More than 90 heads of state are descending on New York City. Here's what to watch for

The meeting room inside United Nations headquarters as seen on 4 September 2015 in New York, United States.

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

    This year, a mysterious attack on Saudi oil fields has provided some late drama to UNGA, as US officials point the finger at Iran, despite its denials and claims of responsibility by Houthi rebels in Yemen. The US, Iran, Saudi Arabia, Yemen and other Middle East players will all be under one roof for days. Some can't st