People tend to notice when James Hansen speaks, and with good reason. Sometimes described as the godfather of climate science, Hansen came to global attention in June 1988 when he was director of the Nasa Goddard Institute for Space Studies.
Invited to speak before a US Senate committee, he warned Nasa was 99% confident that human-made greenhouse gas emissions were already causing global warming, and the likelihood of extreme weather was increasing. Scientists and historians generally agree this forceful testimony 35 years ago was a turning point in mass awareness of a looming climate problem.
Hansen worked at Nasa for more than 40 years and moved towards advocacy and activism as his repeated warnings about the need to act were not properly heeded. Now, at 82, he is an adjunct professor at Columbia University, where he publishes regular analyses of the latest climate evidence.
His most recent commentary, written with three colleagues under the heading “El Niño fizzles. Planet Earth Sizzles. Why?”, looks at why this year is almost certainly going to be the hottest on record. Their most eye-catching conclusion, at least in terms of the global political response to the climate crisis, is that by early next year the world may have reached 1.5C warming above the long-term temperature average.
Trying to limit heating to 1.5C is a key global aspiration inscribed in the landmark Paris climate agreement, and has been widely adopted as a guardrail for avoiding worsening devastation that affects lives, livelihoods and nature.
As the Guardian has reported, the average global temperature last month was what scientists described as a gobsmacking 1.7C or 1.8C above historic levels, and more than 0.5C above the previous hottest September on record.
This doesn’t mean the 1.5C guardrail is gone – that will only happen if temperatures stay that high for an extended period. That has generally been considered to be years away. The Intergovernmental Panel on Climate Change said only that it was likely within the next two decades. Hansen argues recent temperatures challenge that.
For the past four months, the globe has been 0.44C hotter than in 2015, a year chosen because it was the last time we were heading into an El Niño, which tends to warm the planet. The scientists argue if this keeps up until the northern spring (the southern autumn) the 12-month average will be at least 1.6C above temperatures more than a century ago.
They say that would mean 1.5C had been reached “for all practical purposes” and “there will be no need to ruminate for 20 years, as the IPCC proposes” about whether we have actually got there. “On the contrary, Earth’s enormous energy imbalance assures that global temperature will be rising still higher for the foreseeable future,” they say.
Given the consistent emphasis on the 1.5C goal, it is likely some people may interpret this, if it were to happen, as the battle to limit the climate crisis having been lost. Some might be tempted to give up. But this is not the message from the scientific community, and not all climate scientists agree there is enough data to say the pace of warming is accelerating.
Hansen and co dedicate most of their paper to what is causing the recent heat surge. They suggest the current El Niño is a relatively minor player compared to previous similar events, and may ultimately be smaller than the “super” El Niños in the hot years of 1997-98 and 2015-16.
They argue a more likely explanation for a warming spike could be a reduction in human-made aerosols, particularly from power plants and factories in China and the global shipping fleet. Aerosols interact with sunlight and clouds to produce a cooling effect that until recently has offset some of the underlying heat caused by greenhouse gases in the atmosphere. A decline in aerosols pollution, motivated by a desire to improve air quality, could unmask human-induced heating already in the global system.
Hansen has long described this as the Faustian climate bargain. Using fossil fuels releases both aerosols and greenhouse gases, but the cooling effect of the former lasts only days, while the warming of the latter lasts centuries. He says eventually the payment – a rapid increase in warming – was going to come due.
The other area of significant change this year is Antarctica, where warming this year has been off-the-charts. Until recently, there has not been a global heating signal over much of Antarctica despite scientists having long expected that polar regions would warm faster than the rest of the planet. Hansen and co join other scientists who have noted that events this year, particularly a sharp fall in sea ice cover, may indicate this has changed and the continent is becoming an important contributor to global heating.
It is important to stress that not all climate scientists are as pessimistic as Hansen and his colleagues that 1.5C is about to been breached. Debate over whether the guardrail is already lost is not new. The Australian Academy of Science created a wave of controversy in 2021 when it argued that it was “virtually impossible” to stay within that mark.
Michael Mann, a professor at the University of Pennsylvania and celebrated author, is among those who cautions against over interpreting a relatively short period this year that could be explained by an El Niño and natural variability on top of “steady, long-term human-caused warming”.
Mark Howden, a professor at the Australian National University and an Intergovernmental Panel on Climate Change vice-chair, says the most worrying thing about the current heat is that it’s broadly consistent with what scientists have long forecast would happen.
He says the degree to which aerosols are driving record temperatures remains open to debate, noting there was not an equivalent leap when Covid-19 shutdowns caused a significant drop in aerosol use in 2020, and suggests it will be a while before we know if heating has reached 1.5C, given that assessment needs to be based on long-term averages.
But Howden agrees that we are rapidly headed in that direction, and governments are not doing anything like what’s required in response to clear evidence that human and environmental security are at risk.
What does it mean if 1.5C is soon passed? A lot to millions of people and countless species. But it’s worth remembering this threshold was to some extent an arbitrary choice, driven by politics. It could have been 1.2C, 1.4C or 1.6C.
The fundamental message from many scientists is simpler than a guardrail. It’s the same one Howden posted on social media five years ago: every fraction of a degree matters, every year matters, every choice matters.
The question, as always, is: who’s listening?
• Adam Morton is Guardian Australia’s climate and environment editor
