Paris climate deal: How a 1.5 degree target overcame the odds at COP21

Updated

It snuck into the draft climate change agreement earlier this year, and much to everyone's surprise, a 1.5 degrees Celsius target survived the gauntlet of the United Nations climate change negotiations.

Negotiators are catching up on sleep after gruelling talks resulted in 195 nations adopting a an new international agreement to slow global warming.

The target of 1.5C dodged multiple rounds of chops, changes and tweaks to the deal text.

The agreement now has the aim of "holding the increase in the global average temperature to well below 2C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5C above pre-industrial levels, recognising that this would significantly reduce the risks and impacts of climate change".

The newfound enthusiasm for 1.5C reflects a growing realisation that 2C is a figure with very little basis in fact.

Several scientists have told the ABC the 2C degree target is "arbitrary" or "plucked out of the air". None would go on the record admitting that, however.

We can quantify the risks of 1.5 degrees or two or three degrees, but we can never use science alone to justify any target. It's always a question of what values we have. We need to decide as a society how much risk we're willing to take. Climate professor Reto Knutti

But a new paper out this week in Nature Geoscience is not so shy.

"No scientific assessment has clearly justified or defended the two degree target as a safe level of warming," write the authors.

Lead author Reto Knutti is a Swiss professor of climate science.

"We can quantify the risks of 1.5 degrees or two or three degrees but we can never use science alone to justify any target. It's always a question of what values we have," he said.

"We need to decide as a society how much risk we're willing to take.

"Ultimately the two degree target is a political consensus that takes into account what policy makers at that time considered to be both realistically achieve and tolerable. It is high time for a scientific assessment.

"It's a compromise of the best we can hope for and the worse we can tolerate."

The accepted history of the 2C target is that William Nordhaus, a Yale economist, came up with the number in one of the earliest papers on climate change an its economic implications.

In 1975 he wrote: "If there were global temperatures more than 2C or 3C above the current average temperature, this would take the climate outside of the range of observations which have been made over the last several hundred thousand years."

He graphed a rough sketch of historical temperatures and added a label at 2C: "Estimated maximum experienced over last 100,000 years."

Climate science has honed those estimates in the past 40 years. We now know that temperatures have dropped dramatically over 100,000 years - as low as -20 - but in that time they have never been as high as they are now. And they certainly have not surpassed two degrees.

But before scientists had ironed out such details, the 2C target was already making it into official documents. Europe was largely responsible for its ultimate inclusion in the United Nations agreement that came out of Copenhagen in 2009.

Enshrined as an official target, it became a signpost beyond which "dangerous climate change" apparently lay.

Degrees of difference

There is only half a degree between the two targets. Given that humans live in places that experience temperatures as low as -60C and as high as 50C, it seems ludicrous to spend so much energy haggling over half a degree.

However Will Steffan, a member of the Australian Climate Council, said "those temperatures that we scientists use are not to be confused with daily temperature ranges that people experience".

Even though they are both measuring temperatures, he said they are better thought of as a measurement for what the whole climate system is doing.

"A five degree world doesn't just mean it's going to be five degrees warmer in Canberra or Sydney or Darwin or wherever," he said.

"It means is a world that will likely have little or no polar ice. Large areas will be uninhabitable for humans.

"The last ice-age was only five degrees cooler in global average temperatures and yet a lot of north American and northern Europe were under a couple of kilometres of ice."

As scientists have improved our understanding of climate change, there's been a growing realisation that two degrees is likely to be quite dangerous after all.

"At two degrees we'd have several metres of sea level rise," Professor Steffan said.

"Several small island states would not exist."

He said extreme weather such as heatwaves, bushfires, heavy rainfall, and drought would be more common at 2C.

"Coral reefs would almost certainly not be around at 2 degrees. They'd be struggling at 1.5," he said.

Most worrying for Professor Steffan is there is a chance the world would pass certain tipping points that would spell planetary disaster, such as masses of greenhouse gases escaping from melting Siberian tundra pushing climate change beyond anything humans can control.

The rise of 1.5

Howard Bamsey, a veteran negotiator for Australia at the UN climate conferences, said a 1.5C target has always been on the table. When 2C became official in 2009, there was provision for revisiting 1.5C in 2015.

In 2011, the executive secretary of the UN climate agency Christina Figueres said: "Two degrees is not enough – we should be thinking of 1.5 degrees . If we are not headed to 1.5 degrees we are in big, big trouble."

The imprimatur from such a senior climate figure gave the lower target the legs it needed to carry on through several years of lack-lustre negotiations.

Meanwhile continuing science repeatedly found that at 2C, climate change would be more dangerous than might be acceptable.

Mr Bamsey said 1.5C's rise to prominence this year is a result of a long-term push from the nations most vulnerable to climate change: the island nations.

At the beginning at this year's negotiations US president Barak Obama posed for a photo with leaders of some of the least powerful and most vulnerable island nations.

The photo was a strong symbol that the US was willing to side with the tiny nations and their long-held dream of a 1.5 target.

Australia signed on to the 1.5 target when it cut a deal with St Lucia, a Caribbean island nation, to back the target in exchange for being allowed to carry over its savings from the Kyoto Protocol.

With 1.5 now inked as the stretch target, the island nations can walk away from Paris with a sense of victory.

"It's been one that island states have been wanting to see reflected in the outcome of the UN process for some years," Mr Bamsey said.

"I think it's to some extent a product of the persistence of those most vulnerable countries in flagging their concerns that a world that warmed by two degrees would still leave them in a critical situation that would endanger their existence."

Topics: environment, climate-change, 2009-united-nations-climate-change-conference

First posted