Although they cover only 2% of Earth’s surface, cities are major contributors to the climate crisis we’re currently facing. Some cities like Tokyo or Delhi, with populations numbering in the tens of millions, harbor more people than entire medium-sized countries, with greenhouse gas emissions to match. According to a recent analysis, just 25 such mega-cities are responsible for 52% of global urban greenhouse gas emissions (GHGs).

“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 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,” said study co-author Dr. Shaoqing Chen of Sun Yat-sen University, China.

Chen and colleagues conducted a thorough assessment of GHGs on a sector-by-sector basis of 167 major cities in 53 countries, from Durban in South Africa to Milan in Italy. During the analysis, they also tallied progress in terms of emission reductions from 2012 to 2016 and how the cities fared in meeting their short-, mid-, and long-term carbon mitigation goals.

Across the board, both in developed and developing countries, cities are responsible for copious amounts of GHGs emissions. However, Asian mega-cities such as Shanghai and Tokyo were identified as particularly large emitters. On a per-capita basis, however, cities in Europe, the USA, and Australia were found to have higher emissions than the vast majority of cities in developing countries.

The most important source of emissions in most cities was the production of stationary energy (emissions from fuel combustion and electricity use), which was responsible for 60% to 80% of all emissions in North American and European cities. In about a third of all cities included in the assessment, more than 30% of GHGs were due to on-road transportation. Just 15% of total emissions came from railways, waterways, and aviation.

“Breaking down the emissions by sector can inform us what actions should be prioritized to reduce emissions from buildings, transportation, industrial processes and other sources,” said Chen.

The good news is that in at least 30 cities, there was a clear trend of decreasing emissions between 2012 and 2016. The top four cities with the largest per capita reduction were Oslo, Houston, Seattle, and Bogotá. On the opposite end, where emissions per capita increased, were Rio de Janeiro, Curitiba, Johannesburg, and Venice.

In light of the Paris Agreement, 113 of the 167 cities have set their own GHG emission reduction targets, 40 of which claim they want to become carbon neutral by 2050.

 “Cities should set more ambitious and easily-traceable mitigation goals. At a certain stage, carbon intensity is a useful indicator showing the decarbonization 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 said.

The findings appeared in the journal Frontiers in Sustainable Cities.