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Imagine, if you will, two people planning to go on a diet. One of them is a moderately fit but overweight bloke weighing, say, 100 kilograms. The other is morbidly obese, and weighs twice as much.

Time passes and they both shed weight. Now the formerly overweight bloke has reached his ideal weight of 75 kilograms.

The obese guy has lost the same amount, in percentage terms. But he still weighs 150 kilos. He’s still grossly overweight. Clearly, for the sake of his health, he has to lose a lot more.

We use the analogy to point to one of the several specious arguments Prime Minister Tony Abbott has been using to create the impression – the false impression – that his government is serious about acting to limit climate change.

After the announcement of the government’s long-awaited new target for greenhouse gas emissions last week – a reduction of 26 per cent, and possibly 28 per cent on 2005 levels, by 2030 – Abbott fronted the media to insist it was “foursquare in the middle” of the pledges made by other nations ahead of the United Nations climate summit in Paris at year’s end.

“It’s better than Japan,” he boasted. “It’s almost the same as New Zealand. It’s a whisker below Canada. It’s a little below Europe. It’s about the same as the United States. It’s vastly better than Korea. Of course, it is unimaginably better than China.”

Except, of course, none of this is true.

“Bullshit is too kind a description, really.”

The Australian government’s target is not in the middle of the range, and Abbott had no right to be satisfied, for exactly the same reason the hypothetical morbidly obese man above should not be satisfied with his weight-loss achievement.

In greenhouse gas terms, Australia starts out as the obese man in the developed world, and will still be the obese one in 2030.

Let’s go to some of the examples. Japan, as Abbott noted, has set a target slightly less ambitious in percentage terms than Australia’s: 25 per cent by 2030, compared with 26. But when you look a little closer, it’s immediately apparent that the percentage reduction is not the significant measure. What counts is the weight at the end of the process. In 2030, assuming it reaches its goal, Japan will produce the equivalent of eight tonnes of carbon dioxide per capita. If Australia reaches its 26 per cent goal, it will still produce twice as much. Sixteen tonnes.

The comparable figures for other developed countries, according to the Climate Institute, which crunched all the numbers to allow a real, apples-to-apples comparison of carbon dioxide per capita are: New Zealand, 11 tonnes; the European Union, six tonnes; Britain, five tonnes; the United States 10 tonnes. Only Canada will be anywhere close to us, at 14 tonnes.

So, what about the other countries Abbott cited: China and South Korea?

He is right in saying that in percentage terms, our target is more ambitious. Korea’s 2030 goal is to keep its emissions broadly as they were in 2005 – that is somewhere between a 5 per cent decrease and a 1 per cent increase. China looks a whole lot worse. Its emissions are forecast to increase between 72 and 96 per cent.

But once again, as with our hypothetical overweight men, it is instructive to look at their weights at the start and end of the process. In 2005, South Korea generated about 10 tonnes of CO2 per capita. China emitted about four tonnes.

Assuming they meet their 2030 targets, they will still produce only 50 to 60 per cent as much greenhouse gas, per capita, as Australia does.

That’s still a problem for the planet, because the populations of both countries are large. But that’s not really the point.

The point is Australia is not – pardon the pun – pulling its weight.

1 . Deceptive figures

The statistics cited by Abbott in his media conference were not actual lies, but they were deceptive. And the prime minister, his environment minister Greg Hunt, and their supporters in the press, have sought to mislead in other ways, too.

For example, when Abbott says Australia’s target is “about the same as the United States” he relies on another bit of statistical sleight of hand. The US has committed to making its emissions reductions within 10 years, whereas ours are over 15.

“If you take theirs out to 2030, it becomes 41 per cent,” says Will Steffen, a councillor with the Climate Council and emeritus professor of earth systems science at the ANU.

Then there’s the matter of base years.

“By choosing different baselines you can make it appear your effort is much greater or much less,” says Steffen.

“The Europeans choose 1990 because they started acting to reduce their emissions much earlier, and have done a lot of heavy lifting already.

“You can see how significant the choice of base year is by looking at Australia’s current bipartisan target, of reducing emissions 5 per cent on 2000 levels by 2020.

“If you translate that to a 2005 baseline it becomes 13 per cent, because emissions were higher in 2005. If you take 2010, it becomes 8 per cent. But it’s exactly the same amount of emissions.

“So we chose 2005 for our Paris target because our emissions were really high at that time, which makes our effort look greater.

“The point is that as far as the atmosphere is concerned, as far as the climate is concerned, it’s only the actual tonnage of emissions that matters.”

According to the best current climate science, if the world is to have a reasonable – 66 per cent – probability of avoiding an increase of more than 2ºC in average temperatures, we have to stay within a global emissions budget equivalent to 1700 gigatonnes (a gigatonne is a billion tonnes) of carbon dioxide between 2000 and 2050.

To have a 75 per cent probability of staying below 2ºC warming, the global budget drops to about 1500Gt.

That sounds like a huge amount, but it’s not, given the rate at which we are burning fossil fuels.

“We, humanity as a whole, are emitting 30 or 40 billion tonnes a year now, so you can do the sums. We’ve got about 20 years left at current rates. We’ve got to decarbonise, fast,” says Steffen.

And by the Abbott government’s policy, that will not happen.

“We are on course for a three- or four-degree rise,” he says.

That would mean no coral reefs, mass extinctions, the likelihood that Australia could not feed itself, huge global refugee flows, economic collapse.

“At that level of warming you’re talking about a vastly different world and one that humans have never experienced in our evolutionary history,” Steffen says.

2 . International co-operation

The science is clear; the politics isn’t. How do you divide the task among the world’s countries? What allowances should be made for those countries that have only recently industrialised and are emitting a lot now, compared with those that have recently cut their emissions but emitted a lot historically? How do you weigh the needs of poor countries in Africa and Asia that are struggling to drag their people out of poverty? How do you assess capacity to pay?

There are innumerable potential points of dispute.

“Conceptually, the establishment of a formula for the apportionment of a global carbon budget sounds very neat, but practically it’s not,” says the chairman of the government’s Climate Change Authority, Bernie Fraser.

It does, however, provide a reference point for countries in setting their individual targets and justifying them to the world.

“What countries have been asked to do, when they advance their [Paris summit] targets, is show how they are making a contribution towards that commonly agreed two-degree goal,” says Erwin Jackson, deputy CEO of the Climate Institute.

“Unfortunately, some countries, such as Australia, Canada and New Zealand, have ignored it altogether and provided no justification to argue their target is a fair contribution.

“Others like the US, EU, Mexico, South Korea, Norway, Switzerland – a whole range of countries – have actually put forward an argument.”

There are no prizes for guessing why Australia has not done as asked, Jackson suggests: “The Australian government just can’t pretend its target is a fair contribution.”

So, what would be a fair contribution?

Last year – even as the Abbott government was trying unsuccessfully to abolish it – Fraser’s Climate Change Authority gave its judgement.

“The authority believes an emissions budget of 10.1Gt of CO2 2013-2050 (or about 1 per cent of the remaining estimated global budget) would represent an equitable share for Australia,” it said in its targets and progress review.

Allowing that Australia might fairly emit 1 per cent of global carbon dioxide was actually pretty generous, given we have only about 0.3 per cent of world population. To make a contribution consistent with staying below 2ºC, according to the authority, it would require a reduction in our emissions of between 45 and 65 per cent.

The lower end of that range, says Jackson, would give Australia a 66 per cent probability of not exceeding 2ºC warming, while the upper end would give a 75 per cent chance.

“We at the institute advocate a target towards the top of that range,” he says.

But it is a moot point, because the government ignored the advice from scientists and made a decision based on politics. On one hand, the polls show the public wants more action to combat climate change. On the other, the powerful fossil fuel lobby, which contributes handsomely to conservative coffers, demands protection.

3 . Concocted reports

And the politics of this issue are very ugly. When it is not using dodgy statistics to pretend it has set an ambitious target, the government is using equally dodgy numbers to pretend that doing any more would be unaffordable.

The clearest example of this came last week, in a screaming front-page story in the Sydney Murdoch tabloid The Daily Telegraph. Almost every aspect of the story, planted by the government with its favourite partisan media outlet, was either wrong or misleading, starting with the headline: “Labor’s carbon emission plan would strip $600b from economy.”

In reality, the Labor opposition has not yet formulated its target. Nor was there anything secret about the “plan”, as The Telegraph suggested. The modelling, commissioned by the government’s statutory adviser, the Climate Change Authority, was public information. In the skilled hands of The Telegraph, it became public disinformation.

Bernie Fraser, economist, former Treasury head and Reserve Bank governor before he came to the Climate Change Authority, knows a thing or two about the use – and misuse – of economic modelling.

“The Tele report was a concoction,” he says partway through a 10-minute deconstruction of it. “Bullshit is too kind a description, really.”

Space prevents us repeating his detailed critique but should you wish, you can read a swingeing debunking of the story on the academic website The Conversation.

One of the great ironies of contemporary politics is that it was mining baron Clive Palmer who stopped the government from abolishing the Climate Change Authority, in return for his support in getting rid of the previous government’s carbon price.

As a consequence, the authority was charged with delivering three pieces of work.

The first task was to advise what Australia’s future emissions should be. That advice was ignored.

The second, due in December, is to examine the policy options for reducing our emissions. It will recommend an emissions trading scheme, or as Abbott, Hunt and the climate change deniers derisively call it, a carbon tax.

“You can’t get away from a price on carbon,” Fraser says.

The third report, due by the middle of next year, will consider a comprehensive suite of other measures for energy efficiency.

“We’re working on it, despite the dismal atmospherics,” Fraser says in his mild, uninflected voice.

He’s anticipating a lot more “crap” from the Abbott government and parts of the media when those reports come down. All of which means the debate about climate change is going to get a lot hotter in the near future.

Just like Earth itself.

This piece was updated on August 24, 2015, to clarify the global emissions budget.