In protests around the world this year, kids on strike from school have been setting the agenda on climate change.

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And their key message has been that we're facing a "climate emergency" and we need to act now.

They've been accused of overcooking it. They've been told to stay in school and to let the adults sort it out.

But the kids are right. The world is now dangerously close to tipping points that will set in motion unstoppable ecosystem collapses. This is a climate emergency.

That's the message from scientists writing in Nature on Thursday, who say that for some systems, the window to act may have already closed.

A decade ago, it was widely thought that most tipping points wouldn't be reached until around 5 degrees Celsius of warming, but now evidence is mounting that they're more likely to happen at between 1C and 2C above pre-industrial levels, according to Will Steffen from ANU's Climate Change Institute, one of the authors of the paper.

Currently we're at a global average of about 1C degree of warming.

"The more we learn, the riskier it looks," Professor Steffen said.

The scientists warn that the Amundsen Sea ice sheet off West Antarctica may have already reached a tipping point where ice melt is now irreversible.

Once this sector collapses, it will cause a cascade of collapse across the West Antarctic which will add about 3 metres to sea levels over the coming centuries.

Greenland could be 'doomed at 1.5C', by 2030

Models predict that Greenland's tipping point is around 1.5C of warming, which we are on track to reach by 2030. ( Getty Images: Stone )

Data also shows that the Wilkes Basin in the East Antarctic might be unstable. That collapse would add 3-4m to sea level in a timeframe of beyond a century.

"Models suggest that the Greenland ice sheet could be doomed at 1.5C of warming, which could happen as soon as 2030," they wrote.

All the evidence from the geological record shows that the earth doesn't make smooth, slow transitions during times of climate change, according to Katrin Meissner from UNSW's Climate Change Research Centre, who wasn't an author on the paper.

"In the geological record the earth is quite sensitive to small changes," Professor Meissner said.

"We know that some of these things won't happen smoothly, they will jump.

"Ice sheets are a great example where we know that once they start melting, they reach a point where they'll keep melting even if it gets colder."

Although these systems will take between hundreds to more than 1,000 years to fully melt, even a small proportion of that melting over the coming decades will have catastrophic consequences.

Our best case scenario is if we can get to net-zero emissions in 30 years, the authors say. ( ABC North West WA: Susan Standen )

The IPCC predicts that seas could rise by up to 60 centimetres by 2100, even if warming is limited to below 2C.

If the world sticks to its current pledges to reduce emissions, we're on track to hit 3C of warming, the authors say.

"It locks in for future generations that they cannot rely on having stable sea levels for hundreds, if not thousands of years into the future," Professor Steffen said.

"We've really put ourselves against the wall because we haven't done anything for the last couple of decades."

'The schoolchildren have got it right'

Other tipping points that we're fast approaching include the collapse of coral ecosystems, which are likely to contract in diversity by more than 90 per cent as warming approaches 2C.

That in turn is likely to trigger a massive collapse in sea life including fish stocks which depend on those systems.

Melting in Greenland is causing an influx of freshwater into the North Atlantic, which has already slowed circulation in an Atlantic ocean current that feeds rainfall to the Amazon.

If Amazon rainfall decreases, converting it to a drier system, up to 90 gigatonnes of CO2 will be released into the atmosphere.

If we imagine our economies are like big ships, we have to start turning them now in order to avoid a collision in future, the authors write.

"If you look at something down the track and say it could happen in 20 years, but it takes you 30 years to [change course], then you've lost control," Professor Steffen said.

"We're looking at this and saying, 'have we already lost control?' We don't think so, but we're running out of time."

Taking a risk and banking on these things not happening is unacceptable, the authors argue.

Instead, the say we must act immediately to get emissions to zero as soon as possible.

"The intervention time left to prevent tipping could already have shrunk towards zero, whereas the reaction time to achieve net-zero emissions is 30 years at best," they wrote.

It's time to listen to the kids, Professor Steffen said.

"The bottom line is, we're saying the schoolchildren have got it right — this is a climate emergency."