Nonlinear response of summer temperature to Holocene insolation forcing in Alaska

Abstract

Regional climate responses to large-scale forcings, such as precessional changes in solar irradiation and increases in anthropogenic greenhouse gases, may be nonlinear as a result of complex interactions among earth system components. Such nonlinear behaviors constitute a major source of climate "surprises" with important socioeconomic and ecological implications. Paleorecords are key for elucidating patterns and mechanisms of nonlinear responses to radiative forcing, but their utility has been greatly limited by the paucity of quantitative temperature reconstructions. Here we present Holocene July temperature reconstructions on the basis of midge analysis of sediment cores from three Alaskan lakes. Results show that summer temperatures during 10,000-5,500 calibrated years (cal) B.P. were generally lower than modern and that peak summer temperatures around 5,000 were followed by a decreasing trend toward the present. These patterns stand in stark contrast with the trend of precessional insolation, which decreased by ∼10% from 10,000 y ago to the present. Cool summers before 5,500 cal B.P. coincided with extensive summer ice cover in the western Arctic Ocean, persistence of a positive phase of the Arctic Oscillation, predominantly La Niña-like conditions, and variation in the position of the Alaskan treeline. These results illustrate nonlinear responses of summer temperatures to Holocene insolation radiative forcing in the Alaskan sub-Arctic, possibly because of state changes in the Arctic Oscillation and El Niño-Southern Oscillation and associated land-atmosphere-ocean feedbacks.Chironomidae | paleotemperature | climate change | high latitudes | land-ocean-atmosphere interaction A lthough radiative forcing by greenhouse gases is spatially uniform, temperature trajectories have displayed high variability both spatially and temporally during the 20th and 21st centuries. Understanding the patterns and causes of this variability is paramount to anticipating the impacts of climate change at local to regional scales. Among the major contributing factors is nonlinearity in regional temperature responses to large-scale climate forcings. Such nonlinearity may result from land-ocean feedbacks to atmospheric warming and atmosphere-ocean teleconnections associated with climate modes such as the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) (1). For example, rising global mean temperatures are projected to increase prevalence of the AO/NAO positive phase by the end of this century. This change could accentuate the contrast of warming over Eurasia and interior Canada with cooling over northeastern Canada.Paleorecords can help elucidate mechanisms leading to nonlinear and spatially heterogeneous responses of temperature to radiative forcing. During the Holocene, high-latitude insolation varied markedly with the precessional cycle; for example, July short-wave radiation declined by ∼50 W m −2 at 65°N latitude throughout the Holocene (2). Paleoclimate data and model simulations have demonstrated the dominant effect of d...

Orbital-scale benthic foraminiferal oxygen isotope stratigraphy at the northern Bering Sea Slope Site U1343 (IODP Expedition 323) and its Pleistocene paleoceanographic significance

“…This winter northerly wind, linked to location and intensity of the Siberian High and the Aleutian Low (Overland et al, 1999;Rodionov et al, 2007), enhances the southward sea-ice advection in the Bering Shelf (Zhang et al, 2010). This ocean-atmosphere linkage is also important on geological timescales as indicated by data from the Holocene (Clegg et al, 2011;Katsuki et al, 2009;Muhs et al, 2003) and the last 60 kyrs (Rella et al, 2012). The expansion of sea-ice evident in our records suggests that a similar sea-ice atmosphere linkage may have been active at least since 0.9 Ma.…”

Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years

“…In Alaska, lake sediment analyses suggest near-modern temperature during the MCA (28,29) accompanied by relatively arid conditions (29,30), perhaps analogous to the recent trend of diminishing water availability driven by increased evapotranspiration because of warming (31). After the MCA, composite CHAR declines to a brief minimum (1.0) during the Little Ice Age (LIA), which was characterized by cooler (25,28) and wetter (29) conditions in the region. The subsequent sharp rise in CHAR to values similar to the MCA peak corresponds to marked warming and increased forest burning in Alaska during the past few decades (32).…”

“…The expansion of this species and associated increase in landscapescale flammability probably drove the late Holocene rise in FF and overall rates of biomass burning. Furthermore, the midHolocene increase in summer temperature in the region (25) may have played a role in promoting forest burning. (47) and reclassified as coniferous (black; i.e., Picea) or nonconiferous (gray).…”

Variation in the moisture regime of northeastern interior Alaska and possible linkages to the Aleutian Low: inferences from a late-Holocene δ18O record

“…5a). Such large isotopic variations cannot be driven by regional temperature change alone (Dansgaard 1964;Epstein et al 1953), assuming that summer temperature varied by up to 1.5°C in the region over the last 4,000 years (Clegg et al 2011). Although strong evaporative modification of the lake water can result in elevated d 18 O values in endogenic carbonate, at Keche Lake evidence for such isotopic enrichment is lacking in the d 18 O values of Chara stem encrustations, which are of known endogenic origin.…”

“…Recent paleoclimate reconstructions from Alaska provide evidence that the regional climate is highly sensitive to climatic forcings at continental to global scales (Hu et al 2003;Anderson et al 2005;Kaufman et al 2010;Clegg et al 2011). This sensitivity can be attributed in part to the location of the region at the center of several prominent synoptic-scale features of atmospheric circulation.…”