Breakthrough has just released a new discussion paper, Warming has reached 1.5°C. What does that mean for climate advocacy? This blog is part 3 of the paper, on the 1.5 degrees conundrum.
by David Spratt
1.5°C has become the policy-making target
Until 2015, climate-policy making and advocacy was focused on the 2°C goal, which was seen as appropriate to “prevent dangerous anthropogenic interference with the climate system” (Article 2 of the UNFCCC). As an aside, how was 2°C ever considered a reasonable goal? Answering that question — it wasn’t science-based, but first proposed by an economist — may provide insight into why 1.5°C isn’t either.
At the Paris COP in 2015, the overarching goal adopted was to hold “the increase in the global average temperature to well below 2°C above pre-industrial levels” and pursue efforts “to limit the temperature increase to 1.5°C above pre-industrial levels”. Since the Paris Agreement, the focus of advocates has been 1.5°C.
The flawed benchmark of 1.5°C
Sir David King, the former UK Chief Scientist and advisor to three (Labour and Conservative) governments, reflected on his work with AOSIS on 1.5°C in Paris in 2015. The Independent journalist Donnachadh McCarthy reported that King “astounded me by saying he now realised this was wrong, and believes the passing of the Arctic tipping point has been reached... He said the 1.1°C rise that we already have is too dangerous — and candidly admitted he believed US climate professor James Hansen had been right after all in 1988, when he warned the US Congress that we should not pass 350 ppm CO2 (parts per million carbon dioxide). We have now breached 415 ppm and are heading fast towards 500 ppm, Sir David said” (emphasis added).
This is widely understood. The NGO 350.org was explicitly established in 2008 on the view that what was safe was < 350 ppm and < 1°C, a position based on the work of former NASA climate chief James Hansen, sometimes referred to affectionately as the “godfather” of modern climate science.
Likewise, in "A safe operating space for humanity", Rockstrom et al. (including Hansen and Will Steffen), proposed that “human changes to atmospheric CO2 concentrations should not exceed
350 ppm”.
But in current mainstream climate advocacy, < 1°C and < 350 ppm seem to have been dropped to
the wayside.
1.5°C is not a safe limit
Many lines of evidence show that systems have/will have passed their tipping points at 1.5°C, and tipping points are now in play, including at both poles. Coral reef systems have been in a death spiral for more than a decade. Permafrost, boreal forests and the Amazon are becoming net carbon emitters.
In September 2022, Stockholm University’s David Armstrong McKay and his colleagues concluded that even global warming of 1°C risks triggering some tipping points. At 1.5°C, “we're at risk of crossing irreversible thresholds on unique and threatened systems”, says Johan Rockström, director of the Potsdam Institute for Climate Impact Research.
This year, new research has reaffirmed that 1.5°C is too high to prevent tipping points: there is a “significant” risk of large Amazon forest dieback if global warming overshoots 1.5°C. And there is a new scientific warning that “1.5°C is too high for polar ice sheets” and the Paris Agreement target won’t protect them.
As well, Earth may have hit a point of irreversible moisture loss in its soil. And the natural sequestration of CO2 by the terrestrial biosphere peaked in 2008 and is in decline, accelerating climate change.
At a May 2008 climate conference at the Academy of Science conference in Canberra, the international guest speaker was Dr Neil Hamilton, then head of the WWF Arctic Programme. He told a somewhat stunned audience that the WWF was not trying to preserve the Arctic ecosystem because “it was no longer possible to do so”. That is, it had already passed its tipping point, at a time when global average warming was 0.8°C!
Likewise the world’s greatest coral researcher, Australia’s Charlie Veron, told the Royal Society in London in 2009 that a safe boundary for reef systems was 0.5°C.
The evidence grows that the 1.5°C target was never a safe target for humanity.
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| Figure 1: Climate models (CMIP6) and observations (ClimateBrink) |
We are already at 1.5°C.
In 2018, the IPCC mid-range projection for the world to warm to 1.5°C above pre-industrial levels was 2040. But a new World Meteorological Organization report this year indicates that Earth will cross this point in just two years, with a “70% chance that the 2025-2029 five-year mean will exceed 1.5°C above the 1850-1900 average”.
In fact, 2023 was 1.5°C and 2024 reached 1.6°C, and the running average for the last 24 months has been close to 1.6°C. A new paper (currently in pre-print) shows that for one temperature data set, the warming trend reached 1.5°C in 2024, and four other data sets show 2026.
For all practical purposes, the warming trend has hit 1.5°C, as James Hansen has noted: “Averaged over the El Nino/La Nina cycle, the 1.5°C limit has been reached”. This is also consistent with the CMIP6 model projections (Figure 1, produced by Zeke Hausfather).
1.5°C is not a point of system stability
Scientists say it is a big mistake to think we can “park” the Earth System at any given temperature rise – say 2°C – and expect it to stay there. The late Prof. Will Steffen and his coauthors in their widely-read 2018 “Hothouse Earth” paper warned that “even if the Paris Accord target of a 1.5°C to 2°C rise in temperature is met, we cannot exclude the risk that a cascade of feedbacks could push the Earth System irreversibly onto a ‘Hothouse Earth’ pathway”.
In other words, by 1.5°C, there may be so many carbon-cycle and other feedbacks under way and active system instabilities that the climate will not stabilize at 1.5°C, but rather it will move towards a new point of equilibrium at a significantly higher level. The implication is that our choice is either to cool back to Holocene (pre-industrial) conditions, or accept that the next point of system stability may be around 3°C.
This is a point that paleoclimatologists make repeatedly, that the climate’s history over the last 800,000 years shows temperature and CO2 oscillating between two levels, interrupted by abrupt instability, offering critical insights into today's rapidly changing climate. The climate see-sawed between glacials around 180 ppm which were 3-5°C cooler than recent centuries, and the warmer interglacials around 280 ppm (see figure 2). We are now approaching 430 ppm.
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| Figure 2: Earth system stability |
Hansen says we have now left a state of system stability — the Holocene — and are headed into a period of rapid, unstable (non-linear) change driven by record-fast increases in CO2 and amplifying and cascading feedbacks. The current 1.5°C climate is not a point of system stability, which lies at a significantly higher warming when the current system feedbacks have played themselves out to equilibrium. The last time CO2 was around the current level — 3 to 3.3 million years ago — temperatures were around 3°C hotter than pre-industrial and sea levels 25 metres higher.
In other words, 1.5°C is not a destination, but a signpost on a dusty road to somewhere hotter. So it cannot be an advocacy endpoint either, from a “science-based” perspective.
