The world is warmer, by around 2-3 degrees C. The tropics are wetter, sea levels are higher, there is less ice and CO2 levels are rising above present levels. A vision of our future climate, perhaps, but in fact this is Earth just over 3.2 million years ago, in a period of our planet’s history known as the Pliocene. And it’s a window into the past that a group of UK scientists hope could be the best chance so far of revealing the consequences of human-induced climate change.
The team believe they have identified a time in our Earth’s climate history, where everything like the planet’s orbit and the position of the continents closely match conditions of today, but with slightly higher levels of CO2 – those we are predicted to see by the end of the century. Not only that, but for the first time scientists have managed to narrow in on a geologically small timeframe of just 10,000 years. “It’s a unique window into Earth’s past, with the greatest relevance to the future,” says Dr Aisling Dolan at the University of Leeds, who was involved in the study.
The hope is that the geological information within this 10,000-year-timeframe could help scientists improve predictive models for how the Earth will react to an increase in carbon dioxide. Just like weather forecasters use models to make predictions about the weather, climate scientists use models to help them understand more about how our climate is changing, and how the planet is going to react to those changes.
Understanding how the planet reacted to higher CO2 levels in the past is an important step. Throughout its history, Earth’s climate and the amount of carbon dioxide in the atmosphere has changed naturally. “Changes in the levels of CO2 are caused by several unrelated processes,” says Dr James Riding at the British Geological Survey. “Volcanoes, plants and animals, destabilisation of the oceans, methane in the sea floor, these are all natural mechanisms for altering CO2.” However the increase since the industrial revolution has been larger than it should by these natural causes alone. “We know we cannot explain this increase,” says Professor Bette Otto-Bliesner, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado. “There has to be a human influence.”
Historically, the Pliocene has been examined because it is geologically accessible, and also closely matches our current world in terms of plant species and geography. Previous efforts to map the Earth’s climate during this time involved working out the mean climate state over a period of 300,000 years. Geologically, it’s very difficult to get narrow intervals, you need to match up samples from different sites around the world and this involves painstaking research and a large amount of samples.
But the problem with looking across a 300,000-year period is that Earth’s climate changes naturally over this time. As well as naturally varying CO2 levels, our orbit around the sun alters roughly every 100,000 years (because of the gravitational affect of the other planets in our solar system), and coupled with the fact that the planet wobbles on its axis (an affect known as precession), this varies the amount of heat we get from the Sun – and in turn the climate of Earth.
So, the narrower the timeframe in the past, the better the information to put into climate models. “What we had was a shopping list of criteria that needed to be met,” says Professor Alan Haywood, a palaeoclimate modeller of the University of Leeds, who led the research. “By looking at sediment records we are able to understand how the Earth’s climate has changed,” adds Dr Erin McClymont, a physical geographer at the University of Durham, also part of the research team. “To narrow down on the interval, we look at oxygen isotopes in the chemistry of shells.” This is because carbonate built by organisms use the sea to make shells and the chemistry of the sea changes in response to changes in our environment. All this information then had to be combined with models of the Earth’s orbit and things such as stomata in leaves (which react to changes in CO2 levels), to create a more complete global inventory. “It all became a bit of a detective story,” says Haywood.