In person and on screen, world leaders returned to the United Nations’ foremost gathering for the first time in two years on Tuesday with a formidable, diplomacy-packed agenda of escalating crises to tackle, including the still-raging Covid-19 pandemic and a planet grapping with climate change.

Other pressing issues include rising U.S.-China tensions, Afghanistan’s unsettled future under its new Taliban rulers and ongoing conflicts in Yemen, Syria and Ethiopia’s embattled Tigray region.

Last year, no leaders came to the U.N. because the coronavirus was sweeping the globe, so all addresses from leaders were prerecorded, though some resident ambassadors spoke. This year, the General Assembly offered leaders a choice — come to New York or remain online. More than 100 decided to appear in person in the General Assembly hall.

U.N. Secretary-General Antonio Guterres, who opens the weeklong event, “will pull no punches in expressing his concern about the state of the world, and he will lay out a vision to bridge the numerous divides that stand in the way of progress,” U.N. spokesman Stephane Dujarric said. Guterres has already demonstrated that in pointed pre-meeting remarks about the virus and climate change.

“I’m here to sound the alarm. The world must wake up,” Guterres said in his first words after convening the meeting.

By tradition, the first country to speak is Brazil, whose president, Jair Bolsonaro, isn’t vaccinated. He reiterated last Thursday that he doesn’t plan to get the shot any time soon, justifying his refusal by saying he had Covid-19 and therefore has a high level of antibodies.

A key issue ahead of the meetings has been Covid-19 entry requirements for leaders to the United States — and to the U.N. headquarters itself. The U.S. requires a vaccination or a recent Covid-19 test, and the U.N. will operate on an honor system whereby anyone entering the complex attests that they do not have symptoms and have not tested positive in the last 10 days.

The three most closely watched speakers on Tuesday morning are expected to be U.S. President Joe Biden, appearing at the U.N. for the first time since his defeat of Donald Trump in the U.S. election last November; Chinese President Xi Jinping, who in a surprise move will deliver a video address; and Iran’s recently elected hardline President Ebrahim Raisi.

Ahead of the opening of the General Assembly’s annual General Debate, Guterres issued a dire warning that the world could be plunged into a new and probably more dangerous Cold War unless the United States and China repair their “totally dysfunctional” relationship.