C. S. M. Turney & R. T. Jones, J. Quartn. Sci., 25 (2010), Does the Agulhas Current amplify global temperatures during super-interglacials?

Journal of Quaternary Science, 25 (2010) 839–843; ISSN 0267-8179

Does the Agulhas Current amplify global temperatures during super-interglacials?

Chris S. M. Turney* and Richard T. Jones

Climate Change and Sustainable Futures, School of Geography, University of Exeter, Exeter, U.K.

Abstract

Future projections of climate suggest our planet is moving into a ‘super-interglacial.’ Here we report a global synthesis of ice, marine and terrestrial data from a recent palaeoclimate equivalent, the Last Interglacial (ca. 130–116 ka ago). Our analysis suggests global temperatures were on average 1.5 °C higher than today (relative to the AD 1961–1990 period). Intriguingly, we identify several Indian Ocean Last Interglacial sequences that suggest persistent early warming, consistent with leakage of warm, saline waters from the Agulhas Current into the Atlantic, intensifying meridional ocean circulation and increasing global temperatures. This mechanism may have played a significant positive feedback role during super-interglacials and could become increasingly important in the future. These results provide an important insight into a future 2 °C climate stabilisation scenario.

Introduction

Since the late 1980s it has become increasingly recognised that the world climate system is considerably more sensitive to anthropogenic emissions of greenhouse gases (GHG) than previously thought. The Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) projects under a range of emission scenarios that global temperatures will increase over the next century between 1.1 and 6.4 °C, accompanied by a sea-level rise of 18–59 cm (Meehl et al., 2007). Worryingly, the AR4 estimates already appear conservative (Rahmstorf et al., 2007). Critically, past changes in the climate system can provide valuable insights into the future (Jansen et al., 2007; PALSEA, 2010). While past periods may not be complete analogues for anthropogenic-driven climate change, the mechanisms that operated at different times can provide analogues of processes in the future. Future projections of climate driven by anthropogenic emissions of GHG suggest our planet is moving into a ‘super-interglacial’ state: a sustained period warmer than present (Overpeck et al., 2005). There remains, however, considerable uncertainty over the mechanisms of change.

Fortunately, there are several Late Pleistocene super-interglacials preserved in natural archives that can provide valuable insights into a range of processes (Masson-Delmotte et al., 2010). Driven by orbital variations and carbon feedbacks, these periods may provide several important constraints on the future behaviour of the climate system. For instance, one intriguing observation is the apparent decoupling between inferred global temperatures and GHG during greatest warming (Masson-Delmotte et al., 2010). Arguably one of the best superinterglacials for investigating this conundrum is the Last
Interglacial (LIG), spanning the period ca. 130–116 ka ago and characterised by solar insolation anomalies caused by the changing Earth’s orbit (Harrison et al., 1995; Otto-Bliesner et al., 2006). There is some uncertainty, however, regarding the global temperature during this period, with estimates ranging from 0.1 to 2 °C warmer than present (CLIMAP Project Members, 1984; White, 1993; Hansen, 2005; Rohling et al., 2008).

Greatly reduced Arctic sea ice area, changes in ice sheet topography and freshwater influences on the Atlantic Meridional Ocean Circulation (AMOC) have all been proposed as possible feedbacks for driving higher temperatures, but no consensus has been reached (Otto-Bliesner et al., 2006; Masson-Delmotte et al., 2010). Accompanying these changes, recent estimates of sea level corrected for changes in gravity, solid Earth deformation and other effects have suggested the LIG was 6.6–9.4 m higher than today, rising some 6–9 mm a "1 (Kopp et al., 2009) – at least double the current global average.

To better understand the mechanisms and sensitivity of the Earth system to radiative forcing, it is critical that a better constrained temperature estimate is obtained for this period. Here we provide global and regional estimates of temperature during the LIG and present evidence that the Agulhas
Current increased global warming through the enhanced delivery of warm, salty water into the Atlantic Ocean, intensifying AMOC and partially decoupling the climate system from the carbon cycle.

[snip]

A comparison between the reconstructed temperature and d18 O trends in the highest-resolved records preserves a stratigraphic lead in ‘local’ warming over the shift to interglacial conditions around the southern African coastline. These results imply an enhanced leakage of the Agulhas Current into the Atlantic Ocean, injecting warm, saline water into the meridional ocean circulation and amplifying global warming during this superinterglacial. The above observations suggest the LIG can provide important insights into the mechanisms of future climate and whether a 2 °C stabilisation scenario can be considered ‘safe.’

More at this link:  http://www.whatnext.org/Climate/assets/JQS_1.5degrees%20at%20last%20super-interclacial.pdf