Paris Agreement 1.5 °C Temperature Limit

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Head of Climate Science & Impacts

From Paris to the Special Report on 1.5°C and Beyond

Since 2009, more than a hundred Small Island Developing States, Least Developed Countries and many others have been calling for limiting global temperature rise to 1.5°C above pre-industrial levels to prevent the worst of climate change impacts. The inclusion of a 1.5°C temperature limit in the 2015 Paris Agreement was a major victory for vulnerable countries.

In 2018, the Intergovernmental Panel on Climate Change Change published a special report, which outlined climate impacts at 1.5°C of warming, underscoring the urgency for governments to act. It also showed that achieving this goal is feasible, outlining the global emission pathways needed to get there.

This page contains key information on the 1.5°C in the Paris Agreement, how to understand the Paris Agreement temperature goal and Frequently Asked Questions on 1.5°C.

1.5°C Frequently Asked Questions

  • How should the Paris Agreement Temperature Goal be interpreted?

    The Paris Agreement long term temperature goal is set as “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C above pre-industrial level

    This establishes 1.5°C as the long-term warming limit and with respect to emission reduction pathways should be interpreted as pathways that are very likely (i.e. a 90% chance) to hold warming below 2°C. Emission reduction pathways in line with this interpretation fall in the no-or-low overshoot 1.5°C category assessed in the IPCC Special Report on Global Warming of 1.5°C that provides Paris Agreement compatible benchmarks for mitigation efforts. Low overshoot pathways have a limited period of less than 0.1°C overshoot above the 1.5°C limit before returning to below 1.5°C by 2100 at the latest.  Changes in global mean temperature should follow the approach of the IPCC Fifth Assessment Report (AR5).

  • What is the Paris Agreement Long-Term Temperature Goal?

    The long-term temperature goal (Temperature Goal, or LTTG) sets the objective of the Paris Agreement and establishes 1.5°C as the warming limit in the long term. The purpose of the goal is to ‘reduce the risks and impacts of climate change’ as assessed in the science of the time, not to achieve a mere objective in terms of a temperature number.

    See also Schleussner et al. 2016. The Paris Agreement long-term goal architecture consists of the temperature goal in Article 2.1, and the mitigation goal in Article 4.1. Article 2.1a reads: “This Agreement, in enhancing the implementation of the Convention [the UNFCCC], including its objective, aims to strengthen the global response to the threat of climate change, […], including by:

    Holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing 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”
    The Temperature Goal establishes 1.5°C as the limit of long-term warming and accommodates two interpretations: establishing 1.5°C as an upper limit that should not be exceeded, or possibly allowing for a temporary exceedance (overshoot) of the 1.5°C warming level, while warming should always remain “well below 2°C” (see below).

    It should be noted that the Temperature Goal does not speak to temperature stabilisation (see below), but that 1.5°C represents an upper long-term limit. The global emission pathway to meet this goal and to ensure net zero greenhouse emissions are achieved soon enough are set under Article 4.1 of the Paris Agreement (see below). The global emission pathway described in Article 4.1 (peak emissions as soon as possible, rapidly reduce thereafter and achieve net zero greenhouse gas emissions in the second half of the century) is to be operationalised based on the best available science consistent with meeting the long-term temperature goal.

    The IPCC’s Special Report on 1.5°C is the currently best available science and provides benchmarks on when emissions should peak and when they would need to reach net zero to limit warming to 1.5° with no or limited overshoot. The Temperature Goal is explicitly linked to an assessment of climate impacts that it aims to avoid (see below on more background on this assessment) and is forward looking in nature.

  • How should “well below 2°C” be interpreted?

    Holding warming to “well below 2°C” is best understood as having a ‘very likely’ (90%) chance of limiting warming to 2°C. This is in line with no or low overshoot 1.5°C pathways established in the IPCC Special Report on Global Warming of 1.5°C.

    The use of holding warming “well below 2°C” in the Paris Agreement’s Temperature Goal represents a clear strengthening of the “below 2°C” language adopted in 2010 in Cancun. Due to inherent uncertainties linked to feedbacks in the climate system, such as from clouds, the oceans or the global biosphere, the climate response to greenhouse gas emissions is still subject to considerable uncertainties. To link emissions reduction pathways with a specific temperature limit we need to look at the probability of exceeding that limit.

    The science that informed the 2013-2015 Periodic Review and the adoption of the Paris Agreement was the IPCC AR5 (see below). In the AR5, and the wider scientific literature that fed into it, the “below 2°C” Cancun language was interpreted as a 66% probability of staying below 2°C (or “likely” in IPCC terminology). Pathways consistent with this interpretation would show a best estimate peak warming of 1.8°C. At the same time, there would be a probability to+exceed 2°C of warming under these pathways of 33% and an even non-negligible chance of exceeding 2.5°C of warming (6%) would remain.

    This is clearly not in line with the finding of the Periodic Review that establishes 2°C as a “defence line that needs to be stringently defended.” Crucially, these “likely” below 2°C pathways have about an 80% probability of exceeding 1.5°C, which is not compatible with the agreement to pursue efforts to limit warming to 1.5°C. The former goal of holding warming below 2°C with a 66% likelihood cannot be considered Paris Agreement compatible.

    But how should the strengthened “well below 2°C” limit be interpreted? Following IPCC terminology, a “very likely” (or 90%) chance of not exceeding 2°C would be an appropriate interpretation of holding warming to “well below 2°C” and stringently defending the 2°C “defence line.” The IPCC Special Report on Global Warming of 1.5°C, called for in Paris and published in October 2018, provides the most comprehensive assessment to date of greenhouse gas emission pathways that would meet the temperature goal of the Paris Agreement. It identified emission pathways with a “very likely” (90%) probability of not exceeding 2°C above pre-industrial levels. These pathways limit median (best estimate) warming to 1.6°C and are categorised as “no or low overshoot pathways.”

    In IPCC “likelihood” language, these pathways are “as likely as not” to limit warming to below 1.5°C, while a (temporary) overshoot above 1.5°C cannot be ruled out.< It is important to highlight that in some literature, the “well below 2°C” limit is interpreted simply in terms of a temperature difference (for example, a limit to mean warming of 1.7°C or 1.8°C). However, such an interpretation does not account for the geophysical uncertainties in future warming, and therefore does not provide a comprehensive picture. Emissions pathways with a median warming of 1.7 or 1.8°C have a one-in-four, or 25% or around one-in-three, or 33% chance of exceeding warming of 2°C. Hence such limits are not significantly stronger than the pre-Paris Agreement interpretation of holding warming below 2°C, and cannot be classified as “well below 2°C” pathways.

    Paris Agreement-aligned emissions pathways should always be well below 2°C (i.e. “very likely” below 2°C) and allow for, at most, the possibility of a slight overshoot above 1.5°C. Emissions benchmarks for these “no or low overshoot” pathways are available from the IPCC SR1.5.

  • How does the Temperature Goal relate to achieving net zero greenhouse gas emissions?

    The mitigation goal of the Paris Agreement aims to achieve net zero greenhouse gas emissions in the second half of the 21st century with the timing of when this must be achieved linked to meeting the long term temperature goal in accordance with the best available science. Achieving net zero greenhouse gas emissions will lead to long-term decline in warming, in line with pursuing efforts to limit warming to 1.5°C in case of a temperature overshoot.

    Following the IPCC Special Report on Global Warming of 1.5°C limiting warming to 1.5°C implies reaching net zero CO2 emissions globally around 2050 and concurrent deep reductions in emissions of non-CO2 forcers, particularly methane.

    Achieving net zero greenhouse gas emissions as set out in Article 4.1 of the Agreement will then further require net negative CO2 emissions to compensate for emissions of non-CO2 gases considered difficult to fully eliminate, such as methane from farming activities.

    The equivalence between CO2 and non-CO2 emissions is calculated based on the IPCC AR5’s Global Warming Potentials of each greenhouse gas over a period of 100 years (GWP100). This allows each volume of emissions to be quantified in terms of CO2e (CO2-equivalent), so that the balance between emissions and removals required to achieve net zero can be calculated.

    As some gases with a short atmospheric lifetime such as methane also have a strong near-term warming effect, the compensation of their remaining emissions by removals of CO2 (a less strong but long-lived greenhouse gas) will lead to a decline in temperature after the balance is achieved. Such a temperature decline is fully consistent with the continuation of “pursuing efforts” to limit warming to 1.5°C under an overshoot interpretation and establishes 1.5°C as the long-term temperature limit.

    Note that the temperature goal in the Paris Agreement does not speak to stabilisation of temperatures, but rather presents upper limits for warming. The question of temperature stabilisation is not addressed or included anywhere in the Paris Agreement. In fact, a long-term temperature decline would follow from sustained net-zero greenhouse gas emissions as also illustrated in the figure below. Such a temperature decline from peak 21st century levels will be essential for reducing long-term impacts of climate change such as ocean acidification (via reducing CO2 concentrations), sea level rise as well as reducing the risk of triggering tipping points of the earth system (see below). Whilst the use of a temperature stabilisation formulation was proposed at different stages in the Paris Agreement negotiations it was never accepted for inclusion due to considerations relating to the above put forward by vulnerable country parties.

    See also Schleussner et al. 2019.

    lt_warming_outcome_after_net_zero.png

    Figure 1: Illustration of the long-term warming outcome after reaching net zero greenhouse gas emissions. Net-negative CO2 emissions are required to compensate for remaining emissions of non-CO2 like methane from agricultural sources that will not be fully eliminated (Adapted from Schleussner et al. 2019).

  • What temperature metric should be used to assess progress?

    Tracking progress towards the Paris Agreement Temperature Goal should be based on Global Mean Surface Air Temperature relative to the 1986-2005 period. This is the approach used in the IPCC AR5, which forms the scientific basis of the Paris Agreement.

    Global Mean Surface Temperature (GMST) increased by 0.61°C between the 1850-1900 and 1986-2005 periods according to the main observational dataset used in AR5 (HadCRUT4, a global temperature dataset from monthly instrumental records combining sea surface and land surface air temperatures).

    Observations have their limitations: HadCRUT4, like many other observational datasets, does not provide global coverage but only reflects areas for which there are direct measurements and combines air temperature over land and sea-surface temperature over ocean.

    Recent improvements in observational datasets have slightly revised the amount of historical warming. Moreover, Global Surface Air Temperature (GSAT), which is a standard output of climate models and by default is available for the full globe, has not changed in the same way as the observational Global Mean Surface Temperature (GMST) since pre-industrial times. Improvements to the datasets, combined with a move to expressing historical warming in GSAT, could change historical warming estimates by around 0.1°C.

    Also, different Global Mean Surface Temperature datasets, for example from Berkeley Earth or NASA, show more warming over the 20th century between 1850-1900 and 1986-2005.

    However, these datasets have not been used in the IPCC AR5. It is thus problematic to apply them to track progress towards the Temperature Goal without providing an explicit comparison with HadCRUT4 as the temperature metric used in the IPCC AR5.  Any changes to historical warming do not alter the future warming trajectory and impact assessments upon which the Paris Agreement Temperature Goal was based. In particular, they do not affect our understanding of how far away the world was from 1.5°C at the time the Paris Agreement was adopted.

    Tracking progress towards the Temperature Goal should be done in Global Mean Surface Air Temperature relative to the 1986-2005 period. Historic warming should be added as a constant “offset” to warming relative to 1986-2005. If we translate the Paris Agreement Temperature goal into the AR5 approach by subtracting the 0.6°C historical warming estimate, the goal would be to hold warming to well below 1.4°C and pursue efforts to limit the temperature increase to 0.9 °C above 1986-2005 levels. The IPCC AR5 approach is also detailed in the figure below (compare the thermometers on the right and left-hand-sides).

    ipcc_ar5_embers.png

    Figure 1. An illustration of the IPCC AR5 approach to global mean temperature metric assessments relative to 1986-2005 and pre-industrial levels. From the IPCC AR5 Synthesis Report Fig. 1 Box 2.4 (basis for Fig. SPM. 10 and UNFCCC SED Fig. 4)

  • Is the Temperature Goal open for (re)-interpretation with different methodological approaches?

    No. Only methods and approaches from the IPCC Fifth Assessment Report (AR5) should be considered Paris Agreement-compatible. The IPCC AR5 provided the ‘best available science’ at the time of the Paris Agreement and informed the assessment of climate impacts at different temperature levels that underlies the Temperature Goal.

    It is sometimes argued that the Paris Agreement “does not define” what it means by pre-industrial temperature levels, or the temperature metric that should be used to assess warming, and therefore that these questions are open for interpretation.

    But just because a scientific metric is not explicitly defined does not mean it is unknown. On the contrary, the Agreement makes extensive references to (and use of) the “best available science,” providing a clear reference to the Fifth Assessment Reports (AR5) of the Intergovernmental Panel on Climate Change (IPCC), published in 2013 and 2014.

    Key scientific elements of the Paris Agreement’s mitigation architecture, such as the approaches to assessing changes in global mean temperature, the classification of mitigation pathways, and the use of global warming potentials to account for different greenhouse gases, should be interpreted based on the approaches in the IPCC AR5.

    The Temperature Goal is also explicitly linked to assessments of the risks and impacts of climate change undertaken by the UNFCCC’s 2013-2015 Periodic Review on the adequacy of the (previous) long-term goal of holding warming below 2°C (see the outcome of the scientific arm of the review here). The Review concluded that the “guardrail” concept, where up to 2 °C of warming was considered safe, was inadequate and should rather be seen as an upper limit – a line that needs to be stringently defended – and that efforts should be made to “push the defense line as low as possible,” noting that a warming limit of 1.5 °C would come closer to a “safer guardrail.” 

    Based on its assessment of the science as reflected in the Periodic Review, governments decided in Paris, 2015, that the long-term goal of the UNFCCC should be strengthened (see decision 10/CP.21). The impact assessment underlying the adoption of the temperature goal is therefore well-linked to the “best available science” at the time, i.e. as reflected in the IPCC AR5, and metrics and approaches used therein.

    As science progresses, so does our understanding of some of the key scientific concepts reflected in the Paris Agreement. Re-interpreting the Agreement’s goals with metrics that were not used to inform its drafting without providing transparency on how novel approaches differ from those deployed in the IPCC AR5 could lead to a mis-characterisation of the Agreement and its scientific underpinnings. It is of key importance to always compare new metrics with the metrics used in the IPCC AR5 to avoid unwittingly “shifting goalposts” and to maintain a line-of-sight between any new science and the science upon which the Paris Agreement goals are based.

    Since the IPCC AR5, the question of how global mean temperature is assessed in models (Global Mean Surface Air Temperature) and observations (Global Mean Surface Temperature) has received much more attention, and methodological issues with the GMST metric used in the IPCC AR5 in particular have become apparent. These were not well established or addressed in the IPCC AR5.

  • How much has the planet warmed above “pre-industrial” levels?

    The Paris Agreement’s Temperature Goal is based on warming since pre-industrial times (around 1.2°C to date). It should be measured following the approach of the IPCC AR5 report in 2014.

    Setting the guidelines for climate policy, the Temperature Goal is appropriately defined comprising the full scope of the problem at hand – in this case human-made warming – resulting from the greenhouse gas emissions since the industrial revolution.

    Scientifically, however, this is a little more complicated, as temperature measurements even for the early 1900s are scarce, with increased uncertainties around their quality. Scientists generally choose more recent reference periods. In the case of the IPCC AR5 this was 1986-2005, from which they derived temperature differences both forwards and backwards in time.

    The 2014 IPCC AR5, which serves as the scientific basis for the 2015 Paris Agreement, uses the 1850-1900 period as a proxy for the pre-industrial period. It assessed the global temperature increase between 1850-1900 and 1986-2005 to be 0.6°C – based on the HadCRUT4 dataset. This is the relevant reference for the Paris Agreement. A study using proxy-climate indicators has found that some human-made warming might have occurred before that, going back to the 1700s. This number, however, is very uncertain.

  • Have we already reached the Paris Agreement 1.5°C limit?  Or how close are we?

    The simple answer is no. According to the IPCC we might reach or exceed 1.5°C warming between 2030 and 2050 if we follow current emissions trends.

  • Temperatures have already increased by more than 1.5°C above pre-industrial levels in some regions – does this mean that the Paris Agreement’s 1.5°C limit has been reached?

    No. Some regions experience higher warming than others: this is the case for the Arctic, for example. Land area in general experiences stronger warming than the oceans. However, the Paris Agreement temperature goal refers to global average surface-air temperature.  We know land areas will warm faster than the global average, so that  there is a significant chance that many land area regions may exceed 1.5°C warming above pre-industrial levels even though the global average may not.

  • If a single year exceeds 1.5°C warming, is the Paris Agreement 1.5°C limit breached?

    No. The Paris Agreement refers to human-made, long-term temperature change, and not to temperature changes in individual months, seasons, years and not even over a couple of years. When measured over shorter time frames, temperature change is subject to natural climate variations, making it difficult to tease out  human-made changes from natural ones.

    The Paris Agreement, like all international climate agreements, is built on the definitions of the United Nations Framework Convention on Climate Change (UNFCCC) from 1992, which defines climate change as follows:

    “Climate change” means a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.”

    The Paris Agreement 1.5°C limit is clearly defined as human-made warming only, which comes on top of natural, year-to-year variations. On timescales of up to 10 years or more, this natural variability is the dominating force for global mean temperature variations.

    As the world keeps warming and we get closer to the 1.5°C limit, the probability of individual years exceeding that limit is expected to increase. Natural variability in annual temperatures around the long-term trend can reach a couple of tenths of a degree, meaning that an individual year might exceed 1.5°C even at today’s level of human-made warming (around 1.2°C). Nonetheless, should the world reach 1.5°C of global warming, it is expected that every other year would be 1.5°C warmer than pre-industrial levels.

    This dominance of year-to-year variability over short timescales is why climate science in general, and IPCC reports in particular, assess human-made long-term temperature change by averaging global mean temperature over periods of at least 20 years, thus avoiding the confounding influence of these year-to-year natural variations. Alternatively, scientific methods have been established to identify the human-made component in the observed warming signal. Such methods, however, come with uncertainties on the order of 0.1°C. This implies that we will only retrospectively be able to say with certainty if a certain warming level such as 1.5°C has been exceeded.

  • Will we reach the Paris Agreement 1.5°C limit in five years?

    No. Due to natural variability, individual years may exceed 1.5°C above pre-industrial levels, but this does not mean the long-term temperature limit is reached (see previous question). It is unlikely that we will reach the 1.5°C long term limit before 2030.  Given what we know about natural variability there is about a 20% chance that an individual year could exceed 1.5° warming in the next five years, with natural variability in that year adding to the underlying warming from human induced climate change.

  • But I have read that we might reach 1.5°C in the 2020s?

    Such estimates are based on interpretations of the 1.5°C limit that are not consistent with the Paris Agreement Temperature Goal. This can lead to confusing messages.

    The question of when 1.5°C of warming might be reached has recently received increasing attention and different “estimates” have been produced. Different dates do not mean there are necessarily scientific contradictions between the assessments. However, using an interpretation of how to assess 1.5°C above pre-industrial levels that differs from what can be reasonably considered as Paris Agreement-consistent also leads to different estimates of when “1.5°C” is reached. 

    The reasons for different messaging may be: assessments of annual temperatures instead of the human-made long-term warming, use of historical warming estimates different from the IPCC AR5, use of climate projections that overestimate historical warming, or combinations of these factors.

    Being aware of the sensitive policy context of the Paris Agreement 1.5°C limit requires very careful communication on how to interpret it. Not all headlines referring to 1.5°C give it due diligence.

  • But isn’t a new generation of climate models showing higher warming than its predecessors?

    A new generation of climate models (CMIP6) prepared for the IPCC AR6 report shows on average higher warming than previous generations. However, when evaluated against observed warming over recent decades, very high warming models can be ruled out as unrealistic and the projected temperature increase of the refined (observationally constrained) model ensemble is similar to those models used in the IPCC AR5.

    A new generation of climate models (CMIP6) show on average higher warming than previous model generations. This seems to be largely the result of changes in the representation of complex cloud feedbacks in those models.

    However, when evaluating strongly warming models against recent observations, the models that show much stronger warming than previously thought are similarly overestimating historical warming and can therefore be ruled out as too warm.

    The fact that several models overestimate historical warming does not mean that these models are wrong or “worse” than previous generations. There are a lot of useful insights the scientific community can gain from those models, they are just not the best tool to estimate global mean temperature increases (which is also not their sole or primary purpose).

    Observationally constrained model ensembles, however, provide the best estimate for modelled warming and estimate warming outcomes very similar to what the models assessed in the IPCC AR5 estimated.

  • But aren’t we already locked into 1.5°C of warming?

    It is sometimes argued that due to historical emissions of aerosols from air pollution or climate feedbacks, 1.5°C of global mean temperature increase above pre-industrial levels is already locked into the system with the present around 1.2°C warming. The best available evidence in the peer-reviewed scientific literature suggests otherwise.

  • But if we stopped emitting now, wouldn’t temperatures exceed 1.5°C?

    The scenario of zero emissions tomorrow is hypothetical but the IPCC SR1.5 has assessed the effect of a zero emission scenario to look at what warming might be ‘locked’ into the system and found it to be unlikely that warming will exceed 1.5°C.

    In a scenario of sudden net zero emissions, the temperature response and peak warming strongly depend on the interplay between short-lived climate forcers like methane (cooling effect) and aerosols (air pollution, warming effect). Under any mitigation scenario, emissions of both short-lived climate forcers and aerosols would reduce over time while net zero CO2 emissions would lead to no further warming.

    The IPCC SR1.5 has assessed it to be unlikely for warming to exceed 1.5°C under such a zero emissions scenario. It provides an analysis of this question in Chapter 1 and finds a best estimate for a “net zero tomorrow” scenario to lead to a mean additional warming of 0.1°C (see Figure below). Uncertainties remain, of course, particularly in relation to the interplay of different forcers. 

    Further reading: IPCC SR1.5 Chapter 1, Section 1.2.5. Smith et al. 2019

    warming_committment_from_past_emissions.png

    Figure 2: Warming commitment from past emissions of greenhouse gases and aerosols. The bars on the right-hand side indicate the median warming in 2100 and 5%–95% uncertainty ranges (also indicated by the plume around the yellow line). Source IPCC SR1.5 Figure 1.5

  • But what about the aerosols and global dimming?

    Aerosol emissions like sulphate particles or soot have a cooling effect on the climate, sometimes also referred to as “global dimming.” Reducing human aerosol emissions would thus lead to warming, although the exact magnitude is quite uncertain. The effect on peak warming strongly depends on the speed of the phase-out of aerosol emissions and reductions in other emissions such as short-lived climate forcers e.g. methane.

    Stopping all human aerosol emissions would lead to a near immediate warming response. However, like all sudden zero scenarios, this is hypothetical in nature and while aerosol emissions are projected to decline under emission reduction scenarios, they will not reach zero even by the end of the century. More gradual scenarios are thus more realistic.

    At the same time, the effect of other climate gases needs to be accounted for when assessing the effect of aerosols. Reducing short-lived climate forcers such as methane would lead to a net cooling effect. A simple “aerosol warming is locked-in” message thus misses the point.

    This is well illustrated by the effect of the downturn in emissions related to the COVID-19 pandemic that has led to a sudden reduction in global aerosol emissions of about 20%. A potential warming spike in response to reduced aerosol emissions, however, was largely balanced by cooling effects of other climate gases.

  • But wouldn’t we still experience warming if net zero emissions were achieved and atmospheric CO2 concentrations remained constant at today’s levels?

    Yes, we would. But once we reach net zero CO2 emissions, atmospheric CO2 concentrations will not stay constant. They will decline. Thus, the long-term warming under constant atmospheric CO2 levels is not of direct relevance for our ability to reach a certain warming target.

    Not all our human CO2 emissions end up in the atmosphere. In fact, the global carbon project assessed that more than 50% of our emissions are absorbed by the land biosphere and the ocean (leading to ocean acidification). These carbon sinks will be affected by climate impacts and warming, but will not stop operating after human emissions reach zero.

    Atmospheric CO2 concentrations are thus projected to decline once net zero emissions are reached. The magnitude of this effect has been recently assessed in a comprehensive study by the zero-emissions commitment model intercomparison project. While substantial uncertainties remain, including the effect of natural sources of greenhouse gas emissions such as permafrost,  all climate models assessed in the study show net zero emissions leading to a decline in atmospheric CO2 concentrations.

  • So, do we have warming in the pipeline after emissions reach net zero?

    The best scientific estimate of the warming commitment of reaching net zero CO2 emissions is that no further temperature increase would be projected beyond that point.

    A very comprehensive analysis by the zero-emissions commitment model intercomparison project has assessed the question if we have “warming in the pipeline” after net zero CO2 emissions are reached. The answer to this question depends on a complex interplay between different components of the earth system including the carbon cycle, ocean heat uptake and others, and significant uncertainties remain, of course. But the conclusions of this comprehensive assessment is that the warming commitment is most likely close to zero. The temperature response would also depend on the trajectories of aerosols and non-CO2 greenhouse gases.

  • And what about tipping points? Wouldn’t they bring us above the Paris Agreement 1.5°C limit?

    “Tipping elements” of the earth systems such as ice sheets, display unstoppable dynamics (i.e. melting) once dangerous thresholds are crossed. The exact thresholds for such tipping points are still very uncertain and limiting warming to 1.5°C greatly reduces the risks of crossing dangerous tipping points. But warming would also not “spiral out of control” if 1.5°C was exceeded.  

    Tipping elements of the earth system will exhibit highly non-linear or self-amplifying dynamics once dangerous thresholds, so-called tipping points, are crossed. For some tipping points, once triggered, their dynamic will be irreversible over thousands of years.

    There are several tipping points identified in the earth system such as the West Antarctic or the Greenland ice sheet, or the Atlantic overturning circulation (read more about them in the IPCC Special Report on the Ocean and Cryosphere). Crossing tipping points of ice sheets can have far reaching consequences, such as for long-term sea level rise.

    For very high levels of warming (significantly above 2°C), potential tipping points in cloud formations or the Amazon rainforest, or massive permafrost loss could also further amplify climate change globally. The exact threshold for many of those tipping points is not well established nor is the speed of their system response (ice sheets, for example, would melt away over hundreds and thousands of years). Limiting warming to 1.5°C would strongly reduce the risks of crossing such thresholds. But there is no scientific evidence that warming would “spiral out of control” if 1.5°C was exceeded.    We do know however that every increment of warming increases the risks of crossing to points and amplifying feedbacks, and that the risks increase rapidly above 1.5°C warming.

  • Can we still achieve the Paris Agreement 1.5°C limit?

    Very stringent emissions reductions in the coming decade are required in line with pathways that the IPCC Special Report on Global Warming of 1.5°C has assessed in its no or low overshoot 1.5°C pathway category. But it is clear that the world can still get there in particular by utilising the potential of a green recovery from the COVID-19 pandemic.

    However, it is also clear that present policies and commitments for climate action and emission reductions by 2030 are collectively grossly inadequate for staying below the Paris Agreement’s 1.5° limit. If climate action is not really significantly improved in the short-term through substantially strengthened 2030 emissions reduction targets in new and updated nationally determined contributions (NDCs) and full implementation of those NDCs, then meeting the Paris Agreement’s Temperature Goal may become infeasible.

    The purpose of these FAQs is not to provide a detailed assessment of what is required to get the world on a 1.5°C pathway (see the IPCC SR1.5 report or further reading here), but rather to briefly address some of the most common questions in regard to those pathways.  

  • What kind of analysis are these pathways based on?

    The future energy system is modelled by complex energy-economic so-called Integrated Assessment Models. The IPCC has provided an extensive analysis of different pathways from such models that fall into the 1.5°C compatible category. Recently, also the International Energy Agency (IEA) as well as the International Renewable Energy Agency (IRENA)have provided assessments of what it entails to get to net zero by 2050. There is a wealth of information from different sources available analysing the transformation.

  • But aren’t we running out of the remaining carbon budget?

    The remaining carbon budget for limiting warming to 1.5°C is small, and subject to considerable uncertainties. What is clear, however, is that it is continuously depleted and very stringent mitigation is required to stay within it.

    While being key in the long-term, the carbon budget alone is not sufficient to determine the near-term warming trajectory, and all greenhouse gases and climate forcers need to be considered. Furthermore, condensed messages like ‘we have 10 years left’ of carbon budget are scientifically problematic as irreducible uncertainties in carbon budget assessments do not provide the basis for such definite statements.

    The carbon budget is based on the finding that global temperature increase scales approximately linearly with the human CO2 emissions over time. This allows us to derive the remaining CO2 budget until a certain warming would be reached and the IPCC SR1.5 has provided an assessment of remaining budgets from 2018 onwards.

    Notably, carbon budgets are not a pure ‘scientific’ number, but require a range of value judgements to be taken. The most recent estimate by the CONSTRAIN project estimates a remaining carbon budget for 1.5°C (with 50% probability) of around 400 Gt CO2 from the start of 2020.

    While this is a strong indication that time is running out for limiting warming to 1.5°C, necessitating stringent emission reductions, the carbon budget itself does not one-to-one translate into immediate global mean temperature increase, as other anthropogenic emissions such as non-CO2 gases or aerosols affect warming, in particular near-term warming.

    The very large uncertainties with carbon budgets mean that at best they can only provide an orientation around whether or not a given temperature limit can be met, and in addition obscure some key features of the technically and economically feasible pathways which assess how to meet warming limits in practice.

    Exceeding the best estimate for the remaining carbon budget thus does not automatically imply that the temperature level will be exceeded in the near-term. Furthermore, energy economic pathways achieving the Paris Agreement deploy negative CO2 emissions which would bring them back in line with carbon budgets in the longer term, even if they would temporarily exceed them. The amount of carbon dioxide removal required differs strongly between different pathways (see next point).

  • But what about negative emissions?

    Pathways achieving the Paris Agreement net-zero greenhouse gas emissions goal will require some kind of removal of carbon dioxide from the atmosphere. The scale of removals required differs strongly between different approaches and very high levels of deployment have been established as being unsustainable. This rules out so called ‘high overshoot’ pathways relying strongly on negative emissions as an appropriate interpretation of the Paris Agreement temperature goal.

    In order to achieve net-zero greenhouse gas emissions as spelled out in Article 4 of the Paris Agreement (see above), negative CO2 emissions would be required to make up for remaining non-CO2 emissions (for example methane from agricultural production). 

    Pathways based on energy-economic models assessed in the IPCC deploy substantial carbon dioxide removals beyond that point which has rightly raised concerns about the sustainability of those measures. The IPCC SR1.5 has identified a range of different options to achieve negative emissions ranging from technical solutions like direct air capture, or bioenergy in combination with carbon capture and storage to afforestation, reforestation and ecosystem restoration. It has also identified sustainability limits for negative emission technology deployments as well as characteristics of pathways that minimise the requirements of negative emissions.

    The IPCC Special Report on Land further identified best practice deployment strategies for a range of different negative emissions options. These reports find that some amount of negative emissions is possible, but that large scale deployment of negative emission options of any sort would come with substantial risks and negative side effects.

    Limiting the dependence on future carbon dioxide removal is thus paramount and a matter of intergenerational equity. Applying equity considerations to negative emission responsibilities can also highlight the responsibilities for future negative emissions depending on near-term emission reductions. A recent study found that one gigatonne of extra emissions in 2030 could generate about 20–70 additional gigatonnes of negative emission responsibility over the 21st century for big emitting countries like China, the EU, or the USA.

Relevant science on climate impacts and vulnerability

How are we tracking against the Paris Agreement 1.5°C limit?

For more than a decade, the Climate Action Tracker, a collaboration between Climate Analytics and the NewClimate Institute, has been assessing government climate action and measuring it against the globally agreed Paris Agreement aim of “holding warming well below 2°C, and pursuing efforts to limit warming to 1.5°C.”

According to its latest update, published just ahead of the fifth anniversary of the Paris Agreement, if all national governments meet their 2050 net zero emissions targets, warming could be as low as 2.1˚C by 2100, putting the Paris Agreement’s 1.5˚C limit within striking distance.

Its warming projection of its ‘current policies’ scenario has fallen from 3.6˚C in 2015 to 2.9˚C today. This drop has come from governments implementing new policies, increased use of renewable energy, a downturn in the use of coal, and lower economic growth assumptions (both prior to and because of the pandemic).

Achieving the Paris Agreement’s long-term temperature goal of limiting global warming to 1.5°C requires urgent and comprehensive action by all governments and across all sectors. In addition to evaluating the effect of current pledges and policies, the Climate Action Tracker also shows concrete steps governments can take to align their policies with 1.5°C.