For researchers (like me) who are based in Europe, travelling by land doesn’t have to be hard. I talk more about travel further afield below.

The most important resource I’ve found is this one. It’s basically a rail enthusiast network primarily (but not exclusively) focussed on travellers from the UK. It documents how you can get from London to just about anywhere in the world by train, where feasible. Here are some fastest-route rail travel times from London:
I know, 10 hours sounds like a long time on a train. But think of all the grant writing you could get done in that time! With nobody to disturb you…doesn’t sound too bad to me. Many routes also have sleeper trains, meaning that you can travel overnight, arriving in the morning ready for action.
Rail travel is not carbon neutral, but it’s somewhere between 2 and 5 times less costly in terms of emissions than air travel, depending on the distance. This site helps you compared the likely admissions associated with a trip by rail, road and air.
Many of my most important colleagues, collaborators and friends are in the US. So, how is that going to work? Well, of course it’s possible to get the US by boat. The trouble is, of course, that transatlantic travel by cruise ship is arguably more polluting than by plane. There are no obvious answers here, except to go less frequently by plane, combining multiple visits into a single trip, or not to go at all. And what about Asia? Well, luckily there’s the Transiberian, so not impossible. But that’s a pretty long trip…!
Of course, we live in the 21st century, and so it’s also possible to communicate by videoconference. My experience with videoconferencing is mixed. When you have the right facilities, as they do at a research company where I work part time, it can be amazing, especially for presentations and one-on-one discussions. Even without a fancy setup, you can have pretty good meetings using software such as this. However, most universities and research institutes haven’t yet invested heavily in the technology to allow remote talks and meetings. They should. I’m thinking about initiatives that might start to remedy this.
More generally, travelling by land is going to take more time. That means that visits will be longer, meaning time away from home per trip. It’s worth thinking about what impact that might have on families, and whether this will exaggerate the inequality of opportunity that already exists between men and women in science. However, presumably it will also mean fewer trips overall, which is a net positive for families.