A pinboard by
Jessica Amies

PhD Student, Australian National University


With 10% of the world's population living in coastal zones less than 10m above sea level (IPCC Report 2014), sea level rise is one of the major threats a changing climate is bringing to our planet. However, while we know sea levels are rising, we do not know how quickly this will happen. To improve our estimates for the future we must look at how Earth's sea levels have responded in the past to understand how our earth system behaves.

My project is concerned with improving estimates of past sea level. To do this I use sediment cores drilled from the seafloor, which are created from material building up at the bottom of the ocean over millions of years. Sediment cores contain microfossils of plankton, called foraminifera, which individually are smaller than grains of sand. During their lifetimes, foraminifera record information about the seawater around them, and store it in the chemical composition of their shells. When they die, foraminifera sink to the seafloor and are buried, preserving their shells and the information they contain. Over thousands of years, the sediment at the bottom of the ocean builds up, creating a chronological archive of foraminifera shells. By drilling sediment cores and retrieving these shells, we can access millions of years of climatic data. In particular, by measuring the oxygen isotope composition of foraminifera shells, we can reconstruct ice volume, and therefore also sea level, for the past.

My research is focused on sea-level reconstruction over the Mid-Plesitocene Transition (MPT) which occurred between approximately 0.7 and 1.2 million years ago. During the MPT, glacial-interglacials evolved from occurring over 41 thousand year cycles to 100 thousand year cycles, which implies there was a fundamental change in Earth’s climate system, which as yet, remains unexplained.


Relationship between sea level and climate forcing by CO2 on geological timescales.

Abstract: On 10(3)- to 10(6)-year timescales, global sea level is determined largely by the volume of ice stored on land, which in turn largely reflects the thermal state of the Earth system. Here we use observations from five well-studied time slices covering the last 40 My to identify a well-defined and clearly sigmoidal relationship between atmospheric CO(2) and sea level on geological (near-equilibrium) timescales. This strongly supports the dominant role of CO(2) in determining Earth's climate on these timescales and suggests that other variables that influence long-term global climate (e.g., topography, ocean circulation) play a secondary role. The relationship between CO(2) and sea level we describe portrays the "likely" (68% probability) long-term sea-level response after Earth system adjustment over many centuries. Because it appears largely independent of other boundary condition changes, it also may provide useful long-range predictions of future sea level. For instance, with CO(2) stabilized at 400-450 ppm (as required for the frequently quoted "acceptable warming" of 2 °C), or even at AD 2011 levels of 392 ppm, we infer a likely (68% confidence) long-term sea-level rise of more than 9 m above the present. Therefore, our results imply that to avoid significantly elevated sea level in the long term, atmospheric CO(2) should be reduced to levels similar to those of preindustrial times.

Pub.: 08 Jan '13, Pinned: 30 Oct '17

Breathing more deeply: Deep ocean carbon storage during the mid-Pleistocene climate transition

Abstract: The ~100 k.y. cyclicity of the late Pleistocene ice ages started during the mid-Pleistocene transition (MPT), as ice sheets became larger and persisted for longer. The climate system feedbacks responsible for introducing this nonlinear ice sheet response to orbital variations in insolation remain uncertain. Here we present benthic foraminiferal stable isotope (18O, 13C) and trace metal records (Cd/Ca, B/Ca, U/Ca) from Deep Sea Drilling Project Site 607 in the North Atlantic. During the onset of the MPT, glacial-interglacial changes in 13C values are associated with changes in nutrient content and carbonate saturation state, consistent with a change in water mass at our site from a nutrient-poor northern source during interglacial intervals to a nutrient-rich, corrosive southern source during glacial intervals. The respired carbon content of glacial Atlantic deep water increased across the MPT. Increased dominance of corrosive bottom waters during glacial intervals would have raised mean ocean alkalinity and lowered atmospheric pCO2. The amplitude of glacial-interglacial changes in 13C increased across the MPT, but this was not mirrored by changes in nutrient content. We interpret this in terms of air-sea CO2 exchange effects, which changed the 13C signature of dissolved inorganic carbon in the deep water mass source regions. Increased sea ice cover or ocean stratification during glacial times may have reduced CO2 outgassing in the Southern Ocean, providing an additional mechanism for reducing glacial atmospheric pCO2. Conversely, following the establishment of the ~100 k.y. glacial cycles, 13C of interglacial northern-sourced waters increased, perhaps reflecting reduced invasion of CO2 into the North Atlantic following the MPT.

Pub.: 18 Nov '16, Pinned: 30 Oct '17

North American ice-sheet dynamics and the onset of 100,000-year glacial cycles.

Abstract: The onset of major glaciations in the Northern Hemisphere about 2.7 million years ago was most probably induced by climate cooling during the late Pliocene epoch. These glaciations, during which the Northern Hemisphere ice sheets successively expanded and retreated, are superimposed on this long-term climate trend, and have been linked to variations in the Earth's orbital parameters. One intriguing problem associated with orbitally driven glacial cycles is the transition from 41,000-year to 100,000-year climatic cycles that occurred without an apparent change in insolation forcing. Several hypotheses have been proposed to explain the transition, both including and excluding ice-sheet dynamics. Difficulties in finding a conclusive answer to this palaeoclimatic problem are related to the lack of sufficiently long records of ice-sheet volume or sea level. Here we use a comprehensive ice-sheet model and a simple ocean-temperature model to extract three-million-year mutually consistent records of surface air temperature, ice volume and sea level from marine benthic oxygen isotopes. Although these records and their relative phasings are subject to considerable uncertainty owing to limited availability of palaeoclimate constraints, the results suggest that the gradual emergence of the 100,000-year cycles can be attributed to the increased ability of the merged North American ice sheets to survive insolation maxima and reach continental-scale size. The oversized, wet-based ice sheet probably responded to the subsequent insolation maximum by rapid thinning through increased basal-sliding, thereby initiating a glacial termination. Based on our assessment of the temporal changes in air temperature and ice volume during individual glacials, we demonstrate the importance of ice dynamics and ice-climate interactions in establishing the 100,000-year glacial cycles, with enhanced North American ice-sheet growth and the subsequent merging of the ice sheets being key elements.

Pub.: 16 Aug '08, Pinned: 30 Oct '17

Variations in the Earth's Orbit: Pacemaker of the Ice Ages.

Abstract: 1) Three indices of global climate have been monitored in the record of the past 450,000 years in Southern Hemisphere ocean-floor sediments. 2) Over the frequency range 10(-4) to 10(-5) cycle per year, climatic variance of these records is concentrated in three discrete spectral peaks at periods of 23,000, 42,000, and approximately 100,000 years. These peaks correspond to the dominant periods of the earth's solar orbit, and contain respectively about 10, 25, and 50 percent of the climatic variance. 3) The 42,000-year climatic component has the same period as variations in the obliquity of the earth's axis and retains a constant phase relationship with it. 4) The 23,000-year portion of the variance displays the same periods (about 23,000 and 19,000 years) as the quasi-periodic precession index. 5) The dominant, 100,000-year climatic [See table in the PDF file] component has an average period close to, and is in phase with, orbital eccentricity. Unlike the correlations between climate and the higher-frequency orbital variations (which can be explained on the assumption that the climate system responds linearly to orbital forcing), an explanation of the correlation between climate and eccentricity probably requires an assumption of nonlinearity. 6) It is concluded that changes in the earth's orbital geometry are the fundamental cause of the succession of Quaternary ice ages. 7) A model of future climate based on the observed orbital-climate relationships, but ignoring anthropogenic effects, predicts that the long-term trend over the next sevem thousand years is toward extensive Northern Hemisphere glaciation.

Pub.: 10 Dec '76, Pinned: 30 Oct '17