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Reconstructing and Interpreting the Forcing of Climate using Sulfur and Carbon Isotopes

Final Report Summary - REINFORCE (Reconstructing and Interpreting the Forcing of Climate using Sulfur and Carbon Isotopes)

REINFORCE: Reconstructing and Interpreting the Forcing of Climate using Sulfur and Carbon Isotopes

Overview
Climate change is one of the most pressing issues faced by society today, and our ability to project and understand future climate change relies on a firm understanding of the natural forcings, responses, and feedbacks in the natural climate system. Since instrumental records can be geographically limited and only cover the most recent past, it is important to use climate archives in the geological record to understand longer term variations in climate and to identify sensitivities and thresholds in the Earth system.
This project set out to use carbon and sulfur isotopes within geological archives (corals, ice cores, and sediment cores) as a means of reconstructing major forcings on the climate system over a range of timescales. Carbon isotopes in coral skeletons can be used to reconstruct changes in ocean circulation and carbon uptake by the ocean; as the ocean is one of the major sinks for anthropogenic carbon dioxide, it is important to understand its behavior under changing climate conditions. Sulfur isotopes in ice cores can be used to identify the magnitude of volcanic eruptions, which are a major natural forcing of climate. Without an accurate record of the natural forcing of climate by volcanoes, it is impossible to understand either unforced natural climate variability or the sensitivity of the climate system to man-made perturbations, such as greenhouse gases.

Major Results and Conclusions
Over the past four years, this grant has provided funding for several different projects using carbon and sulfur isotopes to investigate past climate forcing over a range of timescales, from the transition out of the last ice age 20,000 years ago when atmospheric CO2 levels were 80 parts per million lower than pre-industrial levels, to major climatic perturbations forced by volcanic eruptions in the 6th century AD. Many of the results from this work have already been published in peer-reviewed journals, and more papers from this work are in preparation for publication in peer-reviewed journals. For instance, in a paper published in Science (Chen et al., 2015), we used deep-sea coral radiocarbon to show that during the transition out of the last ice age, the ocean experienced rapid (centennial scale) ventilation events that released carbon from the ocean into the atmosphere, increasing atmospheric CO2 by more than 10 ppm. (N.B. although this is a rapid change on geological timescales, the current rate of man-made CO2 rise in the atmosphere is about 10 times faster). In another paper published in Paleoceanography that used radiocarbon from corals and sediment cores (Burke et al, 2015), we identified a different ocean circulation structure during the last glacial maximum 20,000 years ago, which allowed for greater storage of carbon in the ocean abyss, helping to explain the lower carbon dioxide concentrations in the atmosphere at this time. In a paper published in Proceedings of the National Acadmeny of Sciences (McConnell et al., 2017), we used sulfur isotopes in ice cores, as well as a number of other geochemical indices, to identify a previously unidentified major long-lasting volcanic eruption right at the start of the last deglaciation. This eruption altered atmospheric chemistry substantially through destruction of ozone, and this could have had knock-on effects on atmospheric circulation and climate change. In a paper that is currently in preparation for submission to a peer-reviewed journal, we use sulfur isotopes from two volcanic eruptions in the mid-6th century AD to show that the first one likely was a high-latitude eruption and was smaller than the larger tropical eruption that followed. This was a surprising result since the big cooling event in the tree ring records was coincident with the earlier eruption and it suggests that the current volcanic forcing records that are used in climate simulations might be overestimating the magnitude of the sulfur loading in the atmosphere for high latitude eruptions.
Finally, this funding has been used by the researcher to establish an active research group (2 post-docs and 4 graduate students) and to set up a new isotope geochemistry laboratory called St Andrews Isotope Geochemistry (STAiG; https://synergy.st-andrews.ac.uk/staig/). The funding supported new and established collaborations, which have since led to successful proposals for additional funding from UK research councils (Standard and Capital Equipment grants from NERC) and charitable trusts (Leverhulme).

Socio-economic impacts of the project
This project funded broader impact activities to increase awareness of climate change by the general public, and to introduce these topics to school children though outreach science events. For instance, the funded researcher has held science “master classes” on Saturdays for primary 1 and 2 students to learn about past climates through fossils and the geological record. The funded researcher also led a Science Taster day to encourage secondary school students from all-girls schools into STEM subjects through hands-on activities and interacting with female scientists from a variety of backgrounds. Additionally, this project funded the employment of a full-time lab technician in the STAiG lab, and led to proposals which have funded two full-time post-doctoral researchers. It has also provided essential laboratory funding and training for PhD students supervised by the researcher.