Writing in the HuffPost Green, Kofi Annan and Ernesto Zedillo welcome commitments to cut emissions, but warn that the shift to a low-carbon economy will not happen without the accurate pricing of carbon.

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Nelson Mandela, who founded The Elders in 2007, believed transformative leaders needed the generosity and courage to see beyond narrow self-interest. No challenge requires these qualities as much, or as urgently, as climate change

As the complex negotiations between 195 member state representatives gather pace in Lima, Peru, at the 20th meeting of the Conference of the Parties (COP20), we once again raise our hopes that this time things will be different, and a draft framework will emerge that will be the basis of a new Climate Agreement that nations sign up to in Paris in 2015.

There has been progress. Commitments by China, the United States and the European Union to cut emissions have given the current UN's climate change talks in Lima momentum. But while an important and welcome step, they do not meet the scale of the reductions needed. Nor have other major economies yet matched their promises.

And while such headline commitments are, of course, essential, they will not be sufficient to protect our world from potential climate disaster. Agreement must also be reached on the mechanisms to deliver these cuts, how they will be assessed, reported and enforced.

There is a huge amount of detailed work required covering a wide range of areas.

We need as well to find the means to help poorer countries cope with climate change which is not just inevitable but is already taking place. Climate change is not an abstract concern or future threat. It is happening now.

According to the UN's World Meteorological Organisation, 2014 is on course to be the hottest on record. It would mean fourteen of the 15 warmest years ever recorded have occurred in the 21st century.

With rising temperatures, just as the experts predicted, has come more frequent and more extreme weather. Droughts are longer, storms are more severe. As always, it is the poorest and most vulnerable who suffer most. Yet we have failed badly so far to face up to either the urgency or magnitude of this crisis.

This is why, building on Lima, a universal climate agreement must be signed at the COP21 talks in Paris next year. Kyoto did not bring about the scale of change we needed. We dare not make the same mistake again. Another failure or further delay would sentence our world to a deeply worrying future.

The over-riding need is agreement on a goal for all nations to reach carbon neutrality by 2050. This must be done in a way which is equitable and supportive of the development aspirations of all countries.

What is also now clear is that at the heart of the process must be a binding international agreement on pricing carbon emissions. Without such an agreement, the change of behaviour to drive the shift to low-carbon economies simply will not happen.

Pricing carbon incorporates the principle that the polluter pays, building incentives and penalties into the process. It enables risk to be quantified and will help deliver the public and private investment needed to transform our economies.

To achieve these goals, the price of carbon needs to be credible and predictable, sufficient to induce the necessary reduction in carbon emissions and encourage investment in climate mitigation. Accurately pricing carbon will accelerate development of alternative sources of energy. In addition, transparent compliance and measurement mechanisms need to be agreed that will hold markets and nations to account and signal progress towards climate goals.

Countries are, of course, at different levels of development and have different responsibilities for the carbon already in our atmosphere. But the principle of 'common but differentiated responsibility' means that, while there must be flexibility on adoption, all countries must be ready to accept carbon pricing and reduce emissions.

Justice, however, also requires richer countries to help those behind them on the development path. They need support to meet the ambitions of their citizens in a sustainable way but also to protect their citizens from the impact of climate change.

At the moment, the Green Climate Fund set up to deliver these goals has only $9.3 billion of the anticipated $100 billion needed each year by 2020. This is hardly likely to encourage developing economies to make the changes needed. Bolstering the fund will require significant private capital and investment that will be fostered by agreeing on a meaningful and equitable price on carbon by the Paris COP in December 2015.

The Elders understand the short-term electoral pressures on the world's political leaders. But now is the time to find the vision and courage -- the 'generous accommodation' of which Mandela spoke -- to raise sights and ambitions.

The costs of failure are immense but so are the rewards. We will not only fulfill our responsibilities to future generations but show how our shared values can be put into action effectively in a globalized economy. Putting a price on carbon is key to this ambition. Efforts to reduce poverty and make the economy greener can, unlike what many suggest, be compatible.