Despite a more relaxed dress code suggested by organizers ahead of time, several heads of state and government donned full business attire as they arrived for a warm welcome from President Luiz Inacio Lula da Silva.

Lula -- himself wearing a dapper suit and collared shirt -- set the tone by forgoing his "lucky tie" in the green, yellow and blue colors of the Brazilian flag, which he wears proudly for most of his international engagements.

As temperatures topped 30 degrees Celsius (86 degrees Fahrenheit) in the humid riverine city on the edge of the Amazon rainforest, many delegates soon followed Lula's lead and ripped off their own ties.

The prime minister of Antigua and Barbuda, Gaston Browne, did so before even leaving the staging area after a light-hearted exchange with Lula.

Last month, COP30 president Andre Correa do Lago announced the Belem Conference of Parties would be "tie-less" to boost delegates' comfort and give the event "a certain Brazilian informality."

As soon as attendees made the long walk Thursday under a blazing sun into the venue, and then realized eating areas and some meetings were another outdoor stroll from the facility, neckwear seemed low priority.

Outside, many summit attendees sported sweaty patches under their arms and on their shirt backs, but inside, most areas of the expansive domed conference center were vigorously air-conditioned.

Brazil has vowed to offset all planet-warming carbon emissions generated as a result of this cooling.

United Nations chief Antonio Guterres and the presidents of Finland, Chile, Mozambique and Colombia were among those to opt for open collars on the first day of a two-day summit preceding the 30th UN climate COP opening Monday.