When scientists trawl through the Earth’s temperature record, they can find only one other time when it changed as rapidly as it is changing today because of human-driven global warming. That was 66 million years ago, and a meteorite the size of a city had just slammed into the planet, vaporising rock and forest and wiping out the dinosaurs.

The dinosaurs didn’t have an early-warning system for their climate-changing crisis, but we do. It’s been going off since at least the 1950s, when the first measurements found too much planet-warming carbon dioxide was being pumped into the atmosphere. And we’ve known how to fix it since the 1980s, with the emergence of renewable energy technology to replace CO2-belching fossil fuels.

Saving the planet now comes down to politics – and accounting. Scientists tell us there is a set amount of carbon dioxide we can burn before things really start to unravel again for the climate. It’s known as the carbon budget and, while there are different calculations set against different thresholds of acceptable temperature increase (your 1.5 degrees versus your 2-degree budget), scientists agree there’s not much left in the war chest. Now, countries opening up more fossil fuel projects, from monster coal mines in Australia to drilling for oil in the Arctic, could blow it even faster.

So, where does the carbon budget stand today? What accounting tricks go into calculating it? And how does the world (and Australia) divvy up a fair share at climate summits such as COP26?

Why do we need a carbon budget?

Meet CO2. It’s a tiny molecule that helps keep our atmosphere warm. A lot of CO2 and you get jungles in the Arctic, not enough and plants that feed on it can’t grow. Until the Industrial Revolution, natural changes in CO2 have driven the Earth’s climate, at times turning it into a hothouse and at others a snowball.

But, for the past 10,000 years, the climate has been remarkably stable. During this period of calm – known as the Holocene – human civilisation has flourished. Now, after just a few decades of pumping CO2 into the atmosphere, humans have ended those millennia of comfort and caused the world to warm by more than 1 degree. The oceans alone are heating so fast it’s as if five atomic bombs are being detonated in the water every second.

About half of the CO2 that humans emit when we burn fossil fuels or chop down trees is taken back out of the air by land sinks, such as forests and the sea. The rest sticks around for hundreds, even thousands, of years. In 2020, although global CO2 emissions saw their biggest slump yet (of about 7 per cent) as travel and industry wound down during pandemic lockdowns, the total amount of CO2 in the atmosphere still reached a record high – 412 parts per million (well beyond what is considered the safe threshold of 350). The last time there was this much CO2 was more than three million years ago and the sea was about 20 metres higher.

The world’s top climate scientists at the UN’s Intergovernmental Panel on Climate Change (IPCC) estimate we have emitted about 2390 gigatonnes of Co2 between 1850 and 2019. One gigatonne means a billion tonnes - that alone is more than the weight of the world’s entire population or a hundred million African elephants, depending on your preferred metric.

Though we’ve known what our excess CO2 is doing to the planet for decades, about half of all we’ve emitted so far was put there just in the past 30 years.

Today we emit more than 40 gigatonnes annually, including from land-use change, such as deforestation. That’s up from about 5 gigatonnes in the 1950s, and puts us on track to hit a disastrous 2.7 of warming by the end of this century. At the Glasgow climate summit in November, new deals were struck to accelerate zero-emissions technology as well as to end deforestation and reduce emissions of another dangerous greenhouse gas, methane by 2030. And more nations have pledged to hit net zero by 2050 (China has committed to be carbon-neutral by 2060 and India by 2070).

That’s shaved the planet’s projected warming down – even to below 2 degrees according to two separate analyses. But going off 2030 pledges, the UN calculates that the world is still on track to hit between 2.4 and 2.7 degrees of warming by 2100. And because this warming is happening so fast, by planetary standards, scientists warn the changes it wreaks might not always happen in a neat, linear line. Temperatures could suddenly jump or ecosystems and weather spiral out of control if we cross climate “tipping points” (more on those later).

The IPCC’s latest report, published in August, was a “code red for humanity”. But it stressed there was still time to avoid disaster. Even under the best-case scenarios modelled – if countries halve emissions by 2030 – scientists say we will reach 1.5 degrees of warming in the next decade or so.

The question now is whether we will hold the line there or continue on to 2 or close to 3 degrees by the end of the century. Even half a degree would mean the difference between hundreds of millions more people displaced by longer droughts, fire seasons, heatwaves, and angrier storms. At 1.5 degrees, for example, about 14 per cent of the world’s population would be exposed to severe heatwaves at least once every five years, but at 2 degrees that number jumps to 37 per cent. The Arctic, crucial to keeping our planet cool, is expected to have ice-free summers once a century at 1.5 degrees; if warming gets up to 2, that becomes every decade. It means most coral reefs, including the Great Barrier Reef, will be gone. And it will be catastrophic for our Pacific Islander neighbours, already inundated by rising seas and storms.

The good news, says IPCC lead author and climate scientist Professor Will Steffen, is that every fraction of a degree of warming we can prevent counts too. Limiting warming to 1.5 degrees was the goal agreed at the last COP in Paris in 2015 (not a binding target itself but part of an agreement between nations to keep warming “well below 2 degrees”), so it’s the number that scientists budget for. If we overshoot it, avoiding 2 degrees could still spare billions of dollars in adaptation funding and help hold together the ecosystems that animals (including humans) need to survive.

What’s left in the carbon budget then?

Every year the shift to clean energy is delayed means we have to make steeper emissions cuts in the future. If, for example, emissions had stayed at 1950s levels, we would still have centuries left before our carbon budget was exhausted. Even cutting CO2 at the turn of the millennium – when the science of what was causing global warming was well established – would have given us many more decades to make the transition.

Since then, we have ramped up emissions so much that the IPCC estimates we only have about 500 gigatonnes left to burn – or about 12 more years of current emissions. Sticking to that budget now gives us a 50-50 shot of holding warming at 1.5 degrees.

To get there, scientists say the world needs deep cuts by 2030 (of at least 45 per cent, which the European Union and countries such as the US, the UK and Japan have already pledged). And we need to collectively hit net zero emissions between 2045-2050. Without rapid action by governments, scientists don’t expect emissions to peak and flatline globally until 2025 “which will make it very hard to get the 50 per cent reduction we need by 2030,” Steffen says.

“This past inaction is costing us dearly, and every year we delay it just gets worse.”

And, as Steffen notes, our 500-gigatonne budget actually started in January 2020, so some of it’s been eaten up already. By next year, with annual emissions already on track to pass 40 billion tonnes again after that 2020 slump, there will be about 420 gigatonnes left. Even then, this budget still only gives us a 50 per cent shot of staying at 1.5 degrees.

Professor Malte Meinshausen, another IPCC lead author, was part of the first team to calculate a comprehensive carbon budget for the world in 2009. He admits no other kind of budget (or policy target) would be built on a 50 per cent chance of success.

“You wouldn’t play Russian roulette with half the barrels loaded. But, fortunately, here it’s not quite the same risk, it’s not a cliff beyond 1.5. But it is a more dangerous world,” he says.

For a budget with a 67 per cent chance of staying at 1.5 degrees, the IPCC calculates we’d have only 400 gigatonnes left as of January 2020, or about 320 gigatonnes from next year, Steffen says. That means the budget would run out in 2030. Staying below 300 tonnes would gets us closer to an 83 per chance of holding warming at 1.5 degrees.

“People like treasurers know how to work with budgets. But the flipside is they also overdraw them, and once you give someone a budget they feel entitled to spend every last bit,” Meinshausen says.

Professor Rob Jackson at Stanford University helps calculate carbon budget numbers each year for the Global Carbon Project. He, instead, imagines it as a bucket we are filling with water. If we let it fill all the way to the brim, there will be a greater chance one little wobble will make it spill over anyway.

What’s carbon drawdown? And what does net zero mean?

Given how much CO2 we are still pumping out, most of the world’s emissions reduction modelling now assumes we will also start sucking carbon back out of the air on a large scale by the middle of the century. This will help us “balance the books” and get to net zero – where no extra CO2 is emitted, but there’s not necessarily a complete end to our emissions either. For parts of the economy that are harder to unchain from fossil fuels, such as steelmaking and air travel, some carbon “offsets” will be used. More trees will be planted or new technology such as specialised fans will draw down the equivalent burned from the air. That’ll even out in the budget, Meinshausen explains, so we will be said to be “carbon neutral”.

Countries still powering ahead with fossil fuel production, such as Saudi Arabia and Australia, have put drawdown technology at the heart of their recently announced net zero plans, including its weak cousin carbon capture (which looks to trap a proportion of emissions – so far, a very small one – as they are produced by power stations and gas fields). But these technologies are unproven at scale. They remain expensive and inefficient. Some haven’t even been invented yet. Natural carbon sinks, such as trees and seaweed hold more promise for large-scale drawdown but also require vast expanses, and so may be vulnerable to extreme weather, from fires to storms.

Drawdown is meant as a last resort, not to avoid decarbonising the sectors we already can, such as energy. The idea is that it will allow for some ongoing emissions of other greenhouse gases posing a threat, namely methane, which we will find harder to stop altogether in a small group of industries such as agriculture, Steffen says.

Switching to renewables fast is by far the cheapest and most effective solution – cheaper now even than the fossil fuels propped up by government subsidies around the world.

What about these tipping points?

One wobble that could cause the carbon bucket to spill over is if we cross a tipping point – the threshold beyond which certain planetary systems spiral out of control, from melting ice sheets to thawing permafrost releasing methane. Some of these feedbacks are already factored into carbon budgets “to some degree”, says Meinshausen, as the science crystallises. But we are not just changing the planet’s temperature. We are burning through its resources at an unprecedented rate too; clearing forests, polluting oceans, and driving an extinction crisis. And that’s making things even less stable.

One of the most vulnerable such tipping points, Steffen says, is the Arctic ice, now melting four times faster today than it was just two decades earlier. The white ice caps at the top and bottom of the world help keep the planet cool by reflecting light. But the more they melt into dark water, the more sun is absorbed as heat in the seas, which in turn melts more of the ice above – a wicked feedback loop. The Greenland ice sheet holds enough water to raise sea levels by more than seven metres. And, remembering our gigatonne metrics, NASA’s satellites show it’s losing 283 billion tonnes of that ice every year.

Scientists are still nervously trying to predict when that melt will become irreversible. “It might be 1.5 degrees, or it might be 1.2 where we are now,” Meinshausen says. Both he and Steffen agree that if the world overshoots its 1.5 degree target for several decades, critical ecosystems might be lost and dangerous tipping points, such as Greenland, set off, even if carbon drawdown does come through and take out enough CO2 to drop the temperature back down by the end of the century.

And, as Steffen explains, setting off one or two major tipping points can trigger others, like a row of dominoes,shaking the whole system. More ice melt can affect the ocean currents which, in turn, affect weather and so can dry out the Amazon, for example. That sprawling jungle acts as the world’s air-conditioning, helping drive rainfall. But, since the 1980s, its dry seasons have been growing longer as climate change and deforestation take their toll.

Almost 20 per cent of the Amazon has been cut down. Some scientists warn just the loss of another 5 per cent could tip half of the remaining jungle into savannah. If that happens, its trees will release centuries of carbon as they die – many gigatonnes. And that could spell serious trouble for our climate accounting.

How does the world divvy up the carbon budget?

When we talk about a carbon budget, we are talking chiefly about burning fossil fuels (although deforestation, agriculture and fires produce emissions too). Meinshausen likens the budget left for world leaders to a cake – every year more is eaten away. “If someone’s already had a big piece of cake, some say they shouldn’t have the same right to what’s left,” he says. “But that’s not completely fair either; our grandmas lived in a time when burning coal was the only way to power homes. Now, because of renewables, it’s an economically stupid thing.”

In fact, last year, the global authority on the energy market, the International Energy Agency (“hardly a radical organisation”, Steffen says) issued a blunt directive: the world cannot afford any new fossil fuel projects. No new coal mines. No new gas or oil. All new cars sold should be electric by 2035 and global electricity renewable by 2040. That calls for a lot of investment in green technologies (for solar power, it means installing the equivalent of the world’s largest solar park every day). But the IEA calculates the shift will actually boost the world’s economy and jobs, as renewables now out-compete fossil fuels.

A recent analysis in Nature found on “conservative estimates” 90 per cent of all coal globally must stay in the ground. For the US and Russia, that’s 97 per cent of their reserves, and for Australia 95 per cent, including one of the world’s largest untapped coal veins being opened up in Queensland’s Galilee Basin by Indian mining giant Adani. Oil-rich nations in the Middle East must not extract two-thirds of their wells. Most of Canada’s sprawling tar sands must not be mined for its high-polluting crude oil and the Arctic must remain untapped for oil (despite Russia’s ambitions to drill further in the vulnerable ecosystem as more ice melts).

Many poorer nations will need help with the shift to green energy, the researchers noted, and so should get more of the carbon budget than developed nations, who have historically emitted more and are in a better position to make a fast shift. The US, once the biggest emitter, has now vowed to double its financial aid for poorer nations to $11.4 billion per year to help them decarbonise and cope with the climate fallout. But, despite a renewed push by the UK, Germany and Canada at COP26, committments still fell short of the unfulfilled $100 billion a year climate fund that rich nations pledged for the developing world between 2020 and 2025, which has become a key impasse at COP negotiations.

Sometimes, for nations still building their economies, it’s easier to leapfrog fossil fuels altogether. Solar and wind are often more practical in Africa than setting up sprawling grid connections for coal and gas, for example. But in India, Indonesia and China (now the world’s top emitter), these old technologies have already arrived large-scale even as investment in renewables starts to drive a shift.

A report in October found that the world still plans to produce more than twice the amount of fossil fuels in 2030 than what we can afford if we are to stay around 1.5 degrees. Even the host of the upcoming COP summit, Britain, which is leading the charge on 2030 targets by pledging to cut 68 per cent of its emissions within a decade, continues to green-light offshore oil drilling. Sprawling multinational companies further complicate the picture. Some estimates say oil and gas giant Shell, for example, has produced about 3 per cent of all global emissions to date just on its own. (In a landmark decision earlier this year, a court ruled Shell must cut its emissions in line with the Paris Agreement too.)

Many experts now say cuts are not enough; governments must commit to ending fossil fuel subsidies and new project approvals too. Some want a global registry of such plans, so the carbon budget can be kept front of mind; others a treaty against new coal, oil or gas in the same way nations have signed agreements against more nuclear weapons. At COP26, more than 40 countries pledged to end coal (including big players such as Germany and Indonesia), but the US, China, India and Australia weren’t among them.

What’s Australia’s fair share?

Australia makes up less than 0.35 per cent of the world’s population but emits about 1. 2 per cent of global emissions, making us one of the biggest per-capita polluters in the world, and about the 14th biggest on the straight numbers. If our fossil fuel exports counted in those emissions tallies, we’d rank about fifth overall, Steffen says. As well as considerable diplomatic clout (which, historically, our governments have often used against the clean-energy transition), we also hold many of the coal and gas reserves that scientists are warning must stay in the ground. And, despite an increasing flow of court challenges – including one ruling that the federal environment minister must consider future climate change impacts on Australia’s children when approving coal mines – the Morrison government plans to keep green-lighting new fossil fuels, even putting methane-producing gas at the heart of our COVID recovery instead of clean energy.

Australia has lobbied hard for special deals at past climate summits (and then pulled off some controversial accounting tricks) to avoid feeling the diplomatic heat so far – most notably securing so-called carryover credits from the Kyoto deal of 1997 that allowed it to keep growing its emissions under reduction pledges thanks to past scale-backs in land-clearing. But, pressure is mounting on Australia to think beyond 2050, phase out coal and gas and lift its 2030 target too (our target currently stands at a cut of between 26 and 28 per cent, though emissions are expected to fall by 30 per cent by 2030. Labor, meanwhile, is taking a 43 per cent by 2030 cut to the next election).

We’ve already burnt through most of our share of the world’s carbon budget (calculated by the government’s own Climate Change Authority as 0.97 per cent). We had roughly 7760 megatonnes (million tonnes) left in 2014, using that modelling methodology, and in a recent report from the Climate Targets Panel, Meinshausen estimated it’s now down to about 3521 megtatonnes. To meet our Paris commitments, Australia needs to cut emissions by at least 50 per cent of 2005 levels by 2030, and reach net zero by 2045, the research concluded. To help hold warming near 1.5 degrees, that 2030 target needs to be even higher; 75 per cent.

“Net zero by 2050 just isn’t enough for us,” says Steffen, another researcher on the report. “Australia can afford to do more and should do more.”

... and what are we doing?

Though our exports are still built around fossil fuels, Australia also stands to gain enormously from the new renewable economy. We live on the sunniest and windiest continent on Earth, after all. Steffen (and Australia’s energy operator) say we could get to 100 per cent renewable energy power in a matter of years, even as soon as 2025. South Australia is getting close, and the ACT is already at “net 100 per cent renewables”, using offsets in other states, and fast electrifying its car fleet. Other experts say Australia could turn its mining and manufacturing expertise to making green steel, and exporting the aluminum, lithium and nickel needed for the world’s clean energy infrastructure.

The Business Council of Australia estimates that ramping up our emissions cuts to between 45 and 50 per cent by 2030 would actually maximise profits, and make the average Australian $5000 a year better off by 2050. Despite concern from the Nationals for regional jobs, those in rural areas would get most of the 200,000 extra jobs expected in the clean energy shift.

But when Australia finally unveiled its new net-zero by 2050 strategy, there was little mention of renewables. Upfront instead was the firm declaration that coal and gas exports would not end (or likely even dip very much) beyond 2050. The “technology, not taxes” plan claims 40 per cent of CO2 cuts will be achieved through “low-emissions technology”, such as clean hydrogen and carbon capture and storage, and 30 per cent through unspecified global tech trends, and “further technology breakthroughs”, though the modelling behind it has not been released.

Indeed, as Prime Minister Scott Morrison boasted that his government had found a way to get to net zero while protecting the economy (the Frank Sinatra approach or the “Australian way”), Treasury admitted it hadn’t modelled the effects of climate change and emissions reductions on the economy for many years. In fact, it had very little involvement in the new net zero strategy, which was prepared with help from consultants.

Meanwhile, the economic winds have shifted, Steffen says, noting China’s recent announcement it will no longer invest in coal mines overseas.And, without support to transition Australian fossil fuel workers to other sectors, as countries such as Germany have done, regional communities are already being left in the lurch when coal mines and power stations abruptly shut their doors.

“Our ministers need to understand that [fossil fuel] markets are going to tumble very fast,” Steffen says. “We’ll be left behind.”

The power of the green economy so far is why Meinshausen says he is still usually the most optimistic of his scientific colleagues. “On the science, we’re confident now that some of the 4-degree nightmare warming scenarios won’t happen [this century]. On jobs and growth, renewables now win,” he says.

But how regular government budgets look in the future – how many billions or trillions more dollars must be spent battling wildfires, heatwaves, drought, storms and other climate nasties – all depends on whether we can keep our carbon accounts out of the red this decade.

This article was published on October 31 and updated after COP26 negotiations wrapped to reflect developments.

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