The Paris Agreement sought to place a lower limit on global warming at 1.5 degrees Celsius. But researchers fear that may already be locked in - even 2 degrees may be overshot. All the more reason for action, many say.
The title of a study published last week in the journal "Nature Climate Change" does not sound optimistic. "Less than 2 degrees Celsius global warming up to 2100 unlikely," it declares.
Researchers from the University of Washington in Seattle say that the chance for humans to limit global warming to no more than 2 degrees Celsius (3.6 degrees Fahrenheit) by 2050 is only 5 percent.
And it is "unrealistic" to expect that global warming can be kept below 1.5 degrees, concludes Adrian Raftery, the lead author of the study.
It is a sobering conclusion, not only in the context of climate change impacts - this has political implications as well.
In the Paris Agreement on climate change, 195 countries agreed to limit the global average temperature rise to well below 2 degrees Celsius compared to the preindustrial level - and if possible, to 1.5 degrees.
A second study in the same issue of the journal reports that warming of 1.1 degrees Celsius is unavoidable, even if humankind were to cease emitting any greenhouse gases from today onward - a rather unlikely scenario.
"Staying under 1.5 degrees of global warming is illusory," said author Thorsten Mauritsen of the Max Planck Institute for Meteorology in Hamburg.
So was the Paris accord only pretending to limit global warming to a maximum of 1.5 degrees? Or were politicians too optimistic? And where do we go from here?
Mauritsen told DW that the scenario painted by the Intergovernmental Panel on Climate Change (or IPCC, the UN scientific body on climate research), makes overly positive assumptions.
"This is a miracle scenario of the IPCC, in which the climate models reach 1.5 degrees Celsius," he said.
That scenario assumes that carbon capture and storage (CCS) technology, which stores carbon dioxide in large quantities underground, is to be used on a large scale.
But this would be far too expensive, Mauritsen added. "The scenario is honestly a bit of self-delusion."
Many researchers have realized that it was extremely unlikely. "The IPCC has not clarified which of its scenarios is realistic at all," says Raftery.
Time delay on warming
The Earth's climate system runs on its own schedule - it will take a while to adjust even after global carbon dioxide emissions go down, explained Thomas Frölicher, a climate scientist at the University of Bern in Switzerland. "Some warming is still in the pipeline of past emissions."
One reason for this is that the oceans absorb and store heat. This heat endures for centuries, until the great masses of water of the deep sea have equalized - until then, temperatures continue to rise.
Fine dust is also responsible for continued heating. If fewer fossil fuels were burned, the amount of particulate matter in the air would decrease rapidly. These particles get washed away - for example, by rain.
Although this sounds positive in terms of improved air quality, it has disadvantages for the climate: Dust particles in the air reflect solar radiation back into outer space, thus cooling the earth. If there were less fossil fuel dust, the Earth would heat up - by several tenths of a degree Celsius per decade, according to a 2012 study.
Mauritsen told DW he was a bit irritated that the Paris Agreement considered the 1.5 degrees target. "At the time, I was of the opinion that this had already been surpassed," he said.
So he and Robert Pincus from the University of Boulder in Colorado got to calculating.
Further warming unavoidable
On the basis of measurements from the last decades, the two researchers calculated that the Earth's average temperature will increase 1.3 degrees Celsius minimum by the year 2100, compared to preindustrial times.
This is, so to speak, the best-case scenario - if humans stopped burning fossil fuels tomorrow. Any heating from additional carbon dioxide emissions would go on top of this.
The seas can also lead to cooling, because the sea absorbs not only heat but also carbon dioxide. The latter would lead to a temperature drop of 0.2 degrees Celsius. Thus, net 1.1 degrees of global warming will remain by the year 2100.
Mauritsen, however, sees this calculation positively. "We still have some freedom of movement until 1.5 degrees Celsius is reached." Although this doesn't look good for the 1.5-degree goal, "there is still hope that we can stay below 2 degrees."
Over 2 degrees likely
Raftery has a different perspective. He and his colleagues have examined how CO2 emissions, and thus also temperatures, will develop to 2100 by analyzing data from 150 countries. He speaks of "predictions," in contrast to the IPCC "scenarios."
The researchers also included into their calculations population growth, economic growth and "carbon efficiency" - that is, the amount of CO2 emitted to achieve a certain gross domestic product.
Their conclusion: The Earth is on track for a very probable temperature rise between 2.0 and 4.9 degrees Celsius.
Carbon dioxide in the atmosphere does not disappear from today to tomorrow, even if we emit less CO2 in the future, Raftery echoed. "It is accumulating."
The increase in the world's population to 11 billion people by the year 2100 is also a major factor in global climate change.
But this prediction shouldn't demotivate people, Raftery stressed: "Even if we miss the 2-degree goal, it is very important to remain as close as possible to this value." The consequences would ultimately be all the more dramatic, as the Earth heats up.
The 2-degree target is widely understood to be the point where the impacts of climate change would still be within a managable range.
But some researchers complain that the 2-degree target is arbitrary and counterproductive. "Climate policy is currently working as an either-or," said Oliver Geden, an energy and climate policy expert at the German Institute for International and Security Affairs, in an interview with the German daily "taz."
"Either we hold to 2 degrees, or climate catastrophe happens," he said.
"My approach is: There is also an in-between space - and that it is better to reach 2.5 degrees or 3, than 4, 5 or 6."
'We have to become more creative'
Delphine Deryng, a scientific advisor at Climate Analytics - an international climate science institute based in Berlin - does not believe that the 1.5 degree target is already a foregone conclusion.
"The two studies involve only the past, but no current developments," Deryng told DW. The boom in renewable energies, for example, is not included in current calculations.
And even if calculations indicate that the 1.5-degree goal would be difficult to achieve, this should only act as a call for more climate action, Deryng told DW.
"We have to become even more creative; perhaps we need a more drastic climate policy. And we should be able to take a closer look at CCS." With too much CO2 in the air, the only way to reduce it is to find ways to capture and store it.
For Raftery, the most important step is further research. "Carbon efficiency has improved in all countries over the last decades, and we must continue to accelerate this." Raftery cites LED technology as an example.
Technical solutions like these are the best way to keep global warming to a minimum, he believes.