Peaking Emissions: An important milestone on the road to Paris

By Michael Doust, C40 Programme Director, Measurement and Planning  

According to the world’s leading scientists, global emissions must peak by 2020 and then begin to decline rapidly if there is any hope of delivering the Paris Agreement and keeping global average temperature to rise to between 1.5 and 2 ̊C. Today, C40 announced that 27 of its cities have reached peak emissions.1 Peaking defines the point in time where emissions switch from increasing to decreasing, and represents a critical turning point in converting the Paris Agreement from aspiration into reality. The longer peaking is delayed, the later global emissions start to decline, and the more difficult it will be to limit global warming. 

Deadline 2020: How cities will get the job done2 concludes that for C40 cities to contribute their fair share of global emissions reduction, aggregate C40 emissions will need to peak in 2020. Considering the UNFCCC’s principle of common but differentiated responsibilities and respective capabilities, the report identifies four trajectories: steep decline, steady decline, early peak and late peak3. On average, cities in the Global North – on the steep and steady decline trajectories – will need to have reached peak emissions by 2020 at the latest, whilst cities in the Global South – on the early and late peak trajectories, and with much lower levels of emissions – may continue to grow their overall emissions until between 2030 and 2035 before they too must peak. 

The number of C40 cities that have already peaked increased from 6 cities by 2000, to 22 by 2010 and 27 cities in 2012, the latest year our methodology allows peaking to be identified4. By the end of 2020, a total of 66 cities are expected to be able to demonstrate that they too have peaked5, representing 69% of C40 city emissions6. A further 22 C40 cities in the Global South have made a commitment to deliver action consistent with the ambitions of the Paris Agreement and reduce their emissions to net zero by 2050. Thus the total number of C40 cities that have either already peaked, are expected to peak in the next few years or have committed to peaking is 88 cities, representing 85% of C40 city emissions.

The analysis shows that – based on the inventory boundary set out in the Global Protocol for Community-scale Greenhouse Gas Emission Inventories4 – cities in the Global North have managed to reduce emissions despite continued economic and population growth. The data, however, suggests no clear relationship between the year when cities peak and when their countries peak (where they have peaked). Some cities peak before their countries do, whilst other cities peak after their countries do. This is driven by a range of factors that require further analysis. 

For example, the city of Portland (2000) peaked emissions seven years before the USA reached the same milestone (2007)7. London, on the other hand, reached peak emissions (2000) nine years after the United Kingdom peaked their emissions (1991)8, 9. Since peaking, emissions in both cities have reduced by approximately 30% whilst experiencing  continued population and economic growth. In the case of Portland “due to a combination of actions, including: improved efficiency in buildings, appliances and vehicles; a shift to lower-carbon energy sources; more walking, biking and transit; and reduced methane emissions from landfills and more recycling”5, and in London “largely due to reduced gas consumption and decarbonisation of the national electricity”10.

Determining when a city has peaked is not always straightforward. While emissions may plateau or decline for a few years, it is no guarantee that this will not reverse in the future. Thus, to provide sufficient confidence that a peak reflects a definitive maximum of emissions, C40 has taken a conservative approach and we only consider a city as having peaked in the past if it meets the following three principal criteria:

  • The emissions reached a maximum level at least 5 years before the most recent inventory year11. If the peak is very recent, a change in emissions could potentially be due to short-term fluctuations caused by extreme weather or an economic downturn rather than indicative of a longer-term trend of reducing emissions.
  • The emissions at peak level are at least 10% higher than the most recent inventory year. Again, this is to eliminate “false” peaks due short-term fluctuations. When looking at the average annual reduction rate in emissions in cities that have peaked, the mean value is 2%, which is equivalent to a 10% reduction over a five-year period.    
  • The city has made a public commitment to deliver further emission reductions.

Cities that do not meet all of the above criteria, or do not have the data to demonstrate this, may well already have peaked their emissions. As a result, the numbers presented are likely to represent an underestimate of the actual number of C40 cities that have already peaked. 

The primary drivers for the 27 cities reaching peak emissions reflect the four action areas identified in Focused Acceleration12 that have the greatest potential to reduce emissions in cities, namely: decarbonising the electricity grid, optimising energy efficiency in buildings, enabling next-generation mobility and improving waste management. 

Cities, however, cannot achieve peak emissions, and the required reductions that follow, by themselves. Coordination and collaboration between different levels of government is critical. In their climate action plan, Portland recognises the important role the state government plays in transportation investments and strengthening building codes14, whilst London’s Environment Strategy calls for greater action from national government to meet electricity grid decarbonisation projections15. In Japan, the closure of large nuclear plants over the last decade and their replacement mainly with natural gas, has increased the emission factor of electricity. As a result, Tokyo, which relies on electricity for approximately 50% for its energy needs, has seen a previous peak reverse despite making significant progress in reducing energy consumption during the same period.


With a limited carbon budget, and a narrow timescale to deliver it, robust and ambitious action is required. Peaking is an important milestone on the road to Paris and as such should be recognised and celebrated. Encouragingly, we are seeing an increase in the number of cities that are peaking emissions, and doing so despite continued economic and population growth. Peaking is, however, only a step towards delivering the Paris Agreement. Just as important is what happens after peaking.

[1] The cities are: Barcelona, Basel, Berlin, Boston, Chicago, Copenhagen, Heidelberg, London, Los Angeles, Madrid, Melbourne, Milan, Montréal, New Orleans, New York City, Oslo, Paris, Philadelphia, Portland, Rome, San Francisco, Stockholm, Sydney, Toronto, Vancouver, Warsaw, and Washington D.C.
[2] Deadline 2020: How cities will get the job done, C40 Cities and ARUP (2016):
[3] Steep Decline” – Cities with a GDP per capita over $15,000 and emissions above the average for C40 (emissions need to be immediately and rapidly reduced and the city is sufficiently developed to do so); “Steady Decline” – Cities with a GDP per capita over $15,000 but emissions lower than the average for C40, (the city is sufficiently developed to immediately reduce emissions, but a less rapid rate of reduction is required than for the Steep Decline group); “Early Peak” – Cities with GDP per capita below $15,000 and higher than average emissions per capita, (an early emissions peak is required, although the city’s development status means that decline cannot be immediate); “Late Peak” – Cities with a GDP per capita below $15,000 and lower than average emissions per capita (a slightly later emissions peak is possible).
[4] An exception was made for Oslo (Norway) – which recorded peak emissions in 2013 – due to the scale of emissions reduction and climate actions taken since peaking.
[5] It will, however, take beyond 2020 to establish this, as peaking can only be concluded on the basis of a historical record of emissions.
[6] Based on reported or estimated emissions in 2015 using the Global Protocol for Community-scale Greenhouse Gas Emission Inventories:
[7] Climate Action Plan, City of Portland (2015):
[8] Delivering London’s Energy Future: The Mayor’s Climate Change Mitigation and Energy Strategy (2011):
[9] Turning Points: Trends in Countries’ Reaching Peak Greenhouse Gas Emissions over Time, WRI (2017):
[10] London Environment Strategy, GLA (2018):
[11] This is consistent with the approach taken by WRI in their working paper Turning Points: Trends in Countries’ Reaching Peak Greenhouse Gas Emissions over Time, WRI (2017):
[12] Focused acceleration: A strategic approach to climate action in cities to 2030, McKinsey Center for Business and Environment and C40 Cities (2017):