What will the US return to the Paris Agreement mean for earth?

By Nick Kilvert, ABC environment reporter

Within hours of becoming president of the US, Joe Biden has moved to recommit his country to the Paris Agreement - the internationally binding treaty to combat climate change.

In 2020, the US became the first country to withdraw formally from the Paris deal, despite being the world's second largest producer of greenhouse gases behind China.

When then- President Donald Trump signalled his intent to withdraw the US from the agreement back in 2017, he said he'd pursue a new deal that "protects our environment, our companies, our citizens and our country".

Now the US now looks set to return to the climate agreement after a 30-day notice period, and is expected to resubmit an emissions reduction target for 2030.

So what does this actually mean for climate change and global emissions targets?

$1.7 trillion over 10 years for green recovery

The Paris Agreement aims to keep the increase in global temperature to less than 2 degrees Celsius above pre-industrial levels, and to pursue efforts to limit that warming to 1.5C.

Since 2015, 190 countries have ratified the agreement, each committing to its own nationally determined contribution (NDC) - an emissions reduction target and a plan to achieve it.

Only seven countries - Iran, Iraq, Libya, Turkey, Eritrea, South Sudan and Yemen - are yet to sign.

The agreement works on a five-year cycle, with more ambitious targets expected each time the participating countries re-convene.

US contribution to emissions

The United States and China are responsible for around 45 percent of global greenhouse gas emissions.

According to the United States Environmental Protection Agency (EPA), the US is responsible for 15 percent of the world's greenhouse gas emissions.

That was equal to around 5.4 gigatonnes of CO2-equivalent emissions in 2018.

Prior to its withdrawal from Paris, the United States aimed to reduce its annual greenhouse gas emissions by 26-28 percent below 2005 levels by 2025, which critics said wasn't ambitious enough to keep warming below 1.5C.

However, given Joe Biden's strong rhetoric on climate change action, including a stated goal of net-zero emissions by 2050, it's likely that he will move to strengthen this 2025 target and submit a much more ambitious goal for 2030.

"The United States urgently needs to embrace a greater ambition on an epic scale to meet the scope of this challenge," his climate plan states.

"Getting to a 100 percent clean energy economy is not only an obligation, it's an opportunity."

For the US to achieve net-zero emissions it will need to overhaul its transportation, electricity and industrial sectors, which are responsible for 28, 27 and 22 percent of emissions each year respectively.

Billed as a green economic recovery, President Biden has already flagged his intention to commit $US1.7 trillion over the next 10 years to help achieve that transformation, while "leveraging additional private sector and state and local investment" for a total of around $US5 trillion.

Impact of US re-entering agreement will 'reverberate around the globe'

But the biggest impact of the US return to the Paris Agreement will not be on the domestic emissions, according to Mark Howden from the Australian National University's Climate Change Institute.

"The message really critically is to other countries. The waves from this change will reverberate around the globe," Professor Howden said.

"With the US coming on board with potentially trillions of dollars, you'll get these private industries coming on board and that will generate new innovations and technologies and prices will come down."

The issue for countries lagging behind in emissions reduction is that they may be penalised in the long term, he said.

"The vast majority of [Australia's] trade will be with countries that have a net-zero policy by 2050. It does raise the possibility of essentially carbon taxes on imports to those countries, and the possibility of a carbon-zero bubble," he said.

"If we're outside that bubble we've got real problems."

The United Nations COP26 climate change conference, slated for November this year in Glasgow, is the next UN platform for countries to announce more ambitious 2025 and 2030 targets.

The Biden administration is also planning its own international climate summit in the next few months, where countries are likely to be encouraged to up their commitments.

What this means for Australia

Australia has been accused of not pulling its weight compared to similar developed nations and by 2030 is aiming for a 26 to 28 percent reduction of annual greenhouse gas emissions, compared to 2005 levels.

Even as the list of countries with net-zero targets grows, Scott Morrison has refused to commit Australia to a hard deadline.

This week Australia's Ambassador to the United States, Arthur Sinodinos, told told ABC's RN Breakfast that the Australian Government will have to "take a look at what it does further on targets, in the context of any global summit on this - and COP 26".

However, when pushed on whether we would come under increasing pressure to present more ambitious emissions reduction goals, Sinodinos said targets aren't everything.

"The point the Government's always made is that targets are important, but what's also important is to have a plan," he said.

"I think in our case it's demonstrating what we have in common with the US - what we're already doing, and what we have to offer in terms of expertise to the US in these areas, including around areas like solar and energy efficiency in building."

But Professor Howden said there's no doubt the pressure will be on countries like Australia.

"The momentum for substantial progress at the forthcoming Glasgow conference really steps up," he said.

"Countries that don't do what is expected in terms of ratcheting up our targets will become increasingly isolated."

Can we still keep to 1.5C?

So if the US decision to recommit to Paris does precipitate increasingly ambitious global emissions targets, what does that mean for climate change?

Currently, a number of countries, including the US, are not on track to reach their commitments, and the world is not on track to keep warming within 2 degrees Celsius, according to the Climate Action Tracker.

The IPCC, the UN's climate science panel, has calculated the global 2030 and 2050 targets needed to give us a "high likelihood" of keeping warming to 1.5 degrees Celsius above pre-industrial levels.

"The pathway to have a high likelihood of landing at 1.5C means we have to have a 40-50 percent [emissions] reduction on 2010 by 2030, and be net-zero by 2050," said Brendan Mackey from Griffith University's Climate Change Response Program.

"If collectively the world community meets those targets, then we'll likely achieve the 1.5C warming goal.

"In which case the US coming back into the agreement is very significant, and greatly increases the likelihood of the 1.5C target being achieved."

But getting to net-zero isn't the only factor: the trajectory by which we get there is also important.

For instance, China has committed to a 2060 goal of achieving net-zero emissions.

If they begin reducing their emissions aggressively now, they will produce significantly less emissions on the way to 2060 than if they coast and make drastic, last-minute cuts.

That's why it's important that countries also meet their interim goals, Professor Mackey said.

"You've got three anchors: 2010, 2030 and 2050. That gives you a curve," he said.

"The 2030 target is critical."

Between 1.5C and 2C of warming, the IPCC predicts that coral reefs will decline from 70-90 percent to more than 99 percent.

Sea level rise will displace a further 10 million people by the end of the century at 2C, compared with 1.5C, and animal extinctions - as well as heat related mortality in people - will be far greater at 2C.

What the US does and how the world responds over the next few years could be the deciding factor.

-ABC