Climate scientists say it's almost certain we'll cross the threshold for a "dangerous" level of global warming as soon as 2038, possibly as early as 2027, if drastic measures aren't taken.
It's a gloomier scenario than the Intergovernmental Panel on Climate Change (IPCC) has put forward, they say; but being based on real-world historical data, sadly perhaps more accurate.
"Climate skeptics have argued that global warming projections are unreliable because they depend on faulty supercomputer models," said Bruno Tremblay, a professor in the Department of Atmospheric and Oceanic Sciences at McGill University and co-author of a new study looking at the future state of the climate.
"While these criticisms are unwarranted, they underscore the need for independent and different approaches to predicting future warming."
Dr Tremblay and his team developed a new climate model they say produces similar results to the IPCC's models, but is about twice as accurate and relies on fewer assumptions.
The threshold for 1.5C of warming will be passed sometime between 2027 and 2042, they say - depending on what actions humanity takes. The IPCC's estimate is 2052.
Under a best-case scenario, where emissions are phased out by 2100, the world has an 80 percent chance of keeping temperature rises below 1.5C this century, and a 97 percent chance of keeping it below 2C.
In the middle scenario - emissions rising to 2050, then falling to about half that level by 2100 - it's almost certain we'll pass 1.5C mid-century, and 94 percent likelihood of breaching 2K by 2100.
The third scenario - emissions keep rising through the 21st century - the 1.5C threshold will be breached in the 2030s or 2040s, and 2C in the 2050s.
"Now that governments have finally decided to act on climate change, we must avoid situations where leaders can claim that even the weakest policies can avert dangerous consequences," said co-author Shaun Lovejoy, also of McGill University.
"With our new climate model and its next generation improvements, there's less wiggle room."
The researchers admit their model has a weakness, but it's not one anyone should be placing hope in.
"It assumes a linear stationary relationship between [climate influences] and temperature, neglecting nonlinear interactions which could arise as the system evolves, as it currently warms," the study reads.
"In particular, so-called tipping points could be reached in the coming century which would lead to a breakdown of the linear model proposed."
Prior studies have outlined some potential feedback loops, for example the melting of polar ice - large white areas reflect heat back into space, and without them, the Earth absorbs more energy and warming speeds up.
The latest study was published in journal Climate Dynamics.