A comment published last week in Nature, a leading scientific journal, has thrown a monkey wrench into hundreds of studies and media stories that previously predicted dire climate consequences in the future due to increased carbon dioxide (CO2) in our atmosphere.

The consequences were predicted by a computer model called Representative Carbon Pathways (RCP), and the worst-case scenario model, RCP8.5, had been cited more than 2,500 times in scientific journals and in hundreds of media stories as the primary need for "urgent action" on climate.  Predictions from the RCP8.5 model suggested maximum global temperature increases of nearly 6°C (10.8°F) by the year 2100, shown in Figure 1.

Figure 1.  Image credit: Neil Craik, University of Waterloo.

In the original scientific paper, RCP8.5 had just a slim 3-percent chance of becoming reality.  Since climate alarmists (and some climate scientists) prefer to preach future doom to spur action, the predictions of RCP8.5 have become known as the "business-as-usual" scenario, even though it was nowhere close to that.

However, in a stunning walk-back, climate scientist Zeke Hausfather of the Breakthrough Institute bucked the climate consensus and said the RCP8.5 worst-case scenario is unlikely to happen.  The reason?  We can't get there, given how much fossil fuel is being used now.  The model assumes a 500-percent increase in the use of coal, which is now considered highly unlikely, since coal use has dropped significantly, as seen in Figure 2.

Figure 2.  Image credit: United States Energy Information Administration (EIA).

With this new information that excludes the worst-case RCP8.5 scenario, rather than predicting a future world that warms by 6°C (10.8°F), climate alarmists will go to the next lower scenario, RCP6, with warming by 2100 around 3°C (5.4°F).

In typical climate alarmist fashion, however, the two authors of this Nature article say the lower temperatures due to this drop-off of coal use and the exclusion of RCP8.5 aren't guaranteed.  What's the reason?  Well, scientists are still uncertain how sensitive global temperatures are to a doubling of CO2 in the atmosphere.  The value showing climate sensitivity, known as the Charney Sensitivity, still isn't known for certain — even though it's more than 40 years after it was first introduced in 1979 by the U.S. National Academy of Sciences (chaired by Jule Charney).  Charney estimated climate sensitivity to be 3°C (5.4 °F), give or take 1.5°C (2.7°F).

Without knowing the true climate warming response to increased CO2, essentially, all climate models become a crapshoot.  It is a glaring illustration of just how imprecise climate science really is.

But get this: new climate models are being used for the next set of major projections due from the Intergovernmental Panel on Climate Change next year, known as AR6.  Those models are said to show that temperatures are more sensitive to CO2 than previously thought.

Because of AR6, the higher numbers of the worst-case scenario are likely to be back on the table, along with continued calls for climate action in the form of reductions, alternate tech, and carbon dioxide taxation.

There is another fly in the ointment: even if the atmosphere turns out to be more sensitive to CO2 than they thought, it is unlikely that the world will ever get to a doubling for CO2 in the atmosphere — the level on which climate sensitivity estimates are based.

Climate scientist Dr. Roy Spencer did a model calculation the same week as this new Nature article was released and discovered something very surprising. Using data from the Energy Information Administration (EIA) projecting that energy-based emissions of CO2 will grow at 0.6 percent per year until 2050, Spencer plugged that data into a climate model.  With the reasonable EIA assumptions regarding CO2 emissions, the climate model does not even reach a doubling of atmospheric CO2, but instead reaches an equilibrium CO2 concentration of 541 ppm in the mid-2200s.

Spencer writes:

[T]he result is that, given the latest projections of CO2 emissions, future CO2 concentrations will not only be well below the RCP8.5 scenario, but might not even be as high as RCP4.5, with atmospheric CO2 concentrations possibly not even reach a doubling (560 ppm) of estimated pre-Industrial levels (280 ppm) before leveling off.  This result is even without future reductions in CO2 emissions, which is a possibility as new energy technologies become available."

The RCP4.5 scenario suggests a range of warming of about 1.7–3.2°C (3–5.8°F), which doesn't constitute a "climate emergency" and may even be beneficial to humankind.  After all, humanity didn't do well during cold periods in history, and another global ice age would certainly be ruinous.

With this broad uncertainty about what the future climate will be, the bottom line on climate science predictions is well served by the great Yogi Berra, who famously said, "It's tough to make predictions, especially about the future."

Anthony Watts (mailto:think@heartland.org) is former television meteorologist and senior fellow for environment and climate at The Heartland Institute.  He operates the most viewed website on climate in the world, WattsUpWithThat.com.