What will rising sea levels do to the world's megacities?

How major cities will be affected by rising sea levels. Image: Statista/CityMetric.

So you know how sometimes people are like, “this is a good news, bad news kind of a thing”? This isn't one of those times. It's bad news that comes with more bad news behind it. It's a bad news sandwich.

This infographic, made for us by the nice people at Statista using Climate Central data, how sea level rises are going to affect a selection of the world's cities. We've focused on the megacities – that is, those with populations of 10m or more – partly because these are where people are mostly likely to be affected in large numbers, and partly just to manage the size of the map.

The bubbles, and the numbers attached to them, represent the share of the cities' 2010 populations that would be below sea level in the event of particular increases in global temperature. The size of the light red bubble represents the percentage of the urban population that could be submerged in the event of a 2°C increase in global temperature (a degree of increase which feels pretty much inevitable); the dark red one is how much people will be affected by a 4°C increase in global temperature (which we could probably still avoid if we tried, but let's be honest, we probably won't try).

The first bit of bad news is that these figures are very obviously awful. London gets off pretty lightly – but a 4°C increase in global temperature would still put 13 per cent of its population under water. In New York, which is bigger, a full quarter of its population lives in areas that may well be underwater.

But these numbers are as nothing towards some of those Asian cities. In Mumbai and Calcutta it's half; in Shanghai, a city of 23m people, it's more than three-quarters.

In 2010, Dhaka, the capital of Bangladesh, contained an estimated 32m people; in the event of the sort of sea level rise that's expected to follow a 4°C increase in global temperature, 38 per cent of them will be underwater.

In other words, what this map shows is that tens of millions of homes are very possibly going to be underwater at some point before the century is out.

Now you may be thinking, well, could be worse. Okay, Asia's in real trouble here (sorry Asia) but much of the rest of the world is getting off relatively lightly.

Ha. No. Remember, this graphic only shows cities of 10m or more. Some of the world's megacities are relatively safe from sea level rises – Moscow and Kinshasa are both well in-land; Sao Paulo isn't, but it is a long way above sea level.

But a lot of other large cities are excluded because they're not quite big enough to make the cut. A 4°C increase in global temperature would put 26 per cent of San Francisco's population under water. In Tampa, Florida, it's 40 per cent. Barisal, a Bangladeshi city of 7m people, would see 88 per cent of its population submerged by the same increase in temperature.

Perhaps the most terrifying of all, though, are the figures that relate to the Netherlands, a name which literally means “lower countries”. In the event of that 4°C increase in temperatures, 98 per cent of the populations of Amsterdam and the Hague will be below sea level.

Now – below sea level doesn't mean “inevitably underwater”, of course. There are mitigation measures cities can take.

But one of the most important mitigation measures is almost certain to be “moving a lot of people to somewhere else”.

We're in trouble, is basically what we're saying here.


Green roofs improve cities – so why don’t all buildings have them?

The green roof at the Kennedy Centre, Washington DC. Image: Getty.

Rooftops covered with grass, vegetable gardens and lush foliage are now a common sight in many cities around the world. More and more private companies and city authorities are investing in green roofs, drawn to their wide-ranging benefits which include savings on energy costs, mitigating the risk from floods, creating habitats for urban wildlife, tackling air pollution and urban heat and even producing food.

A recent report in the UK suggested that the green roof market there is expanding at a rate of 17 per cent each year. The world’s largest rooftop farm will open in Paris in 2020, superseding similar schemes in New York City and Chicago. Stuttgart, in Germany, is thought of as “the green roof capital of Europe”, while Singapore is even installing green roofs on buses.

These increasingly radical urban designs can help cities adapt to the monumental challenges they face, such as access to resources and a lack of green space due to development. But buy-in from city authorities, businesses and other institutions is crucial to ensuring their success – as is research investigating different options to suit the variety of rooftop spaces found in cities.

A growing trend

The UK is relatively new to developing green roofs, and governments and institutions are playing a major role in spreading the practice. London is home to much of the UK’s green roof market, mainly due to forward-thinking policies such as the 2008 London Plan, which paved the way to more than double the area of green roofs in the capital.

Although London has led the way, there are now “living labs” at the Universities of Sheffield and Salford which are helping to establish the precedent elsewhere. The IGNITION project – led by the Greater Manchester Combined Authority – involves the development of a living lab at the University of Salford, with the aim of uncovering ways to convince developers and investors to adopt green roofs.

Ongoing research is showcasing how green roofs can integrate with living walls and sustainable drainage systems on the ground, such as street trees, to better manage water and make the built environment more sustainable.

Research is also demonstrating the social value of green roofs. Doctors are increasingly prescribing time spent gardening outdoors for patients dealiong with anxiety and depression. And research has found that access to even the most basic green spaces can provide a better quality of life for dementia sufferers and help prevent obesity.

An edible roof at Fenway Park, stadium of the Boston Red Sox. Image: Michael Hardman/author provided.

In North America, green roofs have become mainstream, with a wide array of expansive, accessible and food-producing roofs installed in buildings. Again, city leaders and authorities have helped push the movement forward – only recently, San Francisco created a policy requiring new buildings to have green roofs. Toronto has policies dating from the 1990s, encouraging the development of urban farms on rooftops.

These countries also benefit from having newer buildings, which make it easier to install green roofs. Being able to store and distribute water right across the rooftop is crucial to maintaining the plants on any green roof – especially on “edible roofs” which farm fruit and vegetables. And it’s much easier to create this capacity in newer buildings, which can typically hold greater weight, than retro-fit old ones. Having a stronger roof also makes it easier to grow a greater variety of plants, since the soil can be deeper.

The new normal?

For green roofs to become the norm for new developments, there needs to be buy-in from public authorities and private actors. Those responsible for maintaining buildings may have to acquire new skills, such as landscaping, and in some cases volunteers may be needed to help out. Other considerations include installing drainage paths, meeting health and safety requirements and perhaps allowing access for the public, as well as planning restrictions and disruption from regular ativities in and around the buildings during installation.

To convince investors and developers that installing green roofs is worthwhile, economic arguments are still the most important. The term “natural capital” has been developed to explain the economic value of nature; for example, measuring the money saved by installing natural solutions to protect against flood damage, adapt to climate change or help people lead healthier and happier lives.

As the expertise about green roofs grows, official standards have been developed to ensure that they are designed, built and maintained properly, and function well. Improvements in the science and technology underpinning green roof development have also led to new variations on the concept.

For example, “blue roofs” increase the capacity of buildings to hold water over longer periods of time, rather than drain away quickly – crucial in times of heavier rainfall. There are also combinations of green roofs with solar panels, and “brown roofs” which are wilder in nature and maximise biodiversity.

If the trend continues, it could create new jobs and a more vibrant and sustainable local food economy – alongside many other benefits. There are still barriers to overcome, but the evidence so far indicates that green roofs have the potential to transform cities and help them function sustainably long into the future. The success stories need to be studied and replicated elsewhere, to make green, blue, brown and food-producing roofs the norm in cities around the world.

Michael Hardman, Senior Lecturer in Urban Geography, University of Salford and Nick Davies, Research Fellow, University of Salford.

This article is republished from The Conversation under a Creative Commons license. Read the original article.