Taking the long view on climate change

The Earth’s climate has always changed and no doubt always will. However, this alone does not tell us very much about the climate system: we need to be able to say exactly how and why climate can change. Today this requirement has been brought to the fore by the prospect of human-induced global climate change resulting largely from greenhouse gas emissions that arise from our massive and growing appetite for fossil fuels. Thousands of scientists are now striving to predict how a sharp rise in greenhouse gas concentrations will affect the entire climate system, including the ecosystems and societies that it supports. But how can we be sure about our theories of climate change, let alone our theories of ecosystem or market response? Just how important are greenhouse gases in controlling global climate? And what are the timescales and thresholds of climate adjustment? These are just some of the urgent questions that have been raised by the prospect of anthropogenic climate change.

To help answer such questions we can look to the past, at how the Earth’s climate evolved prior to the relative stability that human society has so far enjoyed. Researchers in the Department of Earth Sciences are taking up this challenge, using marine sediments as their lens into the past, and as a guide to the future.

Palaeoclimatology

The study of past climate change – palaeoclimatology – aims to reconstruct what has happened in the past, in the oceans, on the land, in the atmosphere and in ecosystems, and to infer how the global climate system works ‘as a whole’. In the last 20 years of palaeoclimate research, three major questions have emerged that are particularly relevant to modern climate change. First, how did changes in solar radiation (insolation) and atmospheric carbon dioxide (CO2) conspire to trigger massive global climate upheavals such as the glacial–interglacial (‘ice-age’) climate cycles? Second, what regulates atmospheric CO2 concentrations under changing climatic conditions, and what roles can we ascribe to marine biological productivity or ocean circulation changes in particular? And third, how abruptly can regional climate change and with what repercussions for the rest of the world?

All of these questions are interconnected of course, although each bears on a different aspect of the climate system’s ability to pace and amplify climate perturbations through sensitive ‘feedback’ processes.

Past climate by proxy

A central aspect of palaeoclimate reconstructions is the ‘proxy’ character of our observations. Because scientists cannot measure past ocean temperatures directly, they must measure the impacts of past temperature changes instead, usually based on temperature-sensitive organisms or temperature-sensitive chemical constituents in their shells or skeletons.

As a palaeoceanographer, Dr Luke Skinner specifically makes use of marine sediments as a window into the past. Among the many advantages of using marine sediments are that they can be obtained from nearly two thirds of the Earth’s surface, they generally provide unbroken and often very high-resolution records of past conditions, and they contain a diversity of constituents that can be analysed, from tiny fossil shells to grains of sand dropped by passing icebergs.

To reconstruct past climate change, Dr Skinner collects and studies the fossil calcite shells of foraminifera – single-celled blobs of protoplasm – that have accumulated on the sea floor. Using the shells of these tiny creatures, Dr Skinner has been able to generate detailed records of temperature change, both at the sea surface and in the ocean interior. In combination with ice-rafted debris and oxygen- and carbon-isotope records, these reconstructions have helped to demonstrate that the North Atlantic region experienced very intense and abrupt climate swings in the past, involving massive glacier surges as well as drastic changes in the deep ocean circulation system and the Gulf Stream. It has also been possible to show that these same changes in the Atlantic Ocean’s circulation were accompanied by a ‘see-saw’ in temperatures across the hemispheres, with heat pooling in the South to the extent that it was not efficiently delivered to the North. Based on records such as these it is now clear that global change can be heterogeneous and can occur too suddenly to be presaged by obvious warnings.

Perspectives on the future

Although it is clear that no previous climate period can really serve as a blueprint for the future, important lessons can still be learned from the study of the past. One important example is the use of palaeoclimate data to guide the improvement of our climate simulation models. Because numerical and statistical models provide our only means for predicting future climate, it is imperative that they be as general as possible. Studies like those described here are helping to achieve this, by revealing the feedbacks, thresholds and characteristic timescales for climate adjustment, across a wide range of climatic contexts.

In the future, global CO2 levels will only be stabilised if we either drastically cut our emissions or identify, trigger or create a process that ‘mops up’ exactly as much CO2 as millions of consumers are able to produce each day (the basis of carbon capture). The history of climate change tells us that we are going to need as many one-way fluxes out of the atmosphere as we can muster if are going to compete with the ‘leak’ we have created in the Earth’s largest standing carbon reservoir, the solid Earth. We have much to learn about the climate system, both for our own sake and for the sake of knowledge itself.

For more information, please contact the author Dr Luke Skinner (luke00@esc.cam.ac.uk) at the Department of Earth Sciences.