1.5°C of Warming is Closer than We Imagine, Just a Decade Away

April 5, 2018

Global warming of 1.5°C is imminent, likely in just a decade from now. That’s the stunning conclusion to be drawn from a number of recent studies, surveyed below.

Paris Commitments now put the world on a path of 3.4°C of warming by 2100 (Climate Action Tracker)

So how does hitting warming of 1.5°C a decade from now square with the 2015 Paris Agreement’s goal of “holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C”? In two words, it doesn’t.

The Paris text was a political fix in which grand words masked inadequate deeds. The voluntary national emission reduction commitments since Paris now put the world on a path of 3.4°C of warming by 2100 (as illustrated), and more than 5°C if high-end risks including carbon-cycle feedbacks are taken into account.

The Paris outcome is an emissions path continuing to rise for another fifteen years, even though it is clear that “if the 1.5°C limit should not be breached in any given year, the budget (is) already overspent today”. Two years ago, Prof. Michael E. Mann noted: “And what about 1.5°C stabilisation? We’re already overdrawn.”

In fact, the emission scenarios associated with the Paris goal show that warming will “overshoot” the 1.5°C target by up to half a degree, before cooling back to it by the end of this century. Those scenarios rely unduly on unproven Bio-Energy with Carbon Capture and Storage (BECCS) technology in the second half of the century, because the Paris Agreement does not encompass the steep emissions reductions that are required right now.

Average global warming is now 1.1°C above the late nineteenth century, and the rate of warming is likely to accelerate due to record levels of greenhouse gas emissions, and because efforts to clean up some of the world’s dirtiest power plants is reducing the emission of aerosols (mainly sulphates) which have a very short-term cooling impact.

So now, in 2018, the benchmark of 1.5°C of warming is just a decade away or even less, according to multiple lines of evidence from climate researchers:

HENLEY and KING: In 2017, Melbourne researchers  Ben Henley and Andrew King published Trajectories toward the 1.5°C Paris target: Modulation by the Interdecadal Pacific Oscillation on the impact of the Interdecadal Pacific Oscillation (IPO) on future warming. The IPO is characterized by sea surface temperature fluctuations and sea level pressure changes in the north and south Pacific Ocean that occur on a 15-30 year cycle. In the IPO’s positive phase, surface temperatures are warmer due to the transfer of ocean heat to the atmosphere.  The IPO has been in a negative phase since 1999 but recent predictions suggest that it is now moving to a positive phase. The authors found that “in the absence of external cooling influences, such as volcanic eruptions, the midpoint of the spread of temperature projections exceeds the 1.5°C target before 2029, based on temperatures relative to 1850–1900”. In more detail,”a transition to the positive phase of the IPO would lead to a projected exceedance of the target centered around 2026”, and “if the Pacific Ocean remains in its negative decadal phase, the target will be reached around 5 years later, in 2031”.

Projected temperature rises with IPO in positive mode (red) and negative mode (blue)   (Henley and King, 2017)


JACOB et al: A set  of four future emission scenarios, known as Representative Concentration Pathways (RCPs) have been used since 2013 as a guide for climate research and modelling. The four pathways, known as RCPs 2.6, 4.5, 6 and 8.5, are based on the total energy imbalance in the energy system by 2100. RCP8.5 is the highest, and is the current emissions path. In Climate Impacts in Europe Under +1.5°C Global Warming, released this year, Daniela Jacob and her co-researchers found that the world is likely to pass the +1.5°C threshold around 2026 for RCP8.5, and “for the intermediate RCP4.5 pathway the central estimates lie in the relatively narrow window around 2030. In all likelihood, this means that a +1.5°C world is imminent.”

KONG AND WANG: In a study of projected permafrost change, Responses and changes in the permafrost and snow water equivalent in the Northern Hemisphere under a scenario of 1.5 °C warming, researchers Ying Kong and Cheng-Hai Wang use a multi-model ensemble mean from 17 global climate models, with results showing that the threshold of 1.5°C warming will be reached in 2027, 2026, and 2023 under RCP2.6, RCP4.5, RCP8.5, respectively.  On the present, high-emissions RCP8.5 path, the estimated permafrost area will be reduced by 25.55% or 4.15 million square kilometres at 1.5°C of warming.

XU and RAMANTHAN:  A recent study by Yangyang Xu and Veerabhadran Ramanathan, Well below 2 °C: Mitigation strategies for avoiding dangerous to catastrophic climate changes, looked at the high-end or “fat-tail” risks of climate change, in an analysis of the existential risks in a warming world. One of two baseline scenarios used, named Baseline-Fast, assumed an 80% reduction in fossil fuel energy intensity by 2100 compared to 2010 energy intensity. In this scenario, the level of atmospheric carbon dioxide had reached 437 parts per million (ppm) by 2030 and the warming was 1.6°C, suggesting that the 1.5°C would be exceed around 2028. The study is discussed in more detail here.

ROGELJ et al: In Scenarios towards limiting global mean temperature increase below 1.5°C, Joeri Rogelj and co-researchers plot future emissions and warming based on five distinct “Shared Socioeconomic Pathways” (SSPs). These “present five possible future worlds that differ in their population, economic growth, energy demand, equality and other factors”, according to CarbonBrief. The fourth and fifth paths are the world we now live in: SSP4 is a world of “high inequality”, whilst SSP5 is a world of “rapid economic growth” and “energy intensive lifestyles”.  If we look at these paths charted against projected temperatures, then SSP5 exceeds 1.5°C in 2029 and SSP4 by 2031.

Projected global mean temperature for five Shared Socioeconomic Pathways (CarbonBrief)


SCHURER et al:  In Interpretations of the Paris climate target, Andrew Schurer and colleagues demonstrate that the IPCC uses a definition of global mean surface temperature which underestimates the amount of warming over the pre-industrial level. The underestimation is around 0.3°C, and a higher figure includes the effect of calculating warming for total global coverage rather than for the coverage for which observations are 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 would be 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 exceeded three years ago) have been associated with sea levels around 25 metres above the present. And six years ago, Prof. Kenneth G. Miller noted 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 Former NASA climate chief 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 and 2°C targets is that they are far above the Holocene (human civilisation) 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. Hence “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.

David Spratt

David Spratt is a Research Director for Breakthrough and co-author of Climate Code Red: The case for emergency action (Scribe 2008). His recent reports include Recount: It’s time to “Do the math” again; Climate Reality Check and Antarctic Tipping Points for a Multi-metre Sea-level Rise.

Tags: 1.5 degree C., catastrophic climate change