Thanks to global warming, London will be as hot as Barcelona, while 17 US cities including New York, San Francisco, and Washington will face hotter weather than they’ve ever seen before.

Barcelona.

A new analysis carried by researchers at ETH Zurich compared the predicted climates of cities in 2050 with the climates of cities today. The idea behind the study was to help people better visualize the effects of climate change — by describing them not as something which might seem abstract (like an average warming of 1 degree Celsius), but as a comparison. For instance, the study predicts that in 2050, the climate of Madrid in Spain will be like Marrakech, Morocco, is now. Seattle will be like San Francisco is today, while Sweden’s Stockholm will feel like Budapest. Strikingly, London’s climate will become a lot like Barcelona’s.

The approach makes a lot of sense. Cities are hotspots of climate change. They are hotter than their surroundings (due to the urban heat island effect) and also greatly at risk due to the high concentrations of people and infrastructure.

But there’s another reason.

Communicating climate change is notoriously difficult — and city planners are some of the most important people to get the message across to. Even if you do get the general message across, the specifics of how an area is expected to warm can be gimmicky to present.

For instance, since 1901, the planet’s surface has warmed by 0.7–0.9° Celsius. However, that doesn’t say much about particular cities, as the world isn’t heating up uniformly. Some areas are heating more than others, and even if you do get the specifics of a particular city, what does a 0.7 C warming even mean? For most people, that doesn’t exactly come through clearly.

Pairing up cities is a clever idea, as you get a very concrete idea for how things will change. However, it’s also quite a simplistic idea. A city’s climate is complex and depends on the layout, buildings, heat sources, population density, and many other parameters for which there is often no direct analog. So while this is a useful tool, it’s also an approximate one.

Furthermore, there are no good pairings or analogues for around a quarter of the world’s major cities (that is, cities with a population of over 1 million inhabitants). That makes for 115 cities (including Washington and 16 other cities in the US) which will suffer unprecedented climate conditions by 2050. Most of the cities in this category are in the tropics. Metropolises like Kuala Lumpur, Jakarta, Rangoon, and Singapore, will have a climate for which there is just no analogue on Earth at this moment.

In Europe, summers and winters will warm dramatically, with average increases of 3.5°C and 4.7°C, respectively, compared with 2000. Generally speaking, cities in the Northern Hemisphere will have climates similar to that of cities which lie 620 miles (1,000 km) to their south have today,

The study ‘Understanding climate change from a global analysis of city analogues’ was published in PLoS.