When it comes to the climate emergency urbanists are part of the solution, right?

As I think I may have said before in this column, we urbanists think that the answer to any question is the walkable, mixed-use, reasonably dense city. Whether it be happiness and wellbeing, health, community, economic sustainability and mobility the answer is always the same.

We are right of course, but we do need to be careful not to jump to lazy conclusions. For all our talk of the “happy city”, for example, how do we explain that levels of mental illness are often much higher in cities?

This month when we came together as the Academy of Urbanism to discuss how cities, and housing in particular, need to respond to the zero carbon action plans being discussed at COP26, we were careful to show our workings.

We have always assumed that things can be done more sustainably within cities. Indeed I wrote a book making the case for this 20 years ago. One of the most cited studies is a 1989 paper by Newman and Kenworthy showing that the average gasoline consumption of US cities was twice that of Australian cities, four times that of European cities and 10 times higher than Asian cities. Having accounted for price, income and vehicle efficiency they conclude that the variable that really mattered was population density.

If China and India were to urbanise on the US model it would push up global CO2 emissions by 138%, whereas if they were to do so like France the rise would be only 30%

The wider evidence was brought together by Ed Glaeser in his book Triumph of the City. In this he estimates that per-capita CO2 emissions drop by 10% as population density doubles. This matters particularly in countries that are yet to urbanise. He suggests that if China and India were to urbanise on the US model it would push up global CO2 emissions by 138% whereas if they were to do so like France the rise would be only 30%.

But wait. Surely if we are right that life can be lived more sustainability in dense, walkable cities, then urbanising like France should reduce emissions?

This brings us to the rather awkward statistic from UN Habitat that while 55% of the earth’s populations live in cities, they account for 78% of the world’s energy consumption and 60% of greenhouse emissions. Maybe we urbanists are part of the problem rather than the solution?

Before we beat ourselves up too much, I should point out that the UN definition of cities includes suburban sprawl. Much of the rise in energy use is also accounted for by prosperity rather than urban form. People move to cities, especially in the developing world, to improve their prospects and with greater wealth comes increased energy use. The point therefore still stands that the environmental impact of a dense city will be less than a sprawling suburb.

Nevertheless we can’t assume that cities are inherently sustainable and we need to work harder to make our cities zero-carbon-ready. To this end two main themes emerged from the conference.

The first was a need to switch our focus from optimising operational energy use and to look instead at the impact of construction. In a super-efficient modern building the carbon emissions from construction are greater than its lifetime operational emissions. We should worry, as one speaker pointed out, that in three years China pours more concrete than the US did in the whole of the 20^th century.

The second theme was the urgent need to look at our existing building stock because 80% of the buildings that will exist in 2050 (when we plan to be zero carbon) already exist. The work of Pritzker Prize-winning Lacaton & Vassal was much cited by those who said that we should just stop knocking things down. The default position should always be to retain, retrofit and intensify rather than redevelop.

But how do we transform the city from inefficient, low-density sprawl to high-density walkable urbanism without knocking things down? We need to retrofit buildings sure, but we also need to retrofit entire neighbourhoods, towns and cities. How do we create walkable sustainable places without resorting to the wrecking ball? Now that’s an interesting question!