While there is common agreement that Earth's biogeochemical cycles will change profoundly - both directly through accelerated reaction rates and indirectly through climatic influences - the magnitude of potential perturbations remains a topic of considerable debate. The easiest way to find out would be to get on a time machine, hit fast forward into the future, and catch some glimpses of the evolving climate a few hundred or even thousands of years ahead of us. As we all know, there is no time-machine available. We cannot wait hundreds or even thousands of years to see what will happen and then decide how to react. So what can we do now?
The answer is that we can look into the past, by studying the geologic history of our planet. Looking back into climate history will allow an insight into our future. Over the course of almost four decades the most successful international research collaboration in the history of Earth sciences, the (Integrated) Ocean Drilling Program - Deep Sea Drilling Project, has proved to be a fundamental tool for researchers seeking to improve knowledge of Earth's history. The sedimentary successions recovered by the programme have yielded a wealth of information. These document, amongst other things, the evolution in Earth's history from "Greenhouse" to "Icehouse" during the last 65 million years. Basically, the continuous sediment deposits on the seafloor are nothing but innumerable pages of a giant history book of our planet.
Stepping back far into the past provides us with two important messages. Firstly, the current global warming is not unprecedented in Earth's history - that's good news. Secondly, it is unmatched in its magnitude probably for the last 90 million years - that's bad news. Except for one time, more than 55 million years ago.
Warm past
One of the largest and most rapid global warming events ever occurred at the boundary between the Palaeocene and Eocene epochs (roughly 55 million years ago). This event, known as the Palaeocene-Eocene Thermal Maximum (PETM), is a striking example of massive carbon release of still debatable origin. It was related to extreme global climatic warming of 5 to 10°C - even at the poles of our planet. The estimated magnitude of carbon release for this time period is of the order of 4500 Gt of carbon, a similar amount to that associated with greenhouse gas release if we combust all known fossil fuel reserves. What's more, the PETM coincides with the first appearance of modern mammals and primates, suggesting that the warming event also had profound effects on biological evolution. During the past few years, scientists have detected changes in the chemistry of today's oceans similar to those observed at the PETM in response to the actual rise in atmospheric carbon.
The PETM and additional, less severe, global warming events in Earth's history are essential case studies not only as a key for understanding global change mechanisms, but also to illustrate relations between the Earth's systems and patterns of change that may characterize the future. Our research at the MARUM Centre for Marine Environmental Sciences at Bremen University mainly focuses on the timing and detailed chronology of the PETM and other global warming events in the early Cenozoic era in order to help decipher the ultimate causes and consequences of these critical horizons in Earth history. Detailed knowledge of the timing of these events is crucial for understanding the underlying driving mechanisms and the observed climate changes.
To construct a precise timescale we use the well-established fact that climatic rhythms are preserved in the records of deep-sea sediments. On timescales of 20,000-100,000 years climatic variations are the result of variations in the Earth's position in relation to the Sun - the so called Milankovitch cycles. At MARUM we have reconstructed the rhythmic climatic variations for the entire Palaeocene and early Eocene - from ~65 to ~53 million years ago. The basis is an enormous geochemical data set that has been obtained from sediment cores collected by the Ocean Drilling Program in the Atlantic and Pacific Ocean during the last five years. Our work applies the X-ray fluorescence (XRF) Core Scanner, a non-destructive analysis system for relatively fast and closely-spaced analysis of major and minor elements - for example, calcium and iron - by scanning split sediment cores. Because of the technique's high resolution and exceptional data quality, we are able to set up a very accurate and detailed chronology for past global warming events.
Keeping time
Temporal relations are the key to causality arguments in Earth's history. In other words, if we want to evaluate Earth's climate behaviour in the past a highly precise timescale is crucial. Only a very precise timescale can provide us with both the exact dates and rates of climatic processes. The question of rates is especially important because it will give us insight into biological evolution, adaptive radiations and climate change, and a better understanding of how Earth as a complete system evolved.
In other words, a precise timescale is the backbone for the reconstruction of Earth's history. Right now our knowledge about past climate changes is still at an early stage - even though the big picture is known quite well - and the details of potential analogies to present-day conditions still need to be defined. One of the most important challenges is to develop a highly detailed and well-dated geological record back to at last 100 million years ago - a time span for which we have access to the high-quality geologic records needed for this kind of time-series analysis. This very accurate and definite geochronology will not only revolutionize our knowledge about past climate fluctuations, but will also provide new insights into future climate variability.
History is important. Learning from history provides us with information for future decision-making. And climate history is important too. Learning from climate history will allow us a glimpse into the climatic future of our planet. For a confident foretaste a precise geological timescale is essential. If we do not take the chance to learn our lesson from the past, we humans will have enormous difficulty in coping with the consequences of climate change.