They take minutes to send and pollute the planet as a byproduct of their security and decentralization. Anyways, Monero does not contribute much to pollution at all. I guarantee you that you do far more things that pollute the planet in your daily life than a Monero transaction.

MobileCoin is a farcical alternative to Monero, where a few “trusted” corporate nodes are run and 80% of the coin is premined.