feat: add budget constants

[codingfabi]

No description provided.

[han16nah]

Can we add some documentation?
E.g., that the values are provided in tonnes of CO$_2$ (equivalents)?
And the source (i.e., a URL)?

And what does the last value mean (budget_per_person_and_year_long)? Why is it so different for the 1.5° compared to the 2°C limit?

@sarahmadeleine @veitu

[veitu]

Can we add some documentation? E.g., that the values are provided in tonnes of CO$_2$ (equivalents)? And the source (i.e., a URL)?

And what does the last value mean (budget_per_person_and_year_long)? Why is it so different for the 1.5° compared to the 2°C limit?

@sarahmadeleine @veitu

There is a documentation text here that we can add: https://docs.google.com/document/d/1BJl47fkHll8LWOFUh2TaQXWHfAUXATVadKpleb1P8t0/edit#heading=h.jcacpflpeb6u

You can also review the carbon budget calculation in this powerpoint (up to slide 9): https://docs.google.com/presentation/d/1EDV7lMEjx3kl77gFNtEwb96lVdrbLxKB7UqKG4K31Vg/edit#slide=id.g162563edf28_1_0

We calculate the carbon budget per capita based on global carbon budget and world population. The unit is t CO2 (only CO2). The global carbon budgets come from the IPCC: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf

For the world population we use projections from the UN until 2050, but I don't know the exact source.

budget_per_person_and_year is the yearly CO2 budget per person if net-zero is reached in 2045 (Germany's goal).
budget_per_person_and_year_long is the yearly CO2 budget per person if net-zero is reached in 2050 (EU green deal, Canada, etc.)

I don't know where the value for 1.5°C comes from. budget_per_person_and_year_long should actually be 1.1 for 1.5°C.