There's a fatal flaw in the Paris Agreement - no carbon budget.

At COP26 in Glasgow in November, more than 190 countries will line up to offer what they have each, at least for the present, determined is a reasonable contribution on their part to solving the problem of climate change. This is something they agreed to do under an Agreement they all committed to in Paris in 2015.

The central aim of the Paris Agreement is simple. It’s to stop global heating. But the Agreement itself is not geared to stopping this heating, let alone stopping it at 1.5 degrees Celsius above pre-industrial levels, as the signatories clearly preferred when they attended signing ceremonies starting in 2016.

The trouble with the Paris Agreement is that it contains a negotiating mechanism which is more likely to lock in failure than success on the central target of the Agreement – the temperature target.

Relying as it does on continued voluntary contributions of greater and greater reductions in annual emissions of greenhouse gases, the Paris Agreement drags the focus away from the temperature target and onto platforms where political leaders can talk about something other than the fact that their pledges of emission reductions are entirely insufficient for meeting that target.

As a result we now have conference organisers at COP26 who are promoting the notion that if all countries offer to reduce their emissions by 50% by 2030 and reach net zero by 2050, this will “keep 1.5 degrees within reach”. But even the most accommodating scientific models of emissions trajectories necessary for stopping global heating before it gets completely out of control do not support this notion.

A reduction of 50% by 2030 and net zero by 2050 will not “keep 1.5 degrees within reach” if by 2030 or 2050 – or whenever all countries reach net zero – the world has emitted more tonnes of greenhouse gases in total than the atmosphere can hold before it acts as a blanket locking in uncontrollable heating.

It’s the total tonnage emitted that matters, not the percentage reductions in annual emissions that might be offered by opportunistic politicians at one end of the spectrum or desperate ones at the other. Nor does the date by which the emissions are reduced to net zero matter, except that it should be the day before too many tonnes are emitted.

It is a potentially fatal flaw of the Paris Agreement that it confined the basis of ongoing negotiations to voluntary pledges of emissions reductions and provided no basis for discussion of the limit of carbon that can be emitted within a reasonable safety margin.

This limit must be acknowledged sooner or later. We would hope it is acknowledged before the heating gets out of control; but alas the Paris Agreement has no mechanism to guide countries about how close global climate change is to being out of control. And the fact is, it is perilously close.

In the most recent report of the Intergovernmental Panel on Climate Change, the scientific estimate is that the world can emit no more than another 400 billion tonnes of CO2 for a 67% chance of limiting heating to 1.5 degrees Celsius. But if emissions of non-CO2 gases such as methane are reduced more slowly than CO2 emissions and if Earth system feedbacks such as melting permafrost add more CO2 to the atmosphere, the remaining budget for a 67% chance of limiting heating to 1.5 degrees could be as small as 110 billion tonnes.

At the current rate of CO2-e being emitted annually – around 43 billion tonnes – the 400 billion tonne carbon budget would be consumed before 2030 and the 110 billion tonne budget would be consumed in less than three years. Both these estimates should sober the COP26 attendees.

They should also be sobered by the recent release by the United Nations of the Nationally Determined Contributions Synthesis Report. This shows that pledges made thus far, ahead of COP26, are so small as to be an insult to the temperature target. With these pledges the world is on track for heating of up to 4.5 degrees Celsius by 2060. Unless at COP26 there is a miracle increase in pledges, it’s game over in the next five years for climate change. At COP26 we would need to see emission reduction pledges at least quadrupled to keep 1.5 degrees within reach.

If anything, the only emissions target that may hold out some reasonable chance of delivering success on the temperature target in the Paris Agreement is a commitment by all countries to fairly share a mid-range carbon budget – somewhere in between 400 billion tonnes and 110 billion tonnes of greenhouse gases. It would be reasonably safe (given how late all nations have now left the task of reducing to net zero) for all countries to agree to share a carbon budget of 200-250 billion tonnes and to reach net zero not by 2050 but by 2030. That might be seen as a radical target at COP26 but within a year it will become apparent that it is the minimum necessary.

Politicians or their nominees are likely to descend on Glasgow in November to claim that their Nationally Determined Contributions are sufficient to keep 1.5 degrees within reach. However, this will offer no more than a veneer of success – an illusion of safety.

On the other hand, if a discussion can be started at COP26 to acknowledge the fact that there is a finite limit to the tonnage we can still safely emit, there is some hope that the world could be swung back onto a safer path for climate than we are on now. Without that acknowledgement, COP26 will probably offer political wins to countries that in no way deserve it and catastrophic losses for global climate and the planet we depend on.

Australians can find out more about how the world can work together to stop global heating here. We can still fix it; but to do so we must immediately take a position of leadership and cooperation on the international stage.