KARLSRUHE, (Germany): Germany's highest court ruled Thursday that the government's flagship climate protection plan was "insufficient", a major setback for Angela Merkel's right-left coalition in an election year when environmental issues are expected to take centre stage.

In a decision hailed by activists as "historic" and "sensational", Germany's Constitutional Court ruled that Berlin's emissions reduction plan was "incompatible with fundamental rights" because it failed to cover the years beyond this decade.

The current measures "violate the freedoms of the complainants, some of whom are still very young" because they "irreversibly offload major emission reduction burdens onto periods after 2030", the court ruled, partially upholding a series of claims by environmentalists and young people. Under the plan agreed by Merkel's government, Germany will reduce CO2 emissions to 55 percent of 1990 levels by 2030.

The court argued that while the state had not violated its duty to protect citizens against climate change, the government had nonetheless not set out the timeline for further emissions reductions in enough detail.

Berlin must "at the very least determine the size of the annual emission amounts to be set for periods after 2030," the court ruled, saying an improved plan must be put forward by December 31, 2022.

The stunning ruling came at a time when debate over environmental policies was already heating up with the Green party enjoying a surge in popularity and overtaking Merkel's conservatives in several opinion polls over the last week.

Green leader Annalena Baerbock hailed the court's decision as "historic".

"The coming years will be decisive for meaningful action," tweeted the 40-year-old, who is aiming to succeed Merkel and become Germany's first Green chancellor.

Besides an emissions target for 2030, Germany's climate change law introduced in 2019 includes a range of policies, including incentivising renewable energies, expanding electric car infrastructure and carbon trading.

The law was passed in order to comply with the Paris Agreement, under which countries aim to keep the global temperature increase to under two degrees Celsius, and ideally closer to 1.5 degrees, by 2050.

Germany said last month it had met its annual climate goals in 2020, in part due to a reduction of activity during the coronavirus pandemic. Yet activists have long argued that the law does not go far enough, prompting a series of legal claims against the government to toughen the measures.

Supported by Greenpeace and leading German Fridays for Future activist Luisa Neubauer, the claimants included a group of nine young Germans whose families' agricultural or sustainable tourism businesses have been threatened by heatwaves and floods.

"We have won! Climate protection is not a nice-to-have, it's a fundamental right," tweeted Neubauer, 25, in reaction to the ruling, which came on a day when scientists warned Germany could lose its glaciers in 10 years.

The claimants' lawyer Roda Verheyen said that the court had "set a new global benchmark for climate protection as a human right".