Today our atmosphere contains 400 parts per million (ppm) of carbon dioxide. If we carry on as we are, burning the last of our fossil fuel resources, we could reach 1000 ppm by the end of this century.
Before the industrial revolution, it contained only 280 ppm. During the last ice age, it contained only about 200 ppm.
To find a natural Earth with 400 ppm, we must travel back 3 million years, and to find a natural Earth with 1000 ppm, we must travel back nearly 50 million years – to the Eocene, a world of giant anaconda and miniature horses, palm trees and baobab thriving on Antarctica; a world with almost no ice on land and sea levels about 100 metres higher.
But what was that world really like and what does it tell us about the future. It was warmer, but how warm? And where? How was the hydrological cycle different – was it wetter or drier? Was rain less frequent but more intense? And what about the complex interlinked biological, chemical and geological cycles that regulate our environment? Did wetlands spew out more methane? Did more nutrients wash to the ocean causing algal blooms and the harmful anoxia that follows?
This has been the focus of TGRES. We are developing new tools to investigate these issues by creating a global collection of modern wetland sediments; analyses of these will allow us to better understand how temperature, rainfall and methane cycling can be recorded by molecular fossils of plants, bacteria and archaea. These settings are not the only ones of interest but their ancient equivalents are a vital new archive of past climate information because they will greatly expand our understanding of the terrestrial realm.
So far, we have shown that land temperatures during the Eocene (and other greenhouse periods) were markedly higher than they are today, with tropical conditions extending to high latitudes. It also seems that these wetlands were characterized by markedly more intense methane cycling than that of today: as the peat degrades, it released a little bit less carbon dioxide and much more methane, a more potent greenhouse gas. Further work will explore how rainfall changed and how that impacted the chemical breakdown of rocks to form soils. Collectively, we are generating a new understanding of how the Earth System operates when carbon dioxide concentrations are much higher – and by extension gaining insight into the world we are currently creating.