Chironomids can be reliable proxies for Holocene temperatures. A comment on Velle et al. (2010)

Velle et al. (2010) discussed discrepancies between Scandinavian Holocene chironomid-inferred temperature estimates, which they attribute to the response of chironomids to environmental variables other than temperature and to taxonomic shortcomings. They suggest ways in which the reliability of chironomid-based paleotemperature reconstructions could be improved by taking into account ecological complexity. While we agree with many of their recommendations, based on the results of other work, we think their paper is unnecessarily pessimistic regarding the ability of existing chironomid-based temperature inference models to provide reliable estimates of past temperature. We offer a critique of the main points discussed by Velle et al. (2010) and provide evidence that chironomid-based temperature inference models can reliably reconstruct mean July air temperature in the Lateglacial and Holocene over millennial and centennial timescales.

Climate warming and vegetation response at the end of Heinrich event 1 (16 700–16 000 cal yr BP) in Europe south of the Alps

Chironomids preserved in a sediment core from Lago di Origlio (416 m a.s.l.), a lake in the foreland of the Southern Swiss Alps, allowed quantitative reconstruction of Late Glacial and early Holocene temperatures using a combined Swiss-Norwegian temperature inference model based on chironomid assemblages from 274 lakes. We reconstruct July air temperatures of ca. 10 °C between 17 300 and 16 000 cal yr BP, a rather abrupt warming to ca. 12.0 °C at ca. 16 500–16 000 cal yr BP, and a strong temperature increase at the transition to the Bølling/Allerød Interstadial with average temperatures of about 14 °C. During the Younger Dryas and earliest Holocene very similar temperatures are reconstructed as for the interstadial. The rather abrupt warming at 16 500–16 000 cal yr BP is consistent with sea-surface temperature as well as speleotherm records, which indicate a marked Pre-Bølling warming after the end of Heinrich event 1 in southern Europe and the Mediterranean Sea. The pollen record of Origlio and other sites from southern Switzerland and northern Italy indicate an early reforestation of the lowlands prior to the large-scale afforestation at the onset of the Bølling period at 14 700 cal yr BP in Central Europe. Our results suggest that these afforestation processes in the formerly glaciated areas of southern Switzerland and Northern Italy have been promoted by increasing temperatures.