There are not many scenarios that never exceed 1.5°C, nearly all return to below 1.5°C with large-scale CDR.
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The 'virtually impossible' discussion has many layers: 1. The climate response 2. Ability to perform immediate & rapid global emission reductions (still with CDR later) 3. Failing 2, the ability to have CDR at a scale which compensates for failing on 2 (IMHO, highly unlikely)
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'Net' emissions are a slippery slope, but we already deal with net emissions. It is not so scary...
In most Annex I countries LULUCF emissions are a net-sink. The sink is mainly forest regrowth & recovery.
Net emissions have been here since 1990, at least...
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In the EU, most of the sink is increased uptake in existing forests, there is a small part of afforestation (dark green). There are also emission sources, such as from grasslands & new settlements.
Maintaining the sink over time (with climate impacts) could be hard.
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The EU27 now includes the land sink (LULUCF) in its climate targets.
Perhaps this is good? It forces the EU to maintain & expand its sink.
Perhaps this is bad? The EU can now have 'net-zero' emissions in 2050 (though, studies suggest this is mainly agricultural)
Historically, the land & ocean sink have removed about one-half of the anthropogenic CO₂ emissions.
If we mitigate successfully in the future, the sinks will take up less CO₂ since emissions are lower, but they will be replaced by 'engineered sinks'.
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This is a more detailed figure showing the anthropogenic CO₂ emission sources (top), & the land and ocean sinks (with the balance remaining in the atmosphere). Bread & butter carbon cycle...
IPCC: "In model pathways with no or limited overshoot of 1.5°C, global net anthropogenic CO₂ emissions [reach] net zero ~2050 (2045–2055)"
There are likely equally plausible scenarios (shown here) that reach net-zero CO₂ emissions in 2100 with the same 'carbon budget'.
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You don't believe me?
These are the scenarios used for net-zero ~2050 (2045-2055). They basically all cross zero around 2050. This is because they focus on 2100 targets & allow 'overshoot'.
This is a design feature of the scenarios, & are not the only way to get to 1.5°C!
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The temperature response to those scenarios all have a 'peak & decline' shape. Some of the 'peak & decline' is due to CO₂ emissions & some to non-CO₂ emissions (GWP100 confuses this point).
Most modelling uses a 2100 target, allowing overshoot (a cost-effective 'feature').
If we take a remaining carbon budget consistent with 1.5°C, then emissions need to drop rapidly. This curve converges to zero, there is no physical reason to have a straight line to zero.
(I took 580GtCO₂ from SR15 Table 2.2, not adjusted for time past)