Rethinking how we describe "business as usual" on climate change

A new analysis could take a step toward resolving a fiery debate: how to think about — and describe — likely levels of future emissions and warming in light of current trends and planned policies.

Driving the news: Two scientists, in a lengthy post via the Breakthrough Institute, conclude Earth is on track to warm by roughly 3°C above pre-industrial levels by 2100.

• "This is a far cry from the 1.5°C and 2°C targets enshrined in the Paris agreements, but is also well short of the 4°C to 5°C warming in many 'business as usual' baseline scenarios that continue to be widely used," write Zeke Hausfather and Justin Ritchie.

Why it matters: 3°C means a world far hotter than today.

• But it's also far less than would likely be enabled by models using high and even unchecked emissions growth — scenarios that they argue, based on extending the International Energy Agency's 2040 analyses through 2100, are no longer in the cards.
• Hausfather and Ritchie see the most likely outcome from current policies is 2.9°C-3.4°C, but that falls to 2.7°C-3°C if nations meet their existing pledges under the Paris deal.

The big picture: 3°C would mean a lot of damaging outcomes, and even warming to date is causing major harms. A big UN-led report last year explored the consequences of breaching 1.5°C (which is quite likely).

But, but, but: They argue that some commonly cited future "pathways" used in climate literature that bring much higher warming levels are looking quite unlikely for several reasons, including efforts over the past decade to move away from coal.

One level deeper: "Even a current policies scenario where emissions continued to steadily grow rather than leveling off after 2040 would still end up well-below the commonly used RCP8.5 (SSP5-8.5) scenario, which represents the highest end of the range of no-policy baseline scenarios examined in the literature."

The intrigue: Needless to say there's plenty of variables because gaming out the future of technology, policy and economic trends is really dicey. There's also uncertainty about the sensitivity of the climate to rising emissions concentrations.

Go deeper:

• The coming Wall Street battles over climate change
• Fiddling while the planet warms