What is common to the greatest number gets the least amount of care.
Men pay most attention to what is their own; they care less for what is common;
or at any rate they care for it only to the extent to which each is individually concerned.
Even when there is no other cause for inattention,
men are more prone to neglect their duty when they think that another is attending to it. ~ Aristotle

Dear Asher,

Every action has a consequence. Positive actions typically produce positive outcomes and vice versa. But there are many actions for which we do not assess the consequence yet their neutrality is not evident. The Categorical Imperative, as laid out by Immanuel Kant, is the central concept for the Deontological moral philosophy and states, “act only according to the maxim whereby you can, at the same time, will that it should be a universal law.” Each of us impacts the natural environment, some to a lesser extent than others, yet it is not enough to treat our ecological actions as benign since the biosphere belongs to the commons. If we think CO2 emissions offsets for air travel should be universal then we should act accordingly.

The top three contributers of greenhouse gas emissions, whether at home or abroad, are heating and cooling buildings, animal husbandry and transportation – in that order. When traveling in warm climates, it is always good to try to use a fan instead of AC, eat a plant based diet and travel overland as much as possible. So far we are doing great with the first two biggies but it is the air travel which requires reflection.

The journey abroad often begins with a flight. Air travel accounts for 3.5% of all carbon emissions and this is expected to double within the next ten years. Each person emits approximately 92kg of CO2 each hour he is in the air (based on a Boeing 747 at 80% capacity). Conversely, a mature tree absorbs an average of 21kg of CO2 per year. So to offset one hour of flying, each person would need a mature tree working its magic for four years and three months.

For this trip we will fly:
London to Toronto 0.5 hours
Toronto to Houston 3.5 hours

Houston to San Pedro Sula 3.0 hours

Cancun to London 4.5 hours

Total per person 11.5 hours

Total CO2 missions per person

11.5 (hours) X 92kg = 1058 kg

Number of trees required to absorb 1058 kg of CO2 (Assuming the average tree lives for 20 years.)

1058kg (emission) / 21kg (absorption per year) = 50.4 tree years required.

50.4 / 20 (average life of tree) = 2.52 rounded up to 3 trees planted per person.

Therefore, each of us will have to plant three trees and ensure they survive for 20 years in order to offset the CO2 emissions from our flights alone.

Carbon offsetting ought not be a solution on its own. In a way, carbon offsetting is similar to the old Catholic church’s practice of selling indulgences to absolve sins. Rather than actually changing our behavior to a universal imperative, we can continue to engage in the harmful behavior and pay for the offsets. However, in this case, doing something is better than nothing. Currently, we cannot produce zero emissions so as long as we strive for greener technology and offset our emissions when possible, then we can do our part to protect the commons. Before the trip has ended, we will also calculate our overland travel and offset these costs as well. Six trees and counting.

http://www.carbonindependent.org/sources_aviation.html