These current policy or NDC projections have been a mainstay of the @UNEP Emissions Gap Report & @climateactiontr.
These approaches have been based on statistical matching with existing scenario databases.
We wanted to used models to extend 2030 policies & pledges to 2100.
3/
(noting that EGR now does a different approach more similar to our paper, other projects have extended current policies & NDCs such as in CD-LINKS & ADVANCE, the difference is we focus on the issue)
/3b
But, how do you extrapolate policy effort from 2030 to 2100, & in a way that can cover the mechanics of a diverse set of models.
We came up with two approaches, extrapolating based on carbon price extensions & extrapolating based on emission intensity projections.
4/
Even though we harmonised many input data & assumptions across out models, there is a very large spread in temperature outcomes.
We used a diverse set of models, so focused on CO₂ emissions, had to extend some to 2100, & estimated temperature using simplified approaches.
5/
But why the spread in results?
Different models have different baseline 'no climate policy' emissions (red) & different response to climate policy (yellow), & these factors dominated the differences.
6/
We also took some effort to try & explain the results, here for final energy, but without dedicated & specific model-by-model analysis it is perhaps hard to isolate differences. We did not find any strong evidence of model type, structure, etc, pushing results either way.
7/
The first question on many lips is "how much CCS" (well, my first question).
We run each model with current policies, & a carbon price which gives the same emissions as the current policies, & found that the carbon price leads to much greater deployment of CCS.
8/
This indicates that the evolution of the energy system is very dependent on the types of policies implemented, notably whether a carbon price versus a regulation, standard, incentive, etc.
Most scenarios are based around carbon prices, & so high levels of CCS may follow...
9/
Have a look at the paper, there is much more detail than in this thread!
Many interesting aspects to explore. Up next, carbon-climate feedbacks? Maybe...
In case it was not clear, the 2.2-2.9°C range is the median temperature outcome across the models used. If we additionally include climate uncertainty via the TCRE, the range becomes 1.7-3.8°C.
• • •
Missing some Tweet in this thread? You can try to
force a refresh
What do 𝑛𝑒𝑡 CO₂ emissions from land-use change do in 1.5°C scenarios?
On a net basis, CO₂ emissions reach net zero around 2030. But, this is all net, so is that reduction because of reduced deforestation or increased afforestation?
1/
Of the 53 1.5°C scenarios with no or low overshoot, 27 of them report afforestation.
It is unclear if the 'afforestation' variable is defined consistently with the LUC emissions, but if we assume it is then we can therefore define the difference as 'deforestation'.
2/
All scenarios have reduced deforestation, but do not drop to zero (median in 2100 is 0.5GtCO₂/yr). The negative values from C-ROADS are likely a reporting error.
Afforestation has a median of 4GtCO₂/yr, but clearly afforestation is defined differently in models (see 2010).
The US & EU have had large drops in coal CO₂ emissions per capita over the last decades. They have stable energy use & aging coal infrastructure, so new energy sources displace coal.
India is still growing from very low energy per person, & has a young coal fleet.
2/
Oil CO₂ emissions per capita are very low in developing countries, particularly India. The US & EU are struggling to reduce oil use.
You may have heard that after revisions to land-use change emissions, total global CO₂ emissions are approximately flat over the last decade (black line).
But, how much do we revise carbon budget components each year?
Let's have a look...
1/
Fossil CO₂ emissions are revised each year, particularly the last decades. We update data & improve methods. Chinese data has had major revisions & cement was completely revised in 2018, plus lots of smaller improvements.
Land-use change (LUC) emissions are much more uncertain:
* 2014-2015: one bookkeeping model used
* 2016-2019: two bookkeeping models used
* 2020-2021: three bookkeeping models used
* 2021: major update of land-use forcing (change) datasets
After dropping 5.4% in 2020, global fossil CO₂ emissions are expected to increase 4.9% [4.1-5.7%] in 2021, finishing just 0.8% below 2019 emission levels.
"Everyone wants to keep the dream of 1.5°C alive. In every practical sense you are kidding yourself if you think that we are remotely heading towards 1.5°C"
The UNFCCC Synthesis Report says NDCs will lead to a 5% rise in emissions from 2019 to 2030. This is 16% above 2010 levels, as opposed to ~50% below as required for <1.5C. unfccc.int/news/updated-n…