E - Net Zero and Climate change

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E - Net Zero
Net Zero TOC

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2010 Projecting temperatures under constant concentrations vs. net zero
2018 Temperature implications of alternative ways of characterizing net-zero
Even in a world of zero CO2 emissions, however, there are large remaining uncertainties associated with what happens to non-CO2 greenhouse gases (GHGs), such as methane and nitrous oxide, emissions of sulphate aerosols that cool the planet and longer-term feedback processes and natural variability in the climate system.
Finally, while current best estimates suggest that temperatures will stabilise in a zero-emissions world, that does not mean that all climate impacts would cease to worsen. Melting glaciers and ice sheets and rising sea levels all occur slowly and lag behind surface temperature warming. A zero-emissions world would still result in rising sea levels for many centuries to come, with some estimates suggesting that at least 80cm of additional sea level rise is “locked in”.
However, a world of constant concentrations is not one of zero emissions. Keeping concentrations constant would require some continued emissions to offset the CO2 absorbed by the land and oceans. This would amount to around 30% of current global emissions, although the amount needed would fall over time. If emissions are cut to zero, on the other hand, atmospheric concentrations of CO2 would quickly fall, before eventually stabilising at a lower level.
Human emissions of aerosols – tiny particles of sulfur or nitrogen suspended in the atmosphere that reflect incoming sunlight back to space – have a strong cooling effect on the planet, though there are large uncertainties as to exactly how large this effect is. Aerosols also have a relatively short atmospheric lifetime and, if emissions cease, the aerosols currently in the atmosphere will quickly fall back out. As a result, the world would be around 0.4C warmer if CO2 and aerosol emissions go to zero, compared to zero CO2 emissions alone. In this scenario (red line), the world would likely exceed the 1.5C target, reaching around 1.75C by 2100.
Moreover, temperatures are expected to remain steady rather than dropping for a few centuries after emissions reach zero, meaning that the climate change that has already occurred will be difficult to reverse in the absence of large-scale net negative emissions.
Temperature doesn’t fall for a long time even under net zero due to the warming impact of the oceans. The additional surface warming from the oceans continuing to heat up is balanced by the cooling from falling atmospheric CO2.
The best available evidence shows that, on the contrary, warming is likely to more or less stop once carbon dioxide (CO2) emissions reach zero, meaning humans have the power to choose their climate future.
The confusion around the impact of zero emissions is understandable. Even a NASA climate FAQ – last updated in 2007 – still says that “even if we stopped emitting greenhouse gases today, global warming would continue to happen for at least several more decades, if not centuries”. (NASA’s Dr Gavin Schmidt tells Carbon Brief that this wording does not reflect more recent research and an update is in the works).
The studies featured in this piece all look at the effects of zero-emissions scenarios today or in the next few decades. If, however, zero emissions were to occur later in the century, there is the potential to lock in more carbon-cycle feedback processes – such as melting permafrost – than under current global temperature levels. A world that has warmed by 3C or 4C above pre-industrial levels may lock in more committed future warming than today’s world – and more research is needed to explore these effects.
The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) used 18 different ESMs to simulate what would happen in a world where global emissions suddenly ceased after the world had emitted a total 3667 Gigatons of CO2 (or 1000 Gigatons of carbon) – which would be expected to result in warming of around 2C compared to preindustrial levels. It also examined a case where emissions gradually decreased to net zero and found similar results to an abrupt cutoff after net-zero is reached.
These “constant concentration” scenarios showed that there was additional warming “in the pipeline” as the oceans slowly warmed up to reach the same temperature as the atmosphere and brought the Earth back into radiative equilibrium. That is to say, where the amount of energy absorbed by the Earth from the sun is equal to the amount being reradiated back to space. Models tended to suggest 0.4C to 0.5C or so of additional warming would occur over the next few centuries, if concentrations were kept at the same level.
When scientists have pointed this out recently, it has been reported as a new scientific finding. However, the scientific community has recognised that zero CO2 emissions likely implied flat future temperatures since at least 2008. The Intergovernmental Panel on Climate Change (IPCC) 2018 special report on 1.5C also included a specific focus on zero-emissions scenarios with similar findings.

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