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More than half of the world’s most populated cities are getting wetter, according to new research on how water patterns are undergoing dramatic shifts in urban areas as climate change intensifies the atmospheric effects.
The academic study involving Bristol and Cardiff universities, on behalf of aid group WaterAid, found that 52 per cent of cities showed a wetter trend over the past four decades, including Colombo, Mumbai and Kuala Lumpur.
At the same time, some 44 per cent of urban centres were getting drier, including Los Angeles, Riyadh, Paris and Cairo. Some cities have ‘flipped’, going from drier to wetter, such as Lahore, while a smaller number have moved from wetter to drier, such as Madrid and Hong Kong.
To illustrate the wet and dry trends of the 112 cities included in the study, I decided that a map would be the best way to give a global overview and then we could dive into the individual categories used in the study with a series of small multiple charts.
To create the map, Water Aid provided the data in the report along with the population of each city. This data was then loaded into Qgis, the FT’s geographic information systems software of choice.
Sizing the circles by population gave an indication of how many people are affected in each city. Then I coloured them with our usual blue to brown colour ramp, which is intuitive when illustrating wet and dry conditions.
The colour scale is also ideal from an accessibility point of view as it remains virtually unchanged for those readers with red or blue colour vision deficiency.
To overcome the issue of a world map displaying extremely small on our mobile devices — where 70% of our readers view the article — I created two orthographic projections which allow more detail to be seen.
[Nerd note: an orthographic projection depicts a hemisphere of the globe as it appears from outer space, where exactly half of the globe is visible.]
For the charts I wanted to show the top 10 cities in four categories highlighted in the report: intensification of both wet and dry, known as ‘climate whiplash’; the flip to wet; flip to dry; and abatement, which is a decrease in both wet and dry extremes.
The challenge is how to best show 40 charts in a single graphic. My solution was to use small multiple charts and a set of filters. This set up allows you to see the trends across 10 cities at a glance in each category. (We’ve written about the virtues of small multiple before, if you missed it email us at [email protected] and we will forward it to you.)
Past research has found that weather-related disasters such as flooding and drought have increased by 400 per cent in the last 50 years — and 90 per cent of climate disasters are driven by either too little or too much water.
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