Four years after countries struck a landmark deal in Paris to rein in greenhouse gas emissions in an effort to avert the worst effects of global warming, humanity is headed toward those very climate catastrophes, according to a U.N. report issued Tuesday, with China and the United States, the two biggest polluters, having expanded their carbon footprints last year.

“The summary findings are bleak,” the report said, because countries have failed to halt the rise of greenhouse gas emissions even after repeated warnings from scientists. The result, the authors added, is that “deeper and faster cuts are now required.”

The world’s 20 richest countries, responsible for more than three-fourths of emissions, must take the biggest, swiftest steps to move away from fossil fuels, the report emphasized. The ­richest country of all, the United States, however, has formally begun to pull out of the Paris accord altogether.

Global greenhouse gas emissions have grown by 1.5% every year over the past decade, according to the annual assessment, the Emissions Gap Report, which is produced by the U.N. Environment Program. The opposite must happen if the world is to avoid the worst effects of ­climate change, including more intense droughts, stronger storms and widespread food insecurity by midcentury. To stay within relatively safe limits, emissions must decline sharply, by 7.6% every year, between 2020 and 2030, the report warned.

Separately, the World Meteorological Organization reported Monday that emissions of three major greenhouse gases — carbon dioxide, methane and nitrous oxide — have all swelled since the mid-18th century.

Under the Paris agreement, reached in November 2015, every country has pledged to rein in emissions, with each setting its own targets and timetables. Even if every country fulfills its current pledges — and many, including the U.S., Brazil and Australia, are currently not on track to do so — the Emissions Gap Report found average temperatures are on track to rise by 3.2 degrees Celsius from the baseline average temperature at the start of the industrial age.

According to scientific models, that kind of temperature rise sharply increases the likelihood of extreme weather, the accelerated melting of glaciers and swelling seas — all endangering the lives of billions of people.

The Paris accord resolved to hold the increase in global temperatures well below 2 degrees Celsius, or 3.6 degrees Fahrenheit; last year, a U.N.-backed panel of scientists said the safer limit was to keep it to 1.5 degrees Celsius.

There are many ways to reduce emissions: quitting the combustion of fossil fuels, especially coal, the world’s dirtiest fossil fuel; switching to renewable energy like solar and wind power; moving away from gas- and diesel-guzzling cars; and halting deforestation.

In fact, many countries are headed in the wrong direction. A separate analysis released this month looked at how much coal, oil and natural gas the world’s nations have said they expect to produce and sell through 2030. If all those fossil fuels were ultimately extracted and burned, the report found, countries would collectively miss their climate pledges, as well as the global 2 degree Celsius target, by an even larger margin than previously thought.

Diplomats are set to gather in Madrid in December for the next round of negotiations over the rules of the Paris agreement. The world’s biggest polluters are under pressure to raise their pledges.

“This is a new and stark reminder,” Spain’s minister for ecological transition, Teresa Ribera, said of the Emissions Gap Report. “We urgently need to align with the Paris agreement objectives and elevate climate ambition.”

If there is any good news in the report, it is that the current trajectory is not as dire as it was before countries around the world started taking steps to cut emissions. The 2015 Emissions Gap Report said that, without any climate policies at all, the world was likely to face around 4 degrees Celsius of warming.

Coal use is declining sharply, especially in the U.S. and Western Europe, according to an analysis by Carbon Brief. Renewable energy is expanding fast, though not nearly as fast as necessary. And city and state governments around the world, including in the U.S., are rolling out stricter rules on tailpipe pollution from cars.