The following article includes the statement: "The Paris text is vague about the temperature ceilings and does not say whether 1.5°C or 2°C refers to temperatures in one year, over a decade or longer." Perhaps to sustain the integrity of our leaders we could all choose to voluntarily define these limits over the coming 10,000 years (otherwise we will collectively need to admit that we were all just kidding ourselves, and overshoot will become the new normal):
http://www.climatecentral.org/news/as-earth-swelters-global-warming-target-in-danger-20597Extract: "In December, almost 200 nations agreed a radical shift away from fossil fuels with a goal of limiting a rise in average global temperatures to well below 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times while "pursuing efforts" for 1.5°C (2.7°F).
But 2016 is on track to be the hottest year on record, also buoyed by a natural El Niño event warming the Pacific, according to the U.N.'s World Meteorological Organization. The first six months were a sweltering 1.3°C above pre-industrial times.
"It opens a Pandora's box," said Oliver Geden, of the German Institute for International and Security Affairs. "The future debate about temperature targets will be about overshoot."
Many climate scientists say the Paris targets are likely to be breached in the coming decades, shifting debate onto whether it will be possible to turn down the global thermostat.
Climate scientists will meet in Geneva from Aug 15-18 to plan a U.N. report about the 1.5°C goal, requested by world leaders in the Paris Agreement for publication in 2018. Overshoot is among the issues in preparatory documents.
Developing nations see overshoot as a betrayal of commitments by the rich and a recipe to worsen heatwaves such as in the Middle East this year or a thaw of Greenland's ice sheet that could swamp island states by raising global sea levels.
"There is a risk that 'overshoot' is a slippery slope towards lower ambition," said Emmanuel de Guzman, secretary of the Climate Commission of the Philippines, which chairs a group of 43 emerging nations in the Climate Vulnerable Forum (CVF).
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The 1.5°C threshold could be in jeopardy within five years on current trends of world greenhouse gas emissions, led by China and the United States, and 2°C within about 25 years, according to U.N. calculations of the amount of carbon that can be emitted into the atmosphere."