This past April was the second hottest in recorded history, according to data released by the Japan Meteorological Agency. The report found that April 2019 was the second hottest month since records began in 1891, rivaled only by April 2016. Five of the hottest recorded Aprils have occurred in the past decade, according to the agency, with April 2017 and 2018 being the third and fourth hottest months respectively across the globe. April 2014 and 1998 tied for fifth.

For more than 100 days in 2015 and 2016, gas leaked out of the Aliso Canyon Natural Gas Storage Facility near Los Angeles — the largest known leak of methane in United States history. More than 8,300 households were evacuated, and people exposed to the gas reported nosebleeds, dizziness and respiratory problems. This week, California regulators said they now knew why the environmental catastrophe happened. In a 258-page report, investigators said that groundwater had corroded the metal lining of a more-than-50-year-old underground well, leading to its rupture at 892 feet below ground. The report also said that SoCalGas, the company that owns and operates the natural gas well, did not meaningfully investigate or analyze more than 60 previous leaks at the complex. The company did not properly monitor its wells at the site, the report said, adding that “the approach to well integrity at Aliso Canyon had been reactive rather than proactive.”

More than a dozen states are moving to strengthen environmental protections to combat a range of issues from climate change to water pollution, opening a widening rift between stringent state policies and the Trump administration’s deregulatory agenda. In recent months, Hawaii, New York and California have moved to ban a widely used agricultural pesticide linked to neurological problems in children, even as the administration has resisted such restrictions. Michigan and New Jersey are pushing to restrict a ubiquitous class of chemical compounds that have turned up in drinking water, saying they can no longer wait for the Environmental Protection Agency to take action. Colorado and New Mexico have adopted new policies targeting greenhouse gas emissions from fossil fuel drilling and limiting where these operations can take place. And more than a dozen states have adopted policies that would force automakers to produce more fuel-efficient cars than required by federal standards.

The Environmental Protection Agency plans to adopt a new method for projecting the future health risks of air pollution, one that experts said has never been peer-reviewed and is not scientifically sound, according to five people with knowledge of the agency’s plans. The immediate effect of the change would be to drastically lower an estimate last year by the Trump administration that projected as many as 1,400 additional premature deaths per year from a proposed new rule on emissions from coal plants. That, in turn, would make it easier to defend the new regulation, known as the Affordable Clean Energy rule, which is meant to replace former President Barack Obama’s signature climate change measure, the Clean Power Plan. It is not uncommon for a presidential administration to use accounting changes to make its regulatory decisions look better than the rules of its predecessors. But the proposed new modeling is unusual because it discards more than a decade of peer-reviewed E.P.A. methods and relies on unfounded medical assumptions. The five people familiar with the plan, who are all current or former E.P.A. officials, said the new modeling method would be used in the agency’s analysis of the final version of the ACE rule, which is expected to be made public in June. William L. Wehrum, the E.P.A. air quality chief, acknowledged in an interview the new method would be part of the agency’s final analysis of the rule.

Around the country, the EPA under Trump is delegating a widening range of public health and environmental enforcement to states, saying local officials know best how to deal with local problems. Critics contend federal regulators are making a dangerous retreat on enforcement that puts people and the environment at greater risk. One administration initiative would give states more authority over emissions from coal-fired power plants. Another would remove federal protections for millions of miles of waterways and wetlands. Some states and counties say the EPA is also failing to act against threats from industrial polluters, including growing water contamination from a widely used class of nonstick industrial compounds. Michigan, New Jersey and some other states say they are tackling EPA-size challenges — like setting limits for the contaminants in drinking water — while appealing to the real EPA to act. In Houston's oil and gas hub, local officials and residents say a lax EPA response to toxic spills during Hurricane Harvey left the public in the dark about health threats and handicapped efforts to hold companies responsible for cleaning up. Nationwide, EPA inspections, evaluations and enforcement actions have fallen sharply over the past two years, some to the lowest points in decades, or in history. The agency says environmental enforcers remain on the job despite the plunging enforcement numbers.