A new landmark study, the culmination of four years' work by an international team of scientists, has narrowed the range on how hot the planet is likely to become from the effects of human-driven carbon emissions.

The report, published on Wednesday, comes close to quashing the best-case scenarios on global temperatures rises, and means that urgent action on reducing emissions remains vital. But the findings also indicate that the most dire predictions may be less likely.

The team of 25 scientists looked at “equilibrium climate sensitivity” (ECS) – the global temperature rise that would occur following a doubling of CO2 concentration in the atmosphere above pre-industrial levels – a key measurement in climate modelling over the past 40 years.

Since 1979, scientists have stuck to a wide range of possible temperature increases – 1.5C to 4.5C – the basis of the latest UN Intergovernmental Panel on Climate Change report.

By some estimates, at the aspirational 1.5C of warming, severe consequences could be prevented, such as the Arctic region being ice-free in summer and the loss of tropical coral reefs. It could also limit sea-level rise and save many coastal regions and islands. At 4.5C, the predictions are for catastrophic climate breakdown.

The new peer-reviewed study has narrowed the window to a likely ECS range of between 2.6°C and 4.1°C. The best estimate of climate sensitivity is just above 3C.

The report, published in the journal Reviews of Geophysics, also found that there was less than a 5% chance that ECS is below 2C and a 6-18% chance it is above 4.5C.

"It means that the efforts to say that climate sensitivity is so small we don't need to worry, that doesn't pass muster anymore," Dr Gavin Schmidt, a co-author of the report and Director of the NASA Goddard Institute for Space Studies, told The Independent. "But also that the people who say climate sensitivity is 5, 6, 7 degrees, that's not supported either.”

Dr Zeke Hausfather, another co-author, tweeted: "Climate change was always going to be a role of the dice given the complexity of the Earth's climate. What we've effectively done in this new study is found that rolling either a 1 or a 6 is a lot less likely than we previously thought."

The Earth’s temperature is currently around 1.2C (2.2F) above the pre-industrial baseline. Atmospheric concentrations of CO2 are currently around 416 parts per million (ppm), compared to pre-industrial levels of 280ppm.

If emissions trends continue, without significant steps to reduce them, emissions are forecast to hit 560ppm in about four decades.

The team used more separate lines of evidence than any previous study to reach their conclusions, including temperature records since the industrial revolution, paleoclimate records to estimate prehistoric temperatures, satellite data, and the latest climate system models.

They also examined differing climate feedback processes which play a crucial role in how the planet responds to increased levels of CO2. A positive feedback loop accelerates a temperature rise, whereas a negative feedback decelerates it. The varying strands corroborated the same conclusion.

Professor Steven Sherwood, the lead author and University of New South Wales chief investigator with the ARC Centre of Excellence for Climate Extremes, compared the investigative work to trying to solve a crime.

“An important part of the process was to ensure that the lines of evidence were more or less independent,” he said. “You can think of it as the mathematical version of trying to determine if a rumour you hear separately from two people could have sprung from the same source; or if one of two eyewitnesses to a crime has been influenced by hearing the story of the other one.”

Crucially, where the temperature increases end up, hinges on the steps that are taken to dramatically reduce emissions in the coming years.

The report's results are likely to be considered at the 2021 United Nations Climate Change Conference which was postponed due to the coronavirus pandemic, and serve as a stark reminder to national and international policymakers that the window for keeping global heating below the 2C set out in the 2015 Paris Agreement is rapidly closing.

“These results are a testament to the importance of cross-disciplinary research along with slow, careful science and perfectly highlight how international co-operation can unpick our most vexing problems,” said co-author Professor Eelco Rohling from the Australian National University.

“If international policymakers can find the same focus and spirit of co-operation as these researchers then it will give us hope that we can forestall the worst of global warming.”

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