Even before the election of Donald Trump, it was clear that states and cities — what climate nerds call “subnational actors” — are central to tackling climate change. Now that the US federal government is getting out of the climate protection business, at least for four years, subnational actors are more important than ever.

Especially cities.

Cities generate most of the world’s economic activity, innovation, and cultural ferment. They also generate a growing share of its carbon emissions: according to the IPCC, cities are responsible for about 75 percent of global energy-related CO2 emissions. That number will only grow as the world continues to urbanize, especially in fast-growing nations like China and India.

Urban areas are also first in line to feel the effects of climate change. About 90 percent of urban areas in the world are coastal, so if nothing else, they will deal with sea level rise. Some 70 percent already report dealing with climate impacts.

If they hope to avoid worse to come, cities will need to almost entirely rid themselves of carbon over the next few decades. How much could that help in the climate fight? And how can cities go about doing it? Two recent reports attempt to answer these questions.

The opportunities for urban decarbonization are yuge

The first report is a shorter, more theoretical take, from researchers at the Mercator Research Institute on Global Commons and Climate Change. (See this piece from Shobhakar Dhakal, one of the authors, for an accessible overview.)

It highlights two important reasons why decisions about urban infrastructure — buildings, roads, sewers, electricity grids, and the management systems that tie them together — are important to decarbonization.

First, urban infrastructure has a huge influence on greenhouse gas emissions; “differences in the type and shape of the built environment can result in differences in urban transport and residential GHG emissions by a factor of ten.” [my emphasis] There is an enormous difference in emissions between low-carbon urban infrastructure — “relatively high-density households and population; mixed residential use, workplaces, retail, and leisure activities; a high number of intersections; and mobility choices that avoid excessive construction of low-connectivity roads” — and the high-carbon, sprawly kind.

Second, there is enormous inertia in urban infrastructure. “Among all long-lived capital stocks,” the researchers write, “land use, urban form, and road systems stand out for their century-long endurance, exceeding the lifetimes of coal power plants and car fleets.” Once these infrastructure decisions get made, they put in place “boundary conditions” that shape a city’s emissions trajectory for up to a century.

So urban infrastructure decisions matter a lot and last for a long time.

The Mercator crew takes a pass at quantifying the decarbonization possible at the urban level. I’ll skip over the details to the headline finding, per Dhakal:

[B]y building climate-smart urban infrastructure and buildings, we could cut future emissions [from cities] in half from 2040 onwards. We could reduce future emissions by ten gigatonnes per year: almost the same quantity currently being emitted by the United States, Europe and India put together (11 gigatonnes).

While there are substantial opportunities in retrofitting and upgrading existing urban infrastructure, the really big numbers come from smart design of new infrastructure. It’s much cheaper and more effective to do this stuff right the first time.

That means the main priority is guiding the ongoing process of urbanization in Asia, the Middle East, and Africa, where new cities are being built by the dozens.

However, more established cities also have an crucial leadership role to play. Which brings us to the second report.

Big cities across the world now have a roadmap to decarbonization

C40 is a global group of 86 affiliated cities representing 650 million people and a quarter of the world’s GDP. While America was busy electing Trump, C40 was hosting a Mayors’ Summit in Mexico City. There, for the first time, a concrete roadmap was released showing how C40 cities could help meet global climate targets.

Some of the report’s headline findings:

To stay within the more ambitious 1.5 degree temperature increase trajectory that countries agreed to in Paris last year, “average per capita emissions across C40 cities would need to drop from over 5 tCO2e per capita today to around 2.9 tCO2e per capita by 2030.” For many cities in the developing world, this means that the amount of carbon emitted per citizen can remain steady or rise for a while. But for developed cities, it means an immediate, steep reduction. The challenge is ... substantial.

Immediate means immediate. Actions taken by cities in the next four years will determine whether the 1.5 degrees trajectory is possible at all. The report says that “12,000 [carbon mitigation] actions — or on average 143 actions per C40 city — must be initiated by 2017 alone to enable the necessary emissions reductions in later years.” This is all about “bending the curve,” preventing the short-time rise in emissions and setting up larger reductions thereafter.

Between now and 2050, the report says, hitting 1.5 degrees will require $1 trillion in investment across C40 cities — $375 billion in the next four years alone. That investment will be concentrated in East Asia and Europe.

The fastest and biggest reductions over the next four years must come from “wealthier, high carbon cities” with over $15,000 in per-capita GDP — which means, for the most part, developed-world cities.

Mayors themselves can “deliver or influence” only about half of the needed emission reductions in a 1.5 degree scenario. The rest will require larger structural changes, particularly widespread electrification and greening of the electricity grid (“electrify everything; clean up electricity”).

The 12 American cities involved in C40 — cities like Seattle, Washington, DC, and Phoenix -- made a point of issuing a press release in response to the presidential election, vowing to continue the fight against climate change and asking Trump to join.

I doubt he will listen. Here’s hoping they can do a lot on their own.