BERLIN — In a country with ambitious intentions to go green, last week brought an unsettling image for eco-advocates: Germany’s environment minister presiding over the opening of one of the largest coal-fired power plants in the world.

The massive new power station near Cologne is a frontal challenge to efforts to drive down greenhouse gases, and some critics say it embodies a tricky contradiction in Germany’s ambitions to reduce emissions by 40 percent by 2020 and eliminate nuclear power two years later.

A year after German Chancellor Angela Merkel made a surprise decision to speed her country’s phaseout of nuclear power, environmentalists who hailed her plan — a reversal of her previous positions — now worry that her center-right Christian Democrats are too quick to embrace dirtier alternatives.

Germany’s dilemma shows how difficult it is to balance competing environmental priorities, even with vast resources and popular support for the efforts, analysts say. With low-emission nuclear power falling into disfavor in many countries after Japan’s Fukushima Daiichi nuclear meltdown, others may soon have to confront the same questions.

German leaders say that they are finding the right compromise between cost and environmental stewardship and that they will meet their 2020 goals.

“For many years, we will need conventional fossil energy in addition to renewables,” German Environment Minister Peter Altmaier said in an interview. “For 2020, the aim is to have 35 percent renewables, but that means 65 percent fossil electricity.”

He added: “We have to implement two goals at a time, and this can create some problems in the beginning. But we are working very hard, and I am very optimistic that Germany can manage to meet its ambitious CO₂ aims as well as its aims to eliminate its nuclear power.”

The lignite-fired power plant that opened last week may be the embodiment of an environmentalist nightmare. Soft lignite coal is one of the worst offenders for greenhouse gas emissions, although the new plant is replacing older, less-efficient ones. Nuclear power is politically unpopular in Germany, but it is a relatively low-emission source of electricity, meaning that the country has to fill the gap with dirtier sources as renewable energy struggles to keep pace.

Environmentalists fret that Merkel’s allies simply aren’t taking the goals seriously enough. Just months before the Fukushima meltdown in Japan, Merkel moved to postpone the nuclear phaseout as a component of plans to reduce greenhouse emissions in Germany. After the March 2011 earthquake and tsunami caused the nuclear meltdown, Merkel yielded to popular pressure and reverted to the speedier nuclear phaseout planned by her predecessor, center-left Social Democrat Gerhard Schroeder.

But Merkel’s opponents aren’t sure that she really has won the support of her sometimes-fractious coalition.

“They are not convinced” that the nuclear phaseout makes sense, said Oliver Krischer, a member of the opposition Greens in the German parliament and the party’s speaker on energy policy. “They still believe in the old system and aren’t doing the necessary steps which follow the stopping of the nuclear power plants.”