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In a very broad way, climate specialists have laid down a target for climate mitigation: keep global warming below two degrees centigrade and we have a decent chance at avoiding catastrophic climate change.

This target has always troubled me.  Problems include speaking “centigrade” to metrically-challenged Americans; the esoteric nature of “2 degrees” toessentially every thinking person; and the serious uncertainty as to how much risk actually exists.

One particularly troubling element: How do we define “catastrophic”?

• How many species going extinct is acceptable “cost” before it is “catastrophic”?
• How much disrupted agriculture acceptable?
• How much sea rise?
• How much damage before we say it is “catastrophic”?

When it comes to ‘catastrophic’, it seems plausible that reasonable people — if presented with data about climate-change influenced events like Hurricane Sandy and 2012’s drought/heat wave in the United States (with a cost of $30 billion or so) and other climate implications — might see what is occurring as already “catastrophic” with that ‘catastrophic’ getting worse with each passing hour with continued/increasing greenhouse gas emissions.

For decades, scientists have suggested that limiting warming to 2C above pre industrial figures would (likely) be ‘acceptable’, enabling humanity to avoid the worst damages while providing breathing space to wean ourselves from fossil fuels.  While the global economy is not on track to meet this target (actually, on track to blow through it), that 2C target is one for which global politicians, global institutions, and most nations have made some form of commitment to supporting and achieving.

Now, however, as research knowledge advances and we gain a greater understanding of what is happening around the world, an increasing share of the relevant scientific community is rethinking that esoteric and confusing 2C target.  Sadly, for humanity’s future prospects, the scientists don’t seem to be concluding ‘hey, things aren’t so bad and we can acceptably take a lot more pollution and a lot more warming’.  Instead, a group of 18 scientists will publish a paper tomorrow in PLOS One in which they conclude that the 2C target

“would have consequences that can be described as disastrous”

With a 2°C increase,

sea level rise of several meters could be expected …

Increased climate extremes, already apparent at 0.8°C warming, would be more severe. Coral reefs and associated species, already stressed with current conditions, would be decimated by increased acidification, temperature and sea level rise.

The argument is pretty straightforward — and no surprise to readers of this blog:

• humanity has evolved in the Holocene climate …
• a 1C warming keeps us close to the Holocene range and thus not majorly disruptive …
• A 2Ce warming, however, would cause “major dislocations for civilization.”

The title of this paper is far from what one would expect to see in a scientific journal:

Assessing ‘dangerous climate change’: required reduction of carbon emissions to protect young people, future generations and nature

Not too surprising, the climate denial world will almost certainly scream that there aren’t the typical caveats, disclaimers, and otherwise that pepper virtually all scientific literature. These scientists have clearly come to the basic understanding that the uncertainties, which exist, are at the margins of the core issue:  if the worst outcomes (which are plausible and possible) turn out to be true, humanity will face catastrophic implications.

These scientists are — in a sense — changing the climate mitigation scenario in a serious way.  Rather than 1000 gigatons of total emissions (another truly difficult number for the average person/most people to process), we have ‘only’ 500 gigatons.  In essence, we have already burned through the limit and most be quite serious about carbon emissions reductions rather than opening more climate pollution sources with virtually ever passing day.

“Although there is merit in simply chronicling what is happening, there is still opportunity for humanity to exercise free will.”
James Hansen