A man stands in a flooded street as Hurricane Irma hits Miami on Sept. 10, 2017.Matt McClain / The Washington Post

Climate change is already causing staggering impacts on the oceans and ice-filled regions that encompass 80 per cent of the Earth, and future damage from rising seas and melting glaciers is now all but certain, according to a sobering new report from the United Nations.

The warming climate is already killing coral reefs, supercharging monster storms, and fuelling deadly marine heat waves and record losses of sea ice. And Wednesday’s report on the world’s oceans, glaciers, polar regions and ice sheets finds that such effects only foreshadow a more catastrophic future as long as greenhouse gas emissions remain unchecked.

Given current emissions levels, a number of serious impacts are essentially unavoidable, says the report of the Intergovernmental Panel on Climate Change.

Extreme floods that have historically struck some coastal cities and small island nations once every 100 years will become an annual occurrence by 2050, according to the IPCC. In addition, if emissions continue to increase, global sea levels could rise by more than three feet by the end of this century — around 12 per cent higher than the group estimated as recently as 2013. Melting glaciers could harm water supplies, and warming oceans could wreck marine fisheries.

“As a result of excess greenhouse gases in the atmosphere, the ocean today is higher, warmer, more acidic, less productive and holds less oxygen,” said Jane Lubchenco, a former administrator of the National Oceanic and Atmospheric Administration (NOAA). “The conclusion is inescapable: The impacts of climate change on the ocean are well underway. Unless we take very serious action very soon, these impacts will get worse — much, much worse.”

More than 100 scientists from around the world contributed to the latest report by the IPCC, which found that profound and potentially devastating consequences lie ahead for marine life, Arctic ecosystems and entire human societies if climate change continues unabated.

Volunteers travel in a boat along a flooded highway following Tropical Storm Imelda in Fannett, Texas, U.S., on Friday, Sept. 20, 2019. Sergio Flores/Bloomberg

Wednesday’s report comes on the heels of other IPCC warnings about the grave threats climate change poses. Recently, the group detailed how the world’s land and water resources are facing “unprecedented” levels of exploitation and how those changes endanger the global food supply. Last fall, the IPCC also warned that the world must make rapid, sweeping changes to energy, transportation and other systems to hold warming below an increase of 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, a key threshold singled out in the Paris climate agreement.

The findings also come as world leaders gathered this week at the United Nations for a much-anticipated “climate summit” aimed at injecting new momentum into the flagging effort to persuade countries to do more to move away from fossil fuels and toward cleaner forms of energy. While dozens of smaller nations did unveil plans for coming years, the world’s largest emitters have stopped short of committing to transformational changes.

“The climate emergency is a race we are losing — but it is a race we can win if we change our ways now,” U.N. Secretary General António Guterres told world leaders Tuesday in his latest attempt to spur action. “Even our language has to adapt: What once was called ‘climate change’ is now truly a ‘climate crisis.’ . . . We are seeing unprecedented temperatures, unrelenting storms and undeniable science.”

The Washington Post recently detailed how shifting currents and worsening ocean heat events have already triggered die-offs of coastal clam species, worsening algal blooms and shifting fish catches in the South Atlantic along the coasts of Uruguay and Argentina, a hot spot for climate change. Wednesday’s report suggests similar changes are playing out across the world’s oceans – in some areas more than others.

One of the document’s most striking findings involves the rise in sea level, which is now being driven mainly by the rapid melting of ice in Greenland, Antarctica and the world’s smaller glaciers. Sea level rise is accelerating, and the world could see 3.6 feet in total sea level rise by the year 2100 in a very high-emissions scenario. In 2013, the IPCC had estimated that value at slightly over three feet.

We must address root causes of climate change by slowing and eventually stopping accumulation of greenhouse gas emissions

But the truth is that even these estimates may be too small, because when scientists behind the report looked at an alternative method for gauging how much seas could rise – simply canvassing the views of experts – even larger estimates emerged. The group’s findings only highlight “likely” amounts of sea level rise, meaning they do not represent worst-case scenarios.

For some major coastal cities, a historical 100-year flood event will happen annually by the year 2050. That includes large cities such as Jakarta, Indonesia; Manila, Philippines; Bangkok; Lima, Peru; Singapore; Barcelona, Spain; and Sydney, the report says. In the United States, cities facing fast-moving sea-level danger include Los Angeles; Miami; Savannah, Georgia; Honolulu; San Juan, Puerto Rico; Key West, Florida; and San Diego.

Paris Mayor Anne Hildalgo, who chairs C40 Cities, a global group of climate-conscious city leaders, called Wednesday’s report “shocking reading.”

“The world’s coasts provide a home to around 1.9 billion people and over half of the world’s megacities — all of which are in grave danger if we don’t act immediately to prevent rising temperatures and sea levels,” Hildalgo said in a statement. “Extreme high temperatures, coastal flooding, and more frequent natural disasters are becoming the new normal. … This is what the climate crisis looks like now.”

Like coastal cities, various small island nations also face imminent dangers from rising seas and as a result have been among the most vocal in pushing for more-aggressive climate action.

Because sea level-rise greatly amplifies storm surge events, “flood levels are all of a sudden returning in many cases once a year by mid-century, and it just gets worse from there,” said Michael Oppenheimer, a Princeton climate scientist who led the report’s chapter on sea-level rise. “We’re talking about storms that, when they come, result in loss of life, loss of property, shut down cities.”

A firefighter tries to extinguish forest fires at Sebangau National Park area in Palangka Raya, Central Kalimantan province, Indonesia, September 14, 2019. Willy Kurniawan / REUTERS

Granted, the severity of a 100-year flood event varies greatly and will not always be disastrous in any one place, Oppenheimer said. Still, the finding underscores just how big a difference a steady rise in sea level can make – and how soon we are going to start to realize this.

Wednesday’s report also finds that while it may be possible to adapt to rising seas if global emissions are somehow kept low throughout the century, the system could still tip toward very large ice losses from Greenland and Antarctica. If that happens, the rate of sea-level rise could become truly catastrophic, especially by the years 2200 and 2300, when it could exceed 10 feet.

Ice loss is accelerating in Greenland and Antarctica, scientists found. Permafrost, which contains enormous amounts of carbon that can be released as it thaws, has warmed to “record high levels.” Summer Arctic sea ice extent is now probably lower than at any time in “at least 1,000 years,” and the oldest, thickest ice has already declined by 90 per cent.

And then there is the entire world ocean. “Over the 21st century, the ocean is projected to transition to unprecedented conditions,” the report states.

The ocean is losing oxygen, growing more acidic, taking up an increasing amount of heat, and becoming more stratified, with warm water at the surface preventing cooler, nutrient rich waters from rising. All of these changes have profound consequences for marine ecosystems.

One of the most shocking findings involves “marine heat waves,” which have been blamed for mass deaths of corals, kelp forests and other key ocean organisms. The large majority of these events are already directly attributable to climate change, and by the year 2100, they will become 20 times more common in the best case, and 50 times as common in the absolute worst case, compared with the late 1800s, the report found.