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Analysis: Meeting Paris pledges would prevent at least 1C of global warming

Posted on 7 June 2017 by Zeke Hausfather

This is a re-post from Carbon Brief by Zeke Hausfather

President Trump’s decision to withdraw from the Paris Agreement on climate change has raised questions about the effectiveness of the accord, and how that will change without the US.

In his announcement, Trump incorrectly claimed the deal would avoid just 0.2C of warming. In fact, nine separate studies show, on average, that full implementation of current climate pledges would avoid 1C of warming, compared to a business-as-usual world.

Analysis by Carbon Brief finds that if the US reneges on its Paris pledge and takes no action to reduce emissions, it could result in around 0.2C to 0.3C additional warming, whereas a delay in implementation of four or eight years would have minimal impact.

Carbon Brief explains how these temperature estimates are made and explores the impacts of Paris, with and without US participation.

Paris climate impact

The 2015 Paris Agreement was signed by nearly every country on Earth. Countries choose their own nationally determined contribution (NDC) to the accord, with limited enforcement beyond public shaming, for those that fall short.

The first round of NDCs run through 2025 or 2030 and are not sufficient to meet the overall aim of Paris of keeping warming well below 2C above pre-industrial temperatures. As part of the agreement, countries agreed to reconvene every five years to assess progress and ratchet up their efforts.

Nevertheless, the cumulative impact of the NDCs – which reflect both current climate policies and new commitments – is significant. At least nine separate groups have released white papers or published peer-reviewed research since mid-2015 assessing the impact of the Paris Agreement on future warming. A summary of their projections for a business-as-usual world without Paris and a world where countries meet current Paris commitments is shown in the figure below.

Estimates of 2100 temperatures in business-as-usual and Paris commitment scenarios, along with the difference between the two from Climate Action Tracker, the International Energy AgencyFawcett et al 2015Rogelj et al 2016, the European Commission Joint Research CouncilClimate InteractiveMIT, the United Nations Environment Programme, and Lomborg 2015. Adapted and expanded from a prior compilation by Kelly Levin at WRI. Chart by Carbon Brief using Highcharts.

On average, these groups estimate that implementation of current commitments in the Paris Agreement would result in around 1.1C less warming in 2100, with estimates ranging from 0.17C to 1.6C. The groups differ a bit both in estimates of future reductions and of how much warming there would be in a business-as-usual scenario without any climate action. Some include just the NDCs submitted as part of the first round of the Paris Agreement, while others include further commitments made by some countries after Paris, for example additional pledges made at the COP22 UN climate summit in Marrakech last November.

The main challenge of determining the climate impact of the Paris Agreement is how to estimate post-2030 emissions, since the NDCs only extend through 2025 or 2030. Different groups take different approaches, but generally assume that Paris commitments remain in place post-2030 and that emissions continue on a similar path.

The Climate Interactive model, shown below, assumes that countries with absolute emissions reduction targets simply maintain constant emissions after 2030. For example, China’s emissions would peak in 2030 and stay level through 2100. Countries such as India, with commitments to reduce emissions as a percent of GDP, are assumed to resume business as usual emissions rates of change after 2030.

The figure below shows the carbon emissions trajectory in a business-as-usual case, a Paris commitment case, and an emissions trajectory that avoids more than 2C warming.

Projected CO2 emissions under business-as-usual, Paris commitments and 2C trajectories. The 2017-2030 period covered by existing commitments is highlighted, while emission projections after 2030 are shown by dashed lines. Business as usual and Paris trajectories were provided by Climate Interactive from their C-ROADS model; the 2C trajectory is based the IPCC’s RCP2.6 scenario. Chart by Carbon Brief using Highcharts.

Bjørn Lomborg’s 2015 paper in the social science journal Global Policy is the only study that finds a number close to the 0.2C “tiny, tiny amount” mentioned by Trump. Rather than have long-term emissions stay consistent with 2030 goals, Lomborg assumes that countries largely abandon mitigation efforts and that emissions quickly return to only slightly below where they would have been without Paris.

For example, Lomborg assumes that China’s CO2 emissions will continue to rise through 2080 even under the Paris Agreement, despite its pledge to peak emissions by 2030 and growing evidence that emissions may have already peaked.

Other studies, such as those in 2015 by Allen Fawcett and colleagues in Science and in 2016 by Joeri Rogelj and colleagues in Nature find Paris reductions of around 1.2C, similar to most other groups examined.

Impact of US emissions

While dramatic, the decision by the US to withdraw from the Paris Agreement does not necessarily commit the world to any additional warming. Prior to announcing the withdrawal, which will not actually take effect until November 2020, the Trump administration was already moving to gut climate policies put in place by the prior administration. A world where the US remained in the agreement, but failed to put any effort into meeting its targets, would have likely had the same impact on domestic emissions.

What really matters is what the US does in the future: does it completely eschew mitigation for the next 83 years and continue emitting CO2 at its current or even an increasing rate? Or does a future administration bring the US back into the fold and pursue deep reductions in CO2 emissions?

To help answer this question Carbon Brief used a simple climate model to analyse different emission scenarios to try to determine what the Trump administration’s policies will mean for global temperatures at the end of the century and our ability to limit warming to well below 2C in 2100.

Carbon Brief looked at five different scenarios for future US emissions. The first is a world where the US begins an immediate pathway toward deep decarbonisation, meeting its Paris commitments, continuing on to reduce emissions by 80% in 2050, and reaching net-zero emissions by the end of the century.

The second is a variation of this deep reduction scenario where the US abandons climate policies until a new administration in 2021 reverses course and pursues deep reductions. The third is a similar scenario where a new administration in 2025 (rather than 2021) does the same.

The fourth looks at a low business-as-usual scenario where emissions slowly return to where they were back in 2005 and remain roughly constant, and the fifth examines a high business-as-usual scenario where rapid economic and population growth swamp continued reductions in emission intensity, resulting in increased emissions through 2100.

Future CO2 emission scenarios for the US. The “Business-as-Usual High” scenario and “Deep Reduction” scenario were provided by Climate Interactive. The Business-as-Usual Low scenario was based on projections from the IPCC AR5 Scenario Database AMPERE 3 Base GCAM model. Delay scenarios are adapted from the Deep Reduction scenario assuming that the Deep Reduction mitigation trajectory begins at the later date, and that the maximum rate of reductions achieved is maintained until convergence with Deep Reduction. Chart by Carbon Brief using Highcharts.

These scenarios are purely illustrative; models that attempt to predict the future are always wrong but sometimes useful, and these provide a range of possible paths that the US could take.

It is hard to foresee a world where climate policy and mitigation are completely ignored for the rest of the century, and there is a strong argument that existing trends in energy systems and cost reductions in renewables and storage should at least keep emissions flat, if not lead to future declines. Action on a state or local level within the US could also drive additional declines in emissions regardless of policies adopted at the federal level.

To estimate the impact of these scenarios on global temperatures, Carbon Brief assumed the rest of the world pursues policies aimed at limiting warming to 2C following the IPCC’s RCP2.6 scenario. For each scenario, Carbon Brief used a simple climate model to determine the increase in temperatures over time from additional US carbon emissions relative to “deep reduction”.

Estimated additional contribution to global mean surface temperatures due to US emissions relative to the Deep Reductions scenario, assuming that the rest of the world follows a RCP2.6 pathway. Calculated by Carbon Brief using SimMod. Chart by Carbon Brief using Highcharts.

Here, the business-as-usual world where the US continues to increase its emissions results in just under 0.3C additional warming, similar to projections widely reported in the media. A somewhat more plausible scenario where US emissions remain flat through the end of the century yields just under 0.2C additional warming.

If the US delays action for eight years before pursuing deep reductions, there would be around 0.02C additional warming, while delaying only four years results in less than 0.01C additional warming.

While 0.2 to 0.3C additional warming may not sound like much, it would make it nearly impossible for the world to limit warming to 2C. Similarly, a small amount of warming can make a big difference at the margin, as Carbon Brief previously explained when discussing the difference between 1.5C and 2C warming scenarios.

A temporary delay in US emission reduction of a few years would have a much smaller climate impact, assuming that the rest of the world does not follow in the US’s footsteps.


While it is possible to meet a 2C target with temporary delays in US mitigation, the large additional emission reductions needed compared to current Paris commitments are daunting.

The most severe potential impact of a delay in US action on climate change is not that US emissions will be higher for a period of time, but rather that other countries may be less likely to meet their commitments or pursue the deeper reductions needed to set us on a 2C trajectory with the US not participating in the process.

Some businesses are already arguing that the US withdrawal puts them at a competitive disadvantage, and these voices will become louder as countries move closer to the net-zero-emission future required to limit warming to 2C.

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Comments 1 to 7:

  1. At what point in the temperature rise do we have to start accounting for fewer humans on earth. When that happens, less energy will be required to sustain those that survive. Perhaps then earth will find its equilibrium.

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  2. Lombergs study just doesn't make sense. From the article  it just appears to be assumptions that people wont cut emissions or hold to commitments longer term. How does that even qualify as a science study, and get passed peer review? Its vacuous, anyone could have claimed that, and it tells us nothing that we couldn't do just as a maths exercise.

    It's also little more than pessimism based on empty claims about China with no foundation for his claims. Why does he think China will act in that way? If anything, given their form of government it may be easier for China to stay with longer term policies than a democracy.

    He claims other countries will just give up gradually over time, but I see no reason why he would claim that. If you look at other environmental policies it's much less pessimistic. Many policies have stuck in place rather well, like efforts to reduce cfc's, reduce use of asbestos and ddt, etc. Of course there have been failures, and backsliding by some countries, companies, or individuals, but that is definitely not universal or predominant from what I can see.

    Possibly Lomberg thinks countries will see things as all just increasingly too hard for small returns. But he is just speculating, and being a total pessimist. This is not a good way for humanity to think, and we do have choices about how we think

    If you want to predict how countries might react, at least look at history so you have something tangible. One possible scenario in America longer term might be an alternating policy cycle of action and inaction on climate, depending on whether government is dominated by Republicans or democrats, so you get maybe 8 years of inaction, 8 years of action, 8 years of inaction in a repeating cycle. You should calculate that trend of emissions, over time as its the most likely outcome in America.

    In fact real issue in America over climate (and other matters) is a large disconnect between the population and Congress. The population want more done on climate change, and Congress want less done, but given the nature of things this will ultimately force some form of compromise and at least something will be done. In addition the market is moving towards renewable energy regardless of politics.

    The rest of the world seems to be less politically volatile or divided on climate change, eg Germany, Britain, NZ so forward trajectories are likely to be a bit more stable than America.

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  3. Everything I've read about Bjorn Lomborg points to the fact that he is a fraud.  I recall reading (but can't remember where, unfortunately) that he has been funded by some in the fossil-fuel industry to produce "studies" sympathetic to fossil-fuel interests.  I would discount his 2015 paper as worthless.

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    Moderator Response:

    [PS] No accusation of fraud. Please respect our comments policy.

  4. Well, it seems that even assuming the pledges (Paris is no more than that, simple good intentions) will come true, most scenarios above put us well beyond 2 ºC and close or above 3 ºC. That means a big chance of crossing tipping points that will put in motion positive feedbacks that will increase the emissions regardless what we do. So, it becomes somehow irrelevant to say that this or that agreement prevents more or less 0.5 ºC if we have crossed certain temperature thresholds, like 2º C. 

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  5. Paris looks more and more like a political pageant, meant to at least show the hoi polloi it's willing to look like it cares, but any real, substantive and verifiable action is NOT on the table. It all smells very Neoliberal......

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    Moderator Response:

    [JH] Vacuous sloganeering snipped.

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

  6. Recommended supplemental reading:

    Trump used our research to justify pulling out of the Paris agreement. He got it wrong.

    His administration cherry-picked my group’s findings to help make their case.

    Opinion by John Reilly*, Washington Post, June 7, 2017

    As scientists concerned with the very real impact of human activity on climate, my colleagues and I certainly hope our research reaches policymakers at the highest levels of government — which, apparently, it did, when White House officials cited our work to justify President Trump’s decision to pull the United States out of the Paris climate accord.

    Unfortunately, they got it wrong.

    In talking points released along with the president’s withdrawal, the administration referenced an MIT study from the Joint Program on the Science and Policy of Global Change, which I co-direct with Ronald Prinn, to bolster the proposition that even if all signatories to the Paris agreement met their obligations, “the impact on the climate would be negligible.” But that runs counter to the view, held by my colleagues and I, that the Paris agreement’s unprecedented global framework is necessary to address climate change.

    Click here to access the entire article.

    *John Reilly is co-director of MIT’s Joint Program on the Science and Policy of Global Change.

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  7. All right, Mr Moderator, I'll try to express myself more circumspectly:  In view of Bjorn Lomborg's reputation, I don't think any weight should be placed on his 2015 paper.

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