worries a bit. A new peer-reviewed study by Clegg et al. demonstrates that modern global warming is significantly less than the global warming experienced in the higher latitudes, specifically Alaska, during the summers of the last 3,000 years. It demonstrates that the Current Warm Period (CWP) is not unprecedented, at least for Alaska. The authors suggest a tie in to solar variability.
From CO2 science:
What was done
The authors conducted a high-resolution analysis of midge assemblages found in the sediments of Moose Lake (61°22.45’N, 143°35.93’W) in the Wrangell-St. Elias National Park and Preserve of south-central Alaska (USA), based on data obtained from cores removed from the lake bottom in the summer of AD 2000 and a midge-to-temperature transfer function that yielded mean July temperatures (TJuly) for the past six thousand years.
What was learned
The results of the study are portrayed in the accompanying figure, where it can be seen, in the words of Clegg et al., that “a piecewise linear regression analysis identifies a significant change point at ca 4000 years before present (cal BP),” with “a decreasing trend after this point.” And from 2500 cal BP to the present, there is a clear multi-centennial oscillation about the declining trend line, with its peaks and valleys defining the temporal locations of the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age — during which the coldest temperatures of the entire interglacial or Holocene were reached — and, finally, the start of the Current Warm Period, which is still not expressed to any significant degree compared to the Medieval and Roman Warm Periods.
C3 Headlines provided an annotated and reversed graph which you can see below:
The paper title is published in Quaternary Science Reviews
Here’s the abstract:
Six millennia of summer temperature variation based on midge analysis of lake sediments from Alaska
Benjamin F. Clegg, Gina H. Clarke, Melissa L. Chipman, Michael Chou, Ian R. Walker, Willy Tinnere and Feng Sheng Hu
Abstract
Despite their importance for evaluating anthropogenic climatic change, quantitative temperature reconstructions of the Holocene remain scarce from northern high-latitude regions. We conducted high-resolution midge analysis on the sediments of the past 6000 years from a lake in south-central Alaska. Results were used to estimate mean July air temperature (TJuly) variations on the basis of a midge temperature transfer function. The TJuly estimates from the near-surface samples are broadly consistent with instrumental and treering-based temperature data. Together with previous studies, these results suggest that midge assemblages are more sensitive to small shifts in summer temperature (
0.5 °C) than indicated by the typical error range of midge temperature transfer functions (1.5 °C). A piecewise linear regression analysis identifies a significant change point at ca 4000 years before present (cal BP) in our TJuly record, with a decreasing trend after this point. Episodic TJuly peaks (14.5 °C) between 5500 and 4200 cal BP and the subsequent climatic cooling may have resulted from decreasing summer insolation associated with the precessional cycle. Centennial-scale climatic cooling of up to 1 °C occurred around 4000, 3300, 1800–1300, 600, and 250 cal BP. These cooling events were more pronounced and lasted longer during the last two millennia than between 2000 and 4000 cal BP. Some of these events have counterparts in climatic records from elsewhere in Alaska and other regions of the Northern Hemisphere, including several roughly synchronous with known grand minima in solar irradiance. Over the past 2000 years, our TJuly record displays patterns similar to those inferred from a wide variety of temperature proxy indicators at other sites in Alaska, including fluctuations coeval with the Little Ice Age, the Medieval Climate Anomaly, and the First Millennial Cooling (centered around 1400 cal BP). To our knowledge, this study offers the first high-resolution, quantitative record of summer temperature variation that spans longer than the past 2000 years from the high-latitude regions around the North Pacific.
Here’s an excerpt from the conclusion:
Within the limit of chronological uncertainties, some (but not all) of these cooling events at Moose Lake coincide with periods of reduced solar irradiance, such as the solar minima centered on the middle and late LIA (250 and 100 cal BP), 1400 cal BP, and 3400 cal BP (Steinhilber et al., 2009).
Although the co-occurrence of solar minima with cooling during the LIA is well appreciated, the role of solar output in modulating surface temperature remains controversial, partially because the effect of solar activity changes on the surface energy budget is orders of magnitude lower than those of the drivers operating over shorter timescales (e.g., clouds or volcanism) (Damon and Peristykh, 2005). Nonetheless, a number of recent paleoclimate
studies have attributed decadal- to millennial-scale variation to fluctuating solar irradiance in Alaska (Hu et al., 2003; Wiles et al., 2004; Tinner et al., 2008) and elsewhere (Hegerl et al., 2003; Damon and Peristykh, 2005; Eichler et al., 2009). Thus the potential role of solar irradiance in high-latitude climate change remains an issue that warrants further research (MacDonald, 2010). Analysis of midge assemblages in lake-sediment cores
from other sites is necessary to verify our results from Moose Lake and assess the potential linkages of summer temperature variation to fluctuating solar output.
The full paper is available at the Willie Soon’s website at Harvard here (PDF)
Addendum:
Some commenters point out that I did not include this caveat from the paper:
The Moose Lake TJuly record is of limited value for assessing anthropogenic warming in the context of the long-term natural variability because of the relatively coarse temporal resolution and potential impacts of human activity on the lake chemistry. The youngest sample of the record spans the period of AD 1968-1972, falling within the cooler interval of the 20th-century in Alaska (Chapin et al., 2005).
And they have a point, I should have included this. So I’m rectifying that now. They also say:
The inferred TJuly from this sample (13.76 +/- 1.43 °C) compares favorably with the mean of instrumental
July temperatures of the same period (13.77 +/- 1.13 °C,
corrected for a dry adiabatic lapse rate of 9.80 °C per km) as
recorded at a nearby weather station (Gulkana Airport). The relatively coarse resolution of the Moose Lake midge record, along with the brevity of weather-station records from our study region (w50 years), precludes a further assessment of our midge-based TJuly estimates through comparison with instrumental climate data.
However, the midge TJuly estimates of the past 350 years (Fig. 4A) can be compared with treering temperature estimates of the same period from tree line sites in the Wrangell Mountains (Davi et al., 2003; Fig. 4B). In general, the midge temperature inferences parallel the treering temperature patterns. For example, the two records exhibit similar magnitudes of climatic warming after the Little Ice Age (LIA) and both capture low temperatures corresponding to the middle and late phases of the LIA, which also coincide with local advances in valley glaciers within the Wrangell Mountains (Wiles et al., 2004). However, the specific peaks and troughs do not always match between the two records, which is expected given the chronological uncertainties associated with our 210Pb ages and with the age-depth model for samples older than 64 cal BP.
These results contribute to a growing body of evidence
demonstrating the utility of midge assemblages for reconstruction of relatively small TJuly variation on both historic and Holocene timescales (e.g., Heiri et al., 2003; Larocque and Hall, 2003). Together, these studies indicate greater sensitivity of midge assemblages to TJuly variation than implied by reported error envelopes of midge temperature transfer functions.
Clegg et al thinks that the TJuly agrees with a local instrument record, some tree-ring study, and suggest TJuly signal is greater than the error bands. However, this brings up an interesting point.
In a similar midge-paleo study covered on WUWT (Yarrow et al 2009, PNAS, full PDF here, CU-Boulder press release here) the authors of that study say in the press release that:
…changes seen in the sediment cores since about 1950 indicate expected climate cooling is being overridden by human activity like greenhouse gas emissions.
So we have one study, Clegg et al saying that this midge-paleo is too coarse to use for AGW signal determinations, and another similar study Yarrow et al saying midge-paleo (with others) does have enough resolution and it shows a modern impact of humans emitted GHG. Quite the contradiction.
In the Yarrow et al Baffin Island study, they do in fact look at more recent core data than the Clegg et al Alaska lake study. In reading the Clegg et al study, they say:
The youngest sample of the Moose Lake midge record (from 3.0 to 3.5 cm core depth; we did not have adequate amount of sediment from 0 to 3 cm for midge analysis) encompasses sediment deposition of AD 1968-1972.
Yet, in the Yarrow study they apparently did have enough sediment to make a determination and then claim that it shows unprecedented warming and human influence. Interestingly though, they cite a “statistical uncertainty of 2.2 °C”
As with any transfer function, chironomid-inferred temperatures contain some statistical uncertainty (14, 34). Although absolute temperature values have a statistical uncertainty of 2.2 °C, reconstructed trends in past temperature at this site are likely robust because the amplitude of these trends exceeds the statistical uncertainty of the model; furthermore, these trends are supported by many other proxies from the region (36).
So they also compared to other proxies. I find it odd though that Yarrow says this in the CU-Boulder press release here, emphasis mine:
But the cold-adapted midge species abruptly began declining in about 1950, matching their lowest abundances of the last 200,000 years. Two of the midge species adapted to the coldest temperatures have completely disappeared from the lake region, said Axford.
This seems to point to a sample problem for recent layers such as Clegg et al lament. I wonder what chironomid data Clegg et al had from 1972 forward and why they deemed it insufficient.
Apparently though, the lack of certain species wasn’t a problem for Yarrow et al, and they used that to bolster the claim that human caused warming was reflected by that species loss. I pointed to the fact that in Alaska and Canada, post World War II DDT use for mosquito control was the norm, so perhaps the lack of modern midges was a consequence of that DDT use in both cases. It is an uncertainty.
I’m reminded though of the Mann-Briffa Yamal tree ring debacle, where if that data didn’t fit near the present, you throw it out post 1960 and splice on the instrumental temperature record. Yarrow’s insistence that the cold species midge disappearance implies human caused warming is on par with the leap of “Mikes’ Nature trick”. Both ignore other potential influences.
While some commenters complain about the lack of Clegg et al data since 1972, the same posters IIRC did not complain about the proxy data truncation at 1960 and substitution of post 1960 instrumental data in Mann-Briffa’s studies.
While the lack of a useful sample post 1972 may simply be the lake biology, I think I’ll ask Clegg why they decided the post 1972 sample was insufficient and why Yarrow et al 2009 wasn’t referenced in the context of the modern midge data sample, and if they reply, and post a follow up note here.
I’ll close by pointing out Clegg et al’s closing sentence:
Analysis of midge assemblages in lake-sediment cores from other sites is necessary to verify our results from Moose Lake and assess the potential linkages of summer temperature variation to fluctuating solar output.
Replication is the basis of science, it is good to see them calling for that.
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@-Smokey says:
“The problem in Mann ’08 was not that he later corrected his errors. The problem was that he knew before he published that he was using a corrupted proxy. ”
A ‘corrupted proxy’ sounds particluarly evil…. reminds one of a naive young girl in the clutches of Casanova, or what happens to politicians when they gain power…
Corruption is usually a term applied to the ethical actions of people. When applied to inanimate objects or data it is metaphorical and should not carry the implication of moral decay.
But then the data can be more precisely described as an inaccurate proxy for temperature. The data is not corrupt, it is what it is. It just may not be a particularly good proxy for temperature. I am sure that in characterizing the data as (metaphorically) corrupt you did not intend to imply that Mann was ethically corrupt….
Izen,
You may be right. Ms Tiljander unwittingly used a bad proxy, it was not deliberate.
But in the case of Michael Mann, using Tiljander’s proxy after he had been informed that it was no good was deliberate, devious, corrupt, unethical scientific misconduct.
He only issued his correction after he had been caught. But he knew all along that his paper was fraudulent.
@- Smokey
I have no wish to get embroiled in the shallow controversy about the moral probity of personalities in the Mann/Tiljander issue. But the way in which data is used when it is sparse and uncertain is a problem in science – and not just in climate research.
Medical research is notorious for its use of highly uncertain data with small sample sizes and multiple confounding factors pressed into service when the available data is sparse. It increases the uncertainty of the end result, but often it is done just to see if (despite all the problems with the data) it refutes the hypothesis that the papers is advancing.
The paper on past climate derived from the midge deposits in the Moose lake sediments is an interesting paralel to the Tiljander data. Like those data there is significant uncertainty that they can be accurately correlated with the instrumental temperature record. As the authors note there are human impacts on the lake ecology that may reduce or remove the link between temperature and midge numbers that the authors hypothesize was operating before that human influence. At present the lake goes eutrophic in the summer with strong algal blooms.
It is clear from what the authors write in the paper that they have some confidence that the data they have collected indicate the direction and rough magnitude of temperature variation over past millenia at the lake. It is also clear that they have less confidence that the absolute value at any time in that past record can be directly correlated with present temperatures.
So the data may give a robust picture of past anomalies, but cannot be matched to the instrumental temperature to give a comparison between present and past absolute temperature values. The illustration of this is that the authors have to use instrumental data from Gulkana airport Alaska to overlap with the sediment data from Moose lake Alberta despite the fact it is ~200 miles further North, and a thousand miles West in a different ecology and climate area.
In this it shares the problem with the Tiljander data, its information of the pattern of past changes may be good, but it cannot be directly linked to current conditions.
So if any future paper uses this midge data to reconstruct past climate should that paper be rejected because the data cannot be effectively correlated with the instrumental record?
Or would it be sufficient to discuss the problems with ‘joining’ this data set to the instrumental record and show that if it is omitted from the reconstruction it makes no difference to the result?
Izen says:
“I have no wish to get embroiled in the shallow controversy about the moral probity of personalities in the Mann/Tiljander issue.”
It is not a “shallow controversy,” or even a controversy at all. And probity seems to be entirely missing. Mann’s shenanigans go straight to the heart of the corruption endemic to mainstream climate science. See this short article to understand what is happening.
And the Tiljander proxy is not comparable to the midge proxy, because Tiljander was falsified well before Mann’s paper was published.
Other than those two issues, I don’t see anything in your comments that I disageree with.
Regardless of the details of midge taxonomy, I have a question for the AGW faithful: How did polar bears make it through the Minoan Warming?
@ur momisugly – Smokey
Yes, I remember most ofthis malarkey from the reports and articles at the time.
First I would say that I regard the most accurate, and useful, aspect of the original MBH98 paper and the ‘iconic’ graph are the error bars. They make any other claims from the paper moot.
The Tiljander issue is just a replay…
At the risk of exacerbating any strong disagreement rather than reducing it, here’s my take on all this paleoclimate spin.
First, whatever Nature did with the climate in the past is only partially discoverable from the historical record. Strong claims from such evidence is subject to high uncertainty. Its prior distribution should be considered very wide in (rather rough!) Bayesian terms. What various scientists/polemicist THINK it did has absolutely no influence on the reality.
Second , There are two possibilities.-
1) The present warming is unprecedented in human evolutionary history (exclude the PETM and major meteorite impacts) and that exceptionality indicates its anthropogenic origin.
2) The present warming is comparable with past ‘natural’ variation and therefore the rate and magnitude of the warming cannot be used as evidence for its specific anthropogenic cause.
At present the knowledge of the paleoclimate is insufficient to distinguish between these two options unambiguously.
Third…. It doesn’t matter.
The claim that past natural variation is comparable to the present does not refute the hypothesis that the present warming is anthropogenic. It fails to support it, but there are much stronger reasons to attribute the warming to human changes to CO2 than an assumption the warming is exceptional compared to the paleoclimate.
If the present change in climate is similar in past changes then it provides no evidence that the present change is also natural, just that the climate is is capable of variations of that magnitude. Which begs the question – HOW.
And implies a greater degree of sensitivity to small changes, enhanced positive feedbacks, when natural processes do affect the flow of energy through the climate system.
I know it may seem this gives the AGW ‘side’ a win-win scenario on paleoclimate, but I can see no logical alternative given the two options.
If the present warming is exceptional it supports the existing case for its anthropogenic cause.
If the ‘natural’ climate variations can duplicate present observed changes then that has little impact on the other evidence for it present attribution to the CO2 rise, but it does imply a greater sensitivity of the climate system to small changes in the energy balance.
If the MWP really was globally synchronous and comparable with the present temperature then energy was required. Some physical process must have reduced the amount of energy lost from the tropopause to less than that received from the sun. No other mechanism is capable of raising global temperature while the solar output is within the known constraints imposed by Be10 and C14 production (among other evidence).
That requires a climate system that is NOT strongly homeostatic, that small variations within the decadal-century climate processes are amplified to make global temperature changes that have had significant impacts on previous human civilizations.
While many European societies seemed to flourish during the MWP, or Medieval Climate Anomaly as it is more cautiously described, the hotter drier climate on the American continent resulted in the fall of most of the city-state agrarian cultures that had arisen there.
But this strays away from the science of what NATURE is going to do, and into how robust our global society may be in the face of change, natural or anthropogenic.
And that is a whole ‘nother can of worms…..
Izen says:
“There are two possibilities.-
1) The present warming is unprecedented in human evolutionary history…”
Let’s take that one first. The present warming is well within the parameters of the climate during the Holocene. And if we look further back, the parameters are much wider. As you can see, declining temperatures are the real threat.
And as you can see in this chart, based on Phil Jones data, nothing is happening now that hasn’t happened before. In fact, the current *mild* warming cycle of 0.7°C over the past century is much less than many previous warming cycles. That’s why Trenberth is so anxious to get rid of the null hypothesis: it shows that the current cycle is indistinguishable from climate cycles prior to the industrial revolution.
Occam’s Razor says that extraneous variables should be avoided, and that the simplest explanation is generally the best. CO2 is an extraneous variable. Although there is probably some minor warming due to the rise in CO2, it is too insignificant to be measured.
And if the climate had a large sensitivity to that minor trace gas [such as the preposterous 3° – 6° or more opined by the IPCC and others], then temperatures would closely track the CO2 rise. They don’t.
Which brings us to your next point:
“2) The present warming is comparable with past ‘natural’ variation and therefore the rate and magnitude of the warming cannot be used as evidence for its specific anthropogenic cause.”
That is the null hypothesis: the statistical hypothesis that states that there are no differences between observed and expected data.
The null hypothesis has never been falsified. As Dr Roy Spencer puts it: “No one has falsified the hypothesis that the observed temperature changes are a consequence of natural variability.”
The null hypothesis is the hypothesis against which any alternative hypothesis must be tested. If there is no measurable difference, then the alternative hypothesis is falsified.
Next, you ask:
“If the present change in climate is similar in past changes then it provides no evidence that the present change is also natural, just that the climate is is capable of variations of that magnitude. Which begs the question – HOW.”
The ‘magnitude’ is actually quite small. A change of 0.7°C is routine. As Prof Richard Lindzen explains it:
Finally, the MWP has an enormous amount of research supporting its existence. Mann tried to claim that neither the MWP nor the LIA existed, by making the handle of his hockey stick straight. But after the debunking of MBH98/99 by McIntyre, McKittrick and Wegman et al, the IPCC was forced to stop using it in their publications.
But for the sake of argument, and supposing that there was no MWP, there were other warming cycles that exceeded it in magnitude. Which shows that the current climate parameters are nothing new. In fact, right now we are in a “Goldilocks” climate: not too warm, not too cold, but ju-u-u-u-st right.
Try to accept the fact that at current and projected levels, CO2 is a harmless and beneficial trace gas; no global harm due to the rise in CO2 is evident, and increased agricultural production due to the rise in CO2 levels has been repeatedly demonstrated. It’s all good.
OK, but it was fun….
Smokey says:
“The present warming is well within the parameters of the climate during the Holocene.”
Depends on how you define those parameters and how much confidence you have in the paleoclimate reconstructions.
The certainty with which you make that statement indicates you grant more reliability to paleocliamte research than I would. But then I see a lot of comfirmation bias in this area – from both sides.
“Occam’s Razor says that extraneous variables should be avoided, and that the simplest explanation is generally the best. CO2 is an extraneous variable.”
Actually the raised CO2 is the simplest explanation for the observed climate change. Anything else needs additional (undetected) extraneous variables in the ‘natural’ factors that would explain the warming.
I would question the other assertions you make about the supposed insignificance of the warming in terms of energy change and the beneficial effects of raised CO2, but that can wait for threads where such subjects are pertinant.
The fact we live in a ‘Goldilocks’ climate is because we have adapted, and designed our societies and agricultural practises to this climate. How easily we will adapt when ocean acidification or enhanced growth of C3 type ‘weeds’ affecting our food production and the climate changes is uncertain….
Izen,
Fortunately, scientific skeptics are mostly immune from the cognitive dissonance that afflicts true believers in CAGW, because a skeptic’s job is simply to debunk that alarmist conjecture. Skeptics don’t invent fantasies like CO2=CAGW. Skeptics merely ask questions and point out inconvenient facts.
You look at the charts I provided, and immediately go into Orwellian doublethink: black is white, down is up, evil is good, and the climate is controlled by a minor trace gas. Ri-i-i-i-ght.
You have everything wrong, and you can’t see it. The parameters of the Holocene are provided in the very first chart above, which shows that we are currently pretty close to the average temperature for the past ten thousand years; a Goldilocks climate. Yet you inconguously presume that the current benign climate is due to our “societies” and “agricultural practises (sic)”. Pray tell why during the past ten millennia, before the industrial revolution and modern farming methods, the planet’s temperature looks just the same as today? Natural climate variability fully explains that verifiable fact.
I gave you Prof Lindzen’s explanation for the natural warming/cooling cycles… and you let it fly right over your head. You are determined to blame a harmless, beneficial trace gas for your alarmism, and contradictory facts from an internationally esteemed expert don’t matter.
That is not science, that is True Belief; anti-science. Based on his experience and knowledge, Dr Lindzen has probably forgotten more than you will ever learn about atmospheric physics. [Unless, of course, you want to show us what passes for your CV, which I assume consists of reading realclimate a couple of hours a day.]
You wisely avoided the comparison of the repeatedly falsified CO2=CAGW conjecture when compared with the never falsified null hypothesis [unless you claim to know more than another esteemed climatologist, Dr Roy Spencer]. Note that CO2 rises follow temperature rises – not vice-versa. But for those afflicted with doublethink, that contradictory fact doesn’t carry any weight at all.
Nothing is different in today’s climate compared with Holocene averages or parameters. There may be a slight amount of warming from the rise in CO2, but I’ve shown that it is entirely beneficial. Warmth is good. Cold kills. And cognitive dissonance closes your mind to reality: CAGW is an entirely trumped-up scare. Demonizing “carbon” is pablum for the scientifically illiterate.
You say there is confirmation bias on both sides. Not on mine, there isn’t. I’m not frightened by the “carbon” scare, which is based on grant money, university status and international politics. Nothing out of the ordinary is happening, despite decades of failed predictions of doom. Astonishingly, after being proved wrong every step of the way, the alarmist crowd is still convinced that climate catastrophe is right around the corner. Cognitive dissonance.
You’ve drunk the alarmist Kool Aid for too long. Now you’re so hooked on it that perfectly natural cycles take on a sinister meaning. You see what you want to see because so much of your life is invested in the CAGW canard.
As each putative CAGW effect is debunked in turn, your belief system only gets stronger, like the followers of Mrs Keech’s flying saucer cult. The flying saucer didn’t arrive on schedule, so that could only mean it was delayed. It couldn’t possibly mean there aren’t any flying saucers. Being proved wrong only strengthens the belief in those afflicted with cognitive dissonance.
Sea level rise accelerating? Debunked. Global ice cover disappearing? Debunked. Coral bleaching? Debunked. They’ve all been debunked. Yet you continue to believe what Al Gore instructs you to believe. Very sad.
eadler says: January 29, 2011 at 7:56 pm
You are incorrect, science should not be done by subjectively assuming that the discrepancies with your work and others is automatically due to faults in your own work. An objective analysis would simply present the results without interpretation in relation to other work. I have no access to the peer-review process of this paper and thus hold nothing but skepticism based on previous examples involving paleo-climate reconstructions and the fact that this papers cites Mann whose work has nothing to do with midge analysis of lake sediments. Caution can be mentioned in an objective way but that was not done in this paper.
I would like to contribute some brief comments to this thread:
(1) With respect to the value of the Moose Lake record for assessing global warming:
As the name implies, global warming refers to an increase in the heat budget of the entire planet; the global heat budget is affected primarily by radiative controls, including (but not limited to) solar activity and orbital variations that alter how much energy Earth receives, and albedo and greenhouse gas concentration changes that alter rates of energy loss to space. Changes to the global heat budget can only be assessed by hemispheric or ideally global composites of temperature time series. The Mann et al. publication was a pioneering example of this; additional global composite records have since become available.
Local atmospheric conditions are by far the most important controls on temperature trends at an individual site. Recent mean annual temperature rises in Alaska for instance have been attributed to changes in an atmospheric mode known as the North Pacific Index (NPI), for example. Better-known examples on other regions of the globe are the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). However, changes to the globe’s heat budget can significantly alter the frequency or amplitude of these atmospheric circulation patterns.
Recent warming in Alaska, while occurring throughout all months, is dominated by warming in the winter months. Winter is also the season with the strongest atmospheric patterns in each hemisphere due to the intense heat gradient between the tropics and the polar winter. Summer temperatures in Alaska are predominantly governed by cloud radiative forcing today. Comparing modern warming estimates of mean annual temperature with the midge data which only capture mid-summer conditions is a bit like comparing apples and oranges.
The fact that warming predominately occurs in winter is not trivial at these high latitudes, as winter conditions play a large role in influencing permafrost temperatures in summer, and hence can play a role in the development of a thermokarst landscape with potential positive greenhouse gas feedbacks.
(2) A comment on paleotemperature inferences:
Measuring prehistoric temperatures is crucial to gain a context for recent changes, and also to identify natural variations and perhaps even drivers of natural climate variations that play a role on longer timescales. However, it is a bit like measuring with a noisy thermometer. In this case, midge species differ in their temperature preferences, and multivariate analyses show that summer water temperature is one of the dominant factors that explains the geographic distribution of different species of midge larvae. These relationships can be used to get an estimate of the average summer temperature at the time a sediment interval was deposited by identifying its larval midge remains. As might be expected, the precision of that estimate is far lower than reading a temperature value from a thermometer. Additionally, midges are biological organisms that respond to other environmental controls. Water depth, pH, cation concentrations, and nutrient status of lakes are a few examples. A good comparison with instrumental data is an encouraging sign that midges at a particular site are reliable indicators of temperature; other multivariate techniques can also be used to try to identify periods during which the midge assemblages may have been affected by environmental factors other that summer temperature. However, it is important to keep this caveat in mind – it is one of the principle reasons why replication of such studies is crucial. The second paragraph of the paper’s section 4.4 deals with such a possible caveat. The shift to on average higher temperatures between 4000 and 5000 years ago could have been influenced by low lake levels that were indicated by other lines of evidence. Other factors may have contributed during other periods but left no sign in the sediment. The more records that become available in the same region that point to shared temporal trends, the higher the confidence that the patterns represent regional controls (e.g. including temperature), rather more lake or watershed-limited controls, such as changes to the pH or cation composition of the lake’s water.