As they use lightning network the reasoning that there will be high demand for block space is not a valid assumption. Theoretically, it is possible that the government of el salvador is doing a single transaction on the blockchain as channel funding and sends bitcoin via lightning to 6,4 million people from el salvador from that channel without touching the blockchain...

Block space isn't the point. The price of a Bitcoin is. Raise the price, and mining will naturally spend more energy on it. Therefore, anything that raises the price of Bitcoin contributes to Global Warming.

Price of bitcoin * total block reward (new coins + fees)

The total value of the miner reward does increase with price, but because of halvings every 210,000 blocks it's a slower rate than the price increases on its own.

Note here, https://www.blockchain.com/charts/miners-revenue, that the current miner reward (in USD) is currently lower than it was at the peak of the 2017 bull run, despite the unit price being approximately double.

The main impact is still Bitcoin price. Anything that makes Bitcoin's price go up makes the mining energy usage go up too. The fees are just a bonus.

But anyway:

1. A transaction creating a channel factory for 6.4 million different keyholders would be too large to fit in a block.

2. 6.4 million users on LN would be a great target for flood and loot[0].

[0]https://medium.com/blockchains-huji/flood-loot-a-systemic-at...