Whatever climate change does to the world, cities will be hit hardest

More than half of humanity is crowded together in cities. That's about 4 billion people living on top of one another, working, commuting, polluting, and figuring out how to survive. And that proportion will only rise: By the end of the century, about 85% of the world's population will be urban, according to the Organization for Economic Cooperation and Development.

Cities are also incredibly vulnerable to climate change. High temperatures, sea level rise, and extreme weather are all felt more acutely in urban areas. Built on concrete, they absorb solar radiation but not water, making the effects of both heat waves and heavy rains more severe.

If humans keep emitting planet-warming greenhouse gases at our current pace, Earth will warm by at least 3C and as much as 4C by the end of the century compared with preindustrial temperatures, causing much of the planet to become unlivable. Radical emissions cuts could change that forecast if they're made within the next few decades. Lockdowns during the coronavirus led to a record 7.5% drop in emissions in 2020, but the reduction was short-lived. The latest data from the International Energy Agency shows that global emissions were 2% higher in December than at the same point in 2019.

No matter how much the world warms, cities will have it worse, with average summer temperatures rising between 1.9C and 4.4C above current levels, depending on how the future plays out, according to a research team led by Lei Zhao at the University of Illinois. That increase will lead to more deaths in places with scarce resources to adapt and mitigate the effects of a changing climate.

Cities in the northern U.S., southern Canada, central and northern Asia, the Middle East, Europe, and northwestern China will experience the most pronounced warming by the end of the century.

In March, Muscat got a preview of what's coming for cities in the Arabian peninsula when thermometers hit 41.3C (106.3F), a temperature typically reached around June. The Omani capital will be between 1.6C and 3.8C hotter on average by 2100.

The previous images showed the expected temperature change under a middle-of-the road emissions scenario, known as RCP 4.5, which assumes emissions will peak around 2040. These images show the worst-case scenario, RCP 8.5, which is controversial because it factors in unlikely events such as a significant expansion of coal consumption. Scientists use it less and less, but it's still useful to illustrate where the world could wind up without emissions controls.

Summer temperatures in Buenos Aires, meanwhile, are rising faster than the global average, for a total increase of 1.3C over the past three decades. Humidity and the lack of green space will make the heat worse.

Warming in Melbourne will exacerbate extreme events like the heatwaves, bushfires, and rains experienced in recent months. It's likely to take a toll on city dwellers too–during a heatwave in 2009, ambulance calls rose 46%.

Being close to the sea has historically been an advantage, but today some of the world's most densely populated urban areas are suffering disproportionately from rising tides. Coastal areas representing 58% of the world's population have experienced relative sea level increases of 7.8 millimeters to 9.9mm annually, according to scientific research. That compares with an average sea level increase of 2.5mm a year globally for the past two decades.

Multiple reports show that cities such as Jakarta, San Francisco, and Shanghai are sinking even more as groundwater extraction creates underground cavities that begin to collapse under the weight of the concrete and steel above.

Jakarta, Indonesia

Indonesia's capital is sinking at the fastest rate of any major city in the world.

Coastal Flood Risk in 2020

Two-fifths of the city's surface area is now below sea level, with some neighborhoods dropping at a rate of 20 centimeters (8 inches) a year.

Coastal Flood Risk in 2100

Looking at the projections, the Indonesian government decided to move the capital city to higher ground on the forested island of Borneo. The multi-decade, multi-billion-dollar project will have huge environmental consequences.

Alexandria, Egypt

The Mediterranean Sea alongside Egypt's second-largest city, home to more than 5 million people, has been rising at a rate of 3.2 millimeters per year since 2012, local authorities say.

Coastal Flood Risk in 2020

The advancing water has threatened building foundations in the city, founded by Alexander the Great more than 2,300 years ago. Salinity in the Nile Delta is also increasing dangerously and could halve the country's agriculture sector by 2060.

Coastal Flood Risk in 2100

So far, there have been no significant efforts to shield the coast. Residents affected by the rising seas are being forced to move. Much of Alexandria will be flooded by the end of the century if no measures are taken.

Manila, Philippines

The water level in the Manila Bay area is rising more than four times faster than the global average.

Coastal Flood Risk in 2020

Rapid urbanization and out-of-control population growth make mitigation plans more difficult to implement. With almost 13 million residents, the Manila metropolitan region is built in a flood-prone area that has been heavily impacted by cyclones in recent years.

Coastal Flood Risk in 2100

The white-flowered mangroves that once helped shield the coast and gave Manila its name have by now almost disappeared.

When extreme weather events hit, people in the countryside seek safe harbor in cities, which have stronger infrastructure and more hospitals and other essential services. To a storm refugee, that can make the difference between life and death.

Most people displaced by natural disasters eventually return to their place of origin. But not all. Some settle in the suburbs or on the outskirts of cities, adding further stress to urban infrastructure, services, and resources.

Bangladesh's capital, Dhaka, is one of the most densely populated cities on Earth, nearly twice as crowded as the island of Manhattan. About 40% of the city's 14 million residents live in informal settlements, and 70% of those were forced to leave their homes because of phenomena related to climate change, including cyclones and coastal and riverbank erosion.

Displacement doesn't happen only in developing nations. Cities like Buffalo, N.Y., are labeling themselves climate havens and opening themselves up to climate migrants. In the U.S., 13 million coastal residents are expected to be displaced from their homes by the end of the century as sea levels rise, according to a 2020 Plos One study.

Sea levels could rise as much as 6 feet in certain places under the worst-case scenario.

As people flee coastal counties, inland populations will increase. Extreme events such as 2005's Hurricane Katrina or last year's wildfires in California show that a flood of internal migrants can lead to a spike in housing prices in the counties where people resettle.

In many places, heat and flooding aren't either-or. Some cities are or will be affected by both, making adaptation and mitigation efforts even more challenging.

Humans can bear either high temperatures or high humidity, but the two things combined can be deadly. The body copes with heat by sweating, but when the air is damp already, that defense becomes useless.

Climate models project the first occurrences of deadly combined heat and humidity will arrive by mid-century. But scientists have found that such episodes have already taken place at least twice and that the frequency of extreme humidity and heat has more than doubled since 1979. Under business-as-usual emissions scenarios, these conditions will occur several times in the North China Plain between 2070 and 2100, MIT researchers found.

Tianjin, only 80 miles east of Beijing and one of China's largest cities, is likely to be affected by both extreme humidity and heat. The city has a population of 12.8 million and sits at the heart of China's most populous and agriculturally important region.

Sea level rise will wipe out most of Tianjin's current surface area if no adaptation measures are taken. Meanwhile, average temperatures in the city are expected to increase between 2.5C and 5C above current levels.

As the world warms, the effects of climate change will become more unpredictable, with storms, cold spells, and heat waves often matching or even surpassing scientists' worst-case scenarios. Cities and their dwellers will need to spend trillions to mitigate damages, adapt, and survive in a planet that's growing more hostile by the minute.

Disclaimer: This article first appeared on bloomberg.com, and is published by special syndication arrangement.