Last July, Mumbai celebrated the opening of the Bandra-Worli Sea Link, a bridge to connect the island city of Mumbai with its surrounding western suburbs. This feat of construction was built not just to connect two points of land separated by water, but to provide a shortcut around unbearably congested city streets.

The Sea Link is a symbol of the explosive growth in car travel in recent years. This growth is a result of more people, more cars, and more frequent and longer trips. With rising incomes and introduction of low-cost cars, the trend appears set to continue. So what does this mean for the city thirty years from now? Today, the city is crippled by congestion even though only about 5% of trips are made by car. What happens when that figure doubles? What would happen if Mumbaikars drove cars as much as, for example, their European counterparts? And how would this growth impact carbon emissions?

These are some of the questions the Centre for Sustainable Transport in India (CST-India), a member of the EMBARQ Network (the producer of this blog), tried to address in a recent study, which I presented last month at the TRB Annual Meeting in Washington, D.C. (See my Powerpoint presentation here, or at the bottom of this post.) The study considered how CO2 emissions from passenger travel would increase under three different future scenarios, focusing on the Indian cities of Mumbai, Ahmedabad, and Surat. Each scenario imagines a different set of policies that cities would adopt to influence how people travel. For example, in the “Two-Wheeler World” scenario, cities would encourage the use of motorcycles through tax incentives and strategic street design. The analysis then considers how those policies—combined with trends of population growth (including continued in-migration to cities), rising income, longer travel distances, and better fuel efficiency—would influence CO2 emissions.

The magnitude of the results is striking (but of course the magnitude of everything in a megacity like Mumbai can be striking.) For example, under the “Automobility Ubiquity” scenario—in which the city builds roads to accommodate cars and fails to invest in public transport—CO2 emissions would increase by 20 times in by 2040. Even under the “Sustainable Urban Transport” scenario—in which cities build transit, encourage walking and cycling, and promote efficient land use—emissions would increase nearly five-fold.

This seems like a huge increase—and it is—but a look at per capita emissions puts it in some perspective. Under the worst-case scenario (“Automobility Ubiquity”), the average amount of CO2 produced by each person annually would increase from 0.1 tons in 2005 to 1 ton CO2 in 2040 . Compare that with the United States, where today each person is responsible for 2.5 tons of CO2 per year from travel.

Perhaps the most obvious result is the disparity in emissions levels between the various scenarios. This disparity, however, points to the great potential for urban policy to positively influence carbon emissions.

It’s clear that Mumbai and other cities need to act now realize a sustainable future scenario: we need to invest in transit, encourage walking and cycling, and promote land use planning that is consistent with transport goals. These are not new concepts, of course, but studies like this remind us of the magnitude of the challenge and the urgency to act.