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.