Here are the key takeaways from the latest comprehensive climate update that the United Nations Intergovernmental Panel on Climate Change—called the IPCC—released in early August. The new report distils 14,000 individual studies and has more than 200 authors.

The last decade was hotter than any decade in 125,000 years.

Atmospheric carbon dioxide is now at a two-million-year peak.

Greenhouse gas emissions have elevated the global average temperature by about 1.1° Celsius above the pre-industrial late 19th century average. Mankind has already emitted enough greenhouse gases to heat the planet by 1.5°C, but fine-particle pollution is providing a cooling effect.

Scientists can now link specific weather events to human-made climate change. While it was previously impossible to attribute any particular storm or temperature spike to climate change, climate science has advanced since the IPCC's last update in 2013.

Drawing from research on ancient climates and using satellite technology, new models have narrowed the projections of the atmosphere's likely response to industrial emissions and provide a clearer picture of what may result if greenhouse gas emissions are not curbed.

New findings rule out the possibility that unrestricted emissions will have only a mild effect on global temperatures.

Evidence suggests that the cessation of carbon dioxide and other greenhouse gases would reduce heating quickly and allow temperatures to stabilize within a few decades. Some effects will remain irreversible for centuries, including a sea-level rise.

In terms potential impacts on US regulation of greenhouse gases, the IPCC report is expected to form part of the scientific basis for National Environmental Policy Act regulations, inform oil and gas leasing decisions, and possibly accelerate efforts to calculate the social cost of greenhouse gas emissions. Environmental groups can also be expected to use the IPCC report to support climate liability lawsuits against fossil-fuel companies.

Cars and trucks

President Joseph Biden announced a multipronged plan in early August to make cars and light trucks sold in the United States more fuel-efficient and to shift the US toward electric vehicles.

New regulations proposed by the US Environmental Protection Agency and Department of Transportation would require fleet-wide vehicle mileage of 52 miles per gallon by 2026, up from 40 mpg in 2021.

Biden also signed an executive order calling for half of new passenger car sales to be of electric "zero-emission" vehicles powered by batteries and fuel cells or plug-in electric hybrids by 2030.

Falling battery prices mean that larger electric cars will reach price parity with their fossil-fuel counterparts in the US and Europe in 2022, with parity reached in most other segments and regions by 2030. Improvements in battery technology are also expected to boost potential driving ranges.

The administration also called again on Congress to enact incentives to produce zero-emission vehicles and deploy public fast-charging stations.

Executives from various major automotive companies, lawmakers and United Auto Workers members joined Biden at the White House for the announcement.

The rulemaking reverses the freeze on fuel-efficiency standards imposed under Donald Trump, who eased requirements put in place in 2012 under the Obama administration.

Cross-state air pollution

A group of Midwestern utilities has expanded its legal challenges to an EPA cross-state air pollution rule that limits upwind discharges of ozone air pollution that arguably cause downwind states to flunk standards under the Clean Air Act.

The updated EPA rule became effective on June 29, 2021. It tightens national ambient air quality standards under the Clean Air Act for ozone.

It also requires power plants in 12 "upwind" states to reduce emissions of nitrogen oxides significantly starting in the 2021 ozone season.

States must reach a 2008 ozone national ambient air quality standard of 75 parts per billion.

EPA found that nitrogen oxides emissions in Illinois, Indiana, Kentucky, Louisiana, Maryland, Michigan, New Jersey, New York, Ohio, Pennsylvania, Virginia and West Virginia significantly contribute to downwind states' nonattainment and maintenance problems for the 2008 ozone air quality standards under the Clean Air Act.

EPA is issuing new or amended federal implementation plans, or FIPs, in those 12 states that set tighter emissions budgets for power plants. Power plants in eastern states are already allowed to trade emissions allowances as a market mechanism to limit emissions.

A "good neighbor" provision in the Clean Air Act requires states to mitigate their "significant contribution" to the problems of other states in attaining and maintaining compliance with established ambient air quality standards.

Regulators use computer modeling to predict whether states will be in nonattainment with the standards and screen for significant contribution by upwind states to the ozone levels of downwind receptors. EPA then applies a test for cost-effectiveness to decide what emissions reduction measures are required of upwind states.

The litigation brought by a group of utilities and factories calling itself the Midwest Ozone Group says the methodology behind EPA's updated rule is faulty. The group argues that EPA failed to show a sufficient link between emissions in upwind states and resulting ozone problems in downwind states and challenges the standard EPA uses for determining such a link.

The group also says that EPA wrongly targets power plants rather than other emissions sources and requires steps that are not cost effective.

The lawsuit also challenges EPA's threshold for determining "significant contribution" by upwind states to the ozone levels of downwind receptors.

The case is Midwest Ozone Group v. EPA and is before a US appeals court in Washington.

Air toxics rule

EPA submitted proposed revisions to the mercury and air toxics standards—called MATS—for power plants to the White House Office of Management and Budget in early August for regulatory review. This is usually the last step before making them public.

The proposal revises a Trump administration May 22, 2020 rule on MATS.

Specifically, EPA appears likely to eliminate a decision made during the Trump era that it is not "appropriate and necessary" to regulate utility air toxics under section 112 of the Clean Air Act. The Trump EPA found that the agency's prior conclusion that it was "appropriate and necessary" to be incorrect. By removing the finding, the Trump administration removed a legal prerequisite to regulation under section 112. EPA is now expected to reinstate the finding.

The new rulemaking also appears likely to tighten emissions limits. Under a required risk and technology review, EPA must evaluate health risks and assess whether new, cost-effective control technologies are available. Depending on its findings, EPA can tighten mercury and air toxics emissions limits.

The OMB review process could take about 90 days, but comments by Acting EPA air chief Joe Goffman suggest the OMB review is expected to be much quicker.

Greenhouse gas regulation

Industry and various Republican-led states are asking the US Supreme Court to review EPA's authority to regulate greenhouse gases under the Clean Air Act.

They want an immediate review by the high court of a January lower court ruling that set aside a narrow, Trump-era approach to power plant climate rules.

The Biden administration asked the high court to reject the petitions, arguing that it is premature for the court to explore the scope of EPA's authority to regulate carbon emissions from power plants until after the agency decides on a new direction.

EPA says it will issue a new section 111(d) greenhouse gas rule for power plants "after taking into account all relevant considerations, including changes to the electricity sector that have occurred during the last several years."

The Biden administration argues courts should not "speculate" about the potential approach an agency will take to new regulation.

The lawsuits have been consolidated as West Virginia v. EPA.

The issue before the court is a longstanding dispute about whether EPA can ground section 111(d) greenhouse gas emissions targets on actions taken "beyond the fence" of regulated plants—for example, by setting up emissions trading schemes—or whether it is limited to requiring actions to reduce emissions at specific power plants.