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Countries must make sure global climate pollution peaks as swiftly as possible and then steadily declines, rather than overshooting targets for global temperature rise and counting on exotic technologies to draw them back down, a trio of climate scientists concluded in a recent webinar.
With insufficient knowledge on carbon dioxide removal (CDR) techniques meant to reverse overshoot, and solar radiation management (SRM) dangerously ill-advised, rapid decarbonization is the only safe, sustainable path to stabilizing the global climate at livable temperatures, concluded the expert panelists from PROVIDE (Paris Agreement Overshooting Reversibility, Climate Impacts, and Adaptation Needs), an international research team assessing the risks and potentially irreversible impacts of overshooting the targets in the 2015 Paris climate agreement.
Joerj Rogelj, director of research at the Grantham Institute, opened with what he described as a “very bleak picture” of where global warming is heading.
“There is a relatively low chance, given the NDCs [Nationally Determined Contributions, countries’ emission reduction promises under the Paris accord] that are currently on the table, that warming will be limited to below 1.5 or even 2.0°” by the end of the century, Rogelji said, pointing to a yawning “credibility gap” between legally binding policies and net-zero promises.
“To be clear from the start, we are not on track. Emissions reductions need to be strengthened urgently,” he told participants.
But policy-makers now pondering the merits of allowing for “overshoot,” convinced that CDR technologies will bring global temperatures down shortly after they exceed the 1.5°C limit, should think twice, Rogelj warned, not least because it will be a “very long” time before science is in a position to clearly confirm that the 1.5°C threshold has been crossed.
While current best estimates put global heating at around 1.26°C, significant uncertainties around that figure mean humanity may already be committed to more than 1.5° of warming. Or the Earth could be “well beneath” that critical threshold.
That degree of uncertainty means there is “no clear cut-off time when we can say in an easy and straightforward way that we have reached 1.5 and are now clearly beyond it,” he said.
Rogelj cited “three main tasks” for CDR technologies in large-scale emission reduction portfolios. “The first is to help accelerate reductions in the near term, in combination with very stringent reductions in emissions,” a role that would “help us limit maximum warming to as low levels as possible.”
CDR “also helps us achieve net-zero, which is necessary to stop warming from further increasing, and finally, if CDR is further scaled up, we can achieve net negative CO2 emissions, which would lead to a slow reversal in global warming.”
In an overshoot scenario, however, Rogelj said there is “quite a bit of overconfidence” about what CDR technology can achieve, as well as the geophysical limits it will face.
“Very often, when we look at pathways, we just consider the median behaviour of what we know is physically possible,” he warned. But that’s dangerous because “we know that overshoot can be much higher than we are currently planning for. And we also know the reversal or reduction in temperature after overshoot can be much harder than just the median average behaviour that we are currently planning for.”
Rogelj called for a “much more cautious approach” than overshoot-CDR scenarios that amount to “aiming to fail and then hoping for the best.” The only alternative, he said, is “a peak and decline where we first focus on minimizing damages by keeping peak warming as low as we possibly can, then afterwards consider a return to safer pastures by achieving the sustainable levels of net-negative CO2 emissions that we can achieve.”
Sabine Fuss, head of the Sustainable Resource Management and Global Change working group at the Mercator Research Institute on Global Commons and Climate Change, warned that the “scenario space is wide”, meaning that CDR might be called upon to deal with 10 and 20 gigatonnes of carbon dioxide per year in the second half of the century.
If concerns about the sustainability (and viability) of CDR are excluded, the outlook for removals can look quite rosy, Fuss noted, with upper projections by 2050 of 40 gigatonnes for direct air capture, 11 for biomass with carbon capture and storage (BECCS), 10 for afforestation and reforestation, and lesser amounts for soil carbon sequestration, biochar, and enhanced weathering.
But those numbers implode spectacularly once sustainability metrics like land use, water consumption, and environmental justice are factored in, with DAC leading the pack at just five gigatonnes at mid-century.
Faced with such “limited” CDR potential, policy-makers will need to “reduce gross emissions as much as possible” while scientists conduct a great deal more research into what constitutes an optimal CDR portfolio, Fuss said.
That means whatever sustainable CDR is available must be applied to long-term temperature decline, not overshoot scenarios, she added. While 50 years of overshoot above 1.5°C would “commit to around 20 centimetres of additional sea level rise by 2300,” long-term temperature decline could see a reduction by “almost half a metre” by that year.
Benjamin Sanderson, senior research director at the Center for International Climate Research in Oslo (CICERO), declared that there are no merits to SRM. With technologies like atmospheric seeding of sulphate particles using “thousands of aircraft every single day”, the likely result would be catastrophic planetary losses, including the precipitation patterns that sustain the Amazon.
And since SRM amounts to creating another form of climate change, he said it would also result in the catastrophic loss of whatever solid understanding scientists have of the Earth’s climate as it now functions.
It is “quite feasible” that regions currently suffering the impacts of CO2-driven climate change could simultaneously begin to suffer impacts from SRM, he warned, and it would be “quite difficult to disentangle the two.”
