Most think the world will cross 1.5°C global warming between 2026-2030. This is quite defensible, as is 2031-2040.

Though, this all depends on the data set, & how the averaging is done.

IPCC SR15 has current (2017) warming at 1.0°C (running mean), & suggests 1.5°C would be exceeded in 2030-2052 at the current rate.

@hausfath has current (2020) warming at 1.2-1.4°C (not a running mean)

In an earlier analysis he suggested 1.5°C will be crossed around 2030-2032 (median)…

It is worth noting that nearly all 1.5°C scenarios first exceed 1.5°C & then go back below 1.5°C (according to SR15).

Deep mitigation now means we will in any case very likely exceed 1.5°C, unless the climate behaves in unexpected ways.

This 1.5°C exceedance point surprisingly did not come up much in the 'virtually impossible' discussion, except via @Oliver_Geden

There are not many scenarios that never exceed 1.5°C, nearly all return to below 1.5°C with large-scale CDR.

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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More from @Peters_Glen

1 Apr
THREAD "Limiting climate change to 1.5°C is now virtually impossible"

Therefore, a report that focuses on 3°C temperature rise by 2100 (2.7–3.1°C based on current climate policies).

While noting "acting early & urgently reduces the scale of the impacts"…
2. I am not sure what the fuss is about "virtually impossible"? Has anyone read the 'consensus' #IPCC #SR15?

The SPM writes 1.5°C pathways "require rapid & far-reaching transitions in energy, land, urban & infrastructure and industrial systems (𝒉𝒊𝒈𝒉 𝒄𝒐𝒏𝒇𝒊𝒅𝒆𝒏𝒄𝒆)"
3. Current "ambitions would not limit global warming to 1.5°C (𝒉𝒊𝒈𝒉 𝒄𝒐𝒏𝒇𝒊𝒅𝒆𝒏𝒄𝒆)"

Not even 'virtually', just "not" possible!

Noting, that even updated pledges so far lead to a 1% decrease in global emissions, not the required 45% reduction!
Read 12 tweets
26 Mar
'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...

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.

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)

Read 7 tweets
22 Mar
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'.

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...

As we mitigate, the land & ocean sinks take up less, but engineered sinks (eg BECCS) & afforestation increase.

If the land sink is included as an emissions source, then emissions still need to go to zero about the same time (dashed line).

Fig based on…
Read 9 tweets
19 Mar
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'.

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!

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').

Read 4 tweets
18 Mar
THREAD: Net-zero emissions

What is the the different between zero & net-zero? CO₂ or GHG?

Does net-zero mean emissions continue but are offset by Carbon Dioxide Removal (CDR)? (no)

Based on a presentation:…
2. First, why is "net-zero" needed in the first place?

Science shows that the temperature stops rising when CO₂ emissions reach (net-)zero.

There are non-CO₂ emissions, but they have a smaller (secondary) effect & declining emissions may be sufficient…
3. Zero emissions:

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)
Read 14 tweets
17 Mar
What is net-zero? Is it carbon neutral? CO₂ or GHG?

Did you know that 1.5°C scenarios reach net-zero CO₂ emissions around 2050, but net-zero GHG emissions around 2070?

For a likely below 2°C scenario, the net-zero years are shifted back about 20 years.

In fact, many likely below 2°C scenarios don't require net-zero GHG emissions until after 2100.

And why "net" and not just zero?

That is because it is hard to get all emissions completely to zero, & so some carbon dioxide removal is needed to clean up any remaining emissions.

The scale is important. How much CDR is possible?

Read 6 tweets

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