The internationally agreed threshold to prevent the Earth from spiraling into a new superheated era will be “passed for all practical purposes” during 2024, the man known as the godfather of climate science has warned. James Hansen, the former Nasa scientist credited for alerting the world to the dangers of climate change in the 1980s, said that global heating caused by the burning of fossil fuels, amplified by the naturally reoccurring El Niño climatic event, will by May push temperatures to as much as 1.7C (3F) above the average experienced before industrialization. This temperature high, measured over the 12-month period to May, will not by itself break the commitment made by the world’s governments to limit global heating to 1.5C (2.7F) above the time before the dominance of coal, oil and gas. Scientists say the 1.5C ceiling cannot be considered breached until a string of several years exceed this limit, with this moment considered most likely to happen at some point in the 2030s. But Hansen said that even after the waning of El Niño, which typically drives up average global heat, the span of subsequent years will, taken together, still average at the 1.5C limit. The heating of the world from greenhouse gas emissions is being reinforced by knock-on impacts, Hansen said, such as the melting of the planet’s ice, which is making the surface darker and therefore absorbing even more sunlight. “We are now in the process of moving into the 1.5C world,” Hansen told the Guardian. “You can bet $100 to a donut on this and be sure of getting a free donut, if you can find a sucker willing to take the bet.” In a bulletin issued with two other climate researchers, Hansen states that “the 1.5C global warming ceiling has been passed for all practical purposes because the large planetary energy imbalance assures that global temperature is heading still higher”. Hansen has promoted a view, disputed by some other climate scientists, that the rate of global heating is accelerating due to a widening gap between the amount of energy being absorbed by the Earth from the sun and the amount returning to space. Hansen, renowned for his role in publicly revealing the onset of the greenhouse effect to the US Congress in 1988, added that the looming loss of the 1.5C guardrail should provide a jolt the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the foremost body of climate science that has charted pathways to avoid breaching the target. “Passing through the 1.5C world is a significant milestone because it shows that the story being told by the United Nations, with the acquiescence of its scientific advisory body, the IPCC, is a load of bullshit,” Hansen said. “We are not moving into a 1.5C world, we are briefly passing through it in 2024. We will pass through the 2C (3.6F) world in the 2030s unless we take purposeful actions to affect the planet’s energy balance.” Last year was the hottest ever recorded, scientific agencies in the US and the European Union are expected to confirm this week, with the global temperature for 2023 close to being 1.5C above the pre-industrial era. El Niño, which heats up sections of the Pacific Ocean and normally adds to the overall global temperature, is anticipated to be even stronger this year than last, before fading away. Governments meeting at UN climate talks held in Dubai in December reaffirmed the previous commitment, made in Paris in 2015, to strive to restrain the global temperature rise to 1.5C, although scientists have warned the world is well off track to avoid this due to persistently high greenhouse gas emissions and ongoing plans for a massive glut of oil and gas drilling. Carbon emissions from fossil fuels hit another record high last year. While the 1.5C target is a political as much as a scientific one, researchers say there will be worsening impacts in terms of heatwaves, droughts, flooding and other calamities should the world exceed this temperature. For developing countries and small island states at existential risk from sea level rise and extreme weather, the agreed goal is a hard-fought and totemic one, with “1.5 to stay alive” now a common mantra heard at international climate talks. Hansen’s assertion that this year will herald the start of an escalating 1.5C era has received a cautious response from other scientists contacted by the Guardian. Drew Shindell, a climate scientist at Duke University, said this year was an “unusually warm one” due to El Niño and that following years will best judge whether the 1.5C target has vanished. But he added the world was closing in on this point and the 1.5C limit would probably be hit “in the 2020s and not the 2030s any more given recent years have warmed so rapidly, so Jim’s larger point that we’re moving rapidly into the post-1.5C era is correct in my opinion. “To my mind, whether it’s 2024 or 2027 makes little difference in the end to informing our actions – we have to change course immediately or we’ll lose our ability to keep below 2C the same way the 1.5C goal has now become out of reach,” Shindell said. Zeke Hausfather, a climate scientist at Stripe and Berkeley Earth, said “I disagree a bit with Hansen” that global temperatures will not be less than 1.4C above pre-industrial times once there is a countervailing La Niña, a reverse climatic condition to El Niño. “But longer term those sorts of temperatures will no longer be seen as the Earth continues to warm,” Hausfather said, adding that he still expected the long-term average to fully pass 1.5C in the early 2030s. Andrew Dessler, climate researcher at Texas A&M University, said that he also expected it to take “10-ish years” to break the 1.5C barrier, but that Hansen’s views should be taken seriously. “Jim is probably the greatest climate scientist in history, so I am hesitant to disagree with him because perhaps he’ll turn out to be right,” Dessler said. Even if the world’s temperature is to break the 1.5C barrier, researchers stress that this doesn’t mean that all will irretrievably be lost, with every fraction of a degree added, or not, significant in shaping the severity of climate impacts. By current government pledges to cut emissions – if not their actual actions to date – the world is still heading for at least 2.5C (4.5F) warming by the end of this century. “I do think that in worrying about some particular threshold we are addressing the wrong question,” said Kerry Emanuel, a climate and meteorological expert at the Massachusetts Institute of Technology. “There are no magic numbers in climate change, just rapidly growing risks.” Emanuel pointed to recent severe heatwaves, fires and storms that are already being supercharged by global heating of around 1.2C (2.1F) above what it was a little more than a century ago. “Perhaps, once half the population of the planet has experienced at least one of these weather catastrophes, they will get their leaders to act,” Emanuel said. “I hope it doesn’t take that much pain.”