Greta Thunberg. This time last year she was unimaginable. Then, pretty much from nowhere, there she was: small and slight, a girl just turned 16, the way-too-young odd person out on a panel of adults sitting in front of the world’s economic powers at Davos last January. Unshowy and serious, careful, firm, she said it. Our house is on fire.

The ancient Greeks had a word for this: parrhesiastes. It means a person who speaks truth to power: you should not be behaving in this way. Don’t. More specifically it suggests someone in whom directness of expression and access to truth coincide; and it means someone of very little power who’s risking everything – because they can’t not, there’s no option – to speak ethical truth to powers so entrenched that they’re close to tyrannical, because telling this truth is about moral law. “Some people, some companies, some decision-makers in particular know exactly what priceless values they have been sacrificing to continue to make unimaginable amounts of money, and I think many of you here today,” she said to the World Economic Forum conference, “belong to that group of people.”

Thunberg is Swedish; she was born in 2003 and lives in Stockholm with her younger sister Beata, her mother, a singer, and her father, an actor. There’s a picture of the family somewhere online taken when she was much younger; her mother and father are smiling broad old-fashioned smiles at the camera – doing what you do when you have your photo taken. Both the children, only waist- and knee-high, regard the lens with utter seriousness and inquiry. At eight years old, Greta Thunberg heard about climate change. She wondered why no one was doing anything to stop it. At 11 she stopped speaking, in protest; now she sees that “selective mutism” as the first step to understanding the power and necessity of speaking at all.

She was 15 when she began taking days off her education to sit in front of the Riksdag in Stockholm, declaring she’d do this till Sweden reduced carbon emissions in line with the Paris agreement. She gave out leaflets: “I am doing this because you adults are shitting on my future.” She made a couple of speeches internationally that tapped into a profound unspoken understanding in young people; in just six months, nearly one and a half million children across the world were schoolstriking for action on climate change. Her Instagram post on these strikes reads: “Time is much shorter than we think. Failure means disaster. The grown-ups have failed us. And since most of them, including the press and the politicians, keep ignoring the situation, we must take action into our own hands. Starting today. Everyone is welcome. Everyone is needed. Please join in.”

Earlier this month Opec declared Thunberg, and with her the other young climate activists, the “greatest threat” to the fossil fuel industry. Thunberg tweeted them her thanks. “Our biggest compliment yet.” Hers is a voice totally unlike the world’s usual power-cacophony: clean, simple, inclusive, the voice of someone refusing to beguile. She talks ethics to politics without flinching. She cuts through the media white noise and political rabble-rousing to get to the essentials. This is a communal voice and Thunberg is its lightning conductor, and no wonder: when you hear her speak or you read her speeches you know you’re in the presence of the opposite of cynicism – of a spirit, in fact, that rebuffs cynicism and knows that the way we act, every single one of us, has transformatory impact and consequence. “The real power belongs to the people.”

This voice lets us know we’re in disavowal, and that we’d better wake up. Then it tells us, clear as anything, how to do this. Ali Smith

Questions from famous fans

Actor

What can people reading this do today to make an impact?

A great start is to inform yourself. To read and try to understand the problem. It is very depressing but absolutely necessary. Once you fully realise the situation then you will know what to do. And then spread that information to others. Then there are lots of things you can do in your everyday life. Going vegan, stop flying and have shop-stop for instance. Of course we need a system change. But I believe that you cannot have system change without individual change. In today’s debate climate, a lot of people will not listen to you unless you practise as you preach and live by example. And we need everyone to listen. So making some personal changes is very much worth it.

Leader of the Labour party

How will young people today have to live and work differently in the future because of climate change?

In the global north the immediate differences may not be very striking. But we need to rapidly remove all fossil fuels from our everyday life and leave them in the ground. This is what most politicians do not seem to understand. Lowering emissions is not enough. In the global south, however, people already are very much affected. There is an ongoing catastrophe in many parts of the world. And this is already affecting hundreds of millions. There will of course be lots of benefits to changing to a green economy. But we must remember that this is above all an emergency – not primarily an opportunity to create new green economic growth. We need a whole new way of thinking.

Actor and campaigner

How can we most effectively and efficiently support you and your amazing work? (PS: Thank you for your efforts, you’re the most inspiring person.)

Activism seems to be working. So I would encourage people to become activists. And if you have a big platform, then highlight the crisis and communicate the information. There will be general, global climate strikes on 20 and 27 September. We need everyone to participate in these. Even adults and unions and so on. And of course we do this every Friday. Everyone is welcome to join.

Poet laureate

Environmentally speaking, what has been humankind’s most catastrophic invention?

I don’t think there is one singular worst invention. It is all of them combined with our current systems. But the idea that we can dig up astronomical amounts of fossilised, compressed biomass and burn it in the atmosphere without consequences is pretty catastrophic. And also the thought of eternal economic growth on a planet with limited resources.

Model and campaigner

Do you have any sense of what job you would like to do in future – would you be interested in going into politics, for example?

I think I would like work where I feel most needed. I would love to study science but the scientific facts are already there. We now need to communicate those facts and then [take] political action. The politics of today don’t interest me much, however. It’s all about competing, and it’s more important how you say something – the way you say it – than the content of what you actually are saying. That has to change.

Singer and songwriter

Do you ever feel overwhelmed with the huge following you now have? (PS: You are such a remarkable person, and even though things are looking pretty dark, I do hope that you take some time to be proud of how many people you have engaged with your words. Well done!)

Yes, of course. What has happened during the past few months is very hard to grasp. I would never have expected anything close to this.

Labour MP for Tottenham

Can you be politically conservative, ie on the political right, pro-growth, pro-capitalism and pro the for-profit motive, and still support the climate change movement and green issues?

That is not for me to say. I am only communicating the scientific facts. This question is probably not possible to answer without personal opinion and I leave that to others. But I think we can safely say that all ideologies have failed. If some have failed more than others then that is for others to say.

Comedian

You are my new hero and you have really helped me to explain what is going on to my children. How do you keep so calm when people use personal attacks to try and undermine your work?

I had expected that. If you don’t fully understand the ongoing climate breakdown, then what I and the other school strikers do must seem very strange… and since most people are not aware, this is unfortunately what I expected. And I don’t think I can do much about it. Even if I would constantly prove them wrong with facts and arguments, they would make up new things to attack the climate movement or me. But I think that as long as they go after me personally with insults and conspiracy theories then that is good. It proves that they don’t have any arguments. And that they see us as a threat because we are having an impact.

Historian and author

Do you think human beings are generally selfish and focused on their own needs, or do you believe human beings are naturally cooperative, empathetic and creative? Or something in between? And does your view of humanity determine your activism?

I don’t believe humans are naturally evil. But under the wrong circumstances we can be very selfish and greedy, like now. People can be taught to be good, or bad. I think we all need help to focus on the right things to do. With more than 7 billion people on the planet, we need to cooperate. I don’t believe we are continuing to ruin the biosphere because we are evil. I am convinced we are doing it because we are not fully informed of the consequences of our actions. And this is very hopeful to me, because I believe that once we know we will change. This has a huge impact on my activism, because people are adaptable and the changes necessary are not impossible.

Founder of the Jane Goodall Institute and UN messenger of peace

The Jane Goodall Institute’s Roots & Shoots programme involves thousands of young people of all ages in roughly 60 countries. Hundreds of groups are working on projects in connection with the climate crisis. Do you think it is important that young people roll up their sleeves and take action in addition to marching? If so, what would you suggest?

Yes, everything is needed. There are thousands of ways to take action. For example, plant trees, pick up litter, join an organisation or movement that makes a difference and especially try to influence adults and put pressure on people in power. I started my activism at home, changing my parents’ and relatives’ habits and ways of thinking.

Questions from readers

Facebook Twitter Pinterest Greta Thunberg to parents’ generation: ‘Do something, finally, to combat the climate catastrophe!’ - effigy in Dusseldorf in March 2019. The figures the model of Greta is holding are labelled ‘parents’ generation’. Photograph: Lukas Schulze/Getty Images

Do you have days when you feel “there’s no hope, we’re doomed”? If so, where do you get the motivation to keep fighting?

Ali B Kord, Sydney, Australia

Yeah, you have no idea… But that’s not a reason to quit. We must never give up. I have made up my mind and decided to never, ever give up.

I’ve worked in the environmental sector for the past five years. I have gone vegetarian, barely fly (and offset when I do), and cycle to work every day. The problem is I often find it impossible to make others understand the severity of the situation. Any tips on how to effectively persuade people?

Craig Blyth-Moore, Edinburgh

I think a combination of activism works. If more and more people start behaving like we are in a crisis, more people will understand.

People in poverty have little or no concept of climate change because they are busy just surviving. How do we reach the parts of society like this?

Mike Jarrey, Ghent, Belgium

Good question. I think we must make them realise that climate-related issues are the key to other issues as well. Such as equality and social injustice and human rights. The climate issue offers a solid, science-supported black or white issue. The sooner other movements realise this, the better.

Facebook Twitter Pinterest Thunberg with her ‘school strike for climate’ placard outside the World Economic Forum meeting in Davos in January. Photograph: Fabrice Coffrini/AFP/Getty Images

You’ve called for a global general climate strike on 20 September. What do you think about the possibility of a continuous general strike and the impacts it could have on global business-as-usual?

Thomas Kane, Stockport

We children are calling for the adults to join us on 20 and 27 September. To have real impact we must get to the stage where we have continuous general strikes. That will have an enormous impact on business-as-usual. We must get there soon. It would open people’s eyes and put things in perspective.

I’m 21 and I am facing a lot of ageism. I am discriminated against and invalidated because of my age. I’m told I don’t understand “how the world works”, and that my experiences don’t count. How do you push back against ageism and keep raising your voice?

Gayane Aghabalyan, Yerevan, Armenia

I don’t care about age. Nor do I care about those who do not accept the science. I don’t have as much experience, and therefore I listen more. But I also have the right to express my opinion, no matter my age. Also, being young is a great advantage since we see the world from a new perspective and we are not afraid to make radical changes.

As a biology teacher I support your actions because I view the planet from the perspective of ecology. I would like to draw your attention to two important graphs that can be seen online based on the present scientific evidence available. The first is human population growth v global warming and the second is human population growth v extinctions. Both have near perfect positive correlations. So do you think we have a responsibility to manage population growth to save the planet?

Colin Pascoe, Bath

The problem is not the people. It is what we do. But of course it is more difficult to live sustainably with more people on the planet. But these solutions can never be discussed on a personal level – it must be handled on a global level. If we are to control the number of people on Earth then I guess we must start with the high emitters. This debate would take decades and leave no space to solve other problems. It’s simply too big for us individuals. In my experience, the “we are too many people” argument is used as an excuse for not taking action ourselves. Whether we start controlling births or not, we need to get to zero emissions.

How do we get the message across in countries like China and Russia, which do not have the open media that we have?

John Telling, Tunbridge Wells

We do what we can do. I, for instance, can’t vote in China or Russia. Nor do I control media in China or Russia. I can only do what I can. And if enough people stand up for the climate and the environment, that will eventually spread to other countries. Perhaps even to China and Russia. We must focus on what we can do. Not what we can’t do.

Facebook Twitter Pinterest Discussing cross-party action in Westminster with Michael Gove, Layla Moran, Ed Miliband and Caroline Lucas. Photograph: Stefan Rousseau/PA

I am an old man without children. Why should I care about what happens to the planet after my death?

Confundido, online, name not supplied

Maybe you believe in something? Like karma, faith? Or morality? Or just because it’s the right thing to do.

How do you explain that, despite the findings of scientists and the alarming UN report, some people keep on denying or ignoring the climate crisis?

mdivet, online

I don’t believe so much in denial. I think either you don’t fully understand or you don’t accept the facts. This is because the facts just don’t fit your idea of how things are or should be. Your ideology or religion, for example. To accept the climate emergency is to admit that we have all failed, in a major way. And this is not an easy thing to do.

Is it too late?

ID0658798, online

No, most studies show that it is still possible within the laws of physics to avoid the worst scenarios. But it is not possible if we go on like today. And there is a possibility that there are positive feedbacks too, once we start to act. We don’t know.

Is there one achievable issue we should prioritise?

Kim Williams, Birmingham

To treat the crisis as a crisis. Because we can’t solve an emergency without treating it like one. We need to look at the full picture and do all that we can.

• No One Is Too Small to Make a Difference by Greta Thunberg is published by Penguin (£2.99)

• Ali Smith’s latest novel is Spring (Hamish Hamilton)