By Beatrice Ellerhoff and Shirin Ermis.
Since 1990 the United Nations Intergovernmental Panel on Climate Change (IPCC) has meticulously sought to document all available scientific knowledge on climate change: its causes, potential impacts, and options for responses to the global climate crisis. Paleoclimatology, the study of past climates, has, as part of this effort, helped a lot in attributing present climate change to anthropogenic influences. It has shown that the CO2 levels today are higher than at any point in the past 800,000 years. Never in the past 2000 years has the trend in surface temperatures been so uniform across the globe. All this led to the recognized scientific fact, that anthropogenic warming threatens the basis of life for plants, animals, and humans. In order to ensure that Earth remains a habitable planet and to make political action effective, it is necessary to better understand the impacts of global heating on our ecosystems and the climate as a whole. Palaeoclimatologists are in demand to characterise the future course of climate and to look further back into the past, learning from the rich history of the Earth‘s climate.
If we think of the Earth as a sick patient, current scientific methods perform very well in ‘diagnosing’ its major ‘symptoms’: the increase in global temperatures, the melting of glaciers and ice sheets, and rising sea levels can be measured with near certainty. The main causes are proven to be anthropogenic greenhouse gas emissions and atmospheric particulate pollution. However, the ‘prognosis’ still raises questions - for example, how the climate crisis manifests on a local scale and how variable components such as ocean circulations react to it. Answering these questions demands a better grasp of the main drivers of climate variations on Earth. Those often comprise large and complex systems, like the winds and oceans, which are complicated to measure, hard to understand, and even more challenging to model.
In addition, the Holocene (from Greek: ‘holos’ for ‘whole’ and ‘kainos’ for ‘new’) appears as small ripples in the rich history of the Earth which usually swings from ice ages to warm periods. During this epoch of the Holocene, starting 11,700 years ago, humans have had the privilege of living in an extraordinarily stable climate. They began to control the growth and breeding of plants and animals, thereby transforming the abundant environments - first locally, then globally. How sensitive human populations are to rapid climatic change is illustrated by the demise of the Maya civilisation around 900 CE, which is often explained by the onset of severe droughts.
Given the strength of human-caused climate change, the Earth will most likely be faced with temperatures unlike anything experienced during the Holocene. In many ways, the complexity of the challenges ahead only becomes apparent by looking beyond the stability of the Holocene and revealing the strong variability in climate that the planet is able to produce.
Palaeoclimatology takes on the enormous interdisciplinary challenge of decoding the imprints of past climatic states from natural archives, so-called climate proxies, like rocks, ice, cave depositions, fossils, and tree rings. Pollen from sediment cores, for example, hints at plant species and vegetation densities. Gas molecules trapped in ice cores help determine ancient greenhouse gas concentrations. The structure and composition of tree rings and corals contain information about the environmental conditions of their ecosphere. Those traces from the past help to explain major changes in the climate such as the emergence of ice ages and sea level variations on long time scales. This enables researchers to grasp the impact climate events have on flora and fauna, such as well-known mass extinctions and evolutionary changes due to changing ecosystems.
Throughout the history of life on Earth, only a minuscule fraction of climate is captured by direct observations such as temperature records or satellite missions. Before record-keeping, however, the past climate left its mark on various environmental archives such as ocean and lake sediments, ice cores, speleothems, tree rings or corals. These so-called proxies preserve the characteristics of past climatic conditions and thus allow for their reconstruction. In view of the urgency of the climate crisis, palaeoclimatology is a central approach towards complementing our knowledge of complex and changing climatic systems from studying the past.
In palaeoclimatology, climate models complement the study of climate proxies. Much like weather can be forecasted for the next week, climate models can begin from a climate state in the distant past and then be applied to previous time periods in order to ‘hindcast’ the past climate. Because data storage is limited even in high-end science projects, climate models use simplifications of the physical knowledge we have of the climate system and simulate small-scale processes using parameterisations. Challenges here lie, among others, in tuning the key parameters of the models which are a considerable source for uncertainties in climate reconstructions. The data from natural archives can help determine those parameters by providing information on ancient temperatures, precipitation as well as the composition of oceans and atmosphere. Combining the knowledge from empirical data and computer models has therefore become an important task.
The Atlantic Meridional Overturning Circulation (AMOC) is an excellent example of a complex component of the Earth’s climate system which is still inadequately understood and on which both climate models and proxies can help to shed light. This system of ocean currents in the Atlantic runs from the southern ocean to high northern latitudes and transports warm waters northwards. When the waters reach the North Atlantic, they cool and sink before flowing back to the southern ocean. This transport of warm waters into higher latitudes is essential for the temperate climates in Europe and North America. As they sink, the waters store heat and carbon in deep oceans. Over the past decades, oceans have helped offset global heating by absorbing approximately a third of anthropogenic greenhouse gases and large amounts of excess heat.
In recent years many scientists have observed an alarming decline in strength of the AMOC. Freshwater from melting ice sheets, mainly the one covering Greenland, makes the waters in these areas lighter and prevents sinking, thereby affecting the AMOC‘s dynamics. These changes could have strong implications on the surrounding continents and the climate system as a whole. While the immediate response to a decline would be a cooling of climate in Europe and North America, there are also effects that might enforce those of anthropogenic heating, as shown by computer models. Many of these effects remain a question of ongoing research.
A major issue in resolving this question is that measurements of the strength of ocean currents are still sparse. For the AMOC, we have a mere sixteen years of direct observations which are mainly along a single latitudinal band. This absence of long, continuous observations makes the verification of results from computer models difficult and reconstructions of the AMOC indispensable. There are multiple methods in palaeoclimatology which provide an indirect reconstruction of the AMOC. Some scientists use grain sizes of the sand in the sediment of the Labrador Sea between Greenland and Canada to study currents which are part of the AMOC. The chemical composition of ocean sediments can also archive past temperature fluctuations from which we learn about the ability of the waters to sink and form deep water in the northern seas.
Another promising method for reconstructing past behaviour of the AMOC which also uses data from sediment cores is an isotopic analysis. The isotopes of the chemical element Neodymium, which originates in the Earth’s crust, can be distinguished by their masses. The ratio of these isotopes differs across land masses and depends on the age of the rock. For example, northern North America features a lower ratio of Neodymium isotopes compared to the continents on the southern hemisphere. Since the waters in their surroundings adopt these isotope ratios and give them on to ocean sediments, measuring the ratios in sediment cores helps scientists determine the origin of the water masses involved in the AMOC. This way, changes in the underlying dynamics of the ocean can be detected.
(left) Core drilling at a measurement campaign on the Meteor ship in the Mediterranean Sea. (right lower) Sediment core excavated in the North Atlantic used for the reconstruction above. The banding of dark and light layers hints at an alteration of the deep water’s chemical composition. (right upper) Isotope ratios of the chemical element Neodymium extracted from the sediment core. Large parts of northern North America consist of an extremely old continental crust, featuring lower isotope ratios compared to the young mantle-derived rocks from the southern hemisphere. Measuring the mass ratio of different Neodymium isotopes and comparing them to a standard thus allows for tracing water mass exchange between the major ocean basins. Credits to Jörn Lippold for providing the data and pictures (J. Lippold et al., 2016).
The research study from which results are depicted in the figure above, examined the Neodymium ratio between the Holocene and the last glacial maximum - the most recent time when ice sheets were at their greatest extent with much of North America and Europe covered by ice. The Neodymium isotope ratio shows a transition at the end of the last glacial maximum when ice masses started melting, thereby changing the concentration of freshwater in the Atlantic. It suggests that, at the end of the last glacial maximum, the AMOC slowly transitioned from a shallow overturning to an overturning in which water masses sunk much further into the depths of the ocean. This is a crucial insight into the dynamics of the overturning circulation. It shows that, with the climate system, the AMOC can change fundamentally, which in turn can have disruptive effects on the global climate. For the 21st century, scientists expect a decrease in the summer precipitation in northern Europe in the case of a decline in strength of the AMOC. While the area over the North Atlantic is projected to cool, a weaker AMOC could propel other effects of global heating.
Analysing natural archives such as marine sediments therefore has the potential to characterise major components of the Earth’s climate system like the AMOC. To quote the United Nations: “Today’s climate crisis is the defining issue of our time”. Climate science must therefore increasingly lead to political efforts to stop global heating. It may thus seem that paleoclimatology has reached its zenith, but that is far from the case. Given the fact that instrumental records grasp only a brief snapshot, the precious information from natural archives is essential to put observations into context and to characterise the course of future climate change. Although seemingly distant from our current world, palaeoclimatology can hence help us understand the future of our climate system.
Beatrice Ellerhoff is a physicst, jazz pianist, and mountain enthusiast. In her PhD at Heidelberg University, she is currently exploring how temperature variability can be explained across spatial and temporal scales.
Shirin Ermis is a physicist, gardener, and weather enthusiast. She studies for an MSc in Physics at Imperial College London. Her prior research focused on the interconnections between ocean temperatures and currents in the North Atlantic and how likely a "The day after tomorrow" scenario actually is.