How does the Earth stop global warming?

Using metal isotopes to understand climate recovery processes

This page gives an overview of my recent Marie Curie-Sklodowska Action, which was funded by the EU's Horizon 2020 program and ended in May 2020. This page links to my ongoing work in the rest of the website, with more details of specific sub-projects on other pages.

Project Summary

All of us are currently partaking in an Earth system experiment. We are emitting CO2 to the atmosphere at an unprecedented rate, and observing the impacts in real time. From a geological perspective, this is a 'shortcut' of the natural, long term carbon cycle.

The Earth system responds to warming in a variety of ways. Some of these processes, known as negative feedbacks, help re-stabilise the climate. By contrast, positive feedbacks enhance the effects of warming. Projections of the future climate (right) indicate that it will take millions of years for all of the emitted carbon to be removed from the atmosphere, even if we stop today. Our actions today therefore have very long term consequences.

The focus of my research has been on trying to understand how the climate can recover from a global warming event. I study the natural negative feedback mechanisms that operate on long timescales to return the climate to a 'steady state'.

Plausible CO2 emission scenarios from the IPCC 5th report. Although there is uncertainty in our future, whatever scenario happens, the emitted CO2 stays in the atmosphere for thousands to millions of years

Negative feedbacks and environmental hazards

There are a number of geological mechanisms that help to remove CO2 from the atmosphere that operate in response to warming. Many of these also represent very real threats to life on Earth. Warming typically initiates a sequence of events that links processes across the globe. For example, under warmer climates we expect more weathering activity on land. As silicate rocks break down, CO2 is consumed. Weathering also delivers nutrients into the ocean which help to promote primary productivity, again acting to remove carbon from the atmosphere via photosynthesis and storing the carbon as organic matter. The decay of this organic matter by bacteria removes oxygen from the waters, which eventually helps to preserve more of the organic carbon in sediments. In the past this forms organic rich black shales which mark the presence of oceanic anoxia.

On long timescales (millions of years) this removes CO2 from the atmosphere. On short timescales these processes present new risks and challenges to life. Weathering is associated with flooding and siltation, which threaten homes, farm land and marine ecosystems. High nutrient inputs drive cyanobacterial blooms which are damaging to coastal environments. Low oxygen in the waters kills organisms that depend on respiration, which is already causing devastation for fish and seafood farming.

Metal isotopes

My Marie Curie-Sklodowska project aimed to reconstruct different negative feedback processes on a global scale, using different warming events in Earth history. To do this we use a range of metal isotope systems which each tells us a different part of the puzzle.

My primary focus has been on using uranium isotopes to estimate changes in de-oxygenation in the ocean, and to link this back carbon burial processes. Natural uranium is dissolved in seawater when it is oxygenated, but under anoxic conditions it is removed and buried in the sediments. As this happens we observe a change in the isotopic composition of seawater which can be used to quantify the amount of uranium removed, and hence to area of anoxic seafloor. If we measure carbonate sediments, we can reconstruct the changes in seawater isotopic compositions through time.

We are also using zinc and cadmium isotopes to gain information on ocean productivity, and lithium isotopes to tell us about weathering processes. Together these different tools allow us to understand the climate system in the past (right).

Illustrating the environmental cascade that follows CO2 emissions and warming. Some of these processes act to remove CO2 from the atmosphere, but are also threats to life on Earth. In blue are different isotopic systems that can be used to trace these processes. Image credit. M. Clarkson

Photo taken during a sampling trip to the Bremen IODP core repository. With the help of BSc student Maddy Constance, we selected samples from the PETM interval. These deep sea cores extend millions of years into the past and can be accessed by any scientist. Image credit M.Clarkson/ A. Dickson

Estimating anoxia during the PETM

The main component to this work is using uranium isotopes to quantify the global extent of anoxic conditions during the Paleocene Eocene Thermal Maximum. In contrast to the more extensive Mezosoic Oceanic Anoxic Events (OAEs), the PETM shows less evidence for widespread anoxia, but the global extent is still uncertain. Ongoing work using metal isotopes is trying to define an upper limit on the extent of anoxia and make a quantitative comparison to other periods of Earth history.

MOC acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 795722

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