Making an unprecedented call to action, the United Nations' climate panel said that to avoid catastrophe, all countries must change the way their people eat, commute, farm and build - and the changes must kick in right away.
The Intergovernmental Panel on Climate Change (IPCC) looked at the steps needed to limit global warming to 1.5 deg C.
It concluded in a major report released yesterday that deep emission cuts are needed before 2030 to achieve this. Any further delay will condemn economies and ecosystems to deadlier weather extremes, habitat loss, falling crop yields and ever higher sea levels.
"The message is: Over to governments. We have told you the scientific facts, the evidence. It is up to them to decide what to do with it," Dr Jim Skea, one of the report's co-chairs, told reporters in a broadcast from Incheon, near Seoul, where the report was released.
Fellow author Valerie Masson-Delmotte said: "The report shows that we are at a crossroads. And what is going to happen from now until 2030 is critical, especially for CO2 (carbon dioxide) emissions."
Economies would have to shift away rapidly from burning coal, oil and gas and invest heavily in renewable energy. People will have to change their energy use and even the foods they eat.
The cost? Investments in clean energy totalling US$2.4 trillion (S$3.3 trillion) are needed every year from 2016 to 2035, and coal-fired power cut to almost nothing by 2050.
IPCC scientists and officials from 195 member nations met last week in Incheon to haggle over the final wording of a "summary for policymakers" that clearly spells out the immense climate challenge ahead.
Scientists have long warned that mankind should limit warming to 2 deg C to avoid catastrophic climate change. However, the world has already warmed about 1 deg C since pre-industrial times. Even at this level, scientists say climate change is fuelling stronger storms and more extreme floods, while hotter oceans are cooking coral reefs.
"The IPCC report is a wake-up call for slumbering world leaders. We must reduce emissions as quickly as possible to keep 1.5 deg C of warming within reach," said Mr Andrew Steer, president and chief executive of the Washington-based World Resources Institute.
The IPCC study, which took nearly three years to complete and involved 91 authors from 40 countries, is the first to look in detail at the 1.5 deg C limit, which is one of the goals in the 2015 UN Paris Climate Agreement.
The study was urgent because CO2, the main greenhouse gas, reached record levels in the atmosphere last year, and current pledges to cut emissions under the Paris Agreement would lead to warming of about 3 deg C.
The authors said global warming is likely to reach 1.5 deg C as early as 2030 if it continues to increase at the current rate.
In the IPCC's most ambitious pathway, global net human-caused emissions of CO2 would need to fall by about 45 per cent from 2010 levels by 2030, reaching "net zero" around 2050.
While the difference between 1.5 deg C and 2 deg C might seem small, some types of climate-change impact will be less severe by limiting global warming. By 2100, global sea-level rise would be 10cm lower at 1.5 deg C compared with 2 deg C, giving vulnerable low-lying island nations more time to adapt.
Coral reefs would decline by 70 per cent to 90 per cent at 1.5 deg C, whereas virtually all would be lost at 2 deg C.
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