A domino-like cascade of melting ice, warming seas, shifting currents and dying forests could tilt the Earth into a “hothouse” state beyond which human efforts to reduce emissions will be increasingly futile, a group of leading climate scientists has warned.
According to the researchers, keeping global warming to within 1.5-2 degrees Celsius may be more difficult than previously assessed.
The report comes amid a heatwave that has pushed temperatures above 40C (104 Fahrenheit) in Europe this summer, causing drought and wildfires, including blazes in Greece in July that killed 91 people.
Currently, global average temperatures are just over 1C above the pre-industrial period and rising at 0.17C each decade.
Around 200 countries agreed in 2015 to limit temperature rise to “well below” 2C (3.6F) above pre-industrial levels, a threshold believed to be a tipping point for the climate.
"Human emissions of greenhouse gas are not the sole determinant of temperature on Earth," said Will Steffen from the Australian National University.
"Our study suggests that human-induced global warming of two degrees Celsius may trigger other Earth system processes, often called "feedbacks," that can drive further warming - even if we stop emitting greenhouse gases," said Steffen, lead author of the study published an article with the distinctly unexciting title “Trajectories of the Earth System in the Anthropocene,” published in the Proceedings of the National Academy of Sciences. Normally, PNAS.
The scientists wrote: “Our analysis suggests that the Earth system may be approaching a planetary threshold that could lock in a continuing rapid pathway toward much hotter conditions – Hothouse Earth. This pathway would be propelled by strong, intrinsic, biogeophysical feedbacks difficult to influence by human actions, a pathway that could not be reversed, steered or substantially slowed.
“Where such a threshold might be is uncertain, but it could be only decades ahead at a temperature rise of (around) 2C above pre-industrial.”
Commenting on the findings, climate researcher Dr Phil Williamson, from the University of East Anglia, said: “In the context of the summer of 2018, this is definitely not a case of crying wolf, raising a false alarm. The wolves are now in sight.”
Chris Rapley, Professor of Climate Science at University College London said: “Previous research has shown that an increase in the mean global temperature of 11-12C would make more than half of the land area currently occupied by humans uninhabitable. So, a ‘runaway’ warming to a new and uncontrollable hot state would represent an existential threat to humanity and the majority of existing species.”
"What we do not know yet is whether the climate system can be safely 'parked' near 2°C above preindustrial levels, as the Paris Agreement envisages. Or if it will, once pushed so far, slip down the slope towards a hothouse planet. Research must assess this risk as soon as possible."
Maximizing the chances of avoiding a "Hothouse Earth" requires not only reduction of carbon dioxide and other greenhouse gas emissions but also enhancement and/or creation of new biological carbon stores, for example, through improved forest, agricultural and soil management; biodiversity conservation; and technologies that remove carbon dioxide from the atmosphere and store it underground, the paper says. Critically, the study emphasizes that these measures must be underpinned by fundamental societal changes that are required to maintain a "Stabilized Earth" where temperatures are ~2°C warmer that the pre-industrial.
Chris Jones of the UK Met Office in Exeter says that unpublished results suggest the “burn everything” scenario could see atmospheric carbon dioxide levels reach 2000 parts per million – the figure today is 388 ppm. That pulse of CO2 could lead to a global temperature rise of 10 °C.
Temperatures this high were last seen in the Eocene, 34 million years ago, says Paul Pearson of Cardiff University in the UK. Conditions were so different back then that the Canadian High Arctic was populated by plants that are now found in the south-eastern US (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2011.1704).
"Climate and other global changes show us that we humans are impacting the Earth system at the global level. This means that we as a global community can also manage our relationship with the system to influence future planetary conditions. This study identifies some of the levers that can be used to do so," concludes co-author, Katherine Richardson from the University of Copenhagen.