By 2050 there will be more than nine billion people on Earth. To accommodate this jump in population without stoking dangerous climate change, we have no choice but to complete the transition to a low-carbon global economy.
That is what is at stake in the international negotiations on climate change, and that is why the United Nations climate conference in Cancun is important.
An ambitious and legally binding framework for global climate action is needed. The European Union would be ready to agree on this at Cancun.
Regrettably, several other large countries, including the United States and China, are not.
Cancun will therefore not be the end of the road. But nonetheless the conference can still mark a significant step towards a legally binding global climate deal.
It can - and must - deliver progress by agreeing on a politically balanced package of decisions substantial issues that lead to immediate climate action on the ground.
These decisions should capture the progress achieved in the international climate negotiations and lay down some major elements of the "architecture" of the future global climate regime.
They should build on the Kyoto Protocol and incorporate the political guidance of last December's Copenhagen Accord.
In recent preparatory meetings for Cancun, I have seen a hunger for agreement along these lines. With political will, the conference can translate this into a real step forward.
Decisions are within reach on issues such as adaptation to climate change, the fight against deforestation, technology co-operation and governance rules for a new climate fund.
For the EU, a balanced package must include progress on mitigating global greenhouse gas emissions.
In particular, the emission reduction pledges that developed and developing countries have made under the Copenhagen Accord need to be brought into the United Nations framework.
"Anchoring" the pledges in this way will provide a global forum to discuss uncertainties surrounding some of them and to consider ways to make them more ambitious.
The current pledges are a start, but it is clear they are not enough to keep global warming below 2C, as the Copenhagen Accord recognises is necessary.
We also need to see progress in Cancun towards reforming and expanding the international carbon market to capture the huge potential for emission savings in the major emerging economies.
As Europe knows from our own emissions trading scheme, carbon market mechanisms reduce the cost of cutting emissions, can drive investment in innovative low-carbon technologies and can be important sources of funding for future climate action.
It is essential that developed countries deliver on their pledges of "fast start" funding to help the developing world fight climate change.
The EU is doing so.
We have mobilised €2.2 billion in fast-start funding this year as part of our commitment to deliver €7.2 billion over 2010-2012.
In Cancun the EU will give a comprehensive report of how we have implemented our pledge this year.
Building trust also requires greater transparency - transparency in how countries deliver on their emission pledges, and transparency in how developed countries will provide long-term funding to help the developing world tackle climate change.
That is why the EU is pressing for agreement to draw up stronger monitoring, reporting and verification rules.
A set of decisions in Cancun along these lines would be a significant intermediate step towards the robust and legally binding global agreement the world needs.
An ambitious global framework will help accelerate the low-carbon revolution, spurring "greener" growth, creating new jobs and strengthening Europe's energy security.
Achieving a politically balanced package will not be easy, but it is within reach. Failure, on the other hand, would raise the risk of the international climate negotiations losing momentum and relevance.
With political will, Cancun can succeed. Europe will be working throughout the two weeks of the conference to ensure that it does.
* Connie Hedegaard is the European Commissioner for Climate Action.