When you are just one person sitting on a warming planet – when you see economies collapsing, wars raging, and reasons for fear on every corner – how should you react? What can you do? The current cluster of crises has stirred mood-responses that you can hear in every bar and coffee shop. It's worth looking at them, because beyond their siren messages, there is a road to real change that is being neglected.

The first mood is to feel powerless, and to turn this into a defiant pessimism. You know the script. I can't make any difference. It's all going to happen, whatever I do. The political conversation is remote and boring and has nothing to do with me anyway. I'm going to buy an extra-big lock for my door, hug my kids a little tighter, and sit out the storm.

We all have these moods from time to time, but they have now turned into the default mode of citizens in the supposedly advanced democracies. The second mood seems to be the opposite, but is actually its flipside. It says: what we need is a heroic leader who will save us. Enter Barack Obama. He's clever and articulate and has a conscience. He's the photographic negative of George W Bush. He will sort things out. Leave it to him; breathe out at last, and wait for every country to find such a man.

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Both these moods leave you – the ordinary citizen – inert. All you can do is focus on your own personal life and wait, for disaster or salvation. But these twin dispositions leave out the real option that is waiting for you. It is the only one that has ever delivered political change in the past, and it is the only one that will pull us out of the ditch now. It is where ordinary individual citizens – you – come together and raise their voices and offer solutions of their own.

To get there, you have to deal first with the people who say that politics is irrelevant and boring and they don't care. I always offer them one fact. According to the best scientific evidence, if we have five degrees of global warming – which is now a significant possibility in my lifetime, unless we change our behaviour fast – there will be global crop failure. Food will not grow.

Are you bored by this prospect? Is that dull? You won't be bored when you are hungry. Martha Gellhorn, the great war correspondent, said: "People will often say, with pride: 'I'm not interested in politics.' They might as well say, 'I'm not interested in my standard of living, my health, my job, my rights, my freedoms, my future or any future.'" Be serious. It might seem remote; it might seem difficult; it might be a world away from the arcane mumblings of Brown and Cameron; but unless you are a psychopath, you care.

Far from being some dreamy call to kumbaya, collective political action is the single biggest reason your life is incalculably better than that of your great-grandparents. When people first called for equality for women, when people first started to conduct scientific experiments, when people first suggested paid weekends and holidays for ordinary workers, they were greeted by the same glib pessimism we hear today. It'll never happen! What can we do? But ordinary people who believed they were necessary gathered together. They spoke and argued and marched and lobbied in their defence – and they won.

These achievements were never handed down by people at the top. Who was the leader of feminism? Who was the leader of scientific progress? Who was the leader of workers' rights? Sure, there were inspirational individuals along the way. But they happened as a result of millions of ordinary people demanding it, and never giving up. If we had waited for leaders to spontaneously see the light, we would be waiting still. That's why the unquestioning faith in Barack Obama of the past year – now slowly dispersing – has been as disempowering as despair. Both ask nothing of you. In reality, Obama will only be a good President if ordinary people pressure him to be one – if they shove him away from his errors (like aerial bombardment of Pakistan) and push him to pursue his good goals more vigorously (like building universal healthcare at home).

Trusting him to do the right thing is a basic misunderstanding of how progress happens in a democracy. You choose the best leader available within the power structure – which Obama undoubtedly was – and then you pressure him like hell. Great democratic leaders permit the public mood to prevail over the entrenched vested interests blocking their will. It's an art, but it's not the most important art: that lies with you, and me, and all ordinary citizens.

That's why I get angry when I see movies or plays venerating leaders as quasi-messiahs. In the otherwise-excellent new play at London's Trafalgar Studios, The Mountain-Top, Martin Luther King is given a premonition of Barack Obama as The One that will come after him. In the movie Bobby, about the assassination of Robert Kennedy, one character asks in tears: "Jack's dead. Bobby's dead. King's dead. Who's left?" The response is – all of you. Bobby Kennedy's mind was changed on Vietnam by the vast public protests by ordinary people; Martin Luther King had power because he was part of a huge movement of concerned citizens. Neither were lone heroes: there is no such thing in political life.

If you don't turn on to politics, politics will turn on you. In any society, the people who already have power will try to get the state to work in their interests. Every day, the oil companies and the billionaires are lobbying for their interests – and they speak far louder than their numbers, because they have so much hard cash. If you sit back, shrug and say you can't do anything, their interests will prevail over yours.

That's how we got into the credit crunch that endangers your job, and the climate crunch that endangers your ecosystem. Banks spent billions on lobbyists and PR-mongers to make our governments scrap the rules restraining them, so they could then pile up mountains of risky profit. In the end, it caused the financial house to fall down on us all. Similarly, big oil and big coal spend a fortune to stop governments making the urgent transition to clean energy that we need. It will cause the ecological roof to fall in. In both cases, a small concentrated private interest prevailed over the public interest – and you were screwed.

Politicians respond to the pressures put on them. The banks and oil companies and billionaires never stop putting on their pressure, waving their cheques, and making their threats. We need to make sure our collective voices talk louder. The only way to do that is to give your time and energy and dedication to demand genuine democracy.

This isn't something remote. It's very simple and very practical. Choose one or two groups, and donate a few hours of your time a week. There are a thousand brilliant campaigning organisations – I'd recommend Plane Stupid, Greenpeace, End Child Poverty, the Tax Justice Network and the National Secular Society, just for starters. They all have work for you to do, now. If there isn't a group for the cause you most believe in, start your own.

Political change rarely happens in a satisfying orgasmic flash, but if enough of us demand it, it comes in the end. Democracy – real, campaigning democracy, not the dessicated Westminster variety – works like those Push Ha'Penny machines you find in old arcades. You remember: thousands of two pence coins lie on a moving shelf, and you have to drop in coins of your own in the hope it will cause the pennies to tumble down for you to collect. Sometimes it feels like you are wasting your coins and the piles aren't moving even a millimetre – but then a ker-ching landslide happens, often when you least expect it.

You are not powerless. You are surrounded by millions of people who share your frustrations and share your instinct for justice and rationality. It is your job as a citizen to connect with them. Together, you are powerful. If you remain alone and apart and soaked in cynicism, you can be sure the Rupert Murdochs and Wall-Marts and British Petroleums will be fighting for their interests – against yours, and humanity's.

j.hari@independent.co.uk