When Nelson Mandela formed The Elders to promote peace and human rights across the world, he challenged us to be bold and to give a voice to those who have none. No issue demands these qualities more than our collective failure to tackle climate change.

Climate change is the biggest challenge of our time. It threatens the well-being of hundreds of millions of people now and many billions more in the future. It undermines the human rights to food, to water, to health and shelter – causes for which we have fought all our lives.

No one and no country will escape its impact. But it is those with no voice – because they are already marginalized or are not yet born – who find themselves at greatest risk. We have an urgent moral duty to speak out on their behalf.

Given the compelling weight of evidence, it can be hard to understand why we are still dragging our feet on the coordinated action needed to reduce greenhouse gas emissions. The latest report from the expert Intergovernmental Panel on Climate Change sets out clearly that the warming of the climate system is “unequivocal” and that human behaviour is extremely likely the cause.

We have also seen in the last few months – from typhoons in the Philippines, to the polar vortex in North America and widespread floods in Europe – exactly the increase in extreme weather events which experts warn is the inevitable outcome of climate change. The costs are already enormous, which is why the World Bank, the IMF and the International Energy Agency have joined the scientific community in warning about the risks we are running. It is no longer only environmentalists who are ringing alarm bells.

And every year we fail to act brings us closer to the tipping point when scientists fear that climate change may become irreversible. We are taking a terrible gamble with the future of the planet and with life itself.

We know what is needed to prevent this catastrophe. Global temperature rises must be limited to less than 2 degrees above pre-industrial levels. This means turning away from fossil fuels and accelerating the deployment of affordable renewable energies, for instance by putting an internationally agreed price on carbon. The way forward must be to meet the timetable for a new robust, universal and legally binding agreement on climate change next year, under which every country commits to phasing down greenhouse gas emissions.

2014 is a decisive year. The UN secretary-general has called for a climate summit in New York in September. It is imperative that governments and corporate leaders come to this summit with ambitious actions on climate if we are to stand a chance to reach an agreement in 2015 commensurate with the challenge.

Many of the Elders have shouldered the burden of government. We do not make the mistake of believing that addressing climate change is easy. But we know that there are times when, no matter how difficult the immediate landscape, leaders must show courage and boldness. This is such a time.

Our experience has also taught us that if leaders make the right decisions for the right reasons, their voters will support them. By raising their sights – and shrugging off the restraints imposed by vested interests and short-term political considerations – they can also inspire hope, rebuild trust and mobilise action across society.

Solutions to climate change do not just come from research centers and laboratories but also through innovation shown by the people most affected. Many communities and businesses, local and national governments, including in developing countries, are already showing us the way to a carbon-neutral world. These efforts now have to be scaled up.

Climate justice also demands that those richer countries who have done most to cause the buildup of greenhouse gas emissions in our atmosphere – and reaped the benefits - help out the poorer nations to adapt to climate change already underway. 

We have reached a fork in the road. In one direction, a terrible legacy could be handed to our grandchildren and their children. In the other is the opportunity to set our world on the first steps toward a fairer and sustainable future. We do not want the generations to come to say we failed them.

Over the coming months, the Elders will appeal for bold leadership from governments, businesses and citizens to achieve a carbon-neutral world by 2050. Old or young, rich or poor, if ever there was a cause which should unite us all, climate change must be it.