The effects of climate change are bad, and they’re getting worse — especially when it comes to the world’s oceans.

But if unprecedented action is taken soon to reduce planet-warming emissions, it will greatly ease some of the worst impacts and make adaptation less painful.

That’s the underlying message in a landmark report by more than 100 scientists from 36 countries. The Intergovernmental Panel on Climate Change on Tuesday approved the Special Report on the Ocean and Cryosphere in a Changing Climate, which was three years in the works.

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“The open sea, the Arctic, the Antarctic and the high mountains may seem far away to many people,” said Hoesung Lee, IPCC chair, in a statement Wednesday. “But we depend on them and are influenced by them directly and indirectly in many ways — for weather and climate, for food and water, for energy, trade, transport, recreation and tourism, for health and wellbeing, for culture and identity.”

If people prevent the planet from warming more than 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels by 2100 — as world leaders agreed to in Paris in 2015 — the effects of climate change will still threaten livelihoods, alter ecosystems and disrupt weather patterns. But it won’t be nearly as bad as a warmer world.

The latest report says marine heatwaves, for instance, will be 20 times more frequent at 2 C. But it could be 50 times more frequent if greenhouse gas emissions continue to increase.

Marine heatwaves don’t just affect the oceans, though the warmer waters are also putting corals in peril and reducing the abundance of certain fish. They are also making it hotter on land.

Meteorologists blame heatwaves for the record heat Hawaii has experienced this summer, and they may exacerbate wildfires.

In July, firefighters on Maui were responding to their first big wildfire of the season — about 9,000 acres — when they realized the equipment they set out with to tackle the blaze wasn’t going to do the job as it had for similar fires in years past, according to University of Hawaii wildland fire researcher Clay Trauernicht.

That’s because it was so hot out that it had driven the humidity down to a point where the grasses were so crispy that the fire behaved differently, burning up the fuels faster and shifting directions less predictably, he said.

Trauernicht said his biggest takeaway from the IPCC report was the “ginormous” difference between cutting greenhouse gas emissions or continuing to burn fossil fuels when it comes to the severity of the impacts of climate change.

“Whatever extent we can reduce this, we’ve got to do it,” he said.

The report cites marine heatwaves as just one of the impacts of climate change. Scientists project stronger hurricanes, faster eroding coastlines, hotter summers, more flooding, increased sea level rise, more extreme weather and less productive fisheries.

The rate of sea level rise is increasing primarily because glaciers and ice sheets in polar and mountain areas are melting faster than expected. It’s also rising faster because the ocean is expanding as it gets warmer, the report says.

Sea levels rose globally about a half foot last century. Now they’re rising more than twice as fast and getting faster. Sea level could rise 1 to 2 feet by 2100 even if global warming is limited to well below 2 C, but it could rise 2 to 3.6 feet if greenhouse gas emissions continue to escalate.

That’s a big deal for Hawaii, where most of the state’s 1.4 million residents live close to the coast and it’s also where major infrastructure is located. The tourism industry that the overall economy depends on also depends on the coastal environment.

“We are in a critical set of years for the entire history of humanity,” University of Hawaii climate researcher Chip Fletcher said. “But we can never give up. We can never stop. It’s always going to be a crisis until we decarbonize.”

Sea level rise will also increase the frequency of extreme events during high tides and intense storms, the report cautions. With any degree of additional warming, the report says events that happened once per century will happen every year by 2050, heightening the risk for low-lying coastal cities like Honolulu and small islands like those in the northwestern reaches of the Hawaiian Archipelago.

East Island, about 600 miles northwest of Oahu, almost entirely disappeared after an unprecedented hurricane passed over it last year. It was the primary nesting ground for the bulk of the threatened Hawaiian green sea turtle population and a sizable chunk of critically endangered Hawaiian monk seals.

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The IPCC has a reputation for being conservative because it requires 100% agreement, Fletcher said. That tends to water down its reports, compared to peer-reviewed literature that offers the latest science.

But knowing this is the voice of the scientific community speaking is important, he said.

It’s a sentiment shared by Josh Stanbro, who heads Honolulu’s climate change office. He said the IPCC offers an important “rear view mirror approach” whereas the Honolulu climate commission’s appointed members, which include Fletcher, are looking out the front window.

“If there’s anything heartening in this it’s that we have really smart sharp local climate change commissioners giving us good information, making projections and looking at cutting-edge research,” he said.

Still, Stanbro found some aspects of the report to be unnerving. He is concerned about how it leaves as an “x factor” the impact of methane gas that may be released when permafrost thaws. And he was taken aback by how the report underscores how small island nations are exposed to the impacts of climate change.

“We know that as island people,” he said. “But it’s a little unnerving to see that in plain sight where world leaders are saying we’re the canaries in the coal mine.”

Members of the working groups who developed the report said it arms communities and governments with the information they need to act.

It highlights the urgency of “timely, ambitious, coordinated and enduring action,” Ko Barrett, vice chair of the IPCC, told reporters.

Hawaii agencies are among those looking to it for guidance.

Scott Glenn, who heads the state Office of Environmental Quality Control, said he is interested in how it affects baseline assumptions that agencies are making, such as maps showing sea level rise exposure areas.

“With climate change, it’s affecting so many parameters at the same time that it becomes very hard,” he said. “How do you translate that into something like design or engineering if you know there’s this science coming out with these big uncertainty brackets around it?”

IPCC working group members said that reducing carbon emissions enough to matter will require a global effort across all sectors.

“We will only be able to keep global warming to well below 2 C above pre-industrial levels if we effect unprecedented transitions in all aspects of society, including energy, land and ecosystems, urban and infrastructure as well as industry,” Debra Roberts, an IPCC working group co-chair, said in a statement.

The report, which was unanimously approved, provides crucial information going into the United Nations Framework Convention on Climate Change Conference in December in Chile, known as COP25.

Read the full report below.