Using the past to constrain the future: how the palaeorecord can improve estimates of global warming

Climate sensitivity is defined as the change in global mean equilibrium temperature after a doubling of atmospheric CO2 concentration and provides a simple measure of global warming. An early estimate of climate sensitivity, 1.5-4.5{\deg}C, has changed little subsequently, including the latest assessment by the Intergovernmental Panel on Climate Change. The persistence of such large uncertainties in this simple measure casts doubt on our understanding of the mechanisms of climate change and our ability to predict the response of the climate system to future perturbations. This has motivated continued attempts to constrain the range with climate data, alone or in conjunction with models. The majority of studies use data from the instrumental period (post-1850) but recent work has made use of information about the large climate changes experienced in the geological past. In this review, we first outline approaches that estimate climate sensitivity using instrumental climate observations and then summarise attempts to use the record of climate change on geological timescales. We examine the limitations of these studies and suggest ways in which the power of the palaeoclimate record could be better used to reduce uncertainties in our predictions of climate sensitivity.Comment: The final, definitive version of this paper has been published in Progress in Physical Geography, 31(5), 2007 by SAGE Publications Ltd, All rights reserved. \c{opyright} 2007 Edwards, Crucifix and Harriso

The response of the Walker circulation to Last Glacial Maximum forcing: Implications for detection in proxies

International audienceThe response of the Walker circulation to Last Glacial Maximum (LGM) forcing is analyzed using an ensemble of six coordinated coupled climate model experiments. The tropical atmospheric overturning circulation strengthens in all models in a manner that is dictated by the response of the hydrological cycle to tropical cooling. This response arises from the same mechanism that has been found to explain the weakening of the tropical circulation in response to anthropogenic global warming but with opposite sign. Analysis of the model differences shows that the ascending branch of the Walker circulation strengthens via this mechanism but vertical motion also weakens over areas of the Maritime Continent exposed due to lower sea level. Each model exhibits a different balance between these two mechanisms, and the result is a Pacific Walker circulation response that is not robust. Further, even those models that simulate a stronger Walker circulation during the LGM do not simulate clear patterns of surface cooling, such as La Niña-like cooling or enhanced equatorial cooling, as proposed by previous studies. In contrast, the changes in the Walker circulation have a robust and distinctive signature on the tilt of the equatorial thermocline, as expected from zonal momentum balance. The changes in the Walker circulation also have a clear signature on the spatial pattern of the precipitation changes. A reduction of the east-west salinity contrast in the Indian Ocean is related to the precipitation changes resulting from a weakening of the Indian Walker circulation. These results indicate that proxies of thermocline depth and sea surface salinity can be used to detect actual LGM changes in the Pacific and Indian Walker circulations, respectively, and help to constrain the sensitivity of the Walker circulation to tropical cooling

Evidence for extreme floods in arid subtropical northwest Australia during the Little Ice Age chronozone (CE 1400-1850)

Here we report a ~2000-year sediment sequence from the Fortescue Marsh (Martuyitha) in the eastern Pilbara region, which we have used to investigate changing hydroclimatic conditions in the arid subtropics of northwest Australia. The Pilbara is located at the intersection of the tropical Indian and Pacific Oceans and its modern rainfall regime is strongly influenced by tropical cyclones, the Intertropical Convergence Zone (ITCZ) and the Indo-Pacific Warm Pool. We identified four distinct periods within the record. The most recent period (P1: CE ~1990-present) reveals hydroclimatic conditions over recent decades that are the most persistently wet of potentially the last ~2000 years. During the previous centuries (P2: ~CE 1600-1990), the Fortescue Marsh was overall drier but likely punctuated by a number of extreme floods, which are defined here as extraordinary, strongly episodic floods in drylands generated by rainfall events of high volume and intensity. The occurrence of extreme floods during this period, which encompasses the Little Ice Age (LIA; CE 1400-1850), is coherent with other southern tropical datasets along the ITCZ over the last 2000 years, suggesting synchronous hydroclimatic changes across the region. This extreme flood period was preceded by several hundred years (P3: ~CE 700-1600) of less vigorous but more regular flows. The earliest period of the sediment record (P4: ~CE 100-700) was the most arid, with sedimentary and preservation processes driven by prolonged drought. Our results highlight the importance of developing paleoclimate records from the tropical and sub-tropical arid zone, providing a long-term baseline of hydrological conditions in areas with limited historical observations