As Greta Thunberg sets sail across the Atlantic to the United Nations in New York, the backlash against her has been as vicious as it is has been inevitable. According to the Australian conservative climate-change denier Andrew Bolt, she is “deeply disturbed”, “freakishly influential” and “strange”. In a nasty, brutish and short tweet, the former Ukip funder, Arron Banks, simply said, “Freak yachting accidents do happen in August …” Controversialist columnist Brendan O’Neill wrote that there was “something chilling” about Thunberg, who “increasingly looks and sounds like a cult member”.

The attacks on Thunberg look like a kind of displacement activity rooted in fear that what she says might be true

We live in such venomous times that these levels of vitriol are disturbingly common. But it is not at all normal to level such personal attacks at a 16-year-old with multiple mental health issues campaigning to make the world a better place. Banks’s protest that “you lefties have no sense of humour” is as shabby as Donald Trump’s “locker-room banter” defence of his flagrant misogyny. O’Neill says Thunberg is a “poor young woman” whose transformation by others into a “messiah” is “unforgivable”, as though that makes his descriptions of her “monotone voice” and “look of apocalyptic dread” nothing personal.

The ferocity of the attacks demands an explanation, but it also raises questions for those who have built up her up and inadvertently helped make her a target.

Shooting the messenger is, of course, a tried and tested way of dealing with bad news. The case that the anthropogenic climate crisis is real and urgent is now so strong that it’s pretty much the only tactic left for those who refuse to accept it.

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Thunberg is a particularly tempting target. Bolt made much of the fact that she is “someone with disorders which intensify fears”. This is deeply reassuring for people who think anyone who is afraid for the future of the planet is overreacting. “Eco-anxiety” can be dismissed as a pathology rather than as a rational response to a growing emergency.

Thunberg has become exhibit A for those like O’Neill who want to rebrand the environmental cause as “climate-change alarmism”. They see in Thunberg someone who makes this dismissive charge seem plausible. Why try to criticise thousands of sober, grownup scientists when you can have a go at a pig-tailed Young Turk instead?

You wonder about the psychological motivations of those who have set upon Thunberg so thuggishly. It looks like a kind of displacement activity rooted in fear that what she says might be true. Instead of engaging with the argument they wrestle with the arguer. Such speculation is tempting but it’s ultimately beside the point. Getting personal is a sure sign you’re losing the argument so we should not get personal back. Whatever their motives, their actions betray a moral and intellectual bankruptcy.

Play Video 1:40 Greta Thunberg begins zero-carbon Atlantic voyage – video

However, the uncomfortable question for Thunberg’s supporters is whether their virtual canonisation of her has presented a gift to their opponents. Making a young and idealistic teenager the figurehead of a movement makes it too easy to dismiss the campaign as a whole as naive and idealistic. Indeed, the commentator Christopher Caldwell, who is supportive of the cause, worries that the rallying around Thunberg reflects a refusal to engage with complexity. “People have had enough of balance and perspective,” he wrote in the New York Times, “They want single-minded devotion to the task at hand.” That is exactly what Thunberg has come to represent.

The French philosopher Raphaël Enthoven hinted at another problem in a Delphic tweet in which he called her “an anti-product product” that we use to buy virtue with our support and retweets without actually having to do anything ourselves. Many of those who ostentatiously bid her bon voyage across the Atlantic are hardly strangers to airports. “When you consume Greta, you do not help the planet,” wrote Enthoven. “You play the game of the system that destroys it.”

Sanctification also makes reasonable disagreement from people on her side more difficult. Most notably Thunberg has frequently framed the issue as an intergenerational conflict. “Young people must hold older generations accountable for the mess they have created,” she tweeted last December. “We need to get angry, and transform that anger into action.”

Many older people resent being blamed for the climate crisis. Even if you agree that we should take our share of responsibility, what matters most now is clearing up the mess, not squabbling over who caused it.

Thunberg herself should not be blamed for the way in which she has been built up into something bigger than any person can be. On this, she and her fellow campaigners have been more clear-sighted than their adult fans. Before heading to the UN she said of leaders such as Trump that “instead of speaking to me and the school-striking children and teenagers they should be talking to actual scientists and experts in this area”. Mai Sheehan, a 16-year-old fellow activist from Ireland, told the FT that Thunberg “has always been very specific that she is not the leader” and that “all our decisions are made on a consensus basis”.

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Thunberg has been clearer than anyone that it is a mistake to place too much attention on her. “I think there is a lot of focus on me as an individual and not on the climate itself,” she told the German broadcaster Deutsche Welle. “I think we should focus more on the climate issue because this is not about me.”

Thunberg has done infinitely more in a few short years to improve humanity’s prospects than I have done or will do in a lifetime. Lionising her doesn’t help the cause. But the excessive zealotry of some of her supporters is a trifling fault compared to the egregious attacks by critics who would rather take her on than the inconvenient truths she brings.

• Julian Baggini is a writer and philosopher