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

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 demonstrate that the Current Warm Period (CWP) is not unprecedented, at least for Alaska. The authors suggest a tie in to solar variability.

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.

Six millennia of summer temperature variation based on midge analysis of lake sediments from Alaska

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 (T_July) variations on the basis of a midge temperature transfer function. The T_July 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 ( not, vert, similar 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 T_July record, with a decreasing trend after this point. Episodic T_July peaks ( not, vert, similar 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 T_July 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)

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.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.