The world stands on the brink of failure when it comes to holding global warming to moderate levels, and nations will need to take "unprecedented" actions to cut their carbon emissions over the next decade, according to a landmark report by the globe's top scientific body studying climate change.
With global emissions showing few signs of slowing and the United States - the world's second-largest emitter of carbon dioxide - rolling back a suite of Obama-era climate measures, the prospects for meeting the most ambitious goals of the 2015 Paris agreement look increasingly slim. To avoid racing past warming of 1.5 degrees Celsius (2.7 degrees Fahrenheit) over preindustrial levels would require a "rapid and far reaching" transformation of human civilization at a magnitude that has simply never happened before, the group found.
"There is no documented historic precedent" for the sweeping change to energy, transportation and other systems required to reach 1.5 degrees Celsius, the United Nations Intergovernmental Panel on Climate Change, or IPCC, wrote in a report requested as part of the 2015 Paris climate agreement.
At the same time, however, the report is being received with hope in some quarters because it affirms that 1.5 C is still possible - if emissions stopped today, for instance, the planet would not reach that temperature. It is also likely to galvanize even stronger climate action by focusing on 1.5 C, rather than 2 degrees, as a target that the world cannot afford to miss.
Nonetheless, the transformation described in the document raises inevitable questions about its feasibility.
Most strikingly, the document says the world's annual carbon dioxide emissions, which currently amount to more than 40 billion tons per year, would have to be on an extremely steep downward path by 2030 to either hold the globe entirely below 1.5 degrees Celsius, or allow only a brief "overshoot" in temperatures.
Overall reductions in emissions in the next decade would probably need to be more than 1 billion tons per year, larger than the current emissions of all but a few of the very largest emitting countries. By 2050, the report calls for a total or near-total phaseout of the burning of coal.