Over four years in office, the Trump administration has dismantled major climate policies and rolled back many more rules governing clean air, water, wildlife and toxic chemicals.

While other administrations have emphasized cutting regulations, calling them burdensome to industries like coal, oil and gas, the scope of actions under Mr. Trump is “fundamentally different,” said Hana V. Vizcarra, a staff attorney at Harvard Law School’s Environmental and Energy Law Program.

In all, a New York Times analysis, based on research from Harvard Law School, Columbia Law School and other sources, counts more than 70 environmental rules and regulations officially reversed, revoked or otherwise rolled back under Mr. Trump. Another 26 rollbacks are still in progress.

The bulk of the rollbacks identified by the Times have been carried out by the Environmental Protection Agency, which has weakened Obama-era limits on planet-warming carbon dioxide emissions from power plants and from cars and trucks; removed protections from more than half the nation’s wetlands; and withdrawn the legal justification for restricting mercury emissions from power plants.