Lahti, a town in Finland, offering cake, free transport tickets and other rewards to locals who cut their carbon emissions, has developed an app that tracks residents' CO2 outlays based on whether they travel by car, public transport, bike or on foot. The app, called CitiCAP and developed with European Union funds, gives volunteers a weekly carbon quota.

If their allowance is not exhausted, participants get virtual money that can be used to buy bus tickets, access to the swimming pool or a piece of cake.

Ville Uusitalo, the project's research manager, said: "You can earn up to two euros (per week) if your travel emissions are really low. But this autumn, we intend to increase the price tenfold.” Currently, around 44% of trips in the city are considered sustainable.

Head of the project, Anna Huttunen, said: "Lahti is still very dependent on cars. Our goal is that by 2030 more than 50% of all trips will be made via sustainable means of transport”.

On average, a resident Lahti — population 120,000 people — "emits the equivalent of 21 kilograms of CO2 per week", according to Uusitalo. The app challenges users to reduce their carbon emissions by a quarter.

So far 2,000 residents have downloaded the app, with up to 200 of them using it simultaneously.

CitiCAP's developers hope similar tools in the future will help people manage their consumption-related emissions.

"Mobility is only part of our carbon footprint," Uusitalo said .

The town, which is also the EU's 2021 Green Capital, aims to significantly reduce its environmental impact over the next ten years.