Climate change: 1.5°C is closer than we imagine

available, and warming from a true pre-industrial, instead of a late-nineteenth century, baseline. If their finding were applied, warming would now be 1.3°C or more, and hitting the 1.5°C benchmark just half a decade away.

CONSEQUENCES: In their 2017 paper on catastrophic climate risks, Xu and Ramanathan defined 1.5°C as a benchmark for “dangerous” climate change, compared to the convention policy-making mark of 2°C.

But even this lower mark may be too optimistic, given the impacts we have seen at both poles in the last decade.

In any case, it contemplating the imminent reality of the 1.5°C benchmark, it is important to consider what is at stake:

•  In another decade and by 1.5°C, we may well have witnessed an Arctic free of summer sea ice, a circumstance that just two decades ago was not expected to occur for another hundred years. The consequences would be devastating.
•  In 2012, then NASA climate science chief James Hansen told Bloomberg that: “Our greatest concern is that loss of Arctic sea ice creates a grave threat of passing two other tipping points – the potential instability of the Greenland ice sheet and methane hydrates… These latter two tipping points would have consequences that are practically irreversible on time scales of relevance to humanity.” One highly-regarded research paper in 2012 estimated that “the warming threshold leading to a monostable, essentially ice-free state is in the range of 0.8–3.2°C, with a best estimate of 1.6°C” for the Greenland ice sheet.
•  In 2015, researchers looked at the damage to system elements — including water security, staple crops land, coral reefs, vegetation and UNESCO World Heritage sites — as the temperature increases. They found all the damage from climate change to vulnerable categories like coral reefs, freshwater availability and plant life could happen before 2°C warming is reached, and much of it before 1.5°C warming.
•  In 2009, Australian scientists contributed to an important research paper which found that preserving more than 10% of coral reefs worldwide would require limiting warming to below 1.5°C. Recent research found that the surge in ocean warming around the Great Barrier Reef in 2016, which led to the loss of half the reef, has a 31% probability of occuring in any year at just the current level of warming. In other words, severe bleaching and coral loss is likely on average every 3–4 years, whereas corals take 10–15 years to recover from such events.
•  At 1.5°C, the loss of permafrost area is estimated to be four million square kilometres, and there is evidence that a 1.5oC global rise in temperature compared to the pre-industrial level is enough to start a general permafrost melt.
•  The frequency of extreme El Nino events is likely to double by 1.5°C of warming.
• At 1.5°C, it is very likely that conclusions first aired in 2014 –– that sections of the West Antarctic Ice Sheet have already passed their tippings point for a multi-metre sea-level rise –– will have been confirmed. Four years ago scientists found that “the retreat of ice in the Amundsen Sea sector of West Antarctica was unstoppable, with major consequences – it will mean that sea levels will rise 1 metre worldwide… Its disappearance will likely trigger the collapse of the rest of the West Antarctic ice sheet, which comes with a sea-level rise of between 3–5 metres. Such an event will displace millions of people worldwide.” Leading cryosphere researcher Eric Rignot muses: “You look at West Antarctica and you think: How come it’s still there?
•  By 1.5°C, a sea-level rise of many metres, and perhaps tens of metres will have been locked into the system. In past climates, carbon dioxide levels of around 400 ppm (which we exceed three years ago) have been associated with sea levels around 25 metres above the present. And six years ago, Prof. Kenneth G. Miller notes that “the natural state of the Earth with present carbon dioxide levels is one with sea levels about 20 meters higher than at present”.

Clearly, as James Hansen and co-authors wrote last year, “the world has overshot the appropriate target for global temperature”.

They noted a danger of 1.5°C or 2°C targets is that they are far above the Holocene temperature range and if such temperature levels are allowed to long exist they will spur “slow” amplifying feedbacks which have potential to run out of humanity’s control, so “limiting the period and magnitude of temperature excursion above the Holocene range is crucial to avoid strong stimulation of slow feedbacks”.

And in all this evidence, what worries me most?

It is my experience that with few exceptions neither climate policy-makers nor climate action advocates have a reasonable understanding of the imminence of 1.5°C and its consequences.