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Quick action to reduce carbon dioxide and other greenhouse gases in the atmosphere could show “discernible effects” within years, but humanity has only about a dozen years at its current rate of emissions before a narrow window on a 1.5°C future closes, according to carbon budget calculations in yesterday’s science assessment from the Intergovernmental Panel on Climate Change (IPCC).
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“Every tonne of CO2 emissions adds to global warming,” the assessment states. So “from a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net-zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.”
But the IPCC’s recognition of a “near-linear relationship” between humanity’s greenhouse gas (GHG) emissions and global warming comes as the available atmospheric space for further carbon pollution is close to running out.
• To have a 50% chance of hitting 1.5°, carbon emissions from 2020 on would have to be limited to 500 billion tonnes.
Annual global emissions stood at 43 gigatonnes in 2019, the last year before the pandemic produced a small, temporary reduction in GHGs, enough to blow past the 500-Gt limit in less than 12 years unless emissions fall quickly and dramatically.
• For just a one in six chance of holding average global warming to 1.5°C—as Climate Action Network-International Senior Adviser Stephan Singer put it, “playing Russian roulette with the climate but with five bullets”—future emissions would have to be held to 900 billion tonnes of carbon dioxide. That same limit would produce a five in six chance of a 2.0°C future.
Shifts in other greenhouse gases like methane and nitrous oxide could open up more atmospheric space, or leave less of it available.
To improve the odds, the IPCC turns to carbon dioxide removal (CDR) as a strategy to pull carbon pollution out of the atmosphere and “durably store it in reservoirs”. While the report doesn’t get into the politics of CDR, with fossil companies treating it as a lifeline to keep on extracting oil, gas, and coal, the numbers in this report indicate removals are urgently needed on top of drastic reductions in new emissions from all sources.
“CDR aims to compensate for residual emissions to reach net-zero CO2 or net-zero GHG emissions,” the assessment states, or to bring down surface temperatures if removals eventually exceed humanity’s emissions.
But the approach isn’t simple, nor will it address all climate impacts. “CDR methods can have potentially wide-ranging effects on biogeochemical cycles and climate, which can either weaken or strengthen the potential of these methods to remove CO2 and reduce warming, and can also influence water availability and quality, food production, and biodiversity,” the IPCC acknowledges.
Reaching and sustaining net-negative emissions would help reverse ocean acidification, the report states, and “the global CO2-induced surface temperature increase would be gradually reversed, but other climate changes would continue in their current direction for decades to millennia.” Even with carbon removals that far exceeded emissions, for example, “it would take several centuries to millennia for global mean sea level to reverse course”.
But even so, the IPCC points to a stark difference between its low- and high-emission scenarios, underscoring once again that every megatonne of emission reductions is essential and worth fighting for.
Dropping emissions to what the scientists consider low or very low levels “would have rapid and sustained effects to limit human-caused climate change, compared with scenarios with high or very high GHG emissions,” the assessment states, even though it would be hard at first to distinguish those effects from year-to-year variations in different climate impacts, the assessment states. Shifts in global surface temperature “would likely emerge during the near term under a very low GHG emission scenario”, and humanity would see “substantially smaller changes” in a series of “climate-impact drivers” (CIDs) beyond 2040.
“By the end of the century, scenarios with very low and low GHG emissions would strongly limit the change of several CIDs, such as the increase in the frequency of extreme sea level events, heavy precipitation and pluvial flooding, and exceedance of dangerous heat thresholds, while limiting the number of regions where such exceedances occur, relative to higher GHG emissions scenarios.”
Those shifts would also improve air quality and human health, though not enough on their own to meet World Health Organization guidelines in the world’s most polluted regions. Those results will be better, the scientists say, where countries set out to reduce other forms of pollution in tandem with GHGs.
