Just as teen eco-warrior Greta Thunberg was about to make her big entrance at the UN Climate Summit in New York - an event she has campaigned tirelessly for - climate change denier Donald Trump cut in front of her, effectively stealing her moment.

A cascade of emotions paraded across the 16-year-old Swedish activist's face as she watched the US president walk in front of her and speak to reporters at Monday's summit addressing an issue he believes is 'fake news'.

As clips of Thunberg's reaction have gone viral on social media, DailyMail.com spoke to body language expert Patti Wood about the science behind her expressions.

'We saw her face transform. We saw her ready to go be introduced, and then you saw puzzlement, and then you have what I call "dagger eyes",' Wood said, describing how Thunberg's eyebrows tilted inward.

Thunberg then did a 'tongue thrust suppression', pressing her tongue into her cheek.

'She wanted to stick her tongue out but she knew to suppress that,' Wood said.

DailyMail.com spoke to body language expert Patti Wood about the range of emotions that paraded across eco-activist Greta Thunberg's face as President Donald Trump upstaged her big entrance at the UN Climate Action Summit on Monday in New York

Video of the moment Trump upstaged Thunberg has gone viral on social media. Cameras captured the animated activist looking on expectantly as she prepared to enter the summit addressing an issue she's campaigned so hard for

Thunberg's excitement turned to confusion as the looked to her right and spotted Trump - a staunch climate change denier who was not expected to attend the summit

The confusion morphed into anger as Trump cut in front of Thunberg. Body language expert Wood noted how the teen's eyebrows turned inward and she thrust her tongue into her cheek

Wood explained that Thunberg's physical reaction was potentially exaggerated because she has Asperger syndrome, a developmental disorder that often causes difficulty controlling one's emotional state.

'Some people with Asperger's are not as adept at paying attention to other people's social cues,' Wood said.

'It means that [Thunberg] would feel the emotion more intensely and be less likely to suppress it based on what the norm is for any social situation.'

The expert, who has spent years counseling people with Asperger's syndrome on how to better recognize facial cues, said Thunberg's expression 'was not a microfacial cue, it lingered as the anger lingered'.

Wood also noted that the 'lingering' was also likely a result of the fact that Thunberg is only 16.

She said that someone without Asperger's may have suppressed the emotions more quickly, but that the emotions themselves were normal.

'Anybody would feel the emotions she did,' Wood said.

She compared the reaction to being the bride at a wedding and watching someone prance down the aisle ahead of you.

Thunberg glared at Trump as the president addressed a flock of reporters outside the summit

Shortly after the hallway drama, Thunberg delivered a fiery speech in front of the UN Climate Action Summit 2019 in New York, which convened to have nations make plans to confront global warming and rising fossil fuel emissions.

The activist, who is not usually emotional in her speeches, broke down in tears while furiously scolding international delegates.

'I shouldn't be up here. I should be back in school on the other side of the ocean. Yet you all come to us for hope. How dare you. You have stolen my dreams and my childhood with your empty words,' Thunberg said.

'There will not be any solution or plans in line with these figures here today because these numbers are too uncomfortable and you're not mature enough to tell it like it is.

'You are failing us. The young people are starting to understand your betrayal. The eyes of all future generations are upon you. And if you choose to fail us, I say, we will never forgive you.

'Right here, right now is where we draw the line. The world is waking up and change is coming whether you like it or not,' she said to uproarious applause.

Wood suggested that Thunberg's performance was influenced by the upstaging that happened minutes earlier.

'I think it sharpened her. If you look at her messaging in that meeting, she called world leaders immature. [Trump] illustrated that,' Wood said.

Shortly after the hallway drama, Thunberg delivered a fiery speech in front of the UN Climate Action Summit 2019. The activist, who is not usually emotional in her speeches, broke down in tears while furiously scolding international delegates

Trump made an unscheduled stop at the climate summit where he sat through a 15-minute speech by Indian Prime Minister Narendra Modi - but he was not there for Thunberg's blistering attack

The president, who wasn't originally scheduled to attend the summit as he's hosting a session on religious persecution in the same building, left after 15 minutes and closed his eyes during Modi's speech

Although Trump made an unscheduled visit to the climate change summit, he only sat through a speech by Indian Prime Minister Narendra Modi, and missed Thunberg's blistering attack.

During the time he was there though, Trump he sat in the audience wearing headphones and appeared to close his eyes and fall asleep.

The president, who wasn't originally scheduled to attend the summit as he's hosting a session on religious persecution in the same building, left after 15 minutes.

The summit came on the heels of a global Youth Climate Strike on Friday where millions of people in 150 countries took to the streets and demanded faster action to address climate change.

Prior to the strike, Thunberg addressed Congress and criticized Trump's decision to withdraw the US from the Paris Climate Change Agreement early in his tenure.

Thunberg skyrocketed to fame in 2018 when she skipped school to stage a one-person demonstration outside Sweden's parliament in Stockholm.

Her protest ignited a nationwide movement in Sweden thanks to social media and earned Thunberg a Nobel Prize nomination.