The 2015 Paris Climate Accord, signed by 190 nations, set a target of limiting long term global warming to 1.5-2 degrees Celsius. With the United States playing a lead role, the signatories committed to develop action plans for reducing their greenhouse gases (g.h.g.) and rich nations committed massive support for “green development” in poorer regions.

Over the ensuing five years, it has become ever clearer that a human-caused climate crisis is here and now, not in a distant future. Polls show that for most Americans this year’s raging wildfires and record-setting hurricane season have brought this reality home. The emerging scientific consensus is that even limiting warming to the two-degree “upper limit” is increasingly unlikely.

This is the dire setting in which climate-denying President Trump pulled out of the Paris Accord and used executive authority to pursue measures encouraging greater production and consumption of fossil fuels.

Candidate Joe Biden pledged to re-apply to the Paris Accord on “Day One” and reverse a host of Trump’s executive orders. He has laid out an ambitious strategy for investing in climate-friendly infrastructures and renewable energy, promising to stimulate economic growth and create millions of “green jobs.” Appointing John Kerry to oversee an “all of government” strategy underscores these priorities. And analysts at the International Monetary Fund and US financial firms agree with Biden’s optimistic economic assessment.

Jonathan Pershing, a former top US climate negotiator, expects the process of rejoining the Paris Accord to be quick and straightforward: the rest of the world recognizes the crucial importance of bringing the USA back into the fold. After all, America is the world’s second-largest emitter behind China, a potential source of clean energy technology breakthroughs and a key backer of climate-friendly development in poorer countries.

But Pershing stresses that formally rejoining the Paris Accord is the easy part. Tufts Professor Kelly Gallagher concurs: re-establishing America’s credibility will be a far greater challenge. One proof will be rapid and decisive US actions to reduce emissions. But credibility also requires evidence that the US will not reverse course yet again, as it has done twice before, failing to ratify the Kyoto Protocol in the 1990s and then exiting from the Paris Accord.

The Biden administration can take many constructive actions using executive authority. In transportation, for example, the President can re-impose stricter fuel efficiency targets and support state and municipal initiatives encouraging electric vehicles (e.v.s). In the power sector, he can reinstate the Clean Power Plan and license wind and solar arrays on public lands, accelerating the retirement of coal-fired plants.

However, such initiatives could be obliterated by a future climate-denying president. And even before that, the conservative Supreme Court majority could reverse key initiatives. After all, several Obama executive orders – like the Clean Power Plan – were implemented under a Court ruling that g.h.g. are covered by the 1970 Clean Air Act. It is not hard to imagine the current court overturning that precedent.

Prof. Gallagher emphasizes that sustaining an ambitious Biden climate strategy – as well as belief in American reliability – requires more than executive orders. Key commitments must be written into law. There is bipartisan recognition that the United States urgently needs both additional Covid relief and massive infrastructure modernization, so there should be areas for collaboration on climate-friendly legislation, like investment in public transportation, home weatherization, and e.v. charging stations.

But even those piecemeal steps are no sure thing. And more ambitious legislation – like putting a price on carbon emissions, as many other nations have done – will run into America’s most intractable climate challenge: an obstructive US Senate. Resistance is predictable from most Senate Republicans, some Democrats and, crucially, Majority Leader McConnell.

The 2020 election results – so far — do not bode well for ambitious climate legislation. That makes Georgia’s two Senate runoff elections in January critically important – for climate strategy and the entire Biden policy agenda.

David Vail is member of the Economics Policy Network of the Citizens’ Climate Lobby and professor of economics, emeritus, at Bowdoin College.