The world around us is changing. Glaciers are retreating, sea levels are rising, and weather patterns are shifting in ways that affect all of us. Human activities are altering the composition of the atmosphere, and understanding how this connects to the changes we observe – and where it might lead – is one of the most pressing questions of our time.

Earth's climate has shifted before, under conditions not unlike those emerging today. If we could reconstruct those past episodes – how fast things changed, what forces drove them, how ice sheets and oceans responded – we would gain something invaluable: a longer perspective on what is possible.

But how do we read a past that left no written record? These are the questions at the heart of our work. And answering them requires looking further back in time than any thermometer or tide gauge ever could.

This is where proxies come in.

Marine organisms such as foraminifera – tiny single-celled creatures that build intricate shells of calcium carbonate – corals, algae, and shellfish record the conditions of the water in which they live as they grow. The chemistry of these structures carries information about the environment at the time of growth: temperature, acidity, circulation patterns, and more. We call these measurable signals proxies.

Preserved in shells and sediment cores, they allow us to reconstruct conditions across timescales no instrument record could ever reach – from decades to thousands and millions of years into the past.

To make this tangible: foraminifera live in the ocean, and when they die their shells sink to the seafloor and are buried in the sediment. By retrieving these sediments with a coring device – sometimes from thousands of metres water depth – we can bring these shells back to the surface and analyse their chemistry. The ratio of magnesium to calcium in foraminiferal calcite, for instance, is known to correlate with the temperature of the water in which the shell grew. Measure that ratio, and – simplified as this sounds – you have a window into ocean temperature at a moment in the past. Another example is the isotopic composition of boron, which reflects the pH of ancient seawater – a measure of ocean acidity that is closely linked to atmospheric CO₂ levels. These are just two of the signals we work with, each opening a different window into past conditions.

The precise measurements these analyses require – trace element ratios and isotopic compositions at very low concentrations – are carried out in the [ICP-MS Facility], initiated and established by our group and over the years grown into a shared analytical infrastructure used by various research groups across AWI

But this relationship is not the same for every species of foraminifera. Each builds its shell under different physiological conditions, and it is these conditions – not a direct recording of the surrounding seawater – that ultimately determine which elements end up in the shell and in what proportion. The chemical signal we measure is, in a sense, a byproduct of the organism's own biology.

To use a proxy with confidence across different species, time periods and ocean conditions, we need to understand the physiological environment in which the shell forms: how calcium and other ions reach the site of calcification, how the organism regulates the chemistry at that site for its own biological purposes, and how crystals grow layer by layer under these conditions. This understanding allows us to assess where a proxy is reliable and where it reaches its limits – and to extend its application into new contexts with justified confidence rather than assumption. With many years of experience in carbonate mineralogy and crystal chemistry, this mechanistic understanding of biomineralisation is one of the core strengths our group brings to proxy research.

Understanding why these signals vary between species – and under what conditions they can be trusted – is itself an active area of our research. Through a combination of fieldwork and controlled laboratory experiments, we investigate how different organisms incorporate elements into their shells, building the mechanistic foundation that makes our proxies robust across a wide range of applications.

The example above illustrates one end of the timescale we work with – reconstructing ocean conditions thousands to millions of years into the past. But not all questions require such a long view.

Take Atlantification – the process by which warmer, saltier water masses from the Atlantic are pushing further north into the Arctic Ocean through the Fram Strait. This is a recent and ongoing phenomenon, and understanding its pace and extent requires archives with a very different kind of resolution: not broad strokes across geological time, but a detailed year-by-year record of change.

For this we turn to organisms like Clathromorphum compactum, a crustose coralline red alga that grows on Arctic reefs. Like tree rings, these algae lay down a new growth layer every year, and the chemistry of each layer reflects the conditions of that particular year. By reading these chemical signatures layer by layer, we can trace how Arctic waters have changed over the past one to two centuries – precisely the period during which Atlantification has been intensifying.

Many of our palaeoclimate reconstructions are carried out in close collaboration with the Marine Geology section at AWI, combining our proxy expertise with their deep knowledge of sediment archives and palaeoceanographic records.

Two very different archives, then, addressing two very different questions. Together they give us what neither could provide alone: the depth to understand natural climate variability, and the resolution to detect and contextualise the changes happening today.

Across these different timescales and archives, our work helps to place the changes we observe today in a longer context – not to diminish their significance, but to understand them more fully.

The practical value of this is straightforward: the models we use to project future climate need to be tested against the past. If a model cannot reproduce what we know has happened, we cannot trust what it predicts. Proxy records are among the most important tools for this testing – and the more reliable the proxies, the more trustworthy the projections that inform how societies prepare for and respond to a changing climate.

Working group

Dr. Gernot Nehrke
Dr. Albert Benthien
Dr. Jutta Wollenburg
Dr. Grit Steinhöfel-Sasgen
Dr. Gerald Langer
Klaus-Uwe Richter
Anja Terbrüggen
Beate Müller
Kerstin Beyer
Ulrike Richter
Prof. Dr. Jelle Bijma (Senior Advisor)