Thursday, November 29, 2012

New paper finds Greenland was 2–3°C warmer than today 4000 years ago

A paper published today in Quaternary Science Reviews finds west Greenland was "2–3°C warmer than today" from 6,000 to 4,000 years ago and that "summer temperatures were warmer than present by at least 7,100 years ago." According to the authors, "the local Greenland Ice Sheet margin was most retracted behind its present position between 6,000 and 5,000 years ago." The paper corroborates ice core data from central Greenland indicating that Greenland was significantly warmer than the present several times over the past 8,000 years despite "safe" levels of CO2. This paper and many others demonstrate that alarmist claims of a 2°C "tipping point" for the Greenland ice sheet are nonsense.


Holocene temperature history at the western Greenland Ice Sheet margin reconstructed from lake sediments

  • a Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA
  • b Department of Geology, University at Buffalo, 411 Cooke Hall, Buffalo, NY 14260, USA
  • c Department of Geosciences, University of Massachusetts, Morrill Science Center, 611 North Pleasant Street, Amherst, MA 01003, USA
  • d Geography and Environment, University of Southampton, Highfield, Southampton SO17 1BJ, UK
  • e Palaeoecology Laboratory, University of British Columbia, Okanagan Campus, Kelowna, BC V1V1V7, Canada

Abstract

Predicting the response of the Greenland Ice Sheet to future climate change presents a major challenge to climate science. Paleoclimate data from Greenland can provide empirical constraints on past cryospheric responses to climate change, complementing insights from contemporary observations and from modeling. Here we examine sedimentary records from five lakes near Jakobshavn Isbræ in central West Greenland to investigate the timing and magnitude of major Holocene climate changes, for comparison with glacial geologic reconstructions from the region. A primary objective of this study is to constrain the timing and magnitude of maximum warmth during the early to middle Holocene positive anomaly in summer insolation. Temperature reconstructions from subfossil insect (chironomid) assemblages suggest that summer temperatures were warmer than present by at least 7.1 ka (the beginning of the North Lake record; ka = thousands of years before present), and that the warmest millennia of the Holocene occurred in the study area between 6 and 4 ka. Previous studies in the Jakobshavn region have found that the local Greenland Ice Sheet margin was most retracted behind its present position between 6 and 5 ka, and here we use chironomids to estimate that local summer temperatures were 2–3 °C warmer than present during that time of minimum ice sheet extent. As summer insolation declined through the late Holocene, summer temperatures cooled and the local ice sheet margin expanded. Gradual, insolation-driven millennial-scale temperature trends in the study area were punctuated by several abrupt climate changes, including a major transient event recorded in all five lakes between 4.3 and 3.2 ka, which overlaps in timing with abrupt climate changes previously documented around the North Atlantic region and farther afield at ∼4.2 ka.

Highlights

► This paper presents five Holocene lake records from west Greenland. ► Chironomids provide quantitative estimates of summer air temperature anomalies. ► Peak Holocene warmth occurred from 6,000 to 4,000 years ago, with temperatures 2–3° warmer than today. ► A transient climate change is recorded at all five study sites ∼4.2 ka. ► The inferred paleotemperature history agrees well with local glacial geologic records.

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