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Keeping global warming to 1.5°C: really hard, but not impossible

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The Conversation

Source: The Conversation

Source: The Conversation

The Paris climate agreement has two aims: “holding the increase in global average temperature to well below 2℃ above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5℃”. The more ambitious of these is not yet out of reach, according to our new research.

Despite previous suggestions that this goal may be a lost cause, our calculations suggest that staying below 1.5℃ looks scientifically feasible, if extremely challenging.

Climate targets such as the 1.5℃ and 2℃ goals have been interpreted in various ways. In practice, however, these targets are probably best seen as focal points for negotiations, providing a common basis for action.

To develop policies capable of hitting these targets, we need to know the size of the “carbon budget” – the total amount of greenhouse emissions consistent with a particular temperature target. Armed with this knowledge, governments can set policies designed to reduce emissions by the corresponding amount.

In a study published in Nature Geoscience, we and our international colleagues present a new estimate of how much carbon budget is left if we want to remain below 1.5℃ of global warming relative to pre-industrial temperatures (bearing in mind that we are already at around 0.9℃for the present decade).

We calculate that by limiting total CO₂ emissions from the beginning of 2015 to around 880 billion tonnes of CO₂ (240 billion tonnes of carbon), we would give ourselves a two-in-three chance of holding warming to less than 0.6℃ above the present decade. This may sound a lot, but to put it in context, if CO₂ emissions were to continue to increase along current trends, even this new budget would be exhausted in less than 20 years 1.5℃ (see Climate Clock). This budget is consistent with the 1.5℃ goal, given the warming that humans have already caused, and is substantially greater than the budgets previously inferred from the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), released in 2013-14.

This does not mean that the IPCC got it wrong. Having predated the Paris Agreement, the IPCC report included very little analysis of the 1.5℃ target, which only became a political option during the Paris negotiations themselves. The IPCC did not develop a thorough estimate of carbon budgets consistent with 1.5℃, for the simple reason that nobody had asked them to.

The new study contains a far more comprehensive analysis of the factors that help to determine carbon budgets, such as model-data comparisons, the treatment of non-CO₂ gases, and the issue of the maximum rates at which emissions can feasibly be reduced.

Tough task

The emissions reductions required to stay within this budget remain extremely challenging. CO₂ emissions would need to decline by 4-6% per year for several decades.

There are precedents for this, but not happy ones: these kinds of declines have historically been seen in events such as the Great Depression, the years following World War II, and during the collapse of the Soviet Union – and even these episodes were relatively brief.

Yet it would be wrong to conclude that greenhouse emissions can only plummet during times of economic collapse and human misery.

Really, there is no historical analogy to show how rapidly human societies can rise to this challenge, because there is also no analogy for the matrix of problems (and opportunities) posed by climate change.

There are several optimistic signs that peak emissions may be near. From 2000 to 2013 global emissions climbed sharply, largely because of China’s rapid development.

But global emissions may now have plateaued, and given the problems that China encountered with pollution it is unlikely that other nations will attempt to follow the same path. Rapid reduction in the price of solar and wind energy has also led to substantial increases in renewable energy capacity, which also offers hope for future emissions trajectories.

In fact, we do not really know how fast we can decarbonise an economy while improving human lives, because so far we haven’t tried very hard to find out. Politically, climate change is an “aggregate efforts global public good”, which basically means everyone needs to pull together to be successful.

This is hard. The problem with climate diplomacy (and the reason it took so long to broker a global agreement) is that the incentives for nations to tackle climate change are collectively strong but individually weak.

This is, unfortunately, the nature of the problem. But our research suggests that a 1.5℃ world, dismissed in some quarters as a pipe dream, remains physically possible.

Whether it is politically possible depends on the interplay between technology, economics, and politics. For the world to achieve its most ambitious climate aspiration, countries need to set stronger climate pledges for 2030, and then keep making deep emissions cut for decades.

No one is saying it will be easy. But our calculations suggest that it can be done.

Source: The Conversation. Reproduced with permission.  

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  • Ali Pockley

    I reckon that we have well and truly passed this deadline, particularly if you look at mean temperatures for the past year.

  • Joe

    We set new temperature records all the time now and every decade since the 1970’s has been warmer than the decade before. When Julia Gillard / Labor implemented the ETS Australia’s carbon emissions began to fall. But then The Abbott / Liberals repealed the ETS and guess what, our emissions have climbed ever since. Australia’s temperature is up 1 degree Celsius since records began and our carbon emissions are still increasing. With that environmental abomination of Adani soon to be unleashed Australia will be leading the world in burning the planet. Coal is the future…a future Hell.

  • The closest historical analogy to show how rapidly humanity can achieve things is ozone depletion and the Montreal Protocol. That entailed rapid bans and restrictions. Regulations on cigarette smoking are the next closest analogy. These involved progressive restrictions on advertising, usage limits and increases in related taxes. Similar things are coming to a fossil fuel burner near you.

    The problem is there is this disconnect. Take for example this article Brisbane Airport in numbers: Why Australia’s leading airport needs to grow quickly. It highlights the installation of what is claimed to be “the biggest single rooftop site in Australia” and yet the writer fails to mention the growth of emissions from the transport sector. How many times do Australians see fossil fueled vehicles advertised and promoted with no mention of their impact on global heating?

  • Roger M

    Good news compared with IPCC Synthesis report 2015 that gave carbon budget of 1000Gt of CO2 in 2011 to give 66% of models showing less than 2 degrees (from complex models) and 400Gt for 66% of models beating 1.5 degrees.
    You are not very clear about which emissions are included. IPCC (2014) gave Fossil fuel burning/industrial processes 65%, Land use/forestry 11%. Methane/NOx etc at 24%. BOM state of climate 2016 shows atmospheric CO2 equivalent 2015 at 480+ PPM v. CO2 only at 400PPM.
    However with CO2 equivalent emissions at ?50-55 Gt per year and nearly 3 years since the start of 2015 that suggests remaining budget of 750Gt and not much sign of emissions reducing only stabilising.

  • D. John Hunwick

    There is NO chance at ALL of the earth remaining below 1.5 degrees C warming!! We have political systems that are so responsive to coal investment etc that there is no way that they will take either action on time OR large enough to be effective. Give over – here comes a hotter earth for our children and grandchildren and all that come after.

  • WR

    There is no chance of staying below the 1.5C level. The current concentration of greenhouse gases in the atmosphere is enough to produce 1.4C of warming. This is the current increase in the surface air temperature over land.

    It will take less than a decade to reach carbon levels that will produce 1.5C of warming. But it will take a couple more decades for the overall temperature to reach that level because of the lag introduced by the large thermal capacity of the oceans.

    For example, currently the warming over land is 1.4C, while the warming over the oceans is only 0.7C. The oceans warm more slowly than the land. But the ocean would eventually catch up if atmospheric carbon levels didn’t change. Currently the carbon levels are increasing. So the ocean’s temperature hasn’t had a chance to catch up to what is happening over land.

    Of course, by the time the ocean surfaces have reached the 1.5C mark, the atmospheric carbon concentration will probably be at a level that will guarantee well in excess of 1.5C of warming.

  • Bob Fearn

    Yooouwhooo, planet earth calling Dave Frame and H. Damon Matthews. Keeping climate change to a max of 1.5 degrees C is IMPOSSIBLE. Sheeeeeeeeeeeee.