New paleo reconstruction shows warmer periods in Alaska over the past 3000 years

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 original 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 in 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 (especially when referring to global trends) with the midge data which only capture mid-summer conditions is a bit like comparing apples and oranges.

As a side-note: The fact that warming predominately occurs in winter in Alaska 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 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 than 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 including temperature, rather than lake or watershed-limited controls, such as changes to the pH or cation composition of the lake’s water.

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