The scientific community is nervous. It’s not that the climate is changing; we know that already. What has everyone jumpy is new, undeniable evidence that climate change is hitting the global economy as well. Why did Hurricane Sandy put Wall Street under three meters of water instead of bypassing Manhattan and heading out to sea, as East Coast hurricanes have always done?

Rising evidence points to the fact that melting sea ice has collapsed the so-called “Arctic vortex”, which keeps cold air near the poles. It’s likely that the cold, high-pressure air is what drove Sandy straight into New York. Some estimates put the potential global GDP cost of accelerating methane release in the Arctic at US$ 60 trillion – out of a total GDP of $70 trillion. For the first time, Mother Earth is sending invoices back to the economy.

Or consider the Arab Spring: a social revolution led by technologically empowered, well-educated youth reacting to dictatorship – but also to climate change. Oil prices had risen 120%, phosphorus (the most important agricultural fertilizer) by 140%; Russia and Australia had shut their export borders due to forest fires and drought. The fight over fuel subsidies was the spark in the powder keg – and the first socio-ecological crisis of anywhere near its magnitude.

And we’re still only at 0.85 degrees Celsius of warming. As we move from the Holocene period – the 10,000 years since we used agriculture to alter our destiny, and during which there was a maximum temperature variability of about 1 degree – to what has been called the Anthropocene period, of large-scale human intervention in the environment, we face a so-called “3-6-9” world. We are moving towards 3 degrees warming, towards the sixth mass extinction of species, and towards 9 billion people.

This, then, is the troubling context in which the Intergovernmental Panel on Climate Change (IPCC) has released its Fifth Assessment. Like the Fourth Assessment in 2007, which led the IPCC and Al Gore to jointly win the Nobel Peace Prize, the Fifth is the broadest, most cautious summary of the scientific consensus on climate change.

Its conclusions are disturbing. The old news: warming is occurring. There is no discussion to be had. What’s fresh is the conclusion, with 95% certainty – perhaps the highest percentage possible – that human beings are responsible for it. CO2 levels are 40% higher than the pre-industrial 280ppm, and at their highest concentration for 800,000 years; we have hit 450ppm in CO2 equivalence, the number normally portrayed as the ceiling beyond which we should never rise.

If you’re looking for a dramatic new finding, the IPCC report obliges, with news that even Antarctica is losing mass. (This ought to silence one of the main climate-sceptic talking points.) Meanwhile, agriculture remains the single largest emitter of greenhouse gases, via fossil fuel dependence, land-use change and deforestation.

But the Fifth Assessment puts a new emphasis on oceans. These are warming faster than ever, and deeper; 95% of the heat from greenhouse gas emissions is absorbed by the oceans. So the 0.85 degrees warming figure is a bluff: the earth is hiding the heat in her oceans. Like a water furnace, it takes a while to warm up, but when it does it heats your house for a long time.

In fact, the IPCC’s biggest news doesn’t even concern warming at all. It’s a powerful story, with high-school-level chemistry and zero uncertainty. It’s the news that our emission of greenhouse gases is causing the oceans to acidify. As oceans suck up CO2, the CO2 reacts with water – just like in a bottle of Coke – to form carbonic acid. Since we started to emit CO2, the pH level of the oceans has fallen by 30%. Sea water is becoming corrosive to the calcium carbonate that makes up the shells and skeletons of marine life. This is a climate chemical crisis – nothing to do with warming. We are undermining oceanic life support.

Climate scientists now also feel confident saying that climate change has an impact on extreme weather. In 1955, “3-sigma” events – a statistical term for the kind of freak floods and major droughts that ought to happen less than once in a thousand years – affected 1% of the earth’s surface. In 2011, they affected 14.8% of it. We may never know where any given extreme event will hit, but we know they will. In just 50 years, the extremely unlikely has become normal.

The trends do not reassure. Climate models predict an average warming of 4 degrees by 2100: a catastrophic and unacceptable future that we’ve never faced, in 25 million years. And yet this “business as usual” model, in which CO2 emissions hit 1,000 ppm (from 400 today), only accounts for human-caused warming. They assume the earth won’t kick off any “feedbacks” – that the oceans and forests will continue to absorb 60% of all new CO2, that methane sinks won’t be exposed, that the Amazon won’t “flip” and become a savannah, that Greenland won’t melt and drive sea levels up 7 metres.

In other words, the IPCC’s upsetting models are actually very conservative. Until now, the planet has been our best friend. Its resilience has been remarkably high. But we might well push an “on” button and send temperatures skyrocketing even higher: in fact, the IPCC estimates the risk of warming reaching an unimaginable 6 degrees at a scarily imaginable 1.6%. On the other hand, the IPCC says we can release 1,000 gigatons more of CO2 and still maintain a 70% chance of staying below 2 degrees warming. We emit 40 gigatons a year. That leaves us 25 years – with only a 70% chance of success, assuming nature will keep sucking a bit more than 50% of CO2 emissions, and assuming there will be no tipping points. We’re at the beginning of the end of the fossil era.

What can we do? Well, we could transform agriculture, a net emitter of gases, to a net absorber, using more intelligent technologies. We could use biomass to grow a vast carbon sink. We could fit the 363 new coal-fired plants China builds every year with carbon capture and sequestration. We could preserve carbon-rich soils by abolishing the plough and by engineering perennial cereal crops; we could close the loop on nitric phosphorous, which is currently mined in the west Sahara and drains uselessly away into waste water. We could run agriculture on biofuels: transitioning to biogas-fuelled tractors would be an enormous win for the food industry. We eat oil on our plates; we don’t need to.

But the silver bullet may be something we’ve still not tried committedly enough: a carbon tax. Talking to executives at big oil companies such as BP has convinced me that they would prefer not to destroy the environment: they’re in the business of providing cheap energy, which is getting more difficult, as oil sands, offshore drilling and shale gas are proving risky and expensive. Germany, the world’s fourth-biggest economy, already runs 25% of its electricity on renewable energy. A carbon tax could be the incentive to make large-scale renewable energy a profitable option for big business.

So if there’s one thing we can take away from the IPCC’s dire warnings, it’s that the story can be positive. Climate change need not be a burden: it can be an opportunity for a more desirable, less costly future. It has to be. Business as usual just means Mother Nature sending us a horrifying bill.

Author: Johan Rockström is executive director of the Stockholm Resilience Centre, teaches natural resource management at Stockholm University and is participating at the World Economic Forum’s Annual Meeting 2014 in Davos.

Image: Visitors watch water gushing from the section of the Yellow River during a sand-washing operation in Jiyuan, China, 6 July 2013. REUTERS/Stringer