The world’s leading environmental scientists told policymakers and business leaders Sunday that they must invest more to cope with climate change’s immediate effects and hedge against its most dire potential, even as they work to slow the emissions fueling global warming.

The U.N. Intergovernmental Panel on Climate Change said that climate change is already hurting the poor, wreaking havoc on the infrastructure of coastal cities, lowering crop yields, endangering various plant and animal species, and forcing many marine organisms to flee hundreds of miles to cooler waters.

But the Nobel Peace Prize-winning group said that climate change’s effects will grow more severe and that spending and planning are needed to guard against future costs, much as people insure themselves against possible accidents or health problems.

The report said that damage from climate change and the costs of adapting to it could cause the loss of several percentage points of gross domestic product in low-lying developing countries and island states. It added that climate change could “indirectly increase risks of violent conflicts in the form of civil war and inter-group violence” by “amplifying” poverty and economic shocks.

The summary of the report, ratified at a five-day meeting in Yokohama, Japan, avoided specific forecasts or timetables or cost estimates, but it described a range of likelihoods and outcomes in an attempt to give decision-makers the tools to set priorities to combat those effects.

Scientists who helped write the report said that efforts to adapt could include constructing emergency cyclone and flood shelters like those in parts of Bangladesh, moving generators out of New York City basements that flooded during Hurricane Sandy, changing farming techniques to cope with higher temperatures, and conserving water and curbing pollution in areas threatened with more-frequent droughts.

“The focus is as much on identifying effective responses as on understanding challenges,” Chris Field, co-chairman of the IPCC working group writing the report, said in a statement last week. On Monday in Yokohama, he said that “we need to think about reducing risks and building more resilient societies” by drawing on “deep pools” of creativity and innovation.

Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University and one of the report’s authors, said: “There is a more optimistic tone about our ability to adapt to some of these things. We’ve had some bad heat waves and coastal storms, and we have a better idea of what we need to do. Whether we will ever do it, I don’t know.”

But he cautioned that “everyone agrees that if we don’t slow the warming down, our prospects for adaptation are not good.”

The risk-based approach opened the door to discussion in the report of grave climate scenarios, even if their likelihood is relatively remote, just as a company might plan for an extremely rare flood, earthquake or tornado.

An early draft of the report had estimated that governments would need to spend scores of billions of dollars a year on adaptation efforts, according to a person who saw the early version, but the final summary made no mention of how much money might be needed.

“The IPCC’s new report underscores the need for immediate action in order to avoid the most severe impacts of climate change,” said John P. Holdren, President Obama’s science adviser. “It reflects scientists’ increased confidence that the kinds of harm already being experienced as a result of climate change are likely to worsen as the world continues to warm.”

The IPCC report said, “Responding to climate-related risks involves decision-making in a changing world, with continuing uncertainty about the severity and timing of climate-change impacts, and with limits to the effectiveness of adaptation.”

The impact and adaptation report is Part 2 of a four-part assessment by the IPCC. It relied on about 12,000 papers and was written by 309 scientists, who voted on the final version Sunday morning in Japan.

“For the first time, we have measured in terms of risk so each of the climate risks could be weighed against each other and compared with the risk of this versus the risk of something else,” Oppenheimer said.

Jonathan Overpeck, a professor of geosciences at the University of Arizona who also worked on the report, said, “We did that because we deal with day-to-day life by managing risks, and a big part of managing our large corporations is assessing risks and managing those risks.”

The “very high confidence” category of climate-change effects included exacerbating more-intense heat waves and fires, increased food- and water-borne diseases, and a steady rise in sea level in certain regions, such as the East Coast of the United States.

The report reiterated warnings that world leaders and businesses must act to slow climate change, not just adapt to it. “Increasing magnitudes of warming increase the likelihood of severe, pervasive, and irreversible impacts,” the report said. It said that some of the warming could have “cascading effects.”

The report attached a “medium confidence” rating to some of those events, but it highlighted the danger of “abrupt and irreversible regional-scale change” if high temperatures hurt the ability of the Arctic boreal tundra or the Amazon forest to store carbon dioxide, or sped the collapse of a continental ice sheet.

But the most likely damage from climate change will be linked to rising sea levels and temperatures. Those changes could turn the advantages of growing coastal cities into vulnerabilities if interlocking transportation, electrical and information systems fail, Oppenheimer said.

Gary Yohe, an economics and environmental studies professor at Wesleyan University and a co-author of the report, said: “I teach my students that the answer to every economic question is ‘It depends.’ You don’t want to be poor, you don’t want to be young, you don’t want to be old, and you don’t want to live along the coast.”