London is becoming the world’s first National Park City. It wants to support nature using similar principles to the world's national parks, promoting green space as vital to health, pleasure and conservation. Because nature is a “need to have”, not a “nice to have”, according to the National Park City Foundation.

To reach its goal of becoming 50% green space by 2050, London is asking residents to turn a patch of ‘grey space’ into green space. This could be by lifting paving in the driveway, or replacing garden decking with grass. It’s also repeating the call to drill holes in garden fences so that hedgehogs can move around freely. A £12 million Greener City Fund from City Hall includes £5 million on offer to support community tree planting and £1.5 million to create new woodland.

The health benefits of green space in urban areas are numerous, with research showing it lowers the risk of obesity, poor mental health and premature death. Investing in nature pays off fiscally too: For every £1 spent on trees, the UK saves £7 in healthcare, energy and environmental costs, according to Natural England.

At its current 47% green cover, Greater London is already one of the world’s most vegetated cities. One-third of the city is open green space, while an additional 14% of greenery is in private gardens. In comparison, just 10% of Paris and 27% of New York are green, according to the World Cities Culture Forum.

But London faces challenges. Its population is set to swell by 2 million by 2040, to 11 million. It will need to build thousands more homes, while preserving natural habitats. One solution is for housing developers to integrate conservation with construction. Berkeley Homes is one company doing this. Its Kidbrooke Village development includes a 50-acre park, with houses on just 35% of the land. As part of its Woodberry Down complex, finished in 2016, the company also helped fund a nature reserve including 27 acres of wetlands, opened by Sir David Attenborough.

London is also battling toxic air pollution, which contributes to 9,000 premature deaths every year, according to King's College London. The city regularly exceeds EU legal requirements for nitrogen dioxide levels. As part of its greening mission, City Hall is promoting the planting of living walls - vertical vegetation - near busy roads to reduce CO2 emissions and absorb other pollutants.

Green roofs are also part of the plan. Growing plants above our heads can reduce the urban heat island effect, keeping air temperatures down, and cutting energy use for cooling. Vegetated roofs also absorb rainwater, and attract birds and bees.

Half the world lives in urban areas. By 2050, 7 in 10 of us will, according to the United Nations. More urban greenery is a way to keep both us and local wildlife in good health.

London is launching as a National Park City in July, with a week-long festival. The city is encouraging residents to throw their own opening ceremony, which they can publicise on an interactive map. The National Park City Foundation hopes London will be the first of 25 world cities to take up the moniker by 2025.