Just to give some numbers:

The Dutch greenhouse horticulture sector has emitted 6-8 megatons of CO2 per year between 2000-2015 and emissions are expected to decrease to ~4 megatons of CO2 per year in 2020 [1]. 7 megatons is approximately 4% of national emissions [2].

[1] pdf, https://www.wur.nl/upload_mm/c/e/c/f827af2c-9ae6-4bb5-bb47-9...

[2] https://www.cbs.nl/nl-nl/nieuws/2018/37/co2-uitstoot-in-2017...

4% of national emissions is a lot considering the purpose here, and taking into account, as said, that the Netherlands emit quite a bit to begin with because of fossil fuels use (Wikipedia says they emitted twice as much as France per capita in 2014 [1], for example)

[1] https://en.m.wikipedia.org/wiki/List_of_countries_by_carbon_...

> 7 megatons is approximately 4% of national emissions

What makes up the bulk of national emissions?

As of 2nd quarter 2019 [1] rougly:

32% Energy, water and waste services

24.5% Agriculture, mining, industry, construction

12% Other services

13.5% Transport

18% Households

This is a major challenge (not only in the Netherlands) because it appears there is no one "bulk emitter" that must be tackled.

[1] https://www.cbs.nl/nl-nl/nieuws/2019/39/uitstoot-co2-hoger-i...

Maybe they should just pump that CO2 into the greenhouses!

They do. But more commonly, CO2 is intentionally produced in greenhouses by burning natural gas (or propane) for no other reason than to create CO2 for the benefit of the plants:

https://dutchgreenhouses.com/technology/co2-enrichment