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Green areas in big cities perform important ecological functions, such as “filtering dust, absorbing carbon dioxide from the air and improving air quality,” the Convention on Biological Diversity said in its new assessment.

The “Cities and Biodiversity Outlook” is the first global analysis of how urban land expansion will impact biodiversity in the coming decades.

The world’s total urban area is expected to triple between 2000 and 2030, with urban populations set to double to around 4.9 billion in the same period.

Data from the United Kingdom show that a 10 percent increase in tree canopy cover in cities may result in a 3-4 degree Celsius decrease in ambient temperature, thus reducing energy used for air conditioning, the report said.

Urban biodiversity also delivers important health benefits, with studies showing that proximity to trees can reduce the prevalence of childhood asthma and allergies.