There's no doubt our planet is getting hotter and hotter, but the long-term outlook could be even worse than we thought. Scientists are now saying it might already be too late to avoid a temperature rise of up to 7.36 degrees Celsius (13.25 degrees Fahrenheit) above pre-industrial levels by 2100.

That's way above the upper limit of 4.8 degrees Celsius (8.6 degrees Fahrenheit) predicted by the Intergovernmental Panel on Climate Change (IPCC) in 2014, and to make matters worse, a new study suggests that we're underestimating just how sensitive Earth is to greenhouse gases.

Scientists from the US and Germany analysed global mean temperatures on our planet for the last 784,000 years in an attempt to predict how warm conditions will get by 2100.

Based on their results, it appears that higher CO2 levels will cause the atmosphere to heat up more quickly than previously estimated.

"Our results imply that Earth's sensitivity to variations in atmospheric CO2 increases as the climate warms," explains lead researcher Tobias Friedrich from the University of Hawaii at Manoa.

"Currently, our planet is in a warm phase – an interglacial period – and the associated increased climate sensitivity needs to be taken into account for future projections of warming induced by human activities."

As levels of carbon dioxide in the atmosphere rise, more heat is kept in – but the question is, how much more?

Climate change models usually rely on computer simulations to figure out the relationship between carbon dioxide levels and trapped heat. Friedrich and his colleagues tried a different approach by looking at climate history instead.

climate-1New models show the chance of a rise above 7 degrees Celsius. Credit: Tobias Friedrich et al

To chart how Earth's climate had altered over thousands of years, they studied marine sediment cores, ice cores, and planetary movements known as Milankovitch Cycles – how changes in Earth's orbit and tilt affect how the Sun heats it.

They estimated greenhouse gas levels through the air bubbles extracted from ice cores over the last eight glacial cycles (almost 800,000 years).

From this data, they concluded that Earth becomes more sensitive to warming in interglacial warming phases (periods between ice ages), like the one we're now in.

The researchers also calculated there will be a "likely" temperature increase of between 4.78 and 7.36 degrees Celsius (8.6 and 13.25 degrees Fahrenheit) above pre-industrial levels over the next 85 years if greenhouse gas emissions continue at their current rate,.

That means it's absolutely vital that we get those emissions down as quickly as possible.

Because a 7.36-degree Celsius (13.25-degree Fahrenheit) rise would effectively be "game over" for the planet as we know it, climatologist Michael Mann from Penn State University, who wasn't involved in the research, told Ian Johnston at The Independent.

Mann is known for producing the famous 'hockey stick' climate chart, showing a sharp rise in global temperatures starting in the 20th century.

"By 'game over for the climate', I mean game over for stabilising warming below dangerous (ie greater than 2 degrees Celsius [3.6 degrees Fahrenheit]) levels," he said.

One of the researchers behind the new study, Andrew Ganopolski from the Potsdam Institute for Climate Impact Research in Germany, admits that scientists don't always agree on the level of global warming that we can expect.

"In our field of science, you cannot be definite by 100 percent," he told The Independent. "There are always uncertainties and we discuss this in the paper. If we have more and more results of this sort, then we have more reasons to be concerned."

But as far as Friedrich is concerned, there's no doubt about what needs to happen next. "The only way out is to reduce greenhouse gas emissions as soon as possible," he urged.

The study has been published in Science Advances.