The Green New Deal is an international proposal that lays out a grand plan for tackling climate change. The proposal was introduced by Representative Alexandria Ocasio-Cortez of New York and Senator Edward J. Markey of Massachusetts to call on the federal government to wean the United States off fossil fuels and curb planet-warming greenhouse gas emissions across the economy. It also aims to guarantee new high-paying jobs and markets in the clean and green energy industries. Within this context, the resilience of cities facing climate change is increasingly central in urban planning worldwide, under sustained global urbanization trends.
Urban greening and the related green infrastructures represent a potential method to mitigate the effects of climate change at the urban scale, particularly urban heat waves, droughts, and floods. The greening of grey surfaces with a vegetated cover on their top layer can deliver multiple benefits, as a vegetated surface uses rain water for evapotranspiration — the process through which water is transferred from the soil to the atmosphere due to vegetation respiration, gradually releasing it. Additional benefits are also the mitigation of urban heat islands, the reduction of cooling demand in the summer, and the reduction of atmospheric CO2 that is fixated into biomass. Beyond improving the urban habitat, positive ecological and human well-being effects include the reduction of noise in buildings and the creation of spaces for social activities, horticulture, and wildlife habitat, especially for birds and pollinators. The large-scale implementation of urban greening can thus lead to more sustainable city development.
How important are urban greening benefits?
A recent study performed at the Joint Research Centre of the European Commission has proposed an innovative methodology to estimate such benefits in the European context but that can be used as a basis to estimate analogous benefits in other geographic contexts as well.
It was estimated that greening of impervious surfaces can reduce the surface temperature by between 2.5 °C and 6 °C in the summer, causing a reduction of perceptible heat in the atmosphere, a driver of urban heat island effects. If 1% of the impervious surfaces in Europe were greened, the reduction of heat flow to buildings could generate cooling energy savings of about 2.6 TWh/year, reducing cooling costs in the summer. When all the roofs are greened (in Europe, 35% of the impervious surfaces consist of roofs), this would proportionally turn into an energy cost saving whose upper limit is estimated to be around USD $22.15 billion, which is 23.5% of the current cooling expenditure.
According to another recent study, the combined effect of carbon dioxide sequestration by biomass growing on green roofs and energy savings could be up to 1.2% of the 4,500 mega tons of CO2 produced in the EU every year, representing savings of 55.8 mega tons per year and 0.6% of CO2 sequestration from the forests in the world if 35% of impervious surfaces were greened, with a sequestration component of 25.9 mega tons per year, if biomass is removed and does not degrade on the roof.
A green surface would also reduce the rain runoff (i.e., water that flows along impervious surfaces such as streets, reaching canals and sewer systems) by retaining water within the soil and using it for vegetation growth. The runoff reduction is in the order of 17.5%. Considering that pollutant loads associated with rainwater runoff are estimated to be in the order of about 30 million population equivalent for biochemical oxygen demand (BOD), about 18 for nitrogen, and about 6 for phosphorous, this can be a sizable contribution to the treatment of pollution from European urban areas, for a global runoff reduction of about 10 square kilometers per year.
Conclusions
The benefits of urban greening are numerous, and they can lead to the more sustainable development of cities and to a better, more healthy life for humans while supporting the fight against climate change. In particular, up to 55.8 mega tons of CO2 per year could be avoided if 35% of impervious surfaces were greened. The activities related to urban greening can also introduce into the market new companies specialized in greening interventions.
