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grundar t1_jdw5bsu wrote

Reply to comment by snikZero in There Is Still Plenty We Can Do to Slow Climate Change by nastratin

> The (b) table doesn't seem to show a reduction in temperatures even under the most optimistic case.

The (b) table is looking at the change relative to the late 1800s.

For change relative to other periods, look at p.14, Table SPM.1, Scenario SSP1-1.9, Best Estimate:

  • Near term (2021-2040): 1.5C
  • Mid-term (2041-2060): 1.6C
  • Long term (2081-2100): 1.4C
    i.e., 0.2C estimated temperature decrease between mid-term and long term intervals.

Note that SSP1-1.9 reaches net zero CO2 in ~2057 (p.13, Figure SPM.4), so the end of the mid-term interval. In other words, 20-40 years of increasingly net negative CO2 emissions are projected to result in 0.2C lower temperatures.

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snikZero t1_jdwf4fd wrote

Ah, you are correct. This isn't clear from the table on P13, the 'total warming' note underneath makes that ambiguous.

I would note though that SSP1-2.6 also describes a net zero by 2050 followed by negative emissions, but still sees a temperature increase by 2100.

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grundar t1_jdwhh1x wrote

> I would note though that SSP1-2.6 also describes a net zero by 2050

SSP1-2.6 doesn't reach net zero until 2075 (p.13), so it has significant net positive cumulative emissions between 2050 and 2090 (the 2041-2060 and 2081-2100 intervals).

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snikZero t1_jdwka51 wrote

I think you're looking at the CO2 output only graph, i suspect perhaps in aggregate they provide net zero for 2050.

In P12 Box SPM.1.1, and P14 note 25 (explicitly), both state net zero for that date.

It's also possible the graph doesn't align to the notes due to an error.

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grundar t1_jdx1ggj wrote

> In P12 Box SPM.1.1, and P14 note 25 (explicitly), both state net zero for that date.

I believe you're misreading; from p.12 Box SPM.1.1:
>> "scenarios with very low and low GHG emissions and CO2 emissions declining to net zero around or after 2050, followed by varying levels of net negative CO2 emissions23 (SSP1-1.9 and SSP1-2.6), as illustrated in Figure SPM.4."

Both are net zero around or after 2050.

Similarly for p.14 Note 25:
>> "SSP1-1.9 and SSP1-2.6 are scenarios that start in 2015 and have very low and low GHG emissions, respectively, and CO2 emissions declining to net zero around or after 2050, followed by varying levels of net negative CO2 emissions."

Both locations clearly note that the scenario may reach net zero after 2050.

> I think you're looking at the CO2 output only graph, i suspect perhaps in aggregate they provide net zero for 2050.

None of the other GHG graphs reach net zero even by 2100, so net zero GHG emissions always occurs after net zero CO2 emissions.

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snikZero t1_jdxqzsf wrote

You make good points. I considered that the increase in aerosols for that projection might have influenced the net-zero date, but I see from the 1900's comparison graphs that they have a lesser relative effect.

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