What kind of annual sustained global emissions reductions would be necessary to have a 50-50 chance of limiting global warming to 1.5 degrees? [0]

2018 estimates of the carbon budget remaining for a 50% chance of less than 1.5C warming: 200 Gt CO_2-e -- 800 Gt CO_2-e [1]. Lots of uncertainty, I will ignore tails on both sides. Assume current global emissions are around 35 Gt CO_2-e / year [2][3]. Since it is now 2021, not 2018, subtract about 105 Gt CO_2-e from the remaining budget, giving a range of around 95 -- 695 Gt CO_2-e remaining.

Crude estimate of years remaining until carbon budget for 50% chance of limiting warming to 1.5C is exhausted: 2 years -- 20 years, assuming steady 35 Gt CO_2-e / year emissions.

Let's make things easier and relax the target and aim to reduce emissions to 5% of current levels -- a massive drop of 95%, not the full 100%. This relaxed target does not try to achieve net zero emissions, but to merely get us into the ballpark of where that a zero net emissions goal could perhaps be reachable with enough additional effort.

Supposing we are unlucky and reality is consistent with the scenario where we have two years of carbon budget remaining. To stay within the relaxed budget we would need to consistently sustain 78% annual drop in emissions per year for two years.

Supposing we are lucky and reality is consistent with the scenario where there are twenty years of carbon budget remaining. To stay within the relaxed budget we would need to consistently sustain 14% annual drop in emissions per year for twenty years.

What might sustained 14% annual drops in emissions feel like? It seems like world annual emissions dropped 7% in 2020 vs 2019 [3]. In the trailing decade 2010-2019 we were seeing about 1% annual growth in emissions. So if we assume Covid is entirely responsible for that difference, that's a 8% emissions reduction -- perhaps transient that will bounce back -- from a one-off event. If we consistently had two different Covid-scale economic shocks every year for twenty years, each with a corresponding _permanent_ reductions in emissions, that might suffice to maintain the 14% _permanent_ annual drop in emissions necessary to reduce emissions to 5% of current levels by 2041.

To benchmark against reality and pick on my own country: Australia's federal government reckons it is on track to reduce emissions to 480 Mt CO_2-e by 2030, from 540 Mt CO_2-e in 2019 [4]. This corresponds to sustaining a 1.1% annual drop in emissions for eleven years.

[0] I don't think limiting warming to at most 1.5 degrees C is a goal that is intentionally reachable from within the current human system. It is worth doing some back of the envelope accounting exercise every now and again to get a feel for the remaining carbon budget & the scale of the challenge.

[1] https://www.carbonbrief.org/analysis-how-much-carbon-budget-...

[2] I am being optimistic and rounding down to 35 Gt from 36.4 Gt in 2019 to make the numbers easier. Let's ignore 2020's data as not representative of what humans do in a typical year, due to the pandemic shock, although hopefully some of the behaviour change will be sticky (WFH, etc).

[3] https://www.globalcarbonproject.org/global/images/carbonbudg...

[4] https://www.industry.gov.au/data-and-publications/australias...