Middle-ground solution for accuracy

We are currently operating on a simple relationship between flight distances and CO2eq estimated emissions (based on [OWID data](https://ourworldindata.org/travel-carbon-footprint)).

This is unsatisfying because of two main reasons:

1. Estimates are not only much larger than Google Flights estimates, but more importantly the range of discrepancy is very high - see https://github.com/developmentseed/meet-and-greta/issues/9

2. Layovers are not taken into account. This is arguably a showstopper for a lot of flight planning scenarios. It also has the potential to highly impact the final CO2eq estimate (surprisingly, in favor of flights with _more_ layovers)



Which is addressed by:

1. Using a more complex model for calculating GHG emissions that is not solely based on distance

See typical methodologies:

- https://www.myclimate.org/fileadmin/user_upload/myclimate_-_home/01_Information/01_About_myclimate/09_Calculation_principles/Documents/myclimate-flight-calculator-documentation_EN.pdf

- https://www.goclimate.com/blog/wp-content/uploads/2019/04/Calculations-in-GoClimateNeutral-Flight-Footprint-API.pdf

2. Getting data about flight __routes__ (ie "What are the possible flights from airport A to B, including layovers")

With a given flight code, the [Google Travel Impact API ](https://developers.google.com/travel/impact-model)can return a CO2 eq estimate for each travel class.

From there, there are three potential scenarios IMHO:

### 1. Commercial routing API + Google Travel Impact API

Tried in this Observable notebook: [Calculate GHG emissions from airport to airport](https://observablehq.com/@nerik/calculate-ghg-emissions-from-airport-to-airport).
Commercial routing APIs pricing is likely to make this infeasible, but that could use further research.

### 2. In-house solution for routing + Google Travel Impact API

Build a basic routing engine based on open source datasets. [Airports is relatively easy](https://github.com/developmentseed/meet-and-greta/issues/7). Flights, not so sure.
Then call Google's API which is free of charge and under CC-BY-SA.

### 3. In-house solution for routing + in-house CO2eq estimation algo

Develop our own estimation algorithm based on widely documented methodologies and not-so-widely available datasets.

Ping @kathrynberger @developmentseed/data-team