The world could be approaching a “planetary threshold” which, if passed, will turn it into a hothouse, with temperatures stabilising at four to five degrees Celsius higher than preindustrial levels, and oceans rising as much as 60 metres above present levels.
That’s the sobering conclusion from a team of scientists led by Will Steffen of the Australian National University’s Fenner School of Environment and Society and published in the journal Proceedings of the National Academy of Sciences.
Steffen and 15 colleagues use extensive modelling data to explore the possibility that meeting Paris Agreement targets aimed at limiting global warming to within 1.5 to 2°C of pre-industrial levels will not be sufficient to avoid triggering feedback loops and tipping points that could induce a powerful set of warming cycles.
“Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years, and to sea levels significantly higher than at any time in the Holocene,” the researchers warn.{%recommended 6644%}
To reach their conclusions, the researchers develop, as much as is possible, a “deep integration” of biogeophysical data and information describing the development and functioning of human societies. Key to the approach is the recognition that humanity now exerts an influence – albeit unequally – that rivals that of geophysical forces on what they term the Earth System.
So great is the impact of human activity, they suggest, that the timescale remaining in which to head off environmental catastrophe is perilously small.
“Social and technological trends and decisions occurring over the next decade or two could significantly influence the trajectory of the Earth System for tens to hundreds of thousands of years,” they write, “and potentially lead to conditions that resemble planetary states that were last seen several millions of years ago, conditions that would be inhospitable to current human societies and to many other contemporary species.”
The worst-case scenario, in which average temperatures rise 5°C – far higher than the current 1°C above baseline – and ocean levels rise by many metres, will be continuously fuelled and replenished by runaway feedback cycles.
The result, they say, “is likely to be uncontrollable and dangerous to many, particularly if we transition into it in only a century or two, and it poses severe risks for health, economies, political stability (especially for the most climate vulnerable), and ultimately, the habitability of the planet for humans.”
The aim of lowering emissions to put the brake on global warming – the strategy embodied in the Paris Agreement – is worthy, but not necessarily effective. Rather, the scientists conclude, it is based on modelling and predictions that are simplistic.
“Human emissions of greenhouse gas are not the sole cause of temperature changes on Earth,” says Steffen.
“Our study indicates that human-caused global warming of two degrees Celsius may trigger other Earth System processes, often called feedbacks, that can trigger further warming – even if we stop emitting greenhouse gases.”
Steffen and colleagues conclude that further in-depth modelling is required to better understand the risks of the world passing the crucial threshold. Of equal importance, however, is the pressing need to work on urgent strategies to stabilise the climate and limit the long-term damage.
Given the vested interests at work in maintaining adherence to ideas of growth and resource usage, any such strategy is likely to face considerable opposition.
“We suggest that a deep transformation based on a fundamental reorientation of human values, equity, behaviour, institutions, economies, and technologies is required,” they write.
