animals, especially in emerging nations led by China and India where city growth will be fastest, it said.
"Rich biodiversity can exist in cities and is extremely critical to people's health and well-being," wrote Thomas Elmqvist of the Stockholm Resilience Centre, scientific editor of the Cities and Biodiversity Outlook.
The world's urban population is expected to surge from just over 3.5 billion now to 4.9 billion by 2030, according to the assessment by the UN Convention on Biological Diversity.
At the same time, the area to be covered by cities will expand by 150%, it said.
"Most of this growth is expected to happen in small and medium-sized cities, not in megacities," according to the report, issued to coincide with a UN meeting on biodiversity in Hyderabad.
More green spaces in cities can filter dust and pollution and soak up heat-trapping carbon dioxide. Some studies have shown that the presence of trees can help reduce asthma and allergies for children living nearby, it said.
And the study said that cities were also home to a wide range of animals and plants.
More than 65% of Poland's bird species are found in Warsaw. In South Africa, Table Mountain national park, rich in wildlife, is surrounded by the Cape Town municipality. In the United States, Saguaro national park is just outside Tucson.
"Sustainable urban development that supports valuable ecosystems presents a major opportunity for improving lives and livelihoods," said Achim Steiner, head of the UN Environment Programme.
More tree cover in cities can help cool them in summers, meaning less need for air conditioning, it said.
"Recent studies highlight the importance of even small urban gardens in providing habitat for native pollinators such as bees, which have declined alarmingly in recent years," the study added.
And it pointed to real estate arguments for a greener city.
In the United States "city parks increase the value of nearby residential properties by an average of 5 percent; excellent parks can provide a 15 %increase," it said.
Expansion of cities would often displace farms nearby, meaning that agriculture would have to shift elsewhere. That in turn would require care that new farmland did not, for instance, lead to damaging deforestation.
"For the next 40 years urban growth will consume land approximately three times the size of France ... this is often the most prime agricultural land," Elmqvist said.
Many existing cities are trying to get greener. In Bogota, Colombia, residents exercised more after city authorities introduced measures such as closing some roads on weekends and improving bus transport.
In Mexico City, a "Green Roof Program" aims to create 10,000 square meters (107,000 sq ft) of new roof gardens every year.