At the Paris climate summit last December, governments agreed that they would "pursue efforts" to keep warming below this figure.
But a new study shows the limit will be breached over land, even if emissions of warming gases ceased immediately.
The research has been published in the journal Scientific Reports.
Around 180 countries have so far signed the Paris climate agreement in which they pledged not only to limit the global temperature rise since pre-industrial times to "well below" 2C but to do their very best to keep them to 1.5C.
The lower target was crucial in gaining the support of small island states and the poorest developing countries. Rallying around the slogan, "1.5 to stay alive," they have argued their very survival is at stake with the rise in sea levels and the increase in storms that this amount of warming would bring.
But as recorded temperatures this year have edged above 1C, scientists believe we are already "dancing" with the 1.5 degree target.
This new study suggests that it will "almost certainly be surpassed", at least over land, based on the amount of CO2 already in the atmosphere.
"Even if we just had a planet where we could prevent greenhouse gases going up any further, there would be an additional warming over land, in many places if not most it would 1.5 degrees above pre-industrial levels," said lead author Dr Chris Huntingford from the UK's Centre for Ecology and Hydrology.
The researchers say there are two reasons behind the finding.  The first is the fact that much of the heat created in the Earth's greenhouse is currently subsumed into the oceans.
The authors argue that this sea sink will decline as the world cuts emissions of CO2 and other warming gases, putting more heat into the air.
This will combine with the second reason, namely that warming rates over land are far higher than those over the oceans.
"At the moment the oceans are drawing down a vast amount of heat from the atmosphere," said Dr Huntingford, "but as you stabilise the climate you get to the position where these net fluxes of heat actually become zero, and that would correspond to a higher temperature."
Other researchers agree that keeping temperatures at the 1.5 figure is going to be a significant, if not impossible, challenge.
"In my opinion we should work for as low a target as possible," said Dr Gavin Schmidt from Nasa, who was not involved with the study.
"But we should be prepared for worse."
The authors of the latest paper say that it was a good idea to have an "aspirational" 1.5 degree target in the Paris agreement.
Emissions are going down they say and the uptake of renewable and green sources of energy is happening more quickly than expected.
But if the world is to take the 1.5 target seriously, then a serious discussion needs to be held about the implications of that goal.
"I think there needs to be a very thoughtful debate about what's to be gained at these different temperature levels, if approaching the lower levels meant severely damaging the economy," said Dr Huntingford.
"Every climate scientist realises that when you write that we have to get emissions down to hit this target that could potentially push the world into a global recession - so we need to be really clear about what's to be gained by aiming for 1.5 that might be extremely difficult for society rather than 2 degrees."
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Scientists are calling for a "thoughtful debate" about the wisdom of attempts to keep a global rise in temperatures under 1.5C.