A new study shows how a switch in a major climate system could accelerate global temperatures to a 1.5°C limit, but some scientists are challenging the assumptions
May 8, 2017 — In the Brazilian city of São Paulo, more than 80 experts, including dozens of climate scientists, gathered back in March for a giant planning meeting.
As part of the United Nations Intergovernmental Panel on Climate Change (IPCC), the group from 39 different countries were starting their work on a major report that will tell governments and policymakers what kind of impacts they can expect when global warming reaches 1.5°C.
That report is scheduled to be ready in late September 2018 and will assess in detail what’s known about the impacts 1.5°C of global warming could have on societies, ecosystems and efforts to reduce poverty.
But new research published in a leading scientific journal suggests that just eight years after that report is published, the world might have already reached that 1.5°C target – or at least one definition of it (some senior scientists disagree with some of the assumptions in the paper – read on for those important caveats).
Published in the journal Geophysical Research Letters, the research looks closely at the influence of a mechanism in the climate known as the Interdecadal Pacific Oscillation (IPO).
“The IPO is like the long-term version of El Niño – it’s like El Niño’s uncle,” says Ben Henley of the University of Melbourne and the research’s lead author.
When heat gets trapped in deeper layers of the Pacific Ocean, this is known as a negative phase of the IPO.
Since about the year 2000, the IPO hit this negative phase, which tends to slow down the rise in global temperatures that’s being caused by humans burning too many fossil fuels and cutting down forests.
But around 2014, scientists say that this IPO started to shift, possibly towards a positive phase that would act like an accelerator on global warming.
Henley and his colleague Andrew King, also at the University of Melbourne, wanted to know how quick global temperatures might reach 1.5°C, relative to where they were between 1850 and 1900.
According to the paper, the “rate that global temperatures approach the 1.5°C level is likely to be significantly quicker, or slower, depending on the IPO”.
After using the latest computer models of the climate and allowing for the added greenhouse gases in the atmosphere, Henley and King looked to see what the coming decades have in store depending on the phase of the IPO.
If the IPO turns positive, then the average across the models shows that global temperatures hit 1.5°C in around 2026. If the IPO was to turn negative, then this delays the 1.5°C threshold by five years or so.
Henley told me: “Policymakers have to be aware of just how quickly we are approaching this level. But that doesn’t mean that the target is not a sensible thing to have.
“While we might overshoot 1.5°C, stabilising global temperatures at that level still remains a worthwhile goal.”
The paper also gives other projections for breaching 1.5°C based on different assumptions and different models.
For example, if you wait until global temperatures go above 1.5°C over a five-year average period, then some models suggest that if the IPO stays negative for longer, the 1.5°C breach doesn’t happen until around the year 2040 (but this is an outlier in the paper).
Associate Professor Julie Arblaster, a climate scientist at Monash University who was not involved in the research, told me the research “highlights the role of natural or internal variability in the climate system in hitting climate targets”,
Arblaster pointed to one study published last year in the journal Nature Communications that suggested the switch to a positive phase of the IPO might have already happened.
She added: “Other things may also impact the timing of course, such as a large volcanic eruption, but this study helps by providing an estimate of the contribution of the climate system’s internal variability in hitting that target.”
Henley told me a key motivation for doing the sums on the 1.5°C target was to help policymakers understand what kind of timeframe they have to work with.
When countries were negotiating for a new global deal to cut greenhouse gas emissions and slow the impacts of climate change, many smaller and less developed countries were worried that a 2°C global warming target was setting the bar way too high. Some scientists also shared this concern.
So there was a push to have a 1.5°C target included in the text of the deal.
Article 2 of the Paris agreement states that countries agree to keep global warming “well below 2°C above pre-industrial levels” but to also pursue efforts “to limit the temperature increase to 1.5°C … recognising that this would significantly reduce the risks and impacts of climate change”.
Bill Hare is an Australian climate scientist and founder of scientific consultancy group Climate Analytics. Hare is a veteran of the United Nations climate talks.
In an email, Hare said the paper showed “that very ambitious near-term mitigation is required to limit warming to 1.5°C and this analysis underscores this”.
But Hare and his colleagues also had some reservations about the paper’s conclusions.
Hare said the IPCC’s interpretation of when you can say that 1.5°C target is breached was based on much longer time periods than the Henley and King paper.
He also said the assumption in the paper that greenhouse gas emissions would continue to rise under a “business as usual” scenario didn’t reflect how countries were taking action under the Paris agreement. Using more optimistic scenarios could bring temperatures down by as much as 0.2°C by 2030.
Joeri Rogelj of the International Institute for Applied Systems Analysis in Austria, also pointed out that the convention in UN climate negotiations was to refer to targets in terms of longer time frames of 20 or 30 years.
He thought the paper’s assumptions that emissions would remain high were “not compatible” with the agreements countries had made in Paris, but he did agree that the paper demonstrated how urgent the issue was.
Professor Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, is a senior lead author on a chapter of the special report that will look at the impacts of 1.5°C global warming on humans and natural environments.
Hoegh-Guldberg, who was not speaking from an IPCC perspective, told me the research would “wake some people up to the fact that exceeding 1.5°C will happen within the next decade, give or take a couple of years”.
“One unknown that’s associated with the study is the effect that anthropogenic climate change might be having on these long-term climate patterns themselves. Some research groups have provided compelling evidence that patterns associated with El Niño, for example, may actually be amplified by warming.”
He added: “Many scientists have increasingly pointed to the unmanageable ecological and human impacts as average global surface temperatures exceed 1.5°C, and great economic and environmental costs that are likely to be associated.
“Unfortunately, past inaction means that we will exceed 1.5°C no matter what we do. The key issue, however, is what we do next.
“People speak of overshoots as being one of the scenarios that we are likely to face. This is not an escape clause as the overshoot is likely to be catastrophic.”