Four more years of Donald Trump could ‘delay global emissions cuts by 10 years’

Donald Trump’s decision to step back from international climate action pushes any prospect of hitting global emissions targets to “near zero”, according to new analysis.

Modelling suggests there are only very limited circumstances where theParis Agreement’s warming limit of 2C is met – and a US departure from the landmark 2015 deal restricts those options further for every term Trump is in office.

While the immediate effects will be felt at home, ultimately, the knock-on impacts of “the Trump effect” will be “even more detrimental” for international efforts, the study finds.

The research, published inEnvironmental Science and Policy, suggests two presidential terms of US inaction on climate change would create ripple effects across other nations. Ultimately, this could delay global emissions reduction by a decade.

The presence of aclimate scepticin the White House also means other countries would have to pick up the slack to stay within the 2C limit, adding a percentage point of reductions to each of their emissions pledges for every Trump term.

However, scientists not involved in the study have told Carbon Brief that simplistic assumptions in the modelling exclude crucial factors, such as “messy and unpredictable” domestic politics and the emissions cuts being made by individual US states.

造型巴黎

President Trump’sannouncementin 2017 that he intended to withdraw from theParis Agreementwas a significant setback for international climate politics.

Widely viewedas “disastrous”, there was speculation the decision could both lead to higher emissions and trigger an “avalanche” of further departures. However, according to the authors of the new paper, actual analysis of this historic event’s impact has been “sparse”.

Lead authorDr Håkon Sælenfrom theCentre for International Climate Research(CICERO) in Norway tells Carbon Brief they are the first team to investigate the knock-on effects it could have on the “dynamic cooperation process envisioned by Paris”:

“The novelty [of our study] is that it tries to quantify indirect effects, meaning the increase in other countries’ emissions due to the political effects of US withdrawal.”

The researchers used an “agent-based model”, which Sælen describes as “a computerised world where agents [in this case states] interact according to predescribed rules”. Such models are useful for understanding “adaptive systems” like the Paris Agreement that change in response to internal changes.

The model was designed to mimic the Paris Agreement’s key components, including the cycles ofnationally determined contributions(NDCs) in which parties make pledges about future climate action.

With initial emissions based on data from 2015, the model also simulates how countries mightraise the ambitionof their pledges every five years. This ramping up of ambition – the so-called “ratchet mechanism” – ismeant tohappenthis year, but mayface delaysdue to the coronavirus pandemic.

The model ran over one million scenarios based on different combinations of input parameters. The researchers were attempting to capture the full range of possible futures, within the bounds of reality and Paris rules.

The parameters included whether countries do or do not raise their ambition, by a little or a lot, and the influence other countries have on them doing so, as well as other factors including compliance with targets and faith in the overall Paris system.

The Paris Agreementstatesthat each new NDC “will represent a progression” from the existing one. While in principle this should prevent backtracking, for example in response to US departure, the authors note this is ”probably overly optimistic” and, therefore, run two versions of the model – one with this “progression principle” and one without.

The US features even in scenarios where it has left the agreement, butits emissionsremain at 2015 levels for the length of one or two Trump terms. This is basedon workthat suggests emissions will “remain approximately constant” for the length of Trump’s presidency.

The effects of the nation’s withdrawal then play out as other states respond to its static emissions with less ambitious pledges.

Using these components, the researchers determined how many scenarios resulted in a world where emissions remained within thebudget for 2Cof warming, as laid out in the Paris Agreement.

Most of the numbers used in the model are uncertain, so the model was run for multiple different values of those parameters. Sælen is clear that this still represents a very simplified version of the agreement:

“The decision rules must be simplified relative to the complicated decision-making procedures in governments…The model runs should be considered scenarios, not predictions.”

In other words, the model runs are not predictions of how countries will act, but instead reflect the different ways they might respond under various circumstances.

‘The Trump effect’

The researchers wanted to isolate the impact of the Trump presidency, creating three scenarios in which he never assumed office, serves for one four-year term and serves for two terms.

These scenarios assume that whoever was president instead of Trump would have remained part of the Paris Agreement, using the NDC adopted under former president Barack Obama.

When all other factors are kept constant, the researchers are then able to identify “the Trump effect” on other nations’ behaviour, global emissions trajectories and the likelihood of keeping emissions within the 2C target.

Overall, they concluded that the current president’s “laggardship” reduces the prospects for meeting the 2C target to “near zero”. Of their million model runs, only 0.3% stay within their emissions budget for 2C given one Trump term, and 0.1% given two terms.

However, even if the US had remained within the Paris Agreement, the modelling only suggests a maximum of 0.64% of scenarios would remain within the 2C limit.

According toProf Johannes Urpelainen, director at theJohns Hopkins School of Advanced International Studies, who was not involved in the study, this is to be expected. He tells Carbon Brief:

“We’re not on track toward 2C, so a model that correctly describes the current politics and economics of climate change should not lead to 2C.”

The authors acknowledge that “only under a very small set of conditions” do states remain within the budget,let alonethesmaller budgetrequired for 1.5C of warming – the stretch goal of the agreement. They conclude:

“Indeed, these targets might have proved out of reach even without US withdrawal. Nevertheless, US withdrawal will further reduce the prospects for reaching them”.

Pointing out the “substantial” differences between the Trump scenarios, Sælen also notes that not all of the model’s scenarios are equally likely, therefore the percentages of successful runs cannot be seen as strict probabilities.

The modelling suggests that under conditions that are otherwise beneficial for the Paris Agreement, where parties take cues from each other and trust the process, the impact of US withdrawal is highest.

In such scenarios, the 10-year delay in US emissions cuts the researchers expect will result from a longer Trump presidency has far-reaching effects.

It ends up delaying both global emissions reductions and the world’s “net-zero” date by a decade as well. This can be seen in the bottom-right chart below, where a high “reciprocity factor” – meaning countries exert a strong influence on each other – drives a wider gap between the “no Trump” (undotted line) and “Trump 2 periods” (dotted line) scenarios.

As the impact of US withdrawal plays out, the researchers conclude other parties would have to comply “nearly 100%” with their pledges and increase each of their NDC commitments an additional percentage point each for every term Trump spends in office, in order to stay below 2C.

Kelly Levinfrom theWorld Resources Institutetells Carbon Brief the paper shows the far-reaching impact of US inaction, as well as how other states will have to compensate as the Trump administration’s policies “chip away at our ever-shrinkingcarbon budget”.

‘Messy and unpredictable’

Urpelainen, who has previously publishedsimilar work, says while he thinks this is a “very good paper”, he questions the key assumption that US emissions will remain roughly the same due to leaving the Paris Agreement:

“I think really [this is the] lynchpin…Does the agreement have an actual impact on US behaviour, or is it just a way to codify what would happen in any case?”

The US has along historyof non-compliance with international climate agreements and yet there have still been domesticemissions cuts– for example, from the power andtransport sectors.

Another issue is that while the US may be currently on course to withdraw at the end of this year from the Paris Agreement, Levin points out that cities, states and counties accounting for two thirds of the US economy – “equivalent tothe world’s second largest economy” – remain committed to the agreement. This suggests “the worst scenarios in this paper may not materialise,” she says.

In fact, due to the nature of the modelling approach, the complexities of domestic politics are largely excluded in this study, saysDr Sylvia Karlsson-Vinkhuyzen, a political scientist at Wageningen University who was not involved in the work. She tells Carbon Brief:

“The whole modelling exercise assumes that it’s a one-level game, so countries decide based on what other countries do. But, in fact, I think it’s two-level game, it’s at least as much about domestic politics.”

While she says this is “much more messy and unpredictable”, Karlsson-Vinkhuyzen notes this plays out in significant ways – not least President Trump’s decision to leave the agreement in the first place.

Finally, she notes the model is optimistic in that it assumes nations take positive cues from each others’ NDCs.

在现实中,一些国家是莱克阀门ely to be less cooperative, while others may even undertake climate action because it is deemed “the right thing to do”, irrespective of other parties. “The truth is a mixture, there are different drivers for different states at different times,” Karlsson-Vinkhuyzen says.

年代ælen承认这些观点,但contrib指出utions by cities and other smaller actors would be hard for other countries to track if they are not included in an NDC. This means even if their emissions cuts were large the political impact on other nations would not be eliminated.

He says the effects of US withdrawal will be easier to estimate once data on new pledges and post-2020 emissions are available, but notes the postponement of COP26 in Glasgow until next year and the coronavirus pandemic “may take political attention away from updating of NDCs”.