What does “well below 2 C” mean, and what are some of the issues with it?

by | May 14, 2017 | Blog, News and Updates | 0 comments

By Glen Peters, on Cicero, March 2017

The Paris Agreement calls to hold the increase in the global average temperature to “well below 2°C”. What on earth does that mean?

The ambition of the Paris Agreement has become well-known:

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.

If we are to stand any chance of meeting the ambitions of the Paris Agreement, we first need to define exactly what “well below 2°C” means.

Cumulative emissions

In its Fifth Assessment Report, the IPCC placed special emphasis on the concept of so-called cumulative emissions to describe the range of temperature increase we could expect according to how much carbon dioxide is emitted in total.

The concept is attractive for its simplicity: The temperature increase above pre-industrial levels is roughly proportional to the total (cumulative) amount of carbon dioxide emissions. There are nuances, but its simplicity makes the cumulative emissions concept a useful communication tool.

The IPCC assessed that the global average temperature was likely to increase by 0.8 to 2.5 degrees for every 1000 billion tonnes of carbon emitted. The range incorporates uncertainties in the climate system and carbon cycle.

Using this range as the basis for a statistical distribution, it is then possible to estimate the carbon budget with a given probability of keeping the temperature increase below a given level.

A “likely” chance

The IPCC often refer to a “likely chance”, which they define as 66% probability of keeping temperatures below a given level, say 2°C.

When the probability is greater than 50%, it means that the realised temperature will probably be less than 2°C. It turns out that a 66% probability of staying below 2°C gives a median temperature of about 1.6°C.

In a recent article, Rockström and colleagues suggested a 75% probability of staying below 2°C gives a median temperature of about 1.5°C.

What does this mean?

If one interprets “well below 2°C” as a 66% probability, then it is well on the way to “limit the temperature increase to 1.5°C”.

Why is this important?

This means that we can draw on the existing emission scenario literature used in the IPCC Fifth Assessment Report to study the implications of the Paris Agreement, including the 1.5°C ambition level. In other words, we already know what it takes to be “well below 2°C”!

I see this as a rather intriguing coincidence, and not by design. Though, perhaps climate negotiators are smarter than often assumed!

The relationship between the probability of staying below 2°C (horizontal axis) and the median temperature increase (vertical axis). The blue line indicates how these may look for a range of proposed future emission scenarios. The red line assumes the statistical relationship between the temperature increase and cumulative emissions using a normal distribution, as in the IPCC.

An inconsistency?

If a 66% probability of below 2°C is roughly consistent with a 50% probability of 1.6°C, then doesn’t this mean that there is a rather big inconsistency in the carbon budgets?

Yes, there are rather big inconsistencies.

According to the IPCC Synthesis Report, the total carbon budget for a 66% probability of 2°C from 1870 is 2550 to 3150 billion tonnes CO­2 while the budget for 50% 1.5°C is 2300 to 2350 billion tonnes CO2. These ranges clearly do not overlap.

There could be a variety of explanations for this.

First, the 50% probability for 1.5°C is based on two almost identical scenarios from one model.

Second, model variations can be large, and the model used for the 50% 1.5°C budgets, has a lower carbon budget than other models

Third, as I discussed earlier, there is a lot of definitional issues that can lead to differences between carbon budgets.

Forth, non-CO2 emissions take a large share of the carbon budget, and the non-CO­2 emissions could be rather different in 1.5 and 2°C scenarios. My estimates assume the non-CO2 emissions are the same across 1.5 and 2°C scenarios.

The lack of scenarios assessed for 1.5°C may simply mean that we don’t have enough data to estimate a carbon budget for 1.5°C.

A balance of sources and sinks

The Paris Agreement also calls:

…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.

This text has a variety of potential ambiguities, but most significant is the interpretation of anthropogenic sinks.

Carbon cycle scientists generally refer to sources as emissions from land-use change, and sinks from areas not undergoing land-use change.

UNFCCC reporting guidelines are based on managed areas, which may include a part of the land sink that does not undergo land-use change (e.g., forest remains forest).

The two approaches need to be reconciled, but it is unclear what policy makers had in mind when negotiating the Paris Agreement. The implications are potentially significant.

Using the carbon cycle definition, about half of the emission scenarios with a 66% probability of staying below 2°C are below zero by 2100, and all of those cross zero after 2080. This seems somewhat inconsistent with the balance in sources and sinks between 2050 and 2100.

This inconsistency needs to be reconciled.

The total greenhouse gas emissions for scenarios with a 66% probability of staying below 2°C. Only half of these cross the zero line in the period 2050-2100, which is a requirement of the Paris Agreement. This suggests the Paris Agreement is either stronger than a 66% probability or there is an inconsistency in the Paris Agreement.

Take home message

It appears that emission scenarios with a 66% or higher probability of staying below 2°C will be consistent with the Paris Agreement’s “well below 2°C”.

This means we know what it takes to meet the ambition of the Paris Agreement, and there is no need to wait for the IPCC Special Report on 1.5°C.

Planet could breach 1.5C warming limit within 10 years, but be aware of caveats

A new study shows how a switch in a major climate system could accelerate global temperatures to a 1.5C limit, but some scientists are challenging the assumptions

The 2015 United Nations climate talks in Paris delivered a historic climate agreement.
The 2015 United Nations climate talks in Paris delivered a historic climate agreement. Photograph: Francois Mori/AP

In the Brazilian city of São Paulo, more than 80 experts, including dozens of climate scientists, gathered back in March for a giant planning meeting.

As part of the United Nations Intergovernmental Panel on Climate Change (IPCC), the group from 39 different countries were starting their work on a major reportthat will tell governments and policymakers what kind of impacts they can expect when global warming reaches 1.5C.

That report is scheduled to be ready in late September 2018 and will assess in detail what’s known about the impacts 1.5C of global warming could have on societies, ecosystems and efforts to reduce poverty.

But new research published in a leading scientific journal suggests that just eight years after that report is published, the world might have already reached that 1.5C target – or at least one definition of it (some senior scientists disagree with some of the assumptions in the paper – read on for those important caveats).

Published in the journal Geophysical Research Letters, the research looks closely at the influence of a mechanism in the climate known as the Interdecadal Pacific Oscillation (IPO).

“The IPO is like the long-term version of El Niño – it’s like El Niño’s uncle,” says Ben Henley of the University of Melbourne and the research’s lead author.

When heat gets trapped in deeper layers of the Pacific Ocean, this is known as a negative phase of the IPO.

Since about the year 2000, the IPO hit this negative phase, which tends to slow down the rise in global temperatures that’s being caused by humans burning too many fossil fuels and cutting down forests.

But around 2014, scientists say that this IPO started to shift, possibly towards a positive phase that would act like an accelerator on global warming.

Henley and his colleague Andrew King, also at the University of Melbourne, wanted to know how quick global temperatures might reach 1.5C, relative to where they were between 1850 and 1900.

According to the paper, the “rate that global temperatures approach the 1.5C level is likely to be significantly quicker, or slower, depending on the IPO”.

After using the latest computer models of the climate and allowing for the added greenhouse gases in the atmosphere, Henley and King looked to see what the coming decades have in store depending on the phase of the IPO.

If the IPO turns positive, then the average across the models shows that global temperatures hit 1.5C in around 2026. If the IPO was to turn negative, then this delays the 1.5C threshold by five years or so.

Henley told me: “Policymakers have to be aware of just how quickly we are approaching this level. But that doesn’t mean that the target is not a sensible thing to have.

“While we might overshoot 1.5C, stabilising global temperatures at that level still remains a worthwhile goal.”

The paper also gives other projections for breaching 1.5C based on different assumptions and different models.

For example, if you wait until global temperatures go above 1.5C over a five-year average period, then some models suggest that if the IPO stays negative for longer, the 1.5C breach doesn’t happen until around the year 2040 (but this is an outlier in the paper).

Associate Professor Julie Arblaster, a climate scientist at Monash University who was not involved in the research, told me the research “highlights the role of natural or internal variability in the climate system in hitting climate targets”,

Arblaster pointed to one study published last year in the journal Nature Communications that suggested the switch to a positive phase of the IPO might have already happened.

She added: “Other things may also impact the timing of course, such as a large volcanic eruption, but this study helps by providing an estimate of the contribution of the climate system’s internal variability in hitting that target.”

Henley told me a key motivation for doing the sums on the 1.5C target was to help policymakers understand what kind of timeframe they have to work with.

When countries were negotiating for a new global deal to cut greenhouse gas emissions and slow the impacts of climate change, many smaller and less developed countries were worried that a 2C global warming target was setting the bar way too high. Some scientists also shared this concern.

So there was a push to have a 1.5C target included in the text of the deal.

Article 2 of the Paris agreement states that countries agree to keep global warming “well below 2C above pre-industrial levels” but to also pursue efforts “to limit the temperature increase to 1.5C … recognising that this would significantly reduce the risks and impacts of climate change”.

Wrong assumptions?

Bill Hare is an Australian climate scientist and founder of scientific consultancy group Climate Analytics. Hare is a veteran of the United Nations climate talks.

In an email, Hare said the paper showed “that very ambitious near-term mitigation is required to limit warming to 1.5°C and this analysis underscores this”.

But Hare and his colleagues also had some reservations about the paper’s conclusions.

Hare said the IPCC’s interpretation of when you can say that 1.5C target is breached was based on much longer time periods than the Henley and King paper.

He also said the assumption in the paper that greenhouse gas emissions would continue to rise under a “business as usual” scenario didn’t reflect how countries were taking action under the Paris agreement. Using more optimistic scenarios could bring temperatures down by as much as 0.2C by 2030.

Joeri Rogelj of the International Institute for Applied Systems Analysis in Austria, also pointed out that the convention in UN climate negotiations was to refer to targets in terms of longer time frames of 20 or 30 years.

He thought the paper’s assumptions that emissions would remain high were “not compatible” with the agreements countries had made in Paris, but he did agree that the paper demonstrated how urgent the issue was.

Professor Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, is a senior lead author on a chapter of the special report that will look at the impacts of 1.5C global warming on humans and natural environments.

Hoegh-Guldberg, who was not speaking from an IPCC perspective, told me the research would “wake some people up to the fact that exceeding 1.5C will happen within the next decade, give or take a couple of years”.

“One unknown that’s associated with the study is the effect that anthropogenic climate change might be having on these long-term climate patterns themselves. Some research groups have provided compelling evidence that patterns associated with El Niño, for example, may actually be amplified by warming.”

He added: “Many scientists have increasingly pointed to the unmanageable ecological and human impacts as average global surface temperatures exceed 1.5°C, and great economic and environmental costs that are likely to be associated.

“Unfortunately, past inaction means that we will exceed 1.5C no matter what we do. The key issue, however, is what we do next.

“People speak of overshoots as being one of the scenarios that we are likely to face. This is not an escape clause as the overshoot is likely to be catastrophic.”


  • The ‘best available science’ to inform 1.5°C policy choicesGlen Peters
  • Global Carbon Budget 2016Corinne Le Quéré, Robbie Andrew, Josep G. Canadell, Stephen Sitch, Jan Ivar Korsbakken, Glen Peters, Andrew C Manning, Thomas A Boden, Pieter P Tans, Richard A Houghton, Ralph F Keeling, Simone Alin, Oliver D Andrews, Peter Anthoni, Leticia Barbero, Laurent Bopp, Frederic Chevallier, Louise P Chini, Philippe Ciais, Kim Currie, Christine Delire, Scott C Doney, Pierre Friedlingstein, Thanos Gkritzalis, Ian Harris, Judith Hauck, Vanessa Haverd, Mario Hoppema, Kees Klein Goldewijk, Atul K Jain, Etsushi Kato, Arne Körtzinger, Peter Landschutzer, Nathalie Lefevre, Andrew Lenton, Sebastian Lienert, Danica Lombardozzi, Joe R Melton, Nicolas Metzl, Frank Millero, Pedro M S Monteiro, David R Munro, Julia E M S Nabel, Shin-Ichiro Nakaoka, Kevin O’Brien, Are Olsen, Abdirahman Omar, Denis Pierrot, Tsuneo Ono, Benjamin Poulter, Christian Rödenbeck, Joe Salisbury, Ute Schuster, Jörg Schwinger, Roland Seferian, Ingunn Skjelvan, Benjamin D Stocker, Adrienne J Sutton, Taro Takahashi, Hanqin Tian, Bronte Tilbrook, Ingrid T Van Der Laan-Luijkx, Guido R Van Der Werf, Nicolas Viovy, Anthony P Walker, Andrew J Wiltshire, Sönke Zaehle

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