The world has a better chance of saving itself from catastrophic global warming now than at any time over the past two decades, according to the scientist behind some of the most alarming predictions ever made for the planet’s future.

Johan Rockström shocked environmentalists in 2009 when he identified nine categories of Nature that were essential for life as we know it, and warned that we had already crossed into dangerous territory on three of them – including climate change.

Rockström, an environmental science professor at Stockholm University and executive director of the Stockholm Resilience Centre, has since transferred a fourth category, deforestation, to his list of “planetary boundaries” in the danger zone, which threaten irreversible, devastating consequences to the planet.

But he has had a dramatic change of heart over global warming, and is more optimistic that the worst of the threat can be contained than he has been since 1992.

The nine threats In the danger zone Biodiversity loss: the fewer species in an ecosystem, the less healthy it is

Deforestation

Climate change

Eutrophication: Nitrate/phosphate build-up in water (from fertiliser) Other risks to life Ocean acidification

Freshwater consumption

Chemical pollution, such as plastic

Aerosol pollution

Stratospheric ozone depletion

His optimism is founded on the breakneck speed of innovation in wind and solar power in the past two to three years, which means that renewable energy is being deployed on a massive scale and, crucially, at a cost roughly comparable to fossil fuels. Only last week new figures showed that the cost of electricity produced by onshore windfarms in the UK has fallen so much that for the first time it is now cheaper than fossil-fuel energy.

Climate change promise

Rapid improvements in energy efficiency are also key, along with a drive to reduce waste and increase the volume of recycled materials used in manufacturing, he says.

These developments have effectively removed the last major impediment to dramatically reducing greenhouse gases, raising the prospect that even though the planetary boundary has been crossed on global warming, the world may be able to cross back again.

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“We have a paradox unique to our era. On a scientific basis there is more reason to be nervous than ever before. But at the same time there has never before been so much reason for hope,” Professor Rockström told The Independent on Sunday. “The last time I was as optimistic was in 1992, with the Rio conference .... Then we lost 20 years. Now we’re back on a much more hopeful path,” he said.

Professor Rockström is referring to the United Nations meeting in Rio de Janeiro at which world leaders agreed to limit greenhouse gas emissions to a level that would prevent dangerous climate change and set up a framework to do this.

Each year, 192 officials from around the world attend such a summit, often with disappointing results. This year’s event in Paris in December is billed as the most important ever, because world leaders have pledged to agree emissions cuts and other actions that will put the world on a pathway that will eventually limit global warming to 2C. In advance, countries have said how much they are prepared to cut emissions by 2030. While these cuts in themselves fall short of what is needed, the professor is hopeful a more comprehensive deal, involving further cuts beyond 2030, can be struck in Paris – meaning the summit will achieve its goal.

He says the situation regarding climate change is similar to that facing the stratospheric ozone layer, which protects us from the sun’s ultraviolet radiation, towards the end of last century. This was being so badly depleted by chlorofluorocarbons (CFCs) used in fridges and aerosols that the world had crossed “planetary boundaries”, threatening to dramatically increase cases of skin cancer. Then a cheaper alternative to CFCs was invented, the 1987 Montreal Protocol banned CFC use and the ozone layer recovered.

But Professor Rockström warns:“The negatives remain. The world’s coral reefs are so worryingly close to collapse, while the Arctic and Antarctic are deteriorating so rapidly they could hit tipping points that are irreversible ... it’s now or never towards tipping the world to a very costly, very devastating future, versus tipping ourselves towards a sustainable future.