From The World’s Biggest Gamble by Johan Rockstöm et al. (here)
The scale of the decarbonisation challenge to meet the Paris Agreement is underplayed in the public arena. It will require precipitous emission reductions and a new carbon sink on the scale of the ocean sink within 40 years. Even then, the world is extremely likely to overshoot. A catastrophic failure of policy, for example waiting another decade for transformative policy and full commitments to fossil-free economies, will have irreversible and deleterious repercussions for humanity’s remaining time on Earth. Only a global zero carbon roadmap will put the world on a course to phase-out greenhouse gas emissions and create the essential carbon sinks for Earth-system stability, without which, world prosperity is not possible.
The 450 Scenario
Background for this and other such climate alarms is an activist assumption not often openly proclaimed because it is so clearly a leap of faith, not science.
Now we come to an interesting bait and switch. Since Cancun, IPCC is asserting that global warming is capped at 2°C by keeping CO2 concentration below 450 ppm. From Summary for Policymakers (SPM) AR5
Emissions scenarios leading to CO2-equivalent concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels. These scenarios are characterized by 40 to 70% global anthropogenic GHG emissions reductions by 2050 compared to 2010, and emissions levels near zero or below in 2100.
Thus is born the “450 Scenario” by which governments can be focused upon reducing emissions without any reference to temperature measurements, which are stubbornly failing to rise alarmingly.
Within the international expert community, “2 degrees” is generally used as shorthand for a low carbon scenario under which CO2 concentrations in the earth’s atmosphere are stabilized at a level of 450 parts per million (ppm) or lower, representing approximately an 80% reduction in greenhouse gas emissions from current levels, which according to certain computer simulations would be likely to limit warming to 2 degrees Celsius above pre-industrial levels and is considered by some to reduce the likelihood of significant adverse impacts based on analyses of historical climate variability.
Clever as it is to substitute a 450 ppm target for 2°C, the mathematics are daunting. Joe Romm:
We’re at 30 billion tons of carbon dioxide emissions a year — rising 3.3% per year — and we have to average below 18 billion tons a year for the entire century if we’re going to stabilize at 450 ppm. We need to peak around 2015 to 2020 at the latest, then drop at least 60% by 2050 to 15 billion tons (4 billion tons of carbon), and then go to near zero net carbon emissions by 2100.
And the presumed climate sensitivity to CO2 is hypothetical and unsupported by observations:
Calvin Beisner at CFACT (here) provides a full rebuttal of the alarmist logic embodied in the above “Gamble” paper.
First, predictions of rapid warming from enhanced atmospheric greenhouse gas concentration are based on computer climate models that “run hot,” two to three times the warming actually observed over relevant periods, and therefore provide no rational basis for predicting future GAT.
Second, nobody—but nobody—has demonstrated scientifically that global average temperature (GAT) is optimal up to 2°C above the pre-industrial average (the limit aimed for by the Paris agreement) , or that GAT higher than that will even be net harmful, let alone catastrophic.
Third, the aim of “Earth-system stability” is scientifically absurd—undefined, unnatural, and unachievable. Natural systems—especially coupled non-linear chaotic fluid-dynamic systems like Earth’s climate—are not, never have been, and never will be stable.
Fourth, it is sheer fear-mongering without a shred of scientific evidence to say that “waiting another decade for transformative policy and full commitments to fossil-free economies” would be a “catastrophic failure of policy” with “irreversible and deleterious repercussions for humanity’s remaining time on Earth”.
Fifth, even if real scientific investigation (which doesn’t stop with modeling but tests models by empirical observation) could tell us what future temperatures will be and the positive and negative impacts, that wouldn’t tell us how we ought to respond, i.e. what policies to pursue. While science can inform policymakers, it can’t determine policy.
Even assuming the IPCC’s exaggerated estimates of CO2’s warming effect and assuming full implementation of all nations’ commitments under the Paris agreement, at a cost of $1 to $2 trillion annually from 2030 onward, Björn Lomborg calculates it would reduce GAT in 2100 by just 0.17° C, an amount too little to be noticed or have significant impact.
It is more evidence of Climateers Tilting at Windmills