How bad could Trump's Paris Agreement withdrawal be? A scientist's perspective

How bad could Trump's Paris Agreement withdrawal be? A scientist's perspective
The U.S. failing to meet its Paris commitment would cause about $100 billion of damage to the global economy. Credit: Cammie Czuchnicki/

Even before the Paris Agreement was signed in December 2015, market forces and policy measures were starting to tilt the world toward a lower-carbon future. U.S. carbon dioxide emissions peaked in 2007, and Chinese emissions may have peaked in 2014. Solar energy, wind and energy storage are expanding rapidly.

Yet as a scientist and a climate policy scholar, I know market forces and current policies are far from adequate to limit the rise in global temperatures, as envisioned in the Paris Agreement.

And so the Trump administration's decision to withdraw from the Paris Agreement could have a range of consequences for the United States and for humanity. But how broad will these impacts be?

Part of the uncertainty stems from how the climate system will respond to humanity's greenhouse gas emissions. If we are lucky, the climate will be less sensitive than scientists think is most likely; if we are unlucky, it will be more sensitive. But most of the uncertainty arises from how the 194 other signatories of the Paris Agreement and the global economy will respond to Trump's decision.

The optimist's case

The Paris Agreement's long-term goal is to limit global warming to 1.5 to 2.0 degrees Celsius (2.7 to 3.6 degrees Fahrenheit) above preindustrial temperatures, or about 0.5 to 1.0 degrees C (0.9 to 1.8 degrees F) above the current global average temperature.

Current policies in the U.S., even without the power plant regulations proposed by the Obama administration, are adequate to reduce greenhouse gas emissions to about 16 percent below 2005 levels by 2020. But significant new policies at the federal and state level are necessary to meet the U.S. commitment under the Paris Agreement to lower its emissions to 26 percent to 28 percent below 2005 levels by 2025. Largely independent of Trump's decision to withdraw from the Paris Agreement, his obstruction of federal policy to cut greenhouse gas emissions means these targets are not likely to be met.

Meanwhile, however, China and Europe appear to be ready to take up the mantle of climate leadership that the U.S. is abdicating. And so if the U.S. departure from the Paris Agreement does not disrupt international progress, then Trump's move may prove largely symbolic. (Indeed, under the terms of the Paris Agreement, the departure will not take effect until November 4, 2020 – a day after the next presidential election.) Nonetheless, U.S. industry may suffer and the U.S. reputation as a reliable diplomatic partner certainly will.

But the planet will not notice much. Over the five years between 2020 and 2025, the U.S. will emit a total of about 2.5 billion more tons of carbon dioxide-equivalent greenhouse gases than it would if it got on a path to meet its 2025 goal. That's about the same as a 6 percent increase in one year's worth of global .

Until recently, the federal government used an estimate of the social cost of carbon dioxide – one way to calculate the damage caused by climate change – of about US$40/ton. Based on that estimate, the additional emissions caused by the U.S. failing to meet its Paris commitment would cause about $100 billion of damage to the global economy – not an insignificant number, but small in comparison to the size of the . If state governments in California and elsewhere pick up some of the slack left by federal abdication, as some governors are pledging they will, the damage will be less.

If, after Trump, the U.S. rejoins a healthy global climate regime and shifts with a few years' delay on to an emissions trajectory consistent with Paris' long-term goals, then the climate will not be much harmed by any transient U.S. lethargy. The main damage will have been to U.S. leadership, in the clean energy industry and in the world at large.

The pessimist's case

However, the Paris Agreement would not have happened without U.S. leadership. Perhaps, despite the efforts of China and Europe, it will fall apart without the U.S.

President Trump has talked often about reopening coal mines. This is unlikely to happen without significant subsidies – coal is in general no longer competitive as an electricity source with natural gas or, increasingly, solar or wind energy.

But if Trump's vision of a "canceled" Paris Agreement and booming coal economy were to be realized, an analysis my colleagues and I did shows that the costs to the U.S. could be severe. As I wrote in August:

"By the middle of the century, climate models indicate that global mean temperature would likely be about 0.5-1.6 degrees F warmer than today under the Paris Path, but 1.6-3.1 degrees F warmer under the Trump Trajectory. The models also show that, by the last two decades of this century, temperatures would have stabilized under the Paris Path, while the Trump Trajectory would likely be about 4.4-8.5 degrees F warmer."

Sea-level projections by the Intergovernmental Panel on Climate Change (IPCC), by our research group and by others indicate that global average sea level at the end of the century would likely be about 1-2.5 feet higher under the Paris path than in 2000.

Emerging science about the instability of the Antarctic ice sheet suggests it might be around three to six feet higher – or even more – under the Trump trajectory. And, due to the slow response of the ocean and ice sheets to changes in temperatures, the Trump trajectory would lock in many more feet of sea-level rise over the coming centuries – quite possibly more than 30 feet.

Quantitative risk analyses show that warming would impose costs on human health, on agriculture and on the energy system. It would increase the risk of civil conflict globally. And rising seas would reshape coastlines around the U.S. and around the world.

The ultra-pessimist's case

The pessimist's case assumes that future catastrophes will come from the climate and its effects. The ultra-pessimist looks elsewhere.

The Paris Agreement is a milestone within a cooperative system of global governance in which organizations like NATO, the United Nations and the European Union play key roles – a system which some of President Trump's key advisers seek to undermine.

If isolationist policies, including pulling out of the Paris Agreement and weakening the Western alliance, lead to a global trade war and thence to an economic depression, the shutdown of significant chunks of the economy could lead to a larger reduction in greenhouse gas emissions than any careful, deliberate decarbonization policy.

The U.S. saw a small version of this between 2007 and 2009, when the economic downturn was the primary driver of a 10 percent drop in U.S. emissions. Most economic models, including those used to produce projections of future , are not capable of modeling abrupt changes such as these.

Ironically, Trump's decision to withdrawal from global governance, including the Paris Agreement, would in this scenario lower emissions. But global depression is one of the most harmful ways possible to do that – one that would inflict great hardship on the American workers Trump purports to help.

Explore further

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Jun 03, 2017
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Jun 03, 2017
Again a number of eminent points in the matter.
Among other things, where, really, is the proof that all the numbers the "scientists" toss about are true? They expect elaborate, major policy to be built on what they say, with no verification offered that they aren't fabrications intended to advance profiteering investments.
Note that "alternative energy" systems, heavily invested in by Democrats, actually do great damage to the environment. Like the "validity" of the numbers "scientists" claim, the "news" does not address the issue of what they do to the environment. Among other things, windmill farms cause air downwind to be abnormally warm, and solar farms create heat island effects.
And, in fact, no agreement like the Paris accord will do any good until it is admitted that chemtrails cause climate change, not "fossil fuels". Frankly, "science" devotees admit this by their refusal to do anything more than ridicule that.

Jun 03, 2017
@bubba However, that heat can be turned away above our atmosphere to abolish climate change.
The heat will be reflected by increased cloud cover, precipitated by many more cosmic rays due to solar wind minimum until 2053

Jun 03, 2017
the rest of the world could leave trump, and its dumb supporters, enjoying their moronic delusional reality. If only the US wouldn't be the second largest polluter in the world

Jun 03, 2017
The proportionate

Except the industrial system that has already started changing the climate with its activity cannot be trusted to work even more rapid changes to our environment.

The science behind these geoengineering interventions is the barest clue to what will really happen. Meanwhile the Greenhouse Effect has been known for over a century while widespread public warnings we're going too far have been shoved in industry's face for decades. Industry has demonstrated it cannot be trusted with the kind of risky operations that geoengineering interventions entail.

We are much more confident that cutting humanity's additional emissions by about 12Gt CO2e annually would allow the carbon cycle to return to the balance today's species evolved to live in, natural systems doing what they've done to maintain that balance. Cutting more of the 39Gt CO2e humans emit would speed the return to balance. Much less risky to just cut back the harm we do.

Jun 03, 2017
The heat

Yes, if some Internet troll without any data or science is correct. But if thousands of climate scientists are correct, then that isn't happening.

Jun 03, 2017
Is today's science

No, that's today's fake science, coming from fake news sources like "UFO Sightings Hotspot" and "Above Top Secret". They're lies to protect petrofuel and other industrial polluters.

Jun 04, 2017
We haven't technically left anything as the treaty has never been approved by the Senate to begin with and Obama never had the authority to sign us in without the Senate.
Trump can talk all he wants about reopening coal mines but economically they are no longer worthwhile to run. Nat gas has taken over and is cheaper. Unless we start exporting more coal that is a pointless arguement.
People can blather on all they want about how we in the US are so polluting but China is not slowing down their coal power plants and won't be for some years yet. They are still building new ones and so is India and quite a few other countries around the world. The US has been cutting pollution for many years now and we are still cutting it.

Jun 04, 2017
We haven't

We haven't technically left anything because the Paris agreement says the US can at earliest now announce in November 2019 it's leaving a year from that time - a couple weeks after the presidential election day. Which would leave Trump campaigning for reelection on that issue, if he even survives until then.

But in fact Obama's executive action agreeing to it was empowered by the 1992 _UN Framework Convention on Climate Change_, negotiated by Bush Sr's admin and ratified by the US Senate like any foreign treaty is. And even Trump's threat to withdraw would require Senate approval to happen in November 2019.

China is in fact slowing its coal power plants, having just cancelled hundreds of them last month. Its emissions peaked in 2014, and it remains committed to the Paris agreement with its scheduled reductions. Unlike the US which is now a rogue nation. Besides, the US emissions aren't OK because of China's.

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