The world has squandered so much time mustering the action necessary to combat climate change that rapid, unprecedented cuts in greenhouse gas emissions offer the only hope of averting an ever-intensifying cascade of consequences, according to new findings from the United Nations.

Already, the past year has brought devastating hurricanes, relentless wildfires and crippling heat waves, prompting millions of protesters to take to the streets to demand more attention to a problem that seems increasingly urgent.

Amid that growing pressure to act, Tuesday’s U.N. report offers a grim assessment of how off-track the world remains. Global temperatures are on pace to rise as much as 3.9 degrees Celsius (7 degrees Fahrenheit) by the end of the century, according to the United Nations’ annual “emissions gap” report, which assesses the difference between the world’s current path and the changes needed to meet the goals of the 2015 Paris climate accord.

As part of that deal, world leaders agreed to hold warming to “well below” 2 degrees Celsius compared with preindustrial levels; the current trajectory is nearly twice that.

Should that pace continue, scientists say, the result could be widespread, catastrophic effects: Coral reefs, already dying in some places, would probably dissolve in increasingly acidic oceans. Some coastal cities, already wrestling with flooding, would be constantly inundated by rising seas. In much of the world, severe heat, already intense, could become unbearable.

Global greenhouse gas emissions must begin falling by 7.6 percent each year beginning 2020 — a rate currently nowhere in sight — to meet the most ambitious aims of the Paris climate accord, the report issued early Tuesday found. Its authors acknowledged that the findings are “bleak.” After all, the world has never demonstrated the ability to cut greenhouse gas emissions on such a scale.

“Our collective failure to act early and hard on climate change means we now must deliver deep cuts to emissions,” Inger Andersen, executive director of the U.N. Environment Program, said in a statement announcing the findings. “We need to catch up on the years in which we procrastinated.”

The sobering report comes at a critical moment, when it remains unclear whether world leaders can summon the political will to take the ambitious action scientists say is essential. So far, the answer has been no.

Global emissions have risen about 1.5 percent annually on average over the past decade. In the coming decade, that trend must reverse — profoundly and rapidly — if world leaders are to limit the Earth’s warming to 1.5 degrees Celsius (2.7 Fahrenheit) or even 2 degrees Celsius (3.6 Fahrenheit) compared with preindustrial levels, scientists say.

The world already has warmed more than 1 degree Celsius.

Tuesday’s report, which is viewed as the benchmark of the world’s progress in meeting its climate goals, underscores how the pledges that nations made years ago in Paris are woefully inadequate to achieving the goals of the accord. To hold warming to “well below” 2 degrees Celsius, the authors found that countries would need to triple the ambition of their current promises. To hit the more ambitious target of no more than 1.5 degrees of warming, they found, nations would need to ramp up their pledges fivefold.

“Every year of delay beyond 2020 brings a need for faster cuts, which become increasingly expensive, unlikely and impractical,” the report states. “Delays will also quickly put the 1.5C goal out of reach.”

A Washington Post analysis this year found that roughly 20 percent of the world has already warmed to troubling levels. Slowing future warming will require monumental changes, such as phasing out gas-powered cars, halting the construction of coal-fired power plants and overhauling how humans grow food and manage land.

But the world’s carbon emissions have moved in the opposite direction. The United States’ energy-related CO2 emissions rose 2.7 percent last year, after a gradual decline. That increase came as the Trump administration continued to roll back Obama-era climate regulations and made clear that the United States, once a leader in pushing for climate action, will withdraw from the Paris accord in 2020.

Investment in renewable energy in the developing world also dropped significantly in 2018, according to an analysis released Monday by BloombergNEF, which tracks worldwide energy trends.

Last year developing countries added “an extra Texas worth of coal generation,” said Ethan Zindler, head of Americas for the group. “And that’s obviously scary.” At the same time, China’s investment in clean energy projects dropped from $122 billion in 2017 to $86 billion in 2018.

On Monday, the intergovernmental World Meteorological Organization reported that levels of greenhouse gases in the atmosphere had hit a record high and that the trend “means that future generations will be confronted with increasingly severe impacts of climate change.”

“There is no sign of a slowdown, let alone a decline, in greenhouse gases concentration in the atmosphere despite all the commitments under the Paris Agreement,” WMO Secretary General Petteri Taalas said in an announcement, noting that the last time the Earth experienced comparable CO2 concentrations, “sea level was 10 to 20 meters higher than now.”

Next month at the annual U.N. climate conference in Spain, representatives from countries around the world will face pressure to ramp up their ambition — not just their rhetoric — over the coming year. So far, only a handful of the world’s largest greenhouse gas emitters have policies in place to meet the promises they made in Paris four years ago, much less more aggressive strategies “for transformative climate commitments at the breadth and scale necessary,” Tuesday’s report says. The report also found that cutting greenhouse gas emissions could do more than just mitigate climate change. It also could reduce air pollution, improve public health and help conserve wildlife.

Niklas Höhne, a German climatologist and founding partner of NewClimate Institute, which created the Climate Action Tracker, a tool that tracks whether nations are meeting their goals, said Tuesday’s report demonstrates in painful detail how past inaction has made the urgency around climate change more dire.

“We are not a little bit off, we are far off from where we should be,” Höhne said in an email. “The longer action is delayed, the higher cuts will be required. We cannot wait another 10 years.”

Still, Höhne and others have continued to find hope that the world might still marshal the action necessary to stave off the worst effects of climate change.

Already, 70 countries have told U.N. officials they plan to craft more ambitious national climate pledges in 2020 — even as some of the world’s largest emitters have yet to follow suit. Scores of private companies have set their own targets, vowing to investors to sharply cut their carbon footprints. A growing list of states and cities have pushed ahead with policies aimed at meeting the goals of the Paris accord, even as the U.S. government remains on the sidelines.

“These examples show that the transition is possible,” Höhne said.

Even the authors of Tuesday’s depressing U.N. report wrote that they see slivers of hope “behind the grim headlines.”

“A number of encouraging developments have taken place,” they wrote, “and the political focus on the climate crisis is growing in several countries, with voters and protesters, particularly youth, making it clear that it is their number one issue.”