The cities of the world become hot and dry islands at the end of the century.



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More than half of the world’s population lives in cities. By mid-century, the proportion may have risen to 70 percent. Metropolitan areas are also getting hotter than the surrounding countryside. Hard asphalt and concrete surfaces absorb more heat, and buildings, people, and traffic heat up even more.

So it is in cities that most of us will be affected by the consequences of climate change. But until now, few or no modeling and calculations of how the climate will develop during the rest of the century have included the urban areas of the world.

“Global climate forecasts are, in fact, mainly forecasts for non-urban areas,” says Lei Zhao, a researcher in environmental technology at the University of Illinois at Urbana in the United States.

It is very understandable that this happened, as cities make up a very small part of the earth’s land.

– Climate models were originally created to understand large-scale dynamics, and urban areas are either too small for resolution in the models, or too small for changes there to be discernible. Cities are only about three percent of the earth’s surface.

But so we can handle and to adapt to a warmer climate, we need to know how cities are affected.

– There is a disproportionately large difference between the size and the effects of the changes. Cities make up only three percent of the earth’s surface, but that’s where the population and infrastructure are concentrated. So even if it is small and small areas, the consequences will be great, says Lei Zhao.

He and his colleagues have built a model for the climate in urban areas. Since then, they have incorporated it with 26 existing climate models to obtain good forecasts of temperature and humidity in the world’s cities by the year 2100.

– We need to close this knowledge gap and obtain specific information about cities.

If we don’t take important steps to limit greenhouse gas emissions, temperatures in cities across much of the world may have risen 4.4 degrees by the end of the century compared to today, according to researchers’ results published this week in the journal Nature Climate Change. .

– Cities are essential for both development and climate adaptation. So in order to plan action and development, you need to have information about what will happen in cities rather than about outer landscapes, says Lei Zhao.

Warming is greatest in the northern hemisphere and the Arctic. In Anchorage, Alaska, climate change is twice as fast as in lower-latitude cities.

According to the researchers’ calculations, daytime and nighttime temperatures will rise the most in the central and northern United States, southern Canada, Europe, the Middle East, northern Central Asia and northwestern China. In the interior of South America, nighttime temperatures will also increase dramatically by the end of the century.

It is important Be aware of uncertainty in forecasting.

– For risk management and planning what measures should be implemented, it is enough not only to know what the models predict, but also how safe the predictions are. It’s like with weather forecasts.

It makes a big difference if we find out that there is a 10 to 90 percent risk of rain tomorrow.

– If there is a ten percent chance of rain, people may not even mind bringing an umbrella. But if it is 90 percent, they will act differently, says Lei Zhao.

In almost all the worldExcept on the coasts, the air in cities will be drier by the end of the century, according to the researchers.

– Humidity is an important factor. It is not just temperature that determines what heat stress we experience or how much the heat torments us.

With high humidity, the heat is felt more because we cannot cool the body as effectively by sweating. In cities with drier air, the effect of planting more trees and other plants is greater, not just because they provide shade.

– Green infrastructure, such as parks and trees, can reduce the temperature by evaporation more effectively with lower humidity, says Lei Zhao.

But he emphasizes that that solution may not always be as simple as it sounds.

– If it is possible to plant more plants in a city is a different story. It depends on whether there are areas available, enough water and other resources. In the article, we have at least calculated what effect to expect from more green infrastructure.

The Model as Lei Zhao and colleagues have developed it can also be used for other cities.

– In the article, we focus on cities. But the model is not necessarily limited to being used just for that. It can develop in other ecosystems, such as glaciers, polar regions, rivers and the like.

Lei Zhao’s wish is that the studio can better prepare us for a warmer future.

– I hope that our results can help local decision makers to design or plan more sustainable cities, making them greener or in other ways, he says.

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