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2˚C or not 2˚C: that is the question

One of the most exciting things to emerge from the climate talks in Paris was the commitment to an unexpectedly ambitious long-term warming target of 1.5˚C. A heroic effort by a group of island nations and less industrialised countries managed to wrestle the diplomatic initiative and insert new text into the treaty, significantly increasing the scale of ambition (despite much foot-dragging by the usual suspects).

But what does that extra half degree mean in practice? Is it achievable, or wildy unrealistic? What kind of policies could produce decarbonisation aggressive enough to keep us under the threshold? Luckily, as part of the Paris deal, the IPCC was commissioned to produce some rigourously researched new reports answering exactly those questions. However, they won't be ready until 2018. In the meantime, this briefing provides a quick introduction to what must be done, in terms of decarbonisation and emissions concentrations, to meet the 1.5˚C goal.

1.5 or 2˚C: Background to the target

One of the few points of consensus to survive the Copenhagen COP of 2009 was an agreement that average global temperatures should not be allowed to rise more than two degrees Celsius over pre-industrial levels. (1)

Since then, study (2) after study (3) has emphasized the dangers of a two-degree-warmed world and the simultaneous difficulty of actually halting climate change there.

The final Paris agreement retains the two-degree target, while recognizing the importance of pursuing 1.5˚C. Article 2, Section 1 deals most directly with temperature limits:

"This Agreement, in enhancing the implementation of the Convention, including its objective, aims to strengthen the global response to the threat of climate change, in the context of sustainable development and efforts to eradicate poverty, including by: (a) Holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change." (4)

The agreement also directed the Intergovernmental Panel on Climate Change (IPCC) to draw up a report by 2018 on how to keep temperature increases below 1.5˚C.

Staying under 1.5˚C: what is required?

Compared to the 2˚C target, there has been comparatively little work done on the rate and extent of decarbonisation required to stay under a 1.5˚C threshold. What is clear, however, is that the extra 0.5˚C of ambition translates into a vastly more formidable challenge for the global economy.

For a start, temperatures have already risen by 1˚C: 2014 saw average global surface temperatures up almost 0.8˚C above pre-industrial levels (5), and in November 2015 the UK Met Office announced that, for the first time, global mean temperature at the Earth's surface had reached 1 °C above pre-industrial levels. (6)

It's also clear that the INDCs announced by the conclusion of the Paris Agreement, even if perfectly implemented, will not be enough to keep warming below 1.5˚C. Analysis from the UN's Environmental Programme released in November 2015 estimated that full implementation of existing INDCs would result in around 3.5°C of warming by 2100. (7)

"Boo, perhaps it isn't fixed after all"

How much carbon can we burn and still stay below 1.5˚C?

The concept of a “carbon budget” - the cumulative total of how much carbon can be emitted into the atmosphere without crossing temperature thresholds – came into prominence in 2012, when the campaign group released the well-publicised “Do The Math” report. This research found that, to stand a good chance of staying below 2.0˚C by 2100 (with”good chance” in this case meaning an 80% probability), the planet had a remaining carbon budget to 2050 of 565 billion tonnes of GHGs. In 2012, global annual emissions stood at 31.6 billion tonnes, and rising at a rate of 3.2% per year; the report thus found that the 2.0˚C carbon budget would be exhausted well before 2030. (8)

Since 2012, much work has been done on the concept of the carbon budget, and all of it points to the fact that, in order to hit a 1.5˚C target, the world's remaining carbon budget is vanishingly small. Indeed, the World Bank argued in a 2014 report that it is already gone entirely, estimating that even if all global GHG emissions stopped tomorrow, previously released carbon dioxide still extant in the atmosphere will continue to warm the planet to 1.5˚C above pre-industrial levels by the end of the century. (9)

Probably the most methodologically rigorous work done on a carbon budget for 1.5˚C was released in November 2014 by the IPCC, as an update to its Fifth Assessment Report. (10) Although often referred to as a carbon budget, the IPCC’s work also included extra warming from other factors like methane, nitrous oxide and fluorinated gases. It expressed the budget across a range of variables: final temperature increase by end of century, with options for 1.5, 2.0, and 3.0˚C of warming; and the probability of achieving such an outcome. The findings of the budget are expressed in the table on the next page.

The IPCC's carbon budget shows that, in order to have a 66% chance of hitting the 1.5˚C target, total emissions over the industrial period (defined as from 1870 onwards) must not exceed 2250 billion tonnes of CO2e. Subtracting carbon already emitted up to 2015, we are left with a global carbon budget of just 240 billion tonnes. With global emissions currently around 40.3 billion tonnes per year (11), the IPCC's figures imply we have just six years before the budget giving us a two-thirds chance of staying below 1.5 degrees is exhausted.

Source: IPCC, Climate Change 2014: Synthesis Report, Table 2.2, page 61

Accepting a lower balance of probability for hitting the 1.5˚C target buys a little more time, but not by much. As the budget above shows, a 50 per cent chance of hitting 1.5˚C would only increase the time scale to ten years at current emissions levels.

Of course, if emissions were to peak in 2015 and then to decrease year on year (which some signs indicate may have already started to happen - 12), the planet could – in theory – keep cumulative gross emissions below the 240 billion tonne red line. However, it would require aggressive decarbonisation on an almost unimaginable scale.

In order to have a 66% chance of staying below 1.5˚C, global emissions would have to peak in 2015 and then fall at a rate of 17% year on year. (13)

Given the intractability of carbon emissions from agricultural, transport, and industrial sectors, this is far beyond the range of feasible possibilities. Amongst other things, such a deep decarbonisation would require the mass adoption of vegetarian diets the world over, the complete phase out of fossil-fired transport within a decade, and a near-complete cessation of concrete production.

We'll need to turn all those off, for a start

Net zero emissions

Given that reducing gross global carbon emissions from energy use, transport, building, and agriculture to zero is, for all intents and purposes, impossible, the inclusion of a 1.5˚C target in the Paris agreement strongly implies that net global emissions must become negative during the second half of the century. In other words, within forty years the world needs to be removing more CO2e from the atmosphere than it is producing.

A recent paper in Nature Climate Change argues that, in order to keep below the 1.5˚C limit, global CO2e emissions must fall to zero no later than 2060, and then become net negative for the second half of the century, in order to compensate for hard-to-decarbonise sectors like transport and industry. (14) UNEP also argued in November 2015 that even staying within 2˚C would require “so-called ‘negative emission technologies’ such as bioenergy combined with carbon capture and storage.”

The Paris agreement itself supports this. Article 4, Section 1 resolves to peak global greenhouse-gas emissions as soon as possible; then, after 2050, it says that all anthropogenic emissions should be balanced with “removal by sinks”:

"In order to achieve the long-term temperature goal set out in Article 2, Parties aim to reach global peaking of greenhouse gas emissions as soon as possible, recognizing that peaking will take longer for developing country Parties, and to undertake rapid reductions thereafter in accordance with best available science, so as to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century, on the basis of equity, and in the context of sustainable development and efforts to eradicate poverty."

Several studies, including the IPCC reports, have also included scenarios where 1.5˚C is not seen as an absolute limit to warming, but rather a long-term target: in other words, temperatures might temporarily overshoot 1.5˚C during the 21st century, but then are brought back down to 1.5 degrees by 2100, making use of negative emissions technologies such as Carbon Capture and Storage (CCS) to scrub carbon dioxide out of the atmosphere.

Best case scenarios for achieving 1.5˚C

Some modelling has been done on how, in theory, the planet could achieve a long-term goal of 1.5˚C, even if that threshold is temporarily breached in the short term. (15) In a paper published in November 2015, the Energy Program of the International Institute for Applied Systems Analysis (IIASA) modelled deep decarbonisation policies consistent with a 1.5˚C goal.

These include:

  • The complete decarbonisation of the electricity system by 2050, and the electricity sector becoming carbon negative thereafter, largely through the widespread deployment of biomass with carbon capture and storage (BECCS) technology.

  • Large increases in electric vehicles and a 40-50% share of residual liquid transport fuel use in 2050 being supplied by biofuels.

  • Heavily curtailed future energy demand, with total demand for energy in 2100 remaining roughly constant with 2015 levels, despite rising populations and growing economies.

  • Global carbon pricing is assumed to have been in place from 2010 onwards in many of the 1.5˚C scenarios.

  • The deployment of negative emissions technologies for removing carbon from the atmosphere on a vast scale. The pace of CO2e sequestration would exceed the combined tonnage of today’s coal and oil industries, and potentially require as much land as one third of the world’s current croplands.

Each of these points presents potentially insurmountable obstacles, but none more so than net-negative emissions technology.

The limits to negative emissions technologies

A recent study in Nature found that relying on “negative emissions technologies” to reduce net emissions to zero, rather than reducing emissions more in the present, is “extremely risky”. (16) The report, a large overview of negative emissions technologies penned by no less than 40 separate authors, looked closely at the four leading ideas for pulling carbon back out of the air — bioenergy combined with carbon capture and sequestration (BECCS), direct air capture of carbon dioxide, massive afforestation, and so-called “enhanced weathering” of rocks – and found considerable drawbacks to each, if implemented on a scale significant enough to achieve negative emissions.

BECCS, for example, involves burning trees or plants to produce carbon neutral electricity and then sequestering carbon and storing it deep in the ground, thus making the process carbon negative. However, the land requirements for implementing BECCS at scale are colossal. In order to remove 3.3 billion tonnes of carbon from the atmosphere annually, the process would involve an area of land equivalent to the entire continental United States. (17)

Furthermore, the leading negative emissions technologies are still stuck on the drawing board. CCS, certainly the most developed of the technologies under consideration, has been around as a concept for decades, but very few scaled up test facilities have yet been built to see if the technology can be developed in practice. As Dr Tim Kruger from the University of Oxford has argued, “there is an implicit reliance on a suite of techniques that are essentially science fiction, and a negligible evidence base with which to determine whether they could be transmuted into scalable fact.”

The concept of “overshoot” is also inherently problematic. For one, the damage inflicted on the global economy by climate impacts of a +1.5˚C increase are likely to be considerable and may hamper the ability of nations to implement challenging and expensive negative emissions technologies.

Furthermore, the risks of crossing so-called “tipping points”, whereby the global climate irreversibly transitions into a radically altered state through feedback loops such as Amazonian dieback, Arctic permafrost methane release, or the collapse of monsoon cycles, increase considerably as global temperatures approach 2.0˚C.


With considerable understatement, the IIASA paper concludes that respecting a 1.5C goal is theoretically not beyond the realms of possibility, but it poses “a very challenging task”, and the “window for achieving this goal is small and rapidly closing”.

It's important to recognise the disconnect between what is being said, and what is being done. The Paris Agreement sets out the ambitions to avoid dangerous climate change, and recognises that bringing emissions down fast, and then absorbing carbon from the atmosphere, will be necessary. But, as yet, no one – not governments, nor private companies, nor international climate bodies – has begun to invest even a fraction of what would be required to translate these grand statements of ambition into reality.

Dr Kruger again: “It’s as if a new disease was discovered and governments around the world committed to its eradication, but failed to provide any of their own resources, or provide incentives to anyone else to mobilise resources, to develop a cure.”

So, while the increase of ambition represented by the new target should certainly be lauded, even a cursory glance at the research reveals how daunting a task getting anywhere near 1.5˚C would be. - Ben Martin, Editor


  1. Pre-industrial baseline: “There is not a reliable indicator of global temperatures back to 1750, which is the era widely assumed to represent pre-industrial conditions. Therefore 1850-1900 is chosen here as the most reliable reference period, which also corresponds to the period chosen by IPCC to represent a suitable earlier reference period.” - UK Met Office

  2. Eg: Turn Down the Heat: Why a 2˚C World Must Be Avoided, World Bank, 2014

  3. Eg: Fifth Assessment Report, Summary for Policy Makers, Intergovernmental Panel on Climate Change, 2013

  4. See COP21, Adoption of the Paris Agreement, UNFCCC

  5. Energy system transformations for limiting end-of-century warming to below 1.5˚C, Rogeli et al, Nature Climate Change, May 2015

  6. Global temperatures set to reach 1 °C marker for first time, UK Met Office, November 2015

  7. The Emissions Gap Report, UNEP, 2015

  8. See Bill McKibben, Global Warmings Terrifying New Math, July 2012

  9. Turn Down the Heat : Confronting the New Climate Normal. World Bank Group. 2014

  10. Climate Change 2014: Synthesis Report. IPCC, 2014

  11. Persistent growth of CO2 emissions and implications for reaching climate targets, P. Friedlingstein et al, Nature Geoscience 7, Sept 2014

  12. University of East Anglia, Global Carbon Project: Global CO2 emissions projected to stall in 2015, December 2015

  13. Energy system transformations for limiting end-of-century warming to below 1.5˚C, Rogeli et al, Nature Climate Change, May 2015

  14. Energy Program of the International Institute for Applied Systems Analysis (IIASA), here

  15. Biophysical and economic limits to negative CO2 emissions, P. Smith et al, Nature Climate Change, December 2015

  16. Scientists just undermined a key idea behind the Paris Climate Talks; Washington Post, December 7 2015

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