Veerabhadran Ramanathan of the Scripps Institute of Oceanography, US, and Yangyang Xu, now at Texas A&M University, focused on the top 1-in-20 low-probability, high impact (LPHI) model outcomes often ignored by other studies. This required a new “unknown” category for climate risk above 5°C, including existential threats to humanity and widespread species extinctions associated with temperatures not seen on Earth for at least 20 million years.

Avoiding “dangerous” warming, greater than 1.5°C, is not possible without aggressive fossil fuel reductions and an immediate curtailing of super-pollutants, the authors stress. These include methane, hydrofluorocarbons (HFCs), tropospheric ozone and black carbon, and are already subject to independent regulation due to their impacts on human health.

If global emissions remain unchecked, there is a significant chance warming could reach catastrophic levels by mid-century. Under these conditions three-quarters of the worlds’ population could be exposed to deadly heat stress, and numerous tipping points in the climate system could be reached, bringing irreversible changes.

To stay below 1.5°C, the Nature Geoscience study, led by Richard Millar, estimates a limit on cumulative global carbon dioxide emissions after 2015 of 200 Gigatonnes of carbon. This represents a marked increase from the IPCC Fifth Assessment Report, which suggested a ceiling of 615 GtC; the total burned already amounts to 545 GtC.

Xu and Ramanathan advised a 2060–70 target for global carbon neutrality, using both energy intensity reduction and decarbonisation. This alone would yield a 50% chance of staying below 2°C of warming, avoiding likely catastrophic climate change.

Although the challenge is great, the authors of both studies emphasize that many of the technologies needed already exist. What’s more, a fall in emissions from both the US and China, as well as an increase in renewable energy production, contributed to a near-zero growth rate worldwide in 2015.

In PNAS, Xu and Ramanathan consider the implementation of carbon extraction and sequestration – essential to adequately mitigate LPHI warming. Many technologies are on the horizon, such as carbon capture from bioenergy, but none have been effectively scaled, and the authors warn that “major breakthroughs are needed”.

A complementary report delivered to the UN as part of Climate Week NYC 2017 expanded this “three-lever” mitigation strategy, along with a more detailed analysis of the implications for society. It was produced as part of a wider collaboration, the Committee to Prevent Extreme Climate Change, which Ramanathan chairs alongside Nobel Prize winner Mario Molina and Durwood Zaelke, who leads the Institute for Governance and Sustainable Development.

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