A new analysis shows how a second Trump presidency could alter the course of U.S. carbon dioxide emissions.

Why it matters: By rolling back many of President Biden's climate policies, a second Trump administration could lead to an estimated extra 4 billion tonnes of CO2 emissions by 2030, compared with a second Biden term.

• These extra emissions could in turn doom the 1.5°C goal under the Paris Agreement, the study finds, especially with the former president reportedly preparing to undo many Biden initiatives.

Zoom in: The analysis from Carbon Brief, shared first with Axios, is based upon modeling scenarios from two prominent U.S. research groups. It takes into account some, but not all, of Trump's stated plans to roll back Biden's climate policies, such as the Inflation Reduction Act.

• Carbon Brief concludes that under Trump, the U.S. would generate extra emissions through 2030 that would rival the combined annual CO2 output of both the European Union and Japan.
• The research also notes a stark difference in U.S. emissions reductions through 2030 under the different candidates; the current target under Biden is a 50% to 52% cut in greenhouse gas emissions by 2030, relative to 2005 levels.
• Under Biden, current policies would get the U.S. close to that goal, at about 43% below 2005 levels by 2030.
• Under Trump, emissions cuts might amount to as little as 28% below 2005 levels by 2030, depending on the extent to which he is able to reverse Biden's climate policies.

Stunning stat: Carbon Brief's report shows that the emissions path during a second Trump term "would negate — twice over — all of the savings from deploying wind, solar and other clean technologies over the last five years" globally.

Between the lines: The analysis conveys the night and day differences on climate policy between the incumbent and presumed GOP nominee, who questions the existence and significance of human-caused climate change.

• The analysis does not include the effects of additional fossil fuel production that Trump might favor, compared with a second Biden term.
• The report also does not include new Biden policies that might be implemented during a second term.

Context: The modeling comes from the Rhodium Group, as well as a study published in the journal Science last year. The research is subject to uncertainties about economic growth, the cost and accessibility of new technologies and other factors.

• Carbon Brief is a U.K.-based climate policy and science publication funded by readers and the European Climate Foundation.

Reality check: While Trump has stated his intent to reverse Biden's policies, many of which are unpopular among Republicans, it is difficult to predict what changes he would actually pursue, and how sweeping they may be.

• Much would depend on which party controls Congress and the outcome of litigation over regulations.
• Also, as the research makes clear, emissions would likely decline under either Trump or Biden.
• The big difference is in the rate and extent of reductions, and how close they'd allow the world to come to meeting the Paris targets.

The big picture: The analysis also points out some other global climate consequences of the upcoming election.

• A Trump win would conceivably dash remaining hopes of limiting global warming to 1.5°C above preindustrial levels, Carbon Brief notes.
• Warming beyond that point boosts the potential for more severe environmental consequences, such as the melting of the West Antarctic Ice Sheet.
• And, using the updated social cost of carbon from the EPA, Carbon Brief calculates that the extra (theoretical) CO2 under a Trump restoration may cause global damages worth more than $900 billion.

What's next: Looking beyond 2030, and assuming no further policy changes, a Trump win could yield an extra 27 billion tonnes of CO2 emissions by 2050, compared with policies that would be implemented under a second Biden administration, the analysis shows.

What they're saying: "Over the past year or so, a bunch of different analysts have tried to understand the emissions-cutting impact of Biden's key climate policies, including the IRA and draft rules on power plants and vehicles," Carbon Brief deputy editor and analysis coauthor Simon Evans told Axios via email.

• "We turned that on its head and asked, what if a second-term Trump manages to gut those rules, as if they never happened? And the answer is it'd make a huge, many-billions-of-tonnes difference to US emissions."
• Evans noted that this analysis omits any potential Trump moves to boost fossil fuel production beyond reversing Biden policies.
• "Trump might be less bad for the climate than these figures suggest — he might only partially repeal the IRA, for example," Evans said. "But relative to another term with fresh Biden climate policies, Trump might actually be even worse than we've estimated."