The cost of travel

Summary

This essay examines the carbon cost of travel using different modes of transport, and compares it to other sources of carbon emissions (eg. electricity). It finds that international air travel is a significant source of carbon emissions, especially for people with an efficient house who fly regularly.
Compared to phasing out coal power (a noble goal), phasing out air travel would have a similar impact per-capita for regular flyers.
If middle-class Australians expect coal communities to make personal sacrifices to reduce the emissions intensity of Australia's electricity, I think it is reasonable to ask middle-class Australians to fly less.

Introduction

In times past, only the wealthy could afford to travel. This was for the simple reason that most people were so poor that moving any significant distance from their home was unaffordable. And, of course, they had to work all the time.

Also energy, or the ability to do work, was so expensive that to travel took a long time (because to travel quickly takes much more energy than to travel slowly). Let's get a perspective on how much energy travel consumes.

This first figure shows the energy output of a human, an ebike, and a horse

Clearly, the ability of a person to do work is tiny compared to a horse -- it takes approximately 10 people to equal the power output of a horse. Horses are pretty powerful! Let's get a bigger perspective:

Suddenly, the horse doesn't look so powerful anymore! Even a small car can produce the power of many horses. The power output of a car is so great that the human output looks like nothing.

Of course, there is a difference -- the power output of both small and large cars is unsustainable (at least for the vast majority of cars which burn fossil fuels), and is causing changes in the Earth's climate that will be paid for by people in the future. Clearly, if we want efficient transport we should avoid cars.

Here we add air travel to the graph. This makes all of the sustainable transport look like it uses no energy at all. Note that for the cars and the plane, I assume that the car has 4 passengers, and the plane is completely full. Also, per-passenger energy consumption of the plane will be larger for business- and first-class passengers (because they take up more space).
Note that it is possible, at least in principle, to electrify car transport. Electrified air travel is currently science fiction. Note also that, currently, more than 60% of Australia's electricity is generated using fossil fuels.

Distance travelled

The figures so far show the energy consumption of different modes of transport, however, clearly people can travel more quickly by air than by foot or bicycle. However, the ability to travel quickly means that people tend to travel more. For example, one would never try to cycle from Sydney to London for a week or two of travel, but with air travel people do precisely this.

The concentration of vast amounts of power to allow air travel is a way for us to cram even more power consumption into our lives.

Let's look at the carbon emissions for different trips to get some perspective:

As before, buses, cars and planes are full and emissions are per-passenger.

Travel vs coal electricity

To reiterate, these numbers are large -- one person flying from Sydney to London produces more than 1.5 tons of emitted carbon. In comparison, in Australia some people (me included) make a fuss about getting rid of coal power because of the environmental impact. The average Australian household uses about 30 kWh per day (this is a huge amount of power), which is just under 11000 kWh per year. The average household has 3 people in it, so that is 3700 kWh/person. Coal power in Australia emits huge quantities of carbon per unit of electricity produced -- it is about 1 kg per 1 kWh. Thus, annual emissions from an average Australian's electricity use is about 3.7 tons.
A one-way flight from Sydney to London produces are much CO2 as 1/3 of annual electricity emissions, if electricity came from 100% coal.

These numbers become even worse if you consider an efficient house (which should be our goal). If you use 8 kWh per day, in a household of three, then one person's annual emissions under a 100% coal scenario are 8 /3 * 365 * 1 = 970 tons/year. A one way flight from Sydney to London is about two years' worth of emissions for this person.

Air travel as a proportion of total carbon emissions

While air travel comprises a relatively small proportion of Australia's total emissions (about 5%), there are hidden costs that are put into other sectors, such as the provision of fuel. Also, the proportion of carbon emissions from air travel is one of the fastest-increasing sectors, and it is also one of the most discretionary -- air travel can be easily reduced. Also, air-travel is a particularly damaging form of carbon emission because of its location in the upper atmosphere.
I have not been able to find statistics, but I suspect that there are a relatively small number of Australians who engage in a relatively large amount of air travel.

Conclusion

There is an increasing push in Australia to phase out coal power and replace it with renewables. This is a good thing. 

However, there is little awareness that carbon emissions from international air travel are similar to the emissions from 100% coal power for average electricity use in Australia. (This is especially true for wealthier Australians that might fly every year).

If middle-class Australians expect coal communities to make personal sacrifices to reduce the emissions intensity of Australia's electricity, I think it is reasonable to ask middle-class Australians to fly less.

References:

https://en.wikipedia.org/wiki/Horsepower
https://en.wikipedia.org/wiki/Human_power
https://www.answers.com/Q/What_is_the_horsepower_of_one_engine_in_a_Boeing_747
https://reneweconomy.com.au/graph-of-the-day-how-green-is-your-electricity-12278/