It was not a very good day for environmentalists.

On Tuesday, President Donald Trump signed an executive order designed to reverse many elements of President Barack Obama's environmental legacy.

The order seeks to scale back federal limits on greenhouse gases, eliminate regulations of the coal industry, and take climate change out of federal agencies' decision-making processes.

There is a chance the actions Trump has ordered could cause enough of a spike in US greenhouse-gas emissions to undo any mitigation efforts from the Obama era.

But that's not the only possible outcome.

As the Trump administration has already seen, presidents' ability to follow through on their agenda is limited by federal bureaucracy, the courts, Congress, and economic forces in the real world.

As federal agencies gear up to put Trump's new order into action, many of those efforts could — and probably will — meet a wave resistance from many angles (though Congress, which is controlled by a Republican majority, will probably be on board).

Here are the battles Trump's order now most likely faces.

Federal bureaucracy

Just by signing his name, Trump did away with numerous hallmark climate efforts of the Obama era. The most significant of these were the Climate Action Plan, the Obama administration's blueprint for mitigating the impact of climate change, and a moratorium on leases for coal companies to mine on federal land.

View photos Scott Pruitt hat happy EPA More

But many of the most consequential targets of Trump's order — like the Clean Power Plan, which limits greenhouse-gas emissions from power plants — are regulations that have already been written into the Federal Register. That means Trump can't kill them on his own.

Instead, Trump's order instructs his agencies to begin the complicated rule-making processes of repealing or replacing the existing rules.

Those processes can often take years and involve numerous contentious decisions for federal agencies.

For example, the Environmental Protection Agency's administrator, Scott Pruitt, will lead the effort to do away with the Clean Power Plan. As Vox's Brad Plumer points out, Pruitt and the EPA have two options: scrap states' greenhouse-gas limits entirely, or rewrite them to be much weaker.

Choices like that will most likely depend on how confident policymakers are about the second hurdle for Trump's executive order: the courts.

The court system

Every rule the EPA, the Department of the Interior, or any other sector of the executive branch puts on the books has to be justified by laws written by Congress.

View photos Elena Kagan More

When agencies run up against the limits of those laws, citizens can fight them in court.

Environmental groups like the National Resources Defense Council and Sierra Club are already up in arms about Trump's new order and are ready to deploy teams of environmental lawyers to fight these rules in court at every opportunity. (Opponents of environmental regulations, like Pruitt, used the same tactic when they thought the Obama administration overstepped its authority.)

Environmental lawyers will most likely argue that the Clean Air Act mandates that the EPA safeguard the atmosphere. So by failing to limit greenhouse gases, they'll say, Pruitt's EPA wouldn't be upholding that responsibility.