Which will have the most impact?

Nothing beats face-to-face contact, but telepresence aims to come exceptionally close. By integrating a set of high-performance visual, audio, and network technologies and services, people who are geographically separated can interact in a way that captures many of the best aspects of an in-person experience.

When it is possible to exist and function remotely, the need to travel becomes less necessary. In a world of global-business footprints and international collaboration, if people can work together without being in the same place, they can dodge a host of travel-related emissions.

Telepresence now comes to life in a variety of ways and a diversity of settings. From companies and schools to hospitals and museums, virtual interaction is opening new possibilities. Using a mobile telepresence robot, a surgeon can advise on a rare procedure in real time, without traveling from Austin to Amman, for example. Or, executives in telepresence conference rooms in Sydney and Singapore can debate a possible acquisition without taking a single flight.

Telepresence affords many other benefits: cost savings from avoided travel, less grueling schedules for employees, more productive remote meetings, the ability to make decisions more quickly, and enhanced interpersonal connection across geographies.

Trains transport 28 billion passengers and more than 12 billion tons of freight annually. Most rely on diesel-burning engines; some tap into the electric grid. Although trains have steadily improved their fuel-use efficiency in recent decades, rail was responsible for 3.5 percent of emissions within the transport sector in 2013.

Railway companies employ a range of technical and operational measures to improve fuel efficiency and reduce costs. As locomotives are retired, more efficient models replace them, many with more aerodynamic designs. In some cases, those models include hybrid diesel-electric engines and batteries, which gain efficiencies similar to those of hybrid cars, saving 10 to 20 percent on fuel.

Better locomotives, more strategically placed, are enhanced by better cars—lighter, more aerodynamic, able to hold more cargo, and equipped with low-torque bearings. The rails themselves can be better lubricated to reduce friction. How a train is driven also remains critical, and software can improve it.

The number of electric trains is increasing. As electricity production shifts to renewables, rail has the potential to provide nearly emissions-free transport.