The voters of Berlin are heading to the polls on Sunday to decide whether they will make the city climate neutral by 2030.

They're voting in a binding referendum that will force the new conservative local government to invest heavily in renewable energy, building efficiency and public transportation.

Climate campaigners gathered over 260,000 signatures in support of the referendum, which could make Berlin one of few major European cities with a legally binding goal to become carbon neutral in seven years.

The European Union last year started a scheme to help 100 cities inside and outside of the bloc become climate neutral by 2030, but the scheme and the financial support it offers are not legally binding.

Legally binding

The referendum's results would show whether Germans, or at least Berliners, want Germany's climate policy, which now aims to make Europe's biggest economy carbon-neutral by 2045, to be more ambitious.

Climate activists who initiated the vote say the government's target is too far in the future to prevent global warming from exceeding 1.5 degrees Celsius.

"At the moment, climate policy is simply not sufficient to ensure a future worth living in our city," according to Jessamine Davis, a spokesperson for Climate New Start Berlin.

Unlike Berlin's previous referendums, including one calling for expropriation of large landlords or on keeping the former Tempelhof airport free from development, Sunday's climate referendum will be legally binding for the government in Berlin.

"The new version will automatically apply if Berlin population votes if favour," Davis said.

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Seven year timetable

The initiative, if approved, will only oblige the local government to achieve climate-neutrality within seven years, but the group says various scientific studies offer a wide range of specific measures to reach that goal.

They include a mandate to install solar panels on all suitable roofs in the city to generate around 25 percent of the city's electricity, in addition to expanding wind power turbines in the neighbouring Brandenburg state to supply the capital.

Installing a large heat pump on the Spree river and renovating buildings across the city to replace oil and gas heaters with efficient heat pumps are also among the measures that will be needed if Berliners back the new 2030 goal.

Germany's capital would also have to expand electric vehicles usage and add bike lanes while making public transport more attractive, the group suggested on its website.

The referendum comes as Germany's conservative CDU party is negotiating a possible coalition with the Social Democrats in the city after its clear victory in a repeat election, driving the environmentalist Greens into opposition.

According to the initiative organizers, around 455,000 Berliners have requested to cast their votes via mail so far. In addition to a majority of positive votes, the initiative needs at least 608,000 'Yes' votes to make the results binding.

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