The United Nations' forecast for global warming is about 15 percent too low, which means that end-of-century temperatures could be 0.5 degrees Celsius higher than currently predicted, according to a study released Wednesday.
This sobering verdict renders the already daunting challenge of capping global warming at "well under" 2.0 degrees Celsius (3.6 degrees Fahrenheit) - the cornerstone goal of the 196-nation Paris Agreement - all the more difficult, the authors say.
"Our results suggest that achieving any given global temperature stabilization target will require steeper greenhouse gas emissions reductions than previously calculated," they wrote.
A half-degree increase on the thermometer could translate into devastating consequences.
With only a single degree Celsius of global warming so far, the planet has already seen a crescendo of deadly droughts, heat waves and superstorms engorged by rising seas.
The U.N. Intergovernmental Panel on Climate Change (IPCC), which provides the scientific foundation for global climate policy, projects an increase in the earth's average surface temperature of about 4.5 Celsius by 2100 if carbon pollution continues unabated.
But there is a very large range of uncertainty - 3.2 to 5.9 degrees Celsius - around that figure, reflecting different assumptions and methods in the dozens of climate models the IPCC takes into account.
By factoring in decades of satellite observations which track how much sunlight gets bounced back into space, the study showed that the more alarming projections are clearly aligned with that data and the warming that has been measured so far.
One scientist not involved in the research described it as a "step-change advance" in the understanding of how hot our planet is likely to become.
"We are now more certain about the future climate," said William Collins, a professor of meteorology at the University of Reading.
"But the bad news is that it will be warmer than we thought."
The study, published in the journal Nature, not only narrows the temperature, but reduces the degree of uncertainty as well.
"If emissions follow a commonly used ‘business as usual' scenario, there is a 93 percent chance that global warming will exceed 4 degrees Celsius by century's end," said co-author Ken Caldeira, also from Stanford.
Up to now, there was barely more than a coin-toss certainty that the earth would breach the 4 Celsius barrier by 2100 under that scenario.