Every September, the world's leaders gather at the United Nations to reaffirm our founding charter -- our faith in fundamental principles of peace, justice, human rights, and equal opportunity for all. We assess the state of the world, engage key issues of the day, and lay out our vision for the way ahead.

But this year is different. The 64th opening of the General Assembly asks us to rise to an exceptional moment. We are facing many crises -- food, energy, recession, and pandemic flu -- occurring all at once. If ever there were a time to act in a spirit of renewed multilateralism, a time to put the "united" back into the United Nations, it is now.

And that is what we are doing, as action on three issues of historic consequence demonstrates.

First, world leaders are uniting to address the greatest challenge we face as a human family -- the threat of catastrophic climate change. Last week, 101 leaders from 163 countries met to chart the next steps toward December's all-important U.N. climate change conference in Copenhagen. They recognized the need for an agreement that all nations can embrace -- in line with their capabilities, consistent with what science requires, and grounded in "green jobs" and "green growth," the lifeline of a 21st century global economy.

We at the U.N. have prepared carefully for this moment. For two and a half years, ever since I became secretary-general, we have worked to put climate change at the top of the global agenda. Today, we have entered a new phase. Last week's summit sharply defined the issue and focused attention in capitals the world over. To be sure, the issues are complex and difficult, especially those of financing adaptation and mitigation efforts in poorer countries. Yet leaders left New York committed to clear and firm instructions for their negotiators: seal a deal in Copenhagen.

Japan issued a challenge, agreeing to cut CO2 emissions by 25 percent by 2020 if other nations follow. China's President Hu Jintao spoke about all that his country is already doing to reduce energy intensity and invest in "green" alternatives. He emphasized that China is prepared to do more under an international agreement, as did U.S. President Barack Obama.

Negotiators gathered for another round of U.N. talks on Sept. 28 in Bangkok, and we are considering a smaller meeting of major emitting and most vulnerable nations in November. We need a breakthrough in this make-or-break year.

We saw another turning point on a second issue of existential importance: nuclear disarmament. Finally, the assumption that such weapons are needed to keep the peace is crumbling. At a special summit called by the President Obama, the Security Council unanimously adopted a resolution that opens a new chapter in the U.N.'s efforts to address nuclear proliferation and disarmament.

That resolution improves prospects for expanding the Nuclear Non-Proliferation Treaty next May, and offers hope for bringing the Comprehensive Nuclear Test Ban Treaty into force. It also establishes the contours of a legal framework for action against misuse of civilian nuclear technology for military purposes and reflects an emerging consensus, seen in meeting after meeting, that the time has come to increase pressure on countries that fail to respect these principles.

The world is united on a third front, as well. Though some may speak of "turning the corner to recovery," we see a new crisis emerging. According to our recent report, "Voices of the Vulnerable," the near-poor are becoming the new poor.

An estimated 100 million people could fall below the poverty line this year. Markets may be bouncing back, but jobs and incomes are not. That is why, earlier this year, the U.N. put forward a Global Jobs Pact for balanced and sustainable growth. It is also why we are creating a new Global Impact Vulnerability Alert System, giving us real-time data and analysis on socio-economic conditions around the world. We need to know precisely who is being hurt by the financial crisis, and where, so that we can best respond.

That is also why, next year at this time, we will convene a special summit on the Millennium Development Goals. We have only five years to meet the targets for health, education, and human security that we set for 2015. At the various G20 summits over the past year, including the latest in Pittsburg, the U.N. has stood firm to speak and act for all those being left behind.

Rhetoric has always been abundant at the General Assembly, action sometimes less so. Yet listening to the world's leaders speak, last week, I was struck by their passion, commitment, and collective determination to turn a page from a past of countries divided by narrow interests to nations united in the cause of a global common good.

From confronting climate change to creating a world without nuclear weapons to building a more equitable and sustainable global economy, I saw a sprit of renewed multilateralism, with the U.N. at the fore. No country can deal with any of these challenges by itself. But as nations united, the United Nations can.