The rhetoric from the Paris climate deal is reassuring, but the atmosphere can't read press releases. The level of carbon in the atmosphere does not respond to declarations of victory. The chief driver of dangerous climate change is the concentration of carbon dioxide in the atmosphere. This is what creates the "greenhouse" effect of global warming.
So first, a quick reality check of where we're up to.
When Dr David Keeling took the first reading of carbon dioxide concentration in the earth's atmosphere at Mauna Loa in Hawaii in 1958, it was 315 parts per million.
A level of up to 350 is safe, we're told by experts. The danger zone begins somewhere in the 400 to 450 range, they tell us.
Beyond 450, the chance of holding global warming to a maximum 2 degrees, the "tipping point" of irreversible change, is only around 50:50, according to the Climate Change Authority. It recommends no more than 415.
By the time the Kyoto Protocol was hailed as humanity's successful response to the challenge of climate change 39 years later, the carbon concentration had reached 363. By the time of the 2009 Copenhagen anticlimax, it was 387.
The level of carbon first moved into the danger zone of 400 ppm on May 9, 2013, according to the US government's National Oceanic and Atmospheric Administration, which operates the Mauna Loa measuring station.
This was "more than 100 ppm higher than at any time in the last one million years", Charles Miller of NASA pointed out, "and maybe higher than any time in the last 25 million years."
A couple of months ago the son of the original Dr Keeling, Ralph Keeling, who has taken up his father's line of work monitoring carbon levels for the Scripps Institution of Oceanography, posed: "Will daily values at Mauna Loa ever fall below 400 ppm again in our lifetimes? I'm prepared to project that they won't."
As delegates in Paris last Friday approached the climax of their negotiations, the Mauna Loa station recorded a carbon concentration of 402 ppm, the latest available at the time of writing.
At the current rate of emissions, the world will reach 450 ppm in 25 years, or 2040, the point where we take our chances on catastrophic climate change, according to the International Renewable Energy Agency.
So with this background, let's look at what the Paris agreement accomplished. It declares an aim to keep warming "well below" 2 degrees Celsius, and to "pursue efforts" to keep it to 1.5 degrees.
First, the bad news. As climate activist and economics professor Jeffrey Sachs says: "Cynics will say the agreement is unenforceable. They are right."
On the commitments in the Paris agreement, the world is heading to 450 ppm and beyond. They sentence the planet to a global average temperate rise of 2.7 degrees celsius above pre-industrial levels, according to the UN and other analysts. That is, the world is still on a trajectory to irreversible climate change.
As the ANU climate economist Frank Jotzo puts it: "It's very obvious that there's a massive commitment gap."
So why all the hoopla and champagne popping in Paris? The first element is relief, relief that hope is kept alive, that countries didn't walk out or talks collapse, that all countries voiced support for the overarching aim of limiting warming.
Second is that the deal creates a pathway to possible future success. As Environment Minister Greg Hunt told me: "The Paris meeting won't deliver 2 degrees, but the Paris process will."
The process is the agreed reviews of the carbon-cutting targets, reviews set to occur every five years starting on 2020.
"What matters is the process," says eminent Melbourne University economist Ross Garnaut who reported on climate change for the Rudd government. "It's clear and strong and it has the unanimous support of the international community."
Further, the rules say that the countries can only commit to doing more to cut carbon output, not less – it's a one-way process.
Still, as Garnaut says: "The trajectory of emissions reduction has to be a steep downwards one after 2025 if we are to meet the 2050 commitment," of zero net emissions by 2050.
"That will be especially challenging for countries that have a slower start, such as Australia." Australia, he says, will have to "run faster than other developed nations after 2020.
"This will be possible with little sacrifice to out standard of living. We have the richest renewable energy resources per capita of all developed countries and we have exceptional opportunities for biological sequestration."
Indeed Garnaut sees Australia has a chance to emerge as a "renewable energy superpower if we are half smart about it."
ANU's Jotzo says that if the gap between hope and reality is to be bridged, it will be done largely by the speed of technological advance: "It's progress on technology that makes it all possible."
Garnaut points out that in his 2008 report, he forecast that the cost of producing solar panel would fall by a few percentage point per year. In reality, the cost has fallen by 80per cent in seven years.
So at the heart of the Paris agreement is a gamble on technological solutions.
It's an enormous chance to take with the planet. The earth's atmosphere will be impressed only if the gamble pays off.
Peter Hartcher is international editor Sydney Morning Herald Illustration: John Shakespeare
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