WITH his tousled but cherubic looks and a voice sounding even younger than his 29 years, Jesse Klaver is the latest standard-bearer of a party that is trying to recapture its idealistic élan, and so inject new life into his country’s politics. “A sense has developed in the Netherlands, in Europe, in the West, that there is nothing we can do about anything, that this is just the way the world is,” exclaims the man who last week became leader of the Dutch GreenLeft party, succeeding a politician 30 years older. “It’s not true. We built this world, brick by brick, and what you build yourself you can change yourself.”

As a rebuilder of his own party’s fortunes, he may succeed. In 2012 the Dutch Greens saw electoral disaster, dropping from ten seats in the 150-seat parliament to just four after a pointless struggle over the party leadership. Now they are staging a comeback: a poll after Mr Klaver’s accession showed them winning 13 seats.

GreenLeft owes much of this bounce to the Dutch Labour party, which joined a coalition led by the centre-right Liberals and has shared responsibility for fiscal austerity. That has created room for Mr Klaver to promote a fiery anti-austerity agenda. He abhors tax evasion by multinationals. He favours a minimum income (see article). And he has invited the French economist Thomas Piketty to address parliament.

Across northern Europe, many Green parties are taking a similar stance: stepping into a radical space left vacant as Social Democrats move to the centre. The German landscape resembles the Dutch: a coalition between centre-left and centre-right has left voters unsure what the Social Democrats (SPD) stand for. In France the Socialists have reversed early promises and squeezed budgets, driving their erstwhile Green coalition partners to quit the government. In Britain, despite Labour’s partial return to leftist roots in the election it lost on May 7th, the Greens defined themselves as the unapologetic party of the left and quadrupled their vote share, to 4%. In Austria reaction against centrist coalitions has pushed the Greens above 10%; they won 12% in parliamentary elections in 2013. In Sweden the Greens are serving in government for the first time.

It is a sweet moment for parties that began on the ideological fringes in the 1980s, but it holds risks. Europe’s Green parties have come far since the 1990s when the pace-setting German Greens, and several others, saw a struggle between radical Fundis and pragmatic Realos. Across Europe, pragmatism prevailed, and most Greens acknowledged the need for markets. That in turn helped them to influence parties on the centre and right—as when Angela Merkel, Germany’s Christian Democratic chancellor, adopted the long-standing Green goal of phasing out nuclear power. If the Greens shift too far left, that could deny them the option of bargaining with the centre. (As of now they share power with the centre-right in one German state; it’s an open question whether this could happen at national level.) Shaky economies and high unemployment also pose risks for parties focused on the environment. Economic pain tends to shift voters from old centrist parties to small ideological ones, but most of the winners have been Eurosceptic and anti-immigrant, such as the UK Independence Party (UKIP) and France’s National Front (FN). Green parties are universally pro-immigrant and pro-European. Of course, that stance scores well with certain voters. Across Europe, Green supporters share a similar profile: educated, urban, cosmopolitan and young. Neil Carter, a political scientist at York University in Britain, terms them “left-libertarians”. That marks a contrast with blue-collar voters who are moving right, especially on immigration. Yet as a tactic, a sharp turn left is no panacea. In many countries the Green parties’ embrace of radical economics faces competition from traditional far-left parties, such as Germany’s Die Linke or the Dutch Socialists. France’s far-left parties are one reason the French Green party (EELV) has never had much luck. Another is that other French parties also claim environmental laurels. In European elections last spring, Green candidates had to differentiate themselves from the right-wing FN, which also hates multinationals and GMOs.

In countries where they have joined coalitions, the Greens have sometimes struggled with the transition from opposition to government. Germany’s Greens had a successful stint in government in the late 1990s and early 2000s, led by Joschka Fischer, who was popular as foreign minister. But France’s EELV were relieved to quit the unpopular Socialist-led government last year in protest at its liberalising reforms. In Sweden the Greens have six portfolios in a Social Democrat-led government, but they seem bogged down in relatively narrow issues, like an effort to close an airport.

Swedish Green leaders are also prone to energy-sapping gaffes. This month the deputy prime minister, Asa Romson, had to apologise for comparing the Mediterranean refugee crisis to Auschwitz. In her apology she used the word “gypsy” instead of “Roma” and had to say sorry again. As Nicholas Aylott of Sweden’s Sodertorn University notes, this is partly a problem of Green voters’ own making: they like impulsive, informal leaders.

The Greens’ capture of left-wing economic territory is largely a northern European phenomenon. In the south, where Green parties have always been weaker, rising left-wing populist parties such as Podemos of Spain have grabbed that space.

Yet the Greens’ dilemma is not only over where to poach votes. They must also fight apathy. Voter turnout is falling across Europe, especially among the young. For Green parties, attracting new voters is vital and this clearly inspired the Dutch leadership change. When Mr Klaver says the world is “ours to change”, and denounces mainstream parties for “economistic” thinking, he sounds immature. But for a party bent on projecting the optimism of youth, he presses the right buttons.