Modelling suggests there are only very limited circumstances where the Paris Agreement’s warming limit of 2C is met — and a U.S. 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 in Environmental Science and Policy, suggests two presidential terms of U.S. inaction on climate change would create ripple effects across other nations. Ultimately, this could delay global emissions reduction by a decade.
The presence of a climate sceptic in 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 U.S. states.
Widely viewed as “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 author Dr Håkon Sælen from the Centre 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 U.S. 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 of nationally 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 might raise the ambition of their pledges every five years. This ramping up of ambition — the so-called “ratchet mechanism” — is meant to happen this year, but may face delays due 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 Agreement states that each new NDC “will represent a progression” from the existing one. While in principle this should prevent backtracking, for example in response to U.S. 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 U.S. features even in scenarios where it has left the agreement, but its emissions remain at 2015 levels for the length of one or two Trump terms. This is based on work that 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 the budget for 2C of 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 U.S. had remained within the Paris Agreement, the modelling only suggests a maximum of 0.64% of scenarios would remain within the 2C limit.
According to Prof Johannes Urpelainen, director at the Johns 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 alone the smaller budget required for 1.5C of warming — the stretch goal of the agreement. They conclude:
“Indeed, these targets might have proved out of reach even without U.S. withdrawal. Nevertheless, U.S. 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 U.S. withdrawal is highest.
In such scenarios, the 10-year delay in U.S. 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 U.S. 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 Levin from the World Resources Institute tells Carbon Brief the paper shows the far-reaching impact of U.S. inaction, as well as how other states will have to compensate as the Trump administration’s policies “chip away at our ever-shrinking carbon budget.”
“Messy and Unpredictable”
Urpelainen, who has previously published similar work, says while he thinks this is a “very good paper,” he questions the key assumption that U.S. 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 U.S. behaviour, or is it just a way to codify what would happen in any case?”
Another issue is that while the U.S. 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 U.S. economy — “equivalent to the 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, says Dr 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.
In reality, some countries are likely 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.
Sælen acknowledges these points, but notes contributions 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 U.S. 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.”