As on my previous post, I once again tried to estimate my carbon footprint in order to consider paying for “carbon offsets”. The whole issue becomes complicated very quickly, like so many other topics when looked into in depth.
Something that I was shocked to learn was the ratio of CO2 emissions of planes versus car usage. While “everyone knows” that planes are a major source of greenhouse gases, great public attention is also paid to low consumption cars. While of course “every little bit helps”, I find it important to be aware of the main sources of greenhouse gases in order to apply an 80:20 approach to reducing the main culprits.
What I found was that one intercontinental return flight produces more CO2e emissions than a modern car during a whole year:
A car that uses an average of 10 liters/100km (23mpg) and is driven 25,000km/year (15,500 miles) will produce roughly 280g/km of CO2 emissions, resulting in a total of 7 tons per year .
A modern (European) car with a CO2 emission of 140g/km will result in a total of 3.5 tons per year.
A return flight from London to San Francisco will be in the air for ca. 24 hours, cover a distance of 17,000km (11,000 miles) and cause a CO2e emission of roughly 4 tons per passenger.
Included in this figure is the fact that emissions at 30,000 feet have a different impact on the environment than emissions on ground level. The exact factor for plane emissions is contended, but is thought to lie between 2 and 4. Above calculation uses a factor of 2.   
Even though take-off uses a lot of fuel and thus long-haul flights are more fuel efficient per kilometer, a good rule of thumb is to calculate with 90kg CO2/hour with a CO2e factor of 2, leading to 180kg CO2e/hour/passenger.
What this means to me is that depending on your car 1-2 intercontinental flights per year will produce more CO2 than driving 70km in your gas guzzler every single day of the year.
The idea of Carbon Offsetting is to pay money to projects whose goal is to reduce carbon emissions or to capture atmospheric CO2, and thus to offset one’s own emissions with counteracting projects.
The concept of Carbon Offsetting is contended, as it is difficult to prove the additionality of the projects, to audit their effectiveness and because it can be used as a fig leaf to excuse unnecessary emissions rather than reduce them.
Nonetheless, I am intent on offsetting my carbon emissions until I can significantly reduce my CO2 emissions. Based on this WWF study, the highest certification standard available today is the Gold Standard, which has a strong focus on ecological impact and additionality of measures. I have therefore already chosen one of the Gold Standard projects, namely the Atmosfair project to purchase offsets from. Unfortunately, Atmosfair is a very expensive program per ton, and I will have a look at other certified programs that promise to offset emissions at much lower prices.
Our Average Carbon Footprint
The average person’s CO2 footprint in Europe lies between 10 and 20 tons CO2e per year depending on life-style     excluding extensive air travel, etc. Thus depending on flight intensity, a person’s total footprint can lie between 20 and 60 tons CO2 per year, if not more for heavy airplane commuters.
Obviously, such behavior cannot be offset on a large scale by planting trees or other ecological measures. According to this article, a forest can sequester 2.2 tons of CO2 per year on an area of 1 hectare (10,000 m2, 2.5 acres). This means that a person with “moderate” travel activity (total footprint of 30 tons/year) would need to plant 150,000m2 (37 acres) of forest that is kept as forest for eternity to sequester the emitted carbon dioxide (bearing in mind that said trees will not continue indefinitely to sequester carbon dioxide).
The total CO2e emissions of the G8 states with their 1 billion inhabitants are 14 billion tons . This volume would require forestation of 45 million km^2 to offset and to keep these forests alive forever. To visualize, the total land area of the earth is 150 million km^2 .
Therefore it is essential that we not only take carbon offsetting as a convenient fig leaf to calm our conscience, but do our utmost to reduce our carbon emissions, lest we end up ruining the only planet we have.