Small number of megacities responsible for majority of urban emissions

Under the Paris Agreement, governments are obliged to take action to cut carbon and other greenhouse gas emissions, with the goal of limiting the average global temperature rise to 1.5°C above pre-industrial levels.

According to the UNEP Emissions Gap Report 2020, current climate mitigation plans are insufficient and will lead to a temperature increase of more than 3°C by the end of the 21st century. Such a significant rise in global average temperature could see major cities largely submerged by rising sea levels (including Shanghai, Miami, Alexandria and Rio de Janeiro); increasing extreme weather including droughts and heatwaves, and the spread of desert and tropical disease to other parts of the globe.

A new study presents the first international 'balance sheet' of greenhouse gases emitted by the world’s largest cities. While cities cover just two per cent of the surface of the Earth, they are disproportionately large contributors to the climate crisis.

“Nowadays, more than 50 per cent of the global population resides in cities. Cities are reported to be responsible for more than 70 per cent of [greenhouse gas] emissions and they share a big responsibility for the decarbonisation of the global economy,” said Dr Shaoqing Chen, co-author of the Frontiers in Sustainable Cities study. “Current inventory methods used by cities vary globally, making it hard to assess and compare the progress of emission mitigation over time and space.”

The researchers conducted sector-level emission inventories of 167 cities representative of the world’s urban spaces, covering 53 countries across six continents, and at various stages of development. Next, they analysed and compared the carbon reduction progress of these cities based on emission inventories recorded from 2012 to 2016. Finally, they assessed the cities’ carbon mitigation goals: 113 of the 167 had greenhouse gas emission reduction targets and 40 have set net-zero carbon goals.

The analysis found that megacities in Asia, such as Shanghai and Tokyo, are very large and significant emitters and that per capita emissions are significantly higher in Europe, the US and Australia than in developing regions. China, which was categorised as a developing country, contains some cities where per capita emissions match those of developed countries (according to UN categorisation). Their analysis showed that just 25 (15 per cent) of the largest cities account for 52 per cent of urban greenhouse gas emissions in the model.

The cities with the largest per capita emissions reductions were Oslo, Houston, Seattle and Bogotá, while those with the largest increases were Rio de Janeiro, Curitiba, Johannesburg and Venice. Of the 42 cities with time-series traceable data available, 30 decreased their annual greenhouse gas emissions during 2012 to 2016.

Stationary energy and transportation were identified as two major sources of urban emissions. Stationary energy (including emissions from fuel combustion and electricity use in most building types) contributed to between 60 and 80 per cent of total emissions in North American and European cities.

“Key emitting sectors should be identified and targeted for more effective mitigation strategies,” said Chen. “For example, the differences in the roles that stationary energy use, transportation, household energy use and waste treatments play for cities should be assessed.”

Chen added that methodologically consistent global greenhouse gas emission inventories are necessary to track climate mitigation action: “Cities should set more ambitious and easily traceable mitigation goals. At a certain stage, carbon intensity is a useful indicator showing the decarbonisation of the economy and provides better flexibility for cities of fast economic growth and increase in emission.

“In the long run, switching from intensity mitigation targets to absolute mitigation targets is essential to achieve global carbon neutrality by 2050.”

In April, EU researchers reported that inconsistencies in how carbon emissions are calculated has led to a massive underestimate in emissions equivalent to excluding the US from global estimates. While some countries monitor their greenhouse gas emissions using methodologies agreed by the Intergovernmental Panel on Climate Change, some use their own models, leading to discrepancies.