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IN ANTARCTICA, the giant Thwaites glacier is in fast retreat. Ditto the Jakobshavn and Zachariae Isstrom glaciers in Greenland. Climate researchers worry they may have passed their tipping points, beyond which change feeds on itself and cannot be stopped. If the three glaciers melted fully, they alone would commit the world to more than 2 metres of sea level rise.

The Intergovernmental Panel on Climate Change could take key Earth systems beyond their tipping points, part of the worst-case scenario of climate change. Tim Lenton of Exeter University, UK, says a threshold was passed in 2007 when the summer melt of Arctic sea ice accelerated. The fear is that, with less ice cover, the ocean will absorb more heat and prevent winter refreeze, locking the system into perpetual decline.

This is not the only system at risk. Historically, as temperatures have gone up, changing amounts of sea ice at the poles have caused ocean circulation to flip. A new flip could lose us the Gulf Stream and collapse the Asian and West African monsoons, affecting the livelihoods of billions. So far, annual changes in sea ice have not disturbed overall global ocean circulation. But the Atlantic leg has already weakened markedly, which Lenton says may mean it is inching towards its tipping point.

The trouble is that although there is plenty of historical evidence that tipping points exist, we don鈥檛 know what the warning signs are. Take the methane trapped in the permafrost of Siberia and North America, both of which are expected to thaw rapidly this century. Methane is a potent greenhouse gas. It doesn鈥檛 stay in the atmosphere for as long as CO₂, but if a large volume of it were released in one go, this could trigger runaway warming. Right now, methane is escaping from the permafrost, but it is minimal and nobody knows whether this is new or normal.

We certainly shouldn鈥檛 be reassured by the apparent lack of runaway change. Experiments with biological and chemical systems show that they become sluggish when they approach tipping points. They also show that tipping points are sometimes passed without immediate impact. Unfortunately, this is an area where the uncertainties are great and the risks much greater. Fred Pearce

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Living with climate change: will we have to geoengineer?

DAVID KEITH is planning one of the first real-world geoengineering experiments next year. Only a decade ago, the idea that we might try to reverse global warming by artificially cooling the atmosphere seemed fanciful and dangerous. To most, it still does, but with the world on track for 3.6掳C of warming even if governments stick to all their promises (see 鈥�Living with climate change: Can we limit global warming to 2掳C?鈥�), researchers say we should at least understand what plan B looks like.

Keith and his colleagues at Harvard University intend to fly a balloon 20 kilometres up in the air, release bursts of tiny particles, and study how they deflect sunlight and interact with ozone. The particles will be benign 鈥� either ice, calcium carbonate or sulphur compounds that are naturally spewed by volcanoes 鈥� and only 100 grams will be released each time. Still, people have the right to feel jittery, says Keith. One concern is that even small-scale geoengineering experiments could send the message that there is a quick fix on the horizon. 鈥淭he oil industry might say, 鈥楽ee, there鈥檚 no problem鈥�.鈥�

As time drags on, the odds that we will reach for plan B get greater. 鈥淚t seems likely to me that we will eventually have to resort to geoengineering because it鈥檚 taking so long for a critical mass of people to take global warming seriously,鈥� says David Mitchell at the Desert Research Institute in Nevada. The risks are not to be sniffed at. Adding particles to the atmosphere to create a sunshade 鈥� a large-scale version of Keith鈥檚 proposed experiment 鈥� could damage the ozone layer up weather systems, causing severe droughts in some regions.

Another option is fertilising the oceans. The idea is that this would trigger algal blooms, which suck CO₂ out of the air during photosynthesis then drag it to the bottom of the ocean when they die. But large blooms could suffocate other marine species. Besides, there are doubts over how much of an impact it would have on the atmosphere.

A cheap, industrial-scale version, in which CO₂ is chemically removed from the atmosphere and stored underground, so far eludes us. Yet without such solutions, limiting warming to 2掳C may be out of reach. 鈥淧eople are trying to figure out a way to [remove CO₂]. But it鈥檚 cheaper not to put it in the atmosphere in the first place,鈥� says Alan Robock at Rutgers University in New Jersey. Alice Klein

This article appeared in print under the headline 鈥淟iving with climate change: Is runaway change likely?鈥�