Scientists have backed away from the worst-case climate scenario — and the best one too

“A team of 25 scientists from around the world recently took a stab at answering the question of how sensitive the Earth’s climate is to carbon dioxide and came up with range of possibilities. Their results, published July 22 in Reviews of Geophysics, showed that the planet would most likely warm on average between 4.7 degrees Fahrenheit and 7 degrees Fahrenheit (2.6 degrees Celsius and 3.9 degrees Celsius) if atmospheric carbon dioxide were to double.

This is still a wide span, but it’s much smaller than prior estimated range of 2.7 and 9.1 degrees Fahrenheit (1.5 and 4.5 degrees Celsius) that had been the reigning benchmark for decades.

The new, narrower estimate for climate sensitivity has huge implications, not just for climate science, but for how humanity prepares for a warming world. It shows that the worst-case-scenario is not as dire as previously thought, but also that the best possibilities are still quite grim. In particular, it means that it will be almost impossible to hit the main target of the Paris climate agreement, limiting warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit) this century, by chance; it will require aggressive action to reduce emissions with even less margin for delay.”

“The resulting estimate of climate sensitivity — 4.7 degrees to 7 degrees Fahrenheit (2.6 degrees Celsius and 3.9 degrees Celsius) — may seem small, but it represents a drastic shift from the world as we know it today. In an editorial in the Hill, Hausfather pointed out that during the last ice age 20,000 years ago, the planet was on average 9 degrees Fahrenheit (5 degrees Celsius) cooler than today. That led to so much ice across the planet that global sea levels were 300 feet lower than they are today.”

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