Climate computer model projections of future man-made warming due to human emissions of carbon dioxide are running too hot, says a fascinating new study in Nature Geoscience. Consequently, researchers reckon that humanity has more time to prevent dangerous future climate change than had been suggested earlier by the U.N.'s Intergovernmental Panel on Climate Change (IPCC).
This is really good news. This new article shows that climate science is not yet "settled science."
Of course, this is just one article among many thousands addressing aspects of man-made climate change. While its authors are members in good standing in the climate science establishment, they could be wrong. In fact, on the same day as the Nature Geoscience study was published, the United Kingdom's Met Office issued a report that says this: "After a period during the early 2000's when the rise in global mean temperature slowed…the long-term rate of global warming has now returned to the level seen in the second half of the 20th century."
The Met Office attributes the temperature slowdown in the early 21st century to natural climate variations. Specifically, the Pacific Decadal Oscillation had flipped to its cool phase, thus masking ongoing man-made global warming between 1999 and 2014. If true, this would suggest that the climate models are right after all about the long-term temperature trends and that the carbon budget is smaller than the new study calculates.
So what did the Nature Geoscience researchers do? They began by calculating what the global carbon budget should be in order to keep future temperatures from rising 1.5 degrees Celsius above the pre-industrial average. Why that level? Because the signatories to the Paris Agreement on climate change committed to "holding the increase in the global average temperature to well below 2 C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 C above pre-industrial levels."
The researchers next pointed out that the IPCC's Fifth Assessment Report, from 2013, estimated that cumulative carbon dioxide emissions since 1870 would have to remain less than 2,260 gigatons of carbon dioxide to stay below the 1.5 C threshold. But as of 2014, cumulative emissions stood at just over 2,000 gigatons of carbon dioxide. Since humanity is currently emitting about 36 gigatons of carbon dioxide annually, that implies that humanity would blow through the remaining IPCC carbon budget around 2021.
Here's where it gets interesting. The average global temperature now stands at about 0.9 C above the pre-industrial baseline, which implies that global temperature would have to increase by 0.6 C between now and 2021 if the IPCC carbon budget calculations were right. This is highly implausible since such an increase would be about 10 times faster than than what has actually heretofore been observed.
"Taking an average across ESMs [Earth systems models] suggests that our cumulative emissions to date would correspond to about 0.3 C more than best estimates of human-caused warming so far," lead author Richard Millar concludes at CarbonBrief. In the London Times another author of the paper—Myles Allen, a professor of geosystem science at the University of Oxford—said, "We haven't seen that rapid acceleration in warming after 2000 that we see in the models. We haven't seen that in the observations."
In other words, climate computer models projected the global average temperature should be about 1.2 C above the pre-industrial baseline for the 2,000 gigatons of carbon dioxide already emitted. Instead, global average temperature is only 0.9 C higher.
Running the models forward from a 2015 baseline yields a carbon budget of around 880 gigatons of additional carbon dioxide before passing through the 1.5 C threshold. That amounts to about 20 years of emissions.
Glen Peters, a senior researcher at the Center for International Climate Research in Norway, draws out some additional implications from the study. "The updated 1.5 C is more like what we expected at 2 C, and thus the updated 2 C carbon budget is probably more like we expected for 2.5 C," he notes. "Given the emissions pledges submitted to Parri Agreement are somewhat around 2.5 C to 3 C across most studies, then the new carbon budgets would imply that 2 C is roughly consistent with the current emission pledges."
But why reuse the models that have already been shown to be off by 30 percent in their projections? Again, the difference between 0.9 C above the preindustrial baseline and the 1.5 C threshold is 0.6 C. According to the National Oceanic and Atmospheric Administration, global average temperature is rising at 0.17 C per decade, suggesting that the 1.5 C temperature threshold might not be passed for 30 years. The satellite temperature measurements find that the globe is warming at the rate of 0.13 C per decade, implying that the 1.5 C threshold might not be passed for 45 years or so.
These rough temperature increase calculations imply an even larger carbon budget. That might mean that humanity could burn significantly more carbon dioxide-emitting fossil fuels without necessarily crossing the 2 C above preindustrial average temperature threshold set out in the Paris Agreement.