By the end of this decade it may be too late to limit global warming to scientifically guided limits, if the infrastructure built in the next four years is constructed along the same lines as currently planned.



Building high-carbon infrastructure – from transport systems predicated on motor car use, to new coal-fired power plants, and buildings that leak energy – effectively “locks in” a future of greenhouse gas emissions that are likely to far exceed the planet’s capacity to absorb carbon.

Though such factors have been the subject of numerous scientific studies in the past decade, countries have been slow to take the implications on board.

The latest warning comes from the C40 group of mayors and city authorities that have signed up to their own limits on carbon and actions to combat climate change and reduce its impacts, often independently of their national or federal governments.

In a new report, entitled Deadline 2020, published on Tuesday and co-authored by the engineering firm Arup, they suggest that the next four years to the end of 2020 will be crucial in determining whether the world is able to limit warming to no more than 2C above pre-industrial levels – as called for in the Paris agreement signed by world governments last year – or busts that goal, with unpredictable and potentially irreversible effects on climate change.

What is the Paris agreement? It's a climate change accord agreed by nearly 200 countries in December 2015, which came into force on 4 November 2016. The agreement commits world leaders to keeping global warming below 2C, seen as the threshold for safety by scientists, and pursuing a tougher target of 1.5C. The carbon emission curbs put forward by countries under Paris are not legally-binding but the framework of the accord, which includes a mechanism for periodically cranking those pledges up, is binding. The agreement also has a long-term goal for net zero emissions which would effectively phase out fossil fuels.





Actions and regulations introduced by cities are increasingly seen as an important alternative to the national targets and measures that have been the main feature of international climate change negotiations, culminating in the Paris agreement. While national governments may be reluctant to make sweeping changes or new regulations, many mayors and local authorities in large cities, in both the developed and developing world, have sufficient autonomy to set their own path.

Such initiatives are seen as having particular potential in the US, where the president-elect Donald Trump has vowed to “cancel” the Paris agreement, but where the political leaders of many large cities are using their powers to reduce greenhouse gas emissions, for instance through improvements to public transport and new regulations on energy.

The authors of the report calculate that city leaders acting alone could deliver roughly half of the carbon savings required to meet the Paris agreement goals. But there is much to do. Megacities, for instance, defined as those with more than 10 million inhabitants, must reduce their carbon emissions from about 5 tonnes per capita per year at present to less than 3 tonnes within the next decade or so.

About $375bn (£301bn) will be required globally by the end of the current decade to meet the goals, the report found, though some of this expenditure may already be included in planned city budgets.

