President-elect Joe Biden will face constraints of both politics and time when it comes to pursuing his aggressive climate-change agenda.Driving the news: Biden will enter a White House after four years of President Trump rolling back climate policies and time running out to substantively address the problem.Where it stands: The highest profile parts of Biden’s agenda, and the ones that will be quickest out of the gate on Inauguration Day, will be initiatives to reverse Trump’s rollbacks on a host of fronts across the environmental and energy spaces.The list is long (more than 100 by The New York Times’ count), but the number of Trump's rollbacks isn’t actually the biggest impact of his presidency. It’s lost time.This temporal hiatus is essential to understanding how Biden is caught between urgency and politics.“The impact of the Trump administration on emissions has been significant, but the actual regulatory rollbacks were only part of it,” said Trevor Houser, partner at the consulting and research firm Rhodium Group. “The bigger impact was four years of lost federal policy action.”The big picture: Climate change, unlike most other public policy problems, is cumulative. The longer we wait, the harder it gets to solve. Trump’s presidency has coincided with rising alarm and evidence of a warming planet, but the world has been methodically cooking itself for decades.“We’ve had 30 years of inadequate response to climate change, and the last four have been dramatic because there was actually an intention to not respond as opposed to just an inability to respond.”Jason Grumet, president of the Bipartisan Policy Center, a centrist think tankHow it works: Biden has two sequential goals to reduce U.S. emissions, which are in line with scientific consensus but which are also both going to be Herculean political tasks to start solving (let alone achieve).His first goal is to have a carbon-free electricity grid by 2035. Right now, a little less than two-thirds of U.S. electricity is powered by natural gas and coal. Biden’s longer-term goal is to have a net zero-carbon economy by 2050. Right now, about 80% of domestic energy consumption and production came from oil, natural gas or coal.Biden is poised to leverage every inch of the federal government to act on climate change. There are likely to be new financial rules, new environmental regulations and unprecedented limits on fossil-fuel leasing on federal lands and waters, to name just a few of the dozens of probable actions.But, but, but: He’s still unlikely to achieve his biggest goals without major new laws passed by Congress.His regulatory actions, which will inevitably be litigated, will face a more conservative judiciary — not just the Supreme Court, but courts throughout the system.“The urgency of climate change has grown. The courts have become more difficult and less hospitable to the ambitious use of regulatory authority and the Congress has gotten ever more partisan.”Jody Freeman, Harvard professor and former Obama administration officialThe state of play: Congress, with Republicans (probably) still controlling the Senate, is unlikely to pass laws any time soon that would get close to achieving Biden’s goals. Additional clean-energy spending and expanded tax incentives are likely the upper limit of what is possible in the short-term, and would likely be incorporated into economic stimulus legislation.A Green New Deal that's anything like the initiatives described in the resolution backed by Democratic Rep. Alexandria Ocasio-Cortez and Sen. Ed Markey has no chance of passing.A carbon tax also faces steep odds, given politicians’ general aversion to new taxes and especially during a recession when so many Americans are suffering economically.The intrigue: One relatively ambitious policy that some Washington insiders believe is possible is a clean energy standard for electricity, which would help achieve Biden’s 2035 carbon-free power goal. A bipartisan version exists in the House, and Republicans in both chambers have increasingly acknowledged the government should do something about climate change.But, but, but: Such a substantive debate on climate policy is unlikely to occur out of the gate of a Biden presidency given the twin health and economic crises. Some experts say that would be a wise move.“I don’t think we want the biggest climate battles to happen in the next six to 18 months,” said Grumet. “We're more likely to be successful once we are through the trauma and fear of the public health and economic crises.”What I’m watching: Climate change is already back on the diplomatic agenda, judging from Biden’s recent calls with world leaders. But the president-elect is entering these conversations after four years of America retreating and much of the rest of the world moving ahead without the U.S.“It appears the international architecture built through the Paris Accord has withstood the last four years,” Houser said. “That’s unclear if that would have been the case with another four years.”Go deeper: Election hardens political limits of Biden's agenda