Reality check: Our global carbon budget is zero

Photo: Dailuo/Flickr CC by 2.0

The discussion around greenhouse gas emissions is very abstract to most people.

So when politicians — or even the big fossil fuel companies — throw around targets like “net zero by 2050” it can appear that they are taking the climate emergency seriously. But they are not.

The word “net” covers up the fact that such targets are actually about putting more greenhouse gases into the atmosphere — up to 2050 and beyond. This is despite the evidence that the current amount of greenhouse gas in the atmosphere is set to warm the planet by 2º Celsius or more.

It’s much easier to understand the situation if we look at it in terms of a carbon budget: how much carbon dioxide (and other greenhouse gases) can be put into the atmosphere before global warming reaches seriously dangerous levels?

A sobering briefing paper prepared by the National Centre for Climate Restoration (NCCR) in February states that our planetary budget for a safe climate is now zero.

To be safe, the world should be reducing the amount of greenhouse gases in the atmosphere from now and, hence, any targets that assume we can keep pumping more greenhouse gases out up until 2050 are extremely reckless.

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, released in 2014, calculated that for a 66% chance of keeping warming to no more than 1.5ºC, the world would need a carbon budget of 400 gigatonne (GT) of carbon dioxide between 2011 and 2100.

But emissions for just 2011–19 (8 years into the budget period) exceeded 400 GT, which leaves a budget of zero to stick to 1.5ºC warming.

According to the NCCR briefing, the most recent climate models show the Earth reaching 1.5ºC warming over 2025–27, regardless of the emission paths taken from now.

It is possible to take action now that can pull the Earth back from exceeding 1.5ºC warming but not before “overshooting”, that is, going over 1.5ºC warming for several decades before returning to a lower level of warming by 2100.

Australia’s terrifying Black Summer bushfire emergency of 2019–2020 and the extended drought that still blights large parts of the continent were just a taste of worse to come.


So, if our global carbon budget is already overdrawn, how has the IPCC responded?

According to the NCCR, the 2018 IPCC Special Report on Global Warming of 1.5ºC increased the 1.5ºC budget by “underestimating the warming date by, and using an estimate of climate sensitivity — the transient climate response to cumulative carbon emissions (TCRE) — which underestimates warming at system equilibrium”.

This created the illusion of a budget when none existed.

The NCCR briefing paper said that the IPCC underestimates current global warming by 0.3ºC.

“Until now, climate models used or projecting future warming and calculating carbon budgets in IPCC reports estimate a warming sensitivity of ~3°C (for doubled CO2), which excludes factors such as ‘slow’ feedbacks (carbon stores, such as permafrost) and albedo changes (reflectivity).”

Carbon-cycle feedbacks could result in up to 25% more warming than in the main IPCC projections, the briefing paper said.

“When the full risks are included, warming may be as high as 5–6°C for a doubling of CO2 for a range of climate states between glacial conditions and ice-free Antarctica.”

The NCCR briefing also argues that the IPCC carbon budgets assume an “unacceptable risk of failure”.

“IPCC carbon budgets are often associated with a 50% or 66% chance of staying below the target, that is, a one-in-two, or one-in-three, chance of failure … We would never accept those risks of failures in our own lives. Why accept them for impacts which may destroy civilisation as we know it?”

If a prudent risk-management approach is taken, the briefing argued, we should accept that there is no global carbon budget left to meet a 2ºC target. If attention is given to the high-damage, high-end possibilities rather than middle-of-the-road probabilities, this is the unavoidable conclusion.

What needs to happen?

Where are we at now and what needs to be done?

The current level of global warming (since the 1890s) is about 1.2ºC. But, the NCCR report added, the presence of short-lived cooling aerosols (a by-product of burning fossil fuels) in the atmosphere may be masking global warming by up to 0.5ºC.

As fossil fuel use declines, so does the aerosol cooling, so for the next two decades lower emissions will not ameliorate the warming trend.

The Earth has already passed several tipping points for coral reefs, Arctic sea ice and some Antarctic glaciers, the NCCR briefing said. “Further tipping points could be triggered at low levels of global warming and a cluster of abrupt shifts could occur between 1.5°C and 2°C.”

“The IPCC has identified a number of ‘Reasons for concern’ including ‘Large-scale singular events’, which may include tipping points for major Earth-system elements — such as polar ice sheets, permafrost, boreal forests and the Amazon — which could trigger irreversible, self-sustaining warming.”

The risk of such “singular events” is assessed as “moderate” — between 1.5–2°C.

“Global temperature is on track for 3–5°C of warming by 2100 and the temperature increase is still on the high-emissions RCP8.5 path.”

A Representative Concentration Pathway (RCP) is a greenhouse gas concentration (not emissions) trajectory used by the IPCC to model warming under various policy settings.

While sometimes presented as an extreme scenario, the NCCR briefing says that RCP8.5 is “the best match to mid-century under current and stated policies”.

Hothouse Earth

The briefing added that 2°C of warming may trigger a “Hothouse Earth” scenario in which “climate system feedback mechanisms and their mutual interaction drive the Earth System climate to a point of no return, whereby further warming would become self-sustaining”.

The alarming current situation is summarised as: “The world is likely entering a period of accelerated warming, and reducing emissions will have little or no effect on that trend for the next 20–25 years. The imminence of further tipping points makes this a wicked problem, because 2°C is far from safe.”

In a twisted way, the dire global warming situation is sometimes used by those wanting countries like Australia to continue extracting and exporting coal and gas to argue that emergency climate action should be abandoned because it is too late anyway and will only hurt the economy.

But that is crazy and it is an irresponsible argument on both counts.

There is no economy on a dead planet and catastrophic climate change will have a catastrophic economic impact. We saw that in the Black Summer bushfire crisis.

Warming is already on course to reach dangerous levels. But, if we do next to nothing — the course we are on — it could get a lot worse.

We need to start reducing the amount of greenhouse gases in the environment from now, and as fast as we can.

Globally, net zero targets need to be brought forward, at the very least to 2030, to have any chance of staying below 2ºC warming. But for that to happen and the minimal global justice to be respected, the richest, most developed countries like Australia, would have to get to net zero before 2030.

If you like our work, become a supporter

Green Left is a vital social-change project and aims to make all content available online, without paywalls. With no corporate sponsors or advertising, we rely on support and donations from readers like you.

For just $5 per month get the Green Left digital edition in your inbox each week. For $10 per month get the above and the print edition delivered to your door. You can also add a donation to your support by choosing the solidarity option of $20 per month.

Freecall now on 1800 634 206 or follow the support link below to make a secure supporter payment or donation online.