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10 December 2015 11:58

Scientists discuss the 1.5C limit to global temperature rise

Carbon Brief Staff

Multiple Authors

Coral reef over/under the Marshall Islands
Global temperatureScientists discuss the 1.5C limit to global temperature rise

One of the major talking points during the negotiations at COP21 in Paris has been whether the international community should aim to limit global temperature rise to the internationally accepted 2C above pre-industrial levels, or a more stringent target of 1.5C.

The draft agreement text published last Saturday gave two options: “below 1.5C” or “well-below 2C”. The updated text issued on Wednesday afternoon added “below 2C” as a third option.

With global temperatures rise set to pass the 1C mark this year, is a 1.5C limit feasible? What would achieving it mean in practice? And how would a 1.5C world compare to a 2C one? Carbon Brief asked scientists here in Paris for their thoughts.

High ambition

A collection of countries looking for an ambitious deal to curb warming at 1.5C has emerged strongly during the Paris conference. Known as the “Coalition of the High Ambition”, the group spans small island states already feeling the effects of climate change through rising seas and extreme weather, to the big emitters such as the EU and the US.

Along with differentiation and climate finance, deciding what the long-term temperature goal should be is one of the three main elements in the text that still need agreement, Laurent Fabius, the French President of COP21, told a press conference on Wednesday.

That 1.5C is garnering so much support is a welcome surprise, says Prof Joanna Haigh, co-director of the Grantham Institute for Climate Change and Environment. She tells Carbon Brief:

I suppose the whole idea that it’s actually being talked about is incredibly encouraging. Here we are coming into COP thinking how are we going to try and get everybody to go to 2C, now they’re talking about 1.5C. I never would have expected that. Of course, the realism of how it might be achieved is very, very difficult.

The growing momentum is the impact of the more inclusive process employed by the COP this time around, says Prof Chris Rapley, professor of climate science at UCL, where countries submitted their pledges in advance of the conference. He tells Carbon Brief:

I think what we’re seeing is the impact of the process by which the INDCs [Intended Nationally Determined Contributions] came forward…The whole topic of conversation has shifted from, “should we be working together, is it right that we should all work in a globally collaborative way” to now “have we set ourselves sufficient ambition, should we crank up the ambition?” This is why I see this as such a positive point in this long, laborious three-decade process. I think that the mood music has completely changed. The zeitgeist has changed.

Scaling up impacts

How different would a 1.5 and a 2C world be in reality? Carbon Brief spoke to Sir David King, the UK Foreign Secretary’s Special Representative for Climate Change:

What every scientist would respond is by saying ‘actually this is all about probabilities’, and so the probability of dangerous events overwhelming societies simply gets bigger the higher the temperature rises since the pre-industrial time.

You can hear more from Sir David in the clip below.

Dr Rachel Warren, a researcher in climate impacts and mitigation at the University of East Anglia, tells Carbon Brief:

In the 5th assessment of the IPCC, when we assessed the reasons for concern about climate change and we considered unique and threatened systems, we found that a transition from moderate to high risk to those systems occurred somewhere between 1.1-1.6C above preindustrial, whereas by 2C the risks to those systems were already high.

Dr. Carl-Friedrich Schleussner, a scientific advisor at Climate Analytics, tackles the differences between 1.5C and 2C of warming in a new paper, which is currently under review in the journal Earth System Dynamics. He tells Carbon Brief:

Climate impacts, such as heat extremes, crop yield reductions in tropical regions and subtropical water scarcity, are projected to rise significantly between 1.5C and 2C. Under a 2C warming, annual water availability in the Mediterranean is projected to reduce by nearly 20%, a doubling compared to 1.5C.

Particularly for developing countries in the tropics, the combination of high exposure and limited capacity to adapt to rising temperatures points towards a “substantial increase in risks posed by climate change” from 1.5C to 2C, he says.

An extra 0.5C of warming could have a big impact on our oceans, too, Schleussner adds:

This difference in warming is very likely to be decisive for the survival of tropical coral reefs, as under 2C nearly 100% of tropical coral reefs are at risk of annual bleaching events.

Dr Richard Betts, head of climate impacts at the Met Office Hadley Centre, tells Carbon Brief one clear difference between 1.5C and 2C is the risk of long-term major sea level rise:

The IPCC 5th Assessment Report described how the long-term consequences of an approximately 2C scenario could potentially be a commitment to a long-term global sea level rise of between 0.5m and 1m over the next 400 years, depending on whether the melting of the Greenland ice sheet passed the point of no return. Keeping warming at levels lower than 2C would be expected to reduce the chances of this, and this is one reason why the Small Island Developing States are pushing for 1.5C rather than 2C.

Negative emissions

While it’s clear that limiting global temperature rise as much as possible is a good thing, emissions cuts pledged by individual countries so far are enough to keep warming somewhere around the 3C mark. So, what would achieving a 1.5C limit mean in practice?

Dr Glen Peters, a senior researcher Center for International Climate and Environmental Research (CICERO) in Oslo, tells Carbon Brief:

To keep below 1.5C with a ‘likely’ chance [implies] a very, very small remaining carbon budget. If you use the IPCC’s cumulative emission budget that they published in their synthesis report, we have about 400bn tonnes of CO2 to emit [before we] go over 1.5C – and that’s starting in 2011. At current emission rates, that budget will go by about 2020 … If you want to emit for longer, then you’ll have to use large amounts of negative emissions.

Carbon budget countdown infographic for different levels of warming

How many years of current emissions would use up the IPCC’s carbon budgets for different levels of warming? Credit: Rosamund Pearce/Carbon Brief.

While there are uncertainties and questions about large-scale negative emissions, talking about a 1.5C limit represents an important scaling up of ambition, adds Rapley:

[A 1.5C limit] has all sorts of implications about biofuels, CCS and so on that have proved quite intractable up until now. But if that comes through as a commitment, that will be a game-changer. That will really push the level of ambition that countries have to aspire to, and that’s great.


How feasible is achieving a 1.5C limit in reality?

Peters explains the most talked-about technology that could achieve negative emissions, burning biomass and capturing the emissions before they reach the atmosphere – a process known as Bioenergy with Carbon Capture and Storage (BECCS). Peters tells Carbon Brief:

If you look at the CCS side, we’re much further behind in terms of progress in terms of where we thought we would be 10 years ago. CCS is a technology we’re having lots of problems [with], both on the technical aspects but also on the political aspect to encourage more research, development and innovation and so on. So CCS is coming along much slower than we thought. And there’s also issues about storage – the feasibility of storage at that scale, public acceptability of storage, and so on – so there’s a whole range of questions.

The second aspect is the supply of bioenergy, and the huge demand that puts on land, says Warren:

In order to achieve [a 1.5C limit] many of the integrated assessment model outputs show that we would need to deploy large amounts of bioenergy with carbon capture and storage – that has a large demand for land. So it is absolutely critical that if we aim for a target like that, that we manage very carefully the allocation of land between growing crops for bioenergy, growing crops for food production and protecting biodiversity.

Betts echoes the point that land-use change associated with BECCS goes beyond just crops:

The implications of devoting very large land areas to the purpose of climate change mitigation would need to be thought through, as we also need land for food production, and people often also value biodiversity and wilderness which further limits the extent to which we can entirely take over landscapes for farming of food and fuel. Basically there are no easy answers!

On the question of whether achieving a 1.5C target is feasible, Warren continues:

I think it’s very difficult to say whether it’s feasible. I understand that there are some economic models which can produce that. The issue really is what assumptions are in those models and how credible is the information in the models about the use of bioenergy with carbon capture and storage in relation to things like how productive will our agricultural systems be in future. Because in order to reduce that issue of competition for land, we will need to make our agriculture much more efficient and productive without using huge amounts of nitrogen fertiliser, which would then increase greenhouse gas emissions.

Peters takes a less optimistic view, telling Carbon Brief:

Bioenergy has questions, CCS has questions, and when you combine them you’re amplifying the challenges and the feasibility of negative emissions … [G]iven the emission reductions required, the scale of negative emissions, and progress based on INDCs that have already been submitted, I think 1.5C is essentially gone.


But the feasibility of 1.5C depends on whether it is defined as a threshold that isn’t to be crossed, or a target that you can overshoot and come back to, says Dr Joeri Rogelj, a research scholar at the Energy Program of the International Institute for Applied Systems Analysis (IIASA):

From an energy system perspective and geophysics perspective, I think it is very hard to imagine that we would keep warming to below 1.5C. There is just so much inertia, and emissions would have to drop so quickly, so rapidly, that actually the vast majority of scenarios that we have assessed basically exceed 1.5C during a certain period and then drop down afterwards. 

So, in the end, it boils down to can we reverse climate change? Can we peak and decline temperatures? That’s basically what feasibility of 1.5C means.

While there are technical challenges around large-scale negative emissions, these aren’t insurmountable, Rogelj tells Carbon Brief:

All the components that we need are there. The technical problems we have to solve, they’re not of the level of finding a new energy source or nuclear fusion – these are incremental, technical problems, which I think over the course of 10 or 20 years are very solvable – if you have the will and there’s funding to do it.

State of the science

It’s clear there’s a lot more research that needs to happen to understand if and how global temperature rise can be maintained at 1.5C in the long term. Until recently, the focus of the scientific community has been on 2C of temperature rise, or more, says Rogelj:

I think [1.5C] is definitely a neglected target, and it’s really a pity.

For example, of the four main scenarios of future greenhouse gas concentrations that the IPCC used in their Fifth Assessment Report (AR5), the most ambitious in terms of cutting emissions limits global average temperatures to below 2C by 2100. There isn’t one that keeps temperature rise to 1.5C, says Rogelj, but this is set to change:

In fact, there is a new scenario for 1.5C that has been developed for climate scientists to use. It’s definitely something that will be taking off more in the coming years.

Here’s more from Rogelj on the “very exciting” research around 1.5C:

Meanwhile, prompted by countries’ support for a 1.5C limit, the UNFCCC has “invited” the IPCC to choose 1.5C as the topic as the next focus of a special reports.

Prof Jim Skea, the Research Council’s UK energy strategy fellow based at Imperial College London and recently elected co-chair of IPCC working group III, told Carbon Brief at a side-event in Paris yesterday:

The IPCC is now dealing with more than 20 requests for special reports over the next cycle, of which 1.5C is one. We will be thinking carefully about which ones we can do. Clearly, if that request comes from the UNFCCC [CB: which the current draft of the Paris deal does – to be delivered by 2018] it’s got to be taken extremely seriously. But we would need to measure it against whether there is the scientific literature that we can assess that actually casts light on the particular proposal.

Informal discussions have been taking place between the co-chairs for the IPCC working groups to assess what literature is available and how quickly it could be generated for the IPCC to assess, says Skea. But it’s likely there will be a new wave of research of the back of the UNFCCC’s request. Skea says:

I think if there’s a very clear message from UNFCCC that they want evidence on 1.5, I imagine people will gear up to it.

Civil society groups at the Paris Climate Conference calling for 1.5C deal.

Numerous civil society groups came together at the Paris Climate Conference to call for a 1.5C limit agreement in the global climate deal. Credit: Takver/Flickr.

Extra quotes from scientists:

The Science Media Centre in the UK has collected comments from scientists about progress at COP21. Here’s a selection of what they had to say on the 1.5C limit:

Prof Piers Forster, professor of climate change at the University of Leeds:

“It appears the ‘below 2C’ option is now back on the table, compared to last weekend’s draft that had ‘well below 2C’ as the weakest long term commitment.  My hunch and hope is that this is a negotiating ploy and we will end up with ‘well below 2C’ in the final draft.”

Prof William Collins, prof of meteorology at the University of Reading:

“If the agreed limit on warming is reduced to 1.5 degrees this will be very challenging to achieve and will require removing carbon dioxide from the atmosphere by the end of the century. The most ambitious scenario assessed by the IPCC had an average warming of 1.6 degrees above pre-industrial and we are currently exceeding the emissions in that scenario.”

Prof Dave Reay, professor of carbon management and education at the University of Edinburgh:

“The overt inclusion of a 1.5 degree C target is good to see, but it puts the big shortfall in collective emissions reductions into even starker relief. Even if every nation delivers on its stated plans – and that’s a big if – we would still be on a 3 degree pathway to dangerous climate change.”

Prof Myles Allen, professor of geosystem science at the University of Oxford:

It is great to see both “net zero” and the possibility of scaling up mitigation efforts to limit the temperature increase to 1.5 degrees. Even if they don’t feature in the final text, the fact that they have made it so far suggests many governments understand that 2 degrees already takes us into uncharted territory, and that the only way to stabilise temperatures, without resorting to the smoke and mirrors of geo-engineering, is to reduce CO2 emissions to zero.

“The IPCC is crystal clear on this — without CCS, we can’t make these ambitious temperature goals. The UK still has a slim chance to take a lead on this critical technology, but that chance took a serious knock with the government’s inexplicable decision to cancel the Peterhead project at the last minute.”

Dr Matthew Watson, reader in natural hazards at the University of Bristol:

“Any agreement to cut emissions and reduce our dependency on carbon-based fuels is a really good thing. However, an agreement to keep to below 1.5 degrees C of warming has significant and, worryingly, unstated consequences.

“Current scientific thinking suggests we cannot limit warming to less than 1.5 degrees without large scale intervention in the climate system.  Implicit in this agreement, then, is acceptance of some form of climate engineering…We must enter into this agreement with our eyes wide open.”

Main image: Coral reef over/under the Marshall Islands. Credit: Luiz A. Rocha/
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