Full climate change report shows bigger challenges than the summary

Source: Chelsea Harvey, E&E News reporter • Posted: Tuesday, October 16, 2018

The landmark report on rising temperatures last week proposed unprecedented actions to slow down climate change. What if it understated the concern among scientists?

There are signs it did. A longer version of the report by the Intergovernmental Panel on Climate Change implies a much greater chance that the world will exceed an increase of 1.5 degrees Celsius — and then have to lower Earth’s temperatures.

The summary for policymakers, an overview of the full report’s scientific findings, notes that global net carbon emissions should shrink to zero by 2050. That means emissions would need to fall by about 50 percent in the next 12 or 13 years to stay on track. The summary outlines a variety of pathways, just about all of which imply some level of carbon dioxide removal — that is, using carbon-absorbing forests or other forms of technology to draw the gas out of the atmosphere.

The summary implies a Herculean effort. It notes that scenarios to avert an overshoot of the 1.5 C target would require “rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems” and would be “unprecedented in scale.”

But some experts have pointed out that the full report, hundreds of pages long in total, may suggest an even greater challenge than the summary would imply. They point to a greater emphasis on carbon dioxide removal and a higher probability of overshooting the 1.5 C threshold. This suggests an even more urgent need for immediate global action to meet the target.

Scenarios to keep the world below the 1.5 C threshold, or that involve only a small amount of overshoot before bringing temperatures back down, are more limited than the summary would suggest, according to Oliver Geden, a climate researcher at the German Institute for International and Security Affairs. In a recent Twitter thread, he elaborated on the problem.

The summary for policymakers only addresses pathways that would be consistent with no or limited overshoot scenarios. But the full report outlines a total of 78 pathways, each ending in net-zero carbon emissions sometime around midcentury. Thirty-six of them result in a big temperature overshoot. Another 37 scenarios result in a smaller overshoot, and just five end in no overshoot at all.

These outcomes are calculated on a 50 percent probability of success. In other words, scenarios classified as “no overshoot” are “more likely than not” to occur, as defined by IPCC terminology.

The full report is fairly transparent about this. It notes that “importantly, 1.5°C-consistent pathways allow a substantial (up to one-in-two) chance of warming still exceeding 1.5°C.” But overall, the odds of avoiding 1.5 C of warming, with no overshoot at all, may be much slimmer than the summary for policymakers might suggest.

“If one would like to exclude overshoot completely, the result would be that 1.5C is infeasible,” Geden said in an email.

The IPCC report acknowledges that exceeding the 1.5 C target, even temporarily, could carry additional climate consequences. These may include higher sea-level rise, as well as changes in weather, agricultural impacts, and threats to coral reefs and other ecosystems. That means attempting to avoid overshoot, or keeping the overshoot as small as possible, is increasingly important.

The IPCC report is also very clear that some degree of carbon dioxide removal, or CDR, is likely. However, the full report could say that more CDR is needed than the summary might imply. That’s important, because many CDR proposals haven’t been proved on a large scale.

“The SPM [summary for policymakers] states that conventional mitigation is not enough, and there’s an additional need for carbon dioxide removal,” Geden said. “Compared to the full report, the SPM paints a picture too rosy on this.”

The summary suggests that anywhere from 100 to 1,000 billion metric tons of carbon dioxide will need to be removed from the atmosphere by the end of the century in order to meet the 1.5 C target with no or limited overshoot. The exact amount depends on the pathway that policymakers choose to follow.

These pathways may include the planting of forests, the use of bioenergy with carbon capture and storage, or other forms of carbon sequestering technology. The report also notes that higher levels of CDR come with more uncertainty about their impacts on the planet — vast bioenergy plantations, for example, might draw large amounts of CO2 out of the air, but they might also require unsustainable levels of land and water resources.

That’s a concern because the full report suggests that a majority of the pathways to 1.5 C require CDR on the higher end of the spectrum of possibilities. While it’s not mentioned in the summary, the full report suggests that the median deployment of CDR across all the pathways is around 770 billion tons. That’s not to say it’s impossible to succeed with lower amounts — the report does include scenarios as low as 100 billion tons of CDR — but more of them rely on higher levels than lower levels.

It’s worth noting that the IPCC report does not predict the actions of individual governments, and it doesn’t account for the possibility of a U.S. withdrawal from the Paris climate agreement. But if the U.S. did fail to start curbing its emissions within the next decade, while the rest of the world took measures to meet the 1.5 C target, then the need for carbon dioxide removal would likely be greater. Currently, the U.S. emits more than 5 billion tons of carbon dioxide each year.

Brazilian presidential candidate Jair Bolsonaro has also threatened to withdraw his nation from the Paris Agreement.

Ultimately, the actions taken to limit global warming will depend on the collective efforts of nations worldwide. The IPCC report simply advises them of their options, and the summary for policymakers is intended to provide an easy overview of the scientific literature assessed in the full report. In this case, though, it may be that some of the challenges to achieving the Paris Agreement’s most ambitious target are either understated or unclear in the summary.

It’s also not the only case in which some experts have suggested the report may be guilty of downplaying the risk of climate change. Some scientists have also argued that the report’s conclusions on the dangers of future warming may have also been too conservative (Climatewire, Oct. 11).