By Juana Summers, CNN

(CNN) -- Scott Pruitt's tenure as the head of the Environmental Protection Agency has been marked by stories scrutinizing his first-class travel on the taxpayer dime, his lease agreement in Washington and his use of a full-time security detail.

But Pruitt has also been a reliable ally for President Donald Trump, who made easing regulations on American industries a central campaign promise, and the EPA administrator has proposed a slew of rollbacks for major environmental rules, including those governing clean air and water and fighting climate change.

Put together, Pruitt's moves -- some of which have been challenged in court -- represent an attempt to fundamentally restructure the EPA's role in Washington, and are a sharp rebuke of the environmental legacy of former President Barack Obama.

Here are some of Pruitt's key changes:

Changing how the EPA approaches climate change

Like others in the Trump administration, Pruitt has repeatedly highlighted uncertainty about the role that humans have played in global warming, a view that is at odds with the vast majority of scientific evidence. Last month, the EPA circulated talking points instructing its staffers to say "clear gaps" exist in understanding whether human activity contributes to climate change. The EPA also removed references to climate change from several webpages, according to an analysis by the Environmental Data and Governance Initiative.

Rolling back car emissions standards

Pruitt announced in early April that he would revise Obama-era greenhouse gas emissions standards for automobiles, a move that undercut a signature part of Obama's efforts to fight climate change.

The weakening of automobile standards, which is a win for car manufacturers and a blow to environmentalists, also sets up a fight between the Trump administration and the state of California, which says it plans to stick with the stricter rules.

Proposing an overhaul of Obama-era initiatives on coal ash

In March, the EPA proposed changes to the Obama-era directive regulating coal ash waste, giving states and utilities more power to manage their own disposal of it. The proposal, according to The Washington Post, includes more than a dozen changes for how coal ash, which remains when coal is burned in power plants and electricity is generated, is stored at coal-fired power plants nationwide, and would let states change how frequently they test for groundwater contamination.

The decision was praised by industry officials who had called on Pruitt to revisit the rules, but environmental groups said he was caving to special interests in the coal industry.

Withdrawing regulations on 'major sources' of pollution

The EPA announced in January that it was withdrawing the "once-in always-in" policy under the Clean Air Act that dictated how major sources of hazardous air pollutants are regulated. The decades-old policy was opposed by fossil fuel companies, but environmental groups argued that its withdrawal would result in more pollution.

Suspending clean water rule

The EPA announced in January that it would suspend the Waters of the United States rule, which was designed to limit pollution in roughly 60% of the country's bodies of water. Critics said the rule was overly broad in classifying waterways that are subject to federal rules, while supporters see it as a necessary protection, for example, preventing fertilizer runoff from tainting streams and drinking water.

Withdrawing from Clean Power Plan

Pruitt announced in October his decision to withdraw from the Clean Power Plan, the Obama-era rule intended to reduce greenhouse gas emissions from power plants across the country. The Clean Power Plan had a goal of decreasing the country's carbon pollution by roughly one-third by 2030, compared with 2005 levels.

Attempting to suspend an Obama-era methane rule

Under Pruitt, the EPA tried to suspend an Obama-era rule to restrict methane emissions from new oil and gas wells. But a federal court ruled in August that the EPA must enforce the Obama administration's regulation.

Pushing to withdraw the US from the Paris climate accord

Pruitt pushed the White House to leave the Paris climate deal, stood beside the President in the Rose Garden when he announced the decision and after the President spoke offered his own remarks. The move was condemned by global leaders and was seen by many as a setback to efforts to address global warming.