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.
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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.
Read more: What is a pre-industrial climate and why does it matter?
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.
Read more: Paris climate targets aren't enough, but we can close the gap
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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.
The Paris Agreement of 2015 has a central aim to keep global temperature rise this century well below 2°C above pre-industrial levels and to “pursue efforts” to limit the temperature increase even further to 1.5°C. This is an ambitious aim – global temperatures are rapidly approaching the 1.5°C target and the 2°C limit is not far away.
The path to 1.5°C requires that the world achieve zero emissions before 2050. It is imperative, therefore, that we stop burning fossil fuels, known as mitigation. However, our present trajectory suggests we’re not on track. COP24 can’t take its eye off this ball –- there is no long-term plan that doesn’t include zero fossil-carbon emissions. The scientific consensus is that we need to reach “net zero” CO₂ emissions by 2050. But to tack closer to a scenario of 1.5°C warming, COP24 should set this target for 2035.
Carbon removal and non-CO₂ emissions
The United Nations, in the IPCC Special Report on Global Warming of 1.5ºC has accepted that there isn’t any obvious pathway to zero emissions in such a short time frame, so they have pegged their hopes on NETs – Negative Emissions Technologies. These approaches include carbon capture and storage (CCS), which involves sucking CO₂ from the air and storing it deep underground.
Carbon removal along these lines is the second imperative for COP24 in Katowice. Globally we emit around 40 billion tonnes of CO₂ annually, so net zero CO₂ by 2050 will require CO₂ removal of this scale, starting immediately.
Read more: Explainer: what is carbon capture and storage?
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But CO₂ isn’t the only problem. We emit other greenhouse gases such as methane, nitrous oxide and chlorofluorocarbons (CFCs) which all contribute to climate change. Methane is on the rise and is 84 times more potent as a greenhouse gas than CO₂.
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It comes from cows, and it leaks from oil wells and coal mines as “fugitive methane”. It is also seeping out of the melting permafrost in the Arctic. This is a worrying form of “positive feedback” where global warming causes the further release of gases that cause further warming.
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Nitrous Oxide, which is 300 times more potent than CO₂, is rising too, caused by modern agriculture. And the concentration of refrigerant gases, such as CFCs, which are thousands of times more potent than CO₂, is not falling as fast as we’d hoped. So COP24 has a third imperative, to prevent the rise of non-CO₂ greenhouse gases. If we can stabilise non-CO₂ greenhouse emissions at present day levels we’ll be doing well, but concentrations are rising fast.
Desperate times, desperate measures
All of this is going to be hard work. We’re failing to cut down our emissions, the technologies for NETs don’t exist at any meaningful scale, yet and there are no political drivers in place to enforce their deployment. There is also a real risk of a dramatic rise in methane in the near future. COP24 will have to consider emergency plans.
One such plan is very controversial. There are so-called “geoengineering” technologies which can be used to cause changes in global temperatures. One of these is Solar Radiation Management (SRM), which involves injecting tiny aerosol particles high in the atmosphere where they reflect sunlight into space.
We know from the eruption of Mount Pinatubo in 1991 that stratospheric aerosols caused a cooling of around 1°C over a year. The northern winter of 1992 saw a dramatic increase in sea ice and a stalling of glacial melting. SRM technologies exist and the first sun-dimming experiments are underway.
There is a realistic possibility that deploying SRM can buy us some time to enact the essential measures needed to stop warming at or before 1.5°C. The discussions at COP24 must keep all options on the table, and as unpalatable as geoengineering technologies might seem, their deployment may prove to be unavoidable.
The indicators are all in the danger zone. We are seeing increasing Arctic temperatures, rapid loss of Arctic sea ice, reduced Arctic reflectivity, rapidly melting ice shelves and methane release from permafrost. These are leading to rapidly rising sea levels, coastal flooding and storm surges, increased hurricane and storm activity, dry and hot conditions conducive to wildfires, and drought and crop failure.
The urgency for decisive action is the imperative for COP24. The UN must press on with four major strands for meeting the Paris 1.5°C target:
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Reduce fossil carbon emissions.
Remove carbon from the atmosphere (NETs).
Halt the rise of emissions of non-CO₂ greenhouses cases (Methane, Nitrous oxide, CFCs).
Investigate techniques for geoengineering, including Solar Radiation Management.
All four of these must proceed simultaneously and in parallel. COP24 must make this perfectly clear. There is utmost urgency and no time to “wait and see”.