The difference between 1.5 and 2 degrees

At the COP21 climate talks in Paris in December, the international community agreed to limit global warming to 2 degrees. They then added an extra target of “pursuing efforts to limit the temperature increase to 1.5C.”

The secondary target is in response to science suggesting that 2 degrees may be too much. It may put us beyond key tipping points. Small island states fear that 2 degrees could still wipe them off the map, and they were among the biggest advocates for the lower target.

But what difference is that extra half a degree of warming actually going to make?  We’re not entirely sure – these are political numbers, not necessarily scientific ones. 2 degrees never was a particularly sound place to draw a line, and 1.5 is similarly the product of negotiators rather than scientists. But last week the journal Earth System Dynamics published a report on the difference between the two targets, and we can begin to see why we need the lower one.

In a world 1.5 degrees warmer, fresh water availability in the Mediterranean would likely be around 7% lower, against a 16% fall with 2 degrees. Global wheat production could fall 9%, or 16% with the higher target. In the non-linear world of climate change, the difference of half a degree makes things twice as bad.

Sea level rise at 1.5 degrees would be 40 cm by 2100, and 50 cm with 2 degrees. I couldn’t tell you which ones and to what degree, but that’s more coastal regions and global cities at risk of flooding.

Perhaps the most striking finding is that the world’s coral reefs would be devastated at 1.5 degrees, and could be lost entirely at 2 degrees. Climate change attacks coral reefs on two fronts. Warmer seas cause coral bleaching, while ocean acidification (also caused by rising CO2 levels) weakens it further. It’s an assault that they may not survive, and the consequences reach considerably further than scuba divers and the tourist industry.

One in four of all marine species depend on reefs, so we are looking at a huge loss in biodiversity. Reefs are nurseries for young fish, so global fisheries will be affected. An estimated 500 million people rely on reefs to earn a living or as a source of food, so food security is implicated too. Island nations like Indonesia or the Philippines are especially vulnerable. It’s almost impossible to quantify the economic value of reefs, but one frequently cited attempt put it at $375 billion a year.

So that extra half a degree could mean a doubling of impact, and a collapse in marine biodiversity. As the paper says, the difference between 1.5 and 2 degrees is the transition from a more variable version of what we have already to a ‘new climate regime’.

Just to remind you, we’ve had 1 degree of warming already. And at current rates of progress, we’re still on track for 4 degrees.


  1. The steep path to 1.5 degrees | Make Wealth History - September 22, 2016

    […] already had one degree of warming, so 1.5 is a much tougher target. As I’ve mentioned before, the difference between the two is much bigger than the fractional increase in heat. Climate change does not progress in a nicely […]

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