THREAD: Does rounding change 1.5°C carbon budgets?
Short answer: Yes
If the temperature is rounded 0.05°C, then the carbon budget changes 100GtCO₂ (~25% of the remaining carbon budget for 1.5°C)…
That is the tweet, now for the explanation…
1/
Short answer: Yes
If the temperature is rounded 0.05°C, then the carbon budget changes 100GtCO₂ (~25% of the remaining carbon budget for 1.5°C)…
That is the tweet, now for the explanation…
1/
2. Global average temperature has a near linear relationship with cumulative CO₂ emissions, with a correction for non-CO₂ emissions.
This means we need net-zero CO₂ emissions.
It is also great for back-of-the-envelope calculations…
cicero.oslo.no/no/posts/klima…
This means we need net-zero CO₂ emissions.
It is also great for back-of-the-envelope calculations…
cicero.oslo.no/no/posts/klima…
3. Equations:
T = α ×ΣCO2+ T(non-CO₂)
α is the Transient Climate Response to CO₂ Emissions (TCRE)
Assume a small change, so that T(non-CO₂) is ~constant
ΔT = α×Δ(ΣCO2)
(here is some explanation, sorry for different notation)
rdcu.be/cdO5Q
T = α ×ΣCO2+ T(non-CO₂)
α is the Transient Climate Response to CO₂ Emissions (TCRE)
Assume a small change, so that T(non-CO₂) is ~constant
ΔT = α×Δ(ΣCO2)
(here is some explanation, sorry for different notation)
rdcu.be/cdO5Q
4. This is the most important equation in climate science…
ΔT = α×Δ(ΣCO2)
rearrange
Δ(ΣCO2) = ΔT / α
α = 0.44°C & 5–95% range of 0.32–0.62 °C per 1000 GtCO₂
nature.com/articles/s4324…
ΔT = α×Δ(ΣCO2)
rearrange
Δ(ΣCO2) = ΔT / α
α = 0.44°C & 5–95% range of 0.32–0.62 °C per 1000 GtCO₂
nature.com/articles/s4324…
5.
Δ(ΣCO₂) = ΔT / α = 0.05/0.44*1000 = 114GtCO₂
If temperature is rounded 0.05°C (is has uncertainty of that size, which it does), then there goes 100GtCO₂
If the remaining carbon budget is 440GtCO₂, then that is 25%... Just in rounding…
nature.com/articles/s4324…
Δ(ΣCO₂) = ΔT / α = 0.05/0.44*1000 = 114GtCO₂
If temperature is rounded 0.05°C (is has uncertainty of that size, which it does), then there goes 100GtCO₂
If the remaining carbon budget is 440GtCO₂, then that is 25%... Just in rounding…
nature.com/articles/s4324…
6. If we have 0.2°C of CO₂ warming to 1.5°C (allowing say 0.1°C for non-CO₂), then the remaining budget is
0.2/0.44*1000 = 450GtCO₂
If α is 0.4 or 0.5, then the remaining carbon budget goes from 400 to 500GtCO₂.
Just throw around 100GtCO₂ in some rounding!
0.2/0.44*1000 = 450GtCO₂
If α is 0.4 or 0.5, then the remaining carbon budget goes from 400 to 500GtCO₂.
Just throw around 100GtCO₂ in some rounding!
7. I usually play with this another way.
If we go from 1.5°C to 2°C, 0.5°C extra, then
0.5/0.44*1000 = 1100GtCO₂ (though, now non-CO₂ becomes relevant)
That is a lot of extra CO₂, 0.5°C makes a huge difference to mitigation!
cicero.oslo.no/no/posts/klima…
rdcu.be/cdO5Q
If we go from 1.5°C to 2°C, 0.5°C extra, then
0.5/0.44*1000 = 1100GtCO₂ (though, now non-CO₂ becomes relevant)
That is a lot of extra CO₂, 0.5°C makes a huge difference to mitigation!
cicero.oslo.no/no/posts/klima…
rdcu.be/cdO5Q
8. Uncertainties for 1.5°C carbon budgets are huge.
No problem with the climate science, we are just so close to 1.5°C, so uncertainties get amplified.
It is not just me saying this, even the experts say this...
nature.com/articles/s4324…
No problem with the climate science, we are just so close to 1.5°C, so uncertainties get amplified.
It is not just me saying this, even the experts say this...
nature.com/articles/s4324…
9. “need to move on from a 𝐮𝐧𝐢𝐪𝐮𝐞 carbon budget & face the nuances” (uncertainties)
rdcu.be/bHT2C
“The high-profile cumulative carbon [budget] concept carries several & significant uncertainties”
rdcu.be/cdO5Q
But, what would I know...
rdcu.be/bHT2C
“The high-profile cumulative carbon [budget] concept carries several & significant uncertainties”
rdcu.be/cdO5Q
But, what would I know...
10. The carbon budget & cumulative emissions concepts are great for elevator pitches, back of envelope calculations, framing discussions, etc. I use them all the time!
But, they have lots of uncertainties that get buried behind "we have X years to death" framings
But, they have lots of uncertainties that get buried behind "we have X years to death" framings
11. As an aside: “If these uncertainties are combined using simple uncorrelated error propagation, the remaining [budget] from 2016 for 2°C with 66% likelihood could be 850±450 GtCO₂ to one standard deviation”
Not bad for 2016 back-of-envelope:
rdcu.be/cdO5Q
/end
Not bad for 2016 back-of-envelope:
rdcu.be/cdO5Q
/end
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