Simulated influence of carbon dioxide, orbital forcing and ice sheets on the climate of the Last Glacial Maximum

Abstract

A coupled atmosphere-ocean-sea-ice model is used to investigate the climate of the Last Glacial Maximum (ϳ21,000 years ago) and the relative climate-forcing effects of atmosphere CO 2 , the Earth's orbital parameters and ice-sheet albedo. Tropical temperatures are found to be ϳ2.2 ЊC less than today's-slightly colder than indicated by the CLIMAP palaeoclimate reconstruction. This result is consistent with a low to medium climate sensitivity to radiative perturbations. Temperatures are colder still in the northern North Atlantic region, owing to a weakening and shallowing of the thermohaline circulation. A sensitivity analysis suggests that changes in ocean circulation since the Last Glacial Maximum have not contributed directly to the global-mean temperature change since that time.Early CLIMAP 1,2 attempts to reconstruct sea surface temperature (SST) for the Last Glacial Maximum (LGM), around 21 kyr ago, have suggested that relative to the present, global SSTs were on average 1.7 ЊC cooler in August and 1.4 ЊC cooler in February. These reconstructions further suggested that tropical SSTs were similar to those of the present climate whereas in the North Atlantic, SSTs were much colder. Although recent alkenone evidence 3-6 also supports tropical SSTs having been only slightly cooler at the LGM, additional evidence is contradictory. For example, coral records form Barbados 7 and the southwest Pacific 8 , ice-core records from Peru 9 , noble-gas measurements in Brazil 10 and ocean core records from the western equatorial Atlantic 11 suggest that LGM tropical temperatures were significantly below the present-day values.Most previous modelling studies of the LGM have fallen into two categories. In the first, ocean-only models have been used in which surface forcing consists of restoring temperature and salinity to LGM reconstructions with specified atmospheric general circulation model (AGCM) LGM surface wind stress fields 12-14 . The second class of studies involves the integration of AGCMs with either fixed SST or mixed-layer ocean models at the lower boundary 15-19 . Many of these modelling studies have found tropical SSTs consistent with CLIMAP, although this often (but not always; see, for example, ref. 15) reflects the fact that CLIMAP data were used as a boundary condition. Webb et al. 20 on the other hand obtained relatively cool tropical SSTs using an AGCM in which present-day oceanic heat transports were maintained. They argued that reduced evaporation in the subtropics with present-day oceanic heat transport led to reduced water vapour in the atmosphere, and hence reduced absorption of outgoing longwave radiation. These workers also suggested that this cooling mechanism was amplified by other processes in the system.Until recently, it has not been possible to run coupled atmosphere-ocean GCM experiments to equilibrium for LGM scenarios owing to the necessity of applying explicit flux adjustments to keep stable the present-day climate. Any meaningful comparison between the simulated present-day and LGM clima...

Altered Ecologies: Fire, climate and human influence on terrestrial landscapes: Terra Australis 32

“…We note that near-Holocene temperatures were reached in many parts of eastern New Zealand by about 14,000 years ago (Turney et al 2003). An early-Holocene thermal optimum, between 1.5°C and 3°C warmer than present, is suggested for the New Zealand region from about 9000 to 7000 yr BP (Weaver et al 1998;Wilmshurst et al 2007).…”

“…The grey-headed flying fox is a canopy feeder, consuming pollen, nectar, flowers, fleshy fruit and occasionally leaves (Eby 1998;Williams et al 2006). Nectar and pollen are most often from sclerophyllous forests (dietary studies indicate a preference for Eucalyptus and Melaleuca), while the fruit component of the diet tends to be provided by rainforest-related taxa, such as figs, and in the modern setting, urban plantings and orchards.…”

“…We note that near-Holocene temperatures were reached in many parts of eastern New Zealand by about 14,000 years ago (Turney et al 2003). An early-Holocene thermal optimum, between 1.5°C and 3°C warmer than present, is suggested for the New Zealand region from about 9000 to 7000 yr BP (Weaver et al 1998;Wilmshurst et al 2007).At Prospect Hill (13 km north and 470 m lower), low-montane forest replaced shrubland at 10,000 yr BP. However, forest arrival at Staces Tarn challenges the presumed early-Holocene warmer temperatures, as trees are not dominant at the site until 7500 yr BP.…”

“…Although a sporadic visitor to Victoria for at least 100 years, a small permanent population became established in Melbourne in 1986. This southwards extension of range was attributed to the development of a warm, humid urban microclimate associated with the growth of the city ( The grey-headed flying fox is a canopy feeder, consuming pollen, nectar, flowers, fleshy fruit and occasionally leaves (Eby 1998;Williams et al 2006). Nectar and pollen are most often from sclerophyllous forests (dietary studies indicate a preference for Eucalyptus and Melaleuca), while the fruit component of the diet tends to be provided by rainforest-related taxa, such as figs, and in the modern setting, urban plantings and orchards.…”

Sensitivity of the inorganic ocean carbon cycle to future climate warming in the UVic coupled model

“…The coupled model has been extensively and successfully evaluated against both contemporary climate observations ) as well as paleo proxy records (Weaver et al, 1998;Meissner et al, 2003;Schmittner et al, 2002a,b). One of the virtues of the coupled model is that we do not need to employ explicit flux adjustments of heat and freshwater to keep the simulation of the present climate stable.…”