The next six months will test international cooperation more severely than at any time since 1945. That may seem strange to say after a year of global crisis that has demanded unity on an immense scale, yet five urgent challenges confront us and we cannot delay our responses.

Crucial meetings this week in New York and Pittsburgh will determine by next spring whether a new era of collaboration is possible.

We cannot solve these problems immediately, but momentous decisions are demanded now toward halting climate change, renewing economic prosperity, fighting terrorism, ending nuclear proliferation and overcoming poverty.

This week starts with efforts to reinvigorate talks to secure a new international agreement on climate change in Copenhagen this December. Progress is too slow and a deal now hangs in the balance. But failure will increase the threat not only of humanitarian and ecological catastrophe but also of economic decline.

Investment in energy efficiency and low-carbon energy sources will help drive economic growth over the next decade — as well as reduce dependence on imported oil and enhance energy security. Millions of jobs stand to be created as this investment expands — the low-carbon sector is now larger than defense and aerospace combined. But it is vital that we give confidence to such investment through a new international climate agreement.

This will not be possible without the cooperation of developing countries. Britain has suggested a program of $100 billion a year by 2020, financed by wealthier countries and the private sector, to help poorer nations develop low-carbon economies.

We must move toward resolving the issues that remain before Copenhagen. If it is necessary to secure agreement, I will personally go to Copenhagen to achieve it.

In London five months ago, the world came together to fight the global recession. And this week the world comes together again, to forge a global plan for jobs and growth.

So far, action taken in concert has stabilized the international banking system and created the foundation for the resumption of economic growth.

But now the world has to decide whether to stay the course and deliver the promised fiscal stimulus this year and in 2010. Attention must also turn to our next common economic goal: a new system of governance. We need a clear commitment from the Group of 20 on a global compact to provide a framework for jobs, growth and stability over the medium to long term — one that perhaps even includes objectives for global growth.

For nearly a decade, the battle against al-Qaida in Afghanistan, Pakistan and elsewhere has united us.

But now we need to move to the next stage. We need to develop the strategy I call Afghanization — building up the Afghan Army, police and civic institutions and handing power to the Afghan people. International agreement — and progress — on Afghanization must be among the most urgent priorities, and it is something that NATO has to address in the context of a new assessment of the war's progress by the top American military commander in Afghanistan, Gen. Stanley McChrystal.

The world is threatened too by a looming new arms race. President Obama is to be thanked for making nuclear proliferation the theme of this week's U. N. Security Council Meeting. It is clear that a new nuclear nonproliferation agreement is needed urgently. To this end, Britain proposes a new and comprehensive grand bargain on nuclear proliferation: access to civil atomic energy via an international uranium bank for states that renounce current or future nuclear arms, together with a reduction of nuclear weapons by nuclear weapons states.

Finally, this week I will be calling on every country in the developed world to help poorer nations trade their way out of recession and deliver essential health care to the most vulnerable. This will not be easy and will take time. But there is one step we can take immediately: to stop charging the world's poorest, particularly pregnant women and children, for medical treatment they cannot afford.

After 1945, the world — fresh from a devastating conflict — summoned its energies to build a new international order. Now we are being tested again. In the days and months ahead, our collective resolve must hold across all the challenges I have outlined. If it can, then something bigger and even more lasting than the great reconstruction of the postwar era is possible: the creation of the first truly global society.

Gordon Brown is the prime minister of Britain. He wrote this article for The New York Times.