New York (CNN) It sounds like a digestive tract flareup. But that's just how insiders pronounce the acronym for the annual United Nations General Assembly taking place this week: UNGA ("UNG-gah"). And when it ends, global leaders and a cast of thousands will indeed feel like taking some kind of medication.

For just one September week each year, everyone from kings, presidents and prime ministers on down to food tasters fly in to New York City and jam into the United Nations headquarters, in what has become a cliched joke about " diplomatic speed dating ." But there's nothing funny about the top priority at this 74th year of UNGA year, which is the world's climate emergency.

The United Nations Climate Summit

The UN has arranged a special Climate Action summit on Monday September 23, before the formal speeches given by heads of state and governments that normally draw the most attention. Breaking with tradition, UN Secretary-General Antonio Guterres has told country leaders not to come to the podium Monday without "concrete and transformative plans" to halt rising global temperatures, achieve carbon neutrality and cut carbon emissions by 45 percent. VIPs with proposals have been told they have just three minutes to speak.

How concrete the results of the summit will actually be is unclear. The UN doesn't have a global climate army to roam the planet stopping pollution, so it's up to the 193 nations of the UN to take action. What the UN will actually do is what some feel the organization sadly does best: collecting the speeches and other environmental brainstorms in a report.

The aim is to pressure world leaders to do more, as the climate emergency worsens. Some have made it a top priority -- German Chancellor Angela Merkel will be in town for the climate summit, even though she's skipping the rest of the week. In contrast, US President Donald Trump will be in town, but he's skipping the climate summit -- instead hosting a session on religious persecution in the same building. Trump has previously avoided international meetings on climate.

Let the General Debate begin

The so-called "general debate" begins on Tuesday September 24, and the first country to speak is Brazil, by longstanding tradition. It'll be the debut UNGA appearance of Brazilian leader Jair Bolsonaro, who recently vowed to attend even if in a wheelchair, following recent surgery. Expect fireworks: After high global concern over massive fires and deforestation in the Brazilian Amazon , the pro-business Bolsonaro is likely to take climate advocates head-on. He has already promised a "speech that is different from those that came before".

Following him will be Donald Trump, who has stuck to the teleprompter previously, and last year drew laughter from delegates while praising his own administration.

The volume of Trump's appearances and his unpredictability have dominated the past two years of Trump at the General Assembly. Last year, he shocked the visiting Chinese Foreign Minister by accusing Beijing of interfering in US elections while both men sat at the Security Council table.

Trump was also the first US President to enter the building and take just a few steps before stopping in front of cameras and shouting reporters, to answer hot topic questions for three days. "He completely sucked the oxygen out of UNGA," Richard Gowan, UN Director at the International Crisis Center says.

The elephant in the secretariat

A senior UN official stated, "tensions in the Gulf are going to be the backdrop to a lot of events and meetings." Which leads us to the biggest question on the eve of UNGA: Will President Trump meet Iranian President Rouhani?

‎Nothing is planned between the two leaders, though both will be in the UN building amid the parade of delegations. The threats and rhetoric lobbed by both sides have been fierce. The US imposed more sanctions on Iran on Friday before the high-level week begins, and the Iranians say they don't need to talk.

It looks like "a zero chance for a meeting," says Gowan. "Instead of a diplomatic breakthrough, we're likely to see a war of words." The UN is meant to be a neutral ground for dueling sides to have dialogue, and chance face-to-face encounters are exactly what this international forum is good for. In 2000, then-US President Clinton and then Cuban President Fidel Castro suddenly stopped briefly for each other in a hallway, when relations between the nations were running cold. There is still a debate whether a handshake took place

Who's coming to UNGA?

As of Friday, 91 heads of state had committed to attend. French President Macron will be in New York offering an expanded French leadership role as a global mediator, as the Trump administration pulls the US back from international forums.

‎With significant angst at home over the UK's planned withdrawal from Europe, Boris Johnson will make his first UNGA appearance in his role as prime minister. Brexit talks are scheduled with Ireland's leader and others.

And Indian leader Narendra Modi and Pakistan's leader Imran Khan are both attending and speaking in the General Assembly. No talks are scheduled between them for now, as tensions remain high over India's treatment of Kashmir, a disputed territory.

And of fresh interest to American audiences will be Ukrainian President Volodymyr Zelensky, who debuts at UNGA right in the middle of a growing controversy: The US Congress wants to know if Trump pressured Zelensky to investigate Joe Biden's son , who had business ties in the country. Biden, of course, is one of the main contenders for the US Democratic primary and could end up challenging Trump for the presidency next year.

There are also UN meetings big and small on Libya, Yemen, Somalia, Sudan, and Myanmar. Apart from marathon hours of speeches, consider this: 630 meetings have been requested. The UN defines a meeting as a gathering of 15 or more people held within the UN for a maximum of three hours.

There are "no less challenges than last year" affecting geopolitics around the world, said British UN Ambassador Karen Pierce. Expect demonstrators near the UN all day -- though the targets of their anger may not even see them through their tinted limousine windows as they rush to the UN, sirens sometimes wailing.

The list of leaders who will not appear this year is also interesting: Russian President Vladimir Putin and Chinese leader Xi Jinping rarely attend, and will not be present this year. Israeli Prime Minister Benjamin Netanayhu has canceled, amid a post-election muddle back home. Netanyahu, a former UN Ambassador, relishes his General Assembly speeches -- often with props of charts and diagrams -- and he has previously made for must-see TV in between lengthy boring speeches by others.

Canada's Prime Minister Justin Trudeau had been listed as a speaker, but is no longer coming in the wake of multiple revelations of black- and brownface photos that surfaced from his past, just before national elections.

President Nicolas Maduro of Venezuela has sent his regards but will not leave his turmoil-filled nation. His rival for the Venezuelan presidency -- National Assembly leader Juan Guaido, whose presidential authority has been acknowledged by more than 50 countries, including the US -- is unlikely to show but representatives will push the UN for future recognition of Guaido's claim to power.

Although the immediate premises of the United Nations are international ground, the United States is still the host. According to the "host country" agreement, visitors traveling for official UN business are not supposed to be barred, and there was once a time when the US would impose a generous 25-mile radius cordon on international visitors it didn't like.

But this year, the US is not holding back. In addition to the movement restrictions on Iran's Zarif, two Cuban diplomats living in New York who had planned to participate in UNGA were ordered expelled by the Trump administration, in a surprise Thursday announcement. And the remaining diplomats at the Cuban mission face the toughest restrictions among all countries: They're not allowed to leave the narrow island of Manhattan.

And with all the traffic and security blockades, New Yorkers and tourists ‎too may feel no one can get off the island during this madhouse week.