What does a country do when rising seas threaten to swamp its coastal capital?

In the case of Indonesia, the country’s leaders are building a new capital city, from scratch, on higher ground. That monumental endeavor was recounted in detail this week by my colleague Hannah Beech, who has covered Asia for more than two decades.

The project poses a big question, Hannah wrote: What happens to the millions left behind in the old capital, Jakarta, once the government moves?

That question resonates well beyond Indonesia. Rising seas are a challenge for two-thirds of the world’s biggest cities, including New York, Mumbai and Shanghai. The U.N. secretary general, António Guterres, has warned that higher sea levels could result in “a mass exodus of entire populations on a biblical scale” if not addressed.

Today, I want to talk about what leaders around the world are doing about the problem and what the future might hold for people in these cities.

How big is the problem?

Global warming raises sea levels in two ways. First, it melts glaciers and ice sheets, adding water to the oceans. Second, higher temperatures make all that water expand, increasing its volume. Sea levels have already risen about 9 inches, or roughly 23 centimeters, since 1880.

How much more will the seas rise? That depends on how much we let global temperatures rise. If climate change is left unchecked, ice sheets in Antarctica and Greenland could eventually shift from gradual melting to collapse. Scientists aren’t sure exactly where that tipping point lies in the future.

Research reported by my colleagues Denise Lu and Chris Flavelle showed that some 150 million people now live on land that will be below the high-tide line by midcentury. That number is expected to rise as people seeking better lives move to big cities, many of them on the coast.

There are ways to adapt.

A.R. Siders, an assistant professor at the University of Delaware who focuses on climate adaptation, told me there are various strategies cities can use to cope with rising seas.

First, officials can avoid building in flood-prone areas. They can also try to hold the water back by building levees and sea walls. They can accommodate higher water levels by, for example, elevating homes.

Another option is to retreat, or move homes and infrastructure out of harm’s way. That’s what Indonesia is doing, though, so far, only for government buildings, not for Jakarta’s more than 10 million residents.

Municipal leaders will have to use their judgment to decide what mix of strategies is right for their cities, Siders said.

Rotterdam, in the Netherlands, uses permanent sand dunes and dikes, as well as water storage spaces. Ho Chi Minh City, in Vietnam, wants to improve drainage systems and relocate freshwater reserves to protect them from encroaching seawater.

Projects like those, of course, need a lot of money. That’s one big reason developing countries are demanding that rich nations, which are largely responsible for causing climate change, keep their commitments on climate aid.

Are we doing enough?

“I think most people’s gut feeling, in this area, is that it’s not enough,” Siders said. “My suspicion is that doing something that is enough in the long term requires something more radical.”

My colleague Chris, who covers adaptation, told me a big challenge is “how to move people so, wherever they end up, there is still a sense of community.”

There are examples of small communities that have done that. Valmeyer, a town of about 1,000 in Illinois, is generally seen as a successful case.

Perhaps the biggest effort to relocate entire cities happened after the 2011 tsunami in Japan, which displaced half a million people. The Japanese accepted a tax increase to, among other things, help pay for tens of thousands of new homes for those who lost everything.

But even that ambitious project doesn’t come close to the challenge of relocating millions.

Planning, and timing, are crucial.

If past experience is any guide, successful relocation requires time. It took four years to move and rebuild tiny Valmeyer, for example.

Understandably, Chris told me, people tend to want to stay in their homes. And in developing countries, where money is short, planning can be shaky and distrust of officials abounds, moving people is even harder.

Finding the right time to propose a move is often key, Chris told me. Do it too early, and people won’t sense danger enough to buy in to a retreat solution. Wait too long, and the community that the government is trying to save may no longer exist in any meaningful way.

“The people who have means to leave will have already left,” Chris said. “And people with less money will end up even worse.”

A lot will change, and a lot will be lost.

Through the process of reporting this newsletter, I couldn’t help thinking of the park by the beach near my home in Rio de Janeiro. It’s one of my favorite places in the world. Decades from now, the sea may swallow it.

Many of you may be mourning the places you love in advance, too. I can’t imagine the pain that people who live in island nations must feel when they think about this future. That’s another level of tragedy.

No doubt there will be loss. There already is. But big cities change all the time. We can, too. It’s really about coming together to choose how.

Introducing the New York Times audio app

We’ve launched a new iOS app for audio journalism and storytelling. It’ll feature our podcasts and narrated articles about climate, politics, sports, food and more. You’ll also find This American Life and stories from a range of leading magazines. New York Times news subscribers can download it here.

Essential news from The Times

Soaring temperatures ahead: Scientists say they expect global heat to set records over the next five years. The repercussions for health, food and security would be far-reaching.

Wildfires menace oil sites: Fires in Canada have driven thousands of people from their homes. Now, they are striking the heart of the country’s oil and gas industry.

Deadly rains in Italy: Widespread flooding in the northern region of Emilia-Romagna killed at least eight people and forced 5,000 from of their homes.

A crackdown on coal: The Environmental Protection Agency is proposing rules on coal ash landfills to prevent heavy metals from seeping into groundwater.

Location, location, location: The organizers of the Burning Man festival have joined conservationists to try to block plans for a geothermal power plant in the Nevada desert.

From outside The Times

• The Future Ecologies podcast explored primordial sounds, including those that emerged as life began on Earth.

• FEMA is struggling to keep up with natural disasters hitting Americans. According to Grist, extremist groups are stepping in to help, and win more followers in the bargain.

• Recycling plastics could be exacerbating pollution problems. A study reviewed by Inside Climate News found that a plant in Britain produces tons of microplastics a year.

• Scientific American showed why studies that attribute specific disasters to climate change could influence court cases, insurance claims and policy.

Before you go: A vanished bird might still be out there

The roller coaster tale of the ivory-billed woodpecker, a majestic bird whose presumed extinction has been punctuated by a string of contested rediscoveries, took a new turn this week when researchers published a study suggesting that a population survives in the swamps of Louisiana.

Want to go birding with The Times?

The New York Times is inviting readers around the world to participate in a community science project with the Cornell Lab of Ornithology. You’d be helping to gather observations about birds that could give researchers a clearer picture of biodiversity in different places. All are welcome: experienced birders and absolute beginners.

Claire O’Neill, Chris Plourde and Douglas Alteen contributed to Climate Forward