You can’t argue with a rock. So ideally we would now be on a field trip. Instead, I offer you a short film made in the field by the Science Museum to accompany a lump of Hertfordshire Puddingstone that is featured in the Atmosphere Gallery, first opened in 2010.
Thanks to geology, the scientific case for human-induced climate change has recently become significantly more plausible. New observational science based on cores taken from deep beneath the floor of the oceans offers crucial support and control for the computer-based forecasts of those creating models of future climate change. Thanks to the work of the late Sir Nick Shackleton and his colleagues, the record of Earth’s past climates recorded in rocks can now be measured with far greater definition than before: divided into thousands rather than millions of years. We can date the Hertfordshire Puddingstone itself only to within a million years or so, but elsewhere changes in climate that took place long ago were recorded in rocks which we can now examine on a near-human timescale.
One of these ancient past changes in climate is a particularly important guide to present-day concerns: the dramatic warming event that took place 55 million years ago (55 Ma) to which I refer in the Hertfordshire Puddingstone video. Comparison of the volume of carbon released to the atmosphere by natural means at 55 Ma (maybe two thousand billion tonnes) and the volume we are now releasing ourselves (some 500 billion tonnes so far, and counting) strongly suggests that we are indeed facing a major global challenge. We are at least a quarter of the way to repeating through our own agency that 55 million-year-old global warming event, which disrupted Earth for over 100,000 years. That event took place long before Homo sapiens was around to light so much as a camp fire. The nature of the trigger of the 55 Ma event continues to be a matter of academic debate: the broad effects of rapid release of such a volume of carbon are clearly seen in the geological record and are established beyond reasonable doubt. 55 million years ago it got hot, global sea-level rose several metres simply by thermal expansion (there were no polar icecaps to melt at that time) and the oceans became notably more acidic. These changes had a predictably unfavourable effect on existing plants and animals.
I discuss that 55 Ma and other warming events in the geological record with Andrew Revkin of the New York Times in a video recorded in 2011:
The numbers involved in discussions of climate-change include billions of tonnes (for example, reserves of fossil fuels) and parts per billion by volume (for example, concentration of methane in Earth’s atmosphere). This range can create problems in debate unless we consider carefully the rate at which things happen. Let us consider the fate of fossil fuels. If we didn’t take coal, gas and oil out of the ground, they wouldn’t stay there forever. Oil escapes to the surface to form Pitch Lake in Trinidad. Gas escapes to the surface to feed the fire-worshippers’ flames burning on the northern flank of Kirmaky Valley in Azerbaijan. Like the burning fires of Babylon, these phenomena tell us that escape of oil and gas is a natural process, proceeding at a rate to which Earth is well used.
We are now interfering markedly with that natural carbon cycle. If we were to burn all the remaining reserves of fossil fuel, be that coal, oil, gas, shale gas, oil shale, or tar sands, we would release several 1000 billion tonnes of carbon to the atmosphere. If we were to release that carbon slowly enough, Earth would barely react. But we are not releasing carbon slowly, just the opposite. We are releasing carbon at a rate exceeding by an order of magnitude anything we can see in the geological record, even in previous major global warming events.
Since the turn of the century, there has been a run of geological publications on the record of past rapid carbon injections into the atmosphere. This work has been summarised in a report issued by the Geological Society in 2010, which presents evidence from the geological record to make an implacable case for concern about human-induced climate change.
The rocks that have been studied with an eye on past climate change include cores of layers of ancient sedimentary rock, taken from beneath the floor of the world’s oceans. These cores include sediments deposited at that crucial time 55 million years ago. We can compare results obtained from those deep-sea sediments with studies of rocks found on land, such as the 55 Ma rocks now exposed at the surface of the Arctic island of Spitsbergen. From these various rocks we can estimate the rate at which the carbon trigger was pulled 55 million years ago, and the volume of carbon thereby released.
The relatively good news is that we are only about a quarter of the way towards repeating the volume of carbon released 55 million years ago. The bad news comes from recent work in Spitsbergen: we are dumping that carbon at a rate that appears to be unprecedented in the geological record.
The results from Spitsbergen suggest that we are depressing the carbon trigger about ten times faster than it was pulled at 55 Ma. This is all a very long way from the rates of routine release of carbon from below Earth’s surface that we can measure in the smelly tar at Pitch Lake and in the perpetual flames of Azerbaijan. It would be wise to pause and consider.