THE end of the world was never foretold with such calm.

In a hushed courtroom, Eoin McCullough, senior counsel, presented what lay ahead.

Intense heatwaves, extended drought, cyclones, wildfires, floods, loss of biodiversity, destruction of ecosystems, widespread displacement of people, catastrophic breakdown of life as we know it - the barrister delivered the climate change forecast in composed, almost reassuring tones.

Not that anyone wanted to be assured of the kind of terrors he outlined.

Especially not when 14-month-old Mary Barry was bum-shuffling her way around the courtroom, blissfully oblivious to the fact that before she is 14 years old, the world will have to drastically change its ways or suffer irrevocable consequences.

Her dad, Connor, brought her to the climate case simply as a concerned citizen and father.

He works in finance and investment management - perhaps not your typical environmental activist - but it's what he sees in his own profession that alarms him.

"Climate change is the number one issue for global business leaders and they're not necessarily thinking about human rights and let's all do nice things for the environment.

"That should be a warning to everyone. Climate change isn't about mild environmental activism. It's a hardcore threat to my economic security, to my daughter's economic security and to the very society she'll grow up in."

Dad and dote made up just two of the 87 members of the public who crammed into Court Number 29 where there were seats for fewer than 30.

There were school uniforms, suits, grey hair, a jolly shade of pink hair - a mix of ages and backgrounds that gave a flavour of the 16,000 people who signed an online petition saying the case taken by Friends of the Irish Environment (FIE) was also in their name.

It is an odd case because, on just about every point, FIE and the State agree.

Carbon emissions cause climate change, they have to be reduced, we're not doing enough to reduce them - all agreed.

So without getting too philosophical, why are we here? That's where the sides differ. FIE says the National Mitigation Plan is weak and vague and won't help us meet any of our carbon reduction targets.

The State says the plan was correctly drawn up and that for an NGO to have the court quash it would be unwarranted interference by the judiciary in the work of the executive.

But yesterday was all about laying it on the line, the thin blue line it turned out, as Mr McCullough presented copious charts and graphs, all with a blue line running through them indicating the tipping point between sustainability and chaos.

Orla Kelleher from Cobh is doing her PhD on climate justice so despite the gloomy predictions, it was a thrill to see the theory in action.

"These are cases that have the potential to drive the action that's essential to addressing climate change so they're really inspiring," she said.

Sadhbh O'Neill of FIE found day one of the case a moving experience.

"We know this evidence so well but when it's read out step by step in this court which has a constitutional remit to oversee our entire democracy, it's very powerful. It's like climate change has a voice in this room."

The State's voice will be heard later today.