Half of urban greenhouse gas emissions produced by 25 megacities 

New data published by Frontiers highlights that 52% of urban greenhouse gas emissions are generated by just 25 megacities. 

2015 saw the adoption of the Paris Agreement by 170 countries worldwide, with the aim of curbing the average global temperature increase to 1.5°C. Subsequently, many countries and cities offered targets for greenhouse gas mitigation.  

However, the UNEP Emissions Gap Report 2020 has indicated that, short of radical and comprehensive actions to alleviate the climate crisis, we are currently moving towards a temperature increase of more than 3°C by the end of the 21st century. 

Studying urban greenhouse gas emissions 

A novel study published in Frontiers Sustainable Cities offers the world’s first global balance sheet of urban greenhouse gas emissions, emitted by the world’s major cities. The objective of the study was to examine the efficiency of historical greenhouse gas reduction policies applied by 167 globally distributed cities that are at different developmental stages.  

Despite occupying only 2% of the Earth’s surface, cities are the biggest contributors to the climate crisis. However, existing urban greenhouse gas alleviation targets are not adequate for achieving global climate change targets by the end of this century.  

“Nowadays, more than 50% of the global population resides in cities. Cities are reported to be responsible for more than 70% of GHG emissions, and they share a big responsibility for the decarbonization decarbonisation of the global economy. Current inventory methods used by cities vary globally, making it hard to assess and compare the progress of emission mitigation over time and space,” explained co-author Dr Shaoqing Chen, of Sun Yat-sen University, China. 

The researchers performed sector-level greenhouse gas emission inventories of 167 cities, ranging from metropolitan areas like Durban, South Africa, to cities such as Milan, Italy. The scientists then investigated and contrasted the carbon reduction progresses of the cities based on the emission inventories documented in different years (from 2012 to 2016). 

Finally, they evaluated the cities’ short-, mid-, and long-term carbon alleviation goals. The cities were selected from 53 countries around the world and were chosen based on representativeness in urban sizes and regional distribution. The degree of development was distinguished based on whether they belonged to developed and developing countries according to the UN classification criteria. 

The study’s core findings 

  1. 25 cities are responsible for 52% of the total urban greenhouse gas emissions.
  2. Cities in the US, Europe and Australia had considerably higher per capita emissions than cities in developing areas.
  3. Stationary energy and transportation were the two biggest causes of emissions.
  4. Out of the 42 cities that had time-series traceable data, 30 reduced the annual greenhouse gas emissions during the study period. However, in several cities, there was an upsurge in emissions.
  5. 113 of the 167 set differing types of greenhouse gas emission reduction targets, while 40 have set carbon neutrality goals.

The greatest polluters 

The team’s findings indicated that developed and developing countries alike have cities with high total greenhouse emissions, but that megacities in Asia, for example, Shanghai in China and Tokyo in Japan, were particularly significant emitters. The inventory of per capita emissions highlighted that cities in Europe, the US, and Australia have greater emissions than most cities in developing countries, but China too has several cities where per capita emissions equalled those of developed countries. It must also be considered that various developed countries outsource high carbon production chains to China, therefore heightening export-related emissions for the latter. 

The research also found significant sources of greenhouse gas emissions: “Breaking down the emissions by sector can inform us what actions should be prioritised to reduce emissions from buildings, transportation, industrial processes and other sources,” explained Chen.  

Stationary energy – such as emissions from fuel combustion and electricity use in residential and institutional buildings, commercial buildings, and industrial buildings – accounted for 60 to 80% of total emissions in North American and European cities. In one onethird of the cities, over 30% of total emissions were from on-road transportation, and less than 15% of emissions were from railways, waterways, and aviation. 

Finally, the results highlight that the levels of emissions rise and decline differed between the cities over the study period. For 30 cities, there was a clear-cut emission decrease between 2012 and 2016. The top four cities with the largest per capita decrease were Oslo, Houston, Seattle, and Bogotá. Meanwhile, the top four cities with the greatest per capita emissions increase were Rio de Janeiro, Curitiba, Johannesburg, and Venice. 

Policy recommendations going forward 

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

Next, is the development of methodologically consistent global greenhouse gas emission inventories to monitor the efficiency of urban greenhouse gas mitigation policies. Finally, “Cities should set more ambitious and easily-traceable mitigation goals. At a certain stage, carbon intensity is a useful indicator showing the decarbonization decarbonisation of the economy and provides better flexibility for cities of fast economic growth and increase in emission. But in the long run, switching from intensity mitigation targets to absolute mitigation targets is essential to achieve global carbon neutrality by 2050,” Chen concluded. 

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