Carbon Offsets


In recognition of the number of meetings I attend associated for my work, I decided to offset the travel-related carbon emissions.  While I understand the critics of a pay to pollute mentality, traveling to science meetings is part of what I do as a scientist.  So, while I do think carefully about my work-related travel, for meetings I choose to attend, I am left with developing some method to offset those travel-related emissions. The idea of flying to a conference to discuss climate change is an issue that many in the field are grappling with, and I expect that advances in video conferencing will eventually help reduce some of our work-related travel. 


I have outlined the air travel miles for each year since I started at SJSU and the estimated the carbon emissions in units of CO2e.  I then describe how I offset those emissions.  In the spirit of improving my own education about carbon offsets, I try to use a different type of offset strategy for each year.


Calculations of distance flown and the resulting emissions (CO2e) were completed using, which has a good flight calculator. Documentation justifying their methods can be found here.  I also did my own calculations and our numbers matched pretty well. 


Academic Year 2007/2008 (On sabbatical)


As you will see, this year included a huge amount of travel, but at least the extra time allowed me time to think more about carbon offsets.


Air travel (km):             54,176

CO2e (kg):                    17,473


Offsets:  This year I decided to buy compact fluorescent light bulbs (CFLs) and give them away to people to replace their energy-inefficient incandescent bulbs.  By reducing their energy use, I can claim the reductions in emissions since I have purchased the lightbulbs.  Here are my calculations.


If one were to replace a 100W incandescent bulb with a 20W CFL, there is an 80W difference.  If the CFL lasts for 8,000 hours, as most modern ones do (5,000-15,000 hours), then over the lifetime of the bulb, one would save 640 kWh of energy use.  In the US today, 1 kWh of electricity emits 0.59 kg of CO2e (other countries are larger or small depending on their mix of energy), so that our lightbulb replacement would reduce 640*0.59=337 kg of CO2e.   If I wanted to offset 17,473 kg, that would be about 50 light bulbs.  However, not all replacements save 80W.  In one place I installed them, they had lots of 60W bulbs, which were replaced with 15W CFL bulbs, thus saving 450 kWh of energy use over their lifetime.  We also replaced a bunch of 40W bulbs with 10W CFL ones (300 kWh).  So, I bought a whole bunch of bulbs, and here is the summary. 


Old Bulb

New Bulb



CO2e reduced (kg)




















17,454 kg of CO2e



Note that the recipients of the bulbs were quite pleased both because they knew their emissions would go down, but more importantly for some, they would save money.  Consider the example of changing a single 100W bulb with a 20W bulb.  If you use the bulb 5 hours day, then over a year you would save 146 kWh/year.  With national prices around $0.10/kWh, this totals $14.60 per year in savings.  While CFL bulbs cost more than incandescent (typical cost ~ $5-7 today), they also last a long longer (5-10 times) and so over the lifetime of the bulb (over 4 years in this case) one CFL bulb would save a person about $60.00.  That is pretty good, especially when I gave the bulbs for free!  For more bulb info, see the article in Popular Mechanics here.


Academic Year 2006/2007


Air travel (km):           16,580

CO2e (kg):                   5,022


Offsets:  Purchased through Bonneville Environmental Foundation (


For this year, I purchased 10 renewable energy certificates (or green tags) through the organization, Bonneville Environmental Foundation. They have been around for a while, and there is plenty of information about their organization here. Each green tag represents 1,000 kWh of renewable energy, and in total, this represents more than 5 tons of CO2e offsets. 


The idea is that they support renewable energy projects like wind and solar, and my purchasing of green tags essentially helps put more renewable energy on the grid.  So when I purchased the green tags, I am purchasing the environmental attributes of a certain amount of renewable energy, while someone else (who I do not know) gets the actual energy.  The way I explain this to my students is as follows.  If coal costs 10c/kWh to produce and wind costs 12c/kWh to produce, then when you buy a green tag, you are paying the difference, 2c/kWh to put this energy on the grid.  Then, you get the goodness of knowing that a certain amount of renewable energy is on the grid, and someone else happens to get the energy.  This way, if I cannot buy renewable energy at my home (or in my case, have to fly), then I can offset my emissions by helping to put extra renewable energy on the grid. 


Academic Year 2005/2006


Air travel (km):            27,690

CO2e (kg):                     8,911


Offsets:  Purchased through Native Energy (


For this year, I purchased carbon credits from the organization, Native Energy, which has a methane cogeneration scheme.  I purchased 9 tons of offsets ($12/ton).  Here is how it works. 


Cattle farmers typically use lagoons to store the manure their cows produce.  In turn, this manure emits methane, which is a strong greenhouse gas.  The projects financially assisted by Native Energy reduce methane emissions through methane digesters that capture the methane and then produce energy from it.  By both reducing existing methane emissions and generating energy, the farm reduces their emissions of heat trapping gases.  So, my $12/ton goes towards these projects.


Academic Year 2004/2005


Air travel (km):           60,050

CO2e (kg):                 18,641


Offsets:  Purchased through (  For 2002-2004, I purchased carbon offsets from a tree conservation organization in Australia.  The idea is that they plant trees (they actually conserve newly planted trees) and take care of them for 100 years, thus ensuring that update of carbon will last at least 100 years.  Although we are aware that trees eventually return some (or all) of their carbon to the atmosphere after they die (or after a forest fire), this organization will maintain these trees for a period of years.  While not guaranteeing a permanent removal of carbon from the atmosphere, the additional benefits of forest land (presuming the planting of native trees) include habitat for animals, birds and insects.  While there have been some critics of tree carbon capture programs, this organization recognizes these and attempts to address them.  Anyhow, it is a nice idea to know that you are responsible for planting at least a few trees. 


To offset 18 tons of CO2 cost me $24/ton. This maintains about 90 trees for 100 years. 



Academic Year 2003/2004


Air travel (km):            2,200

CO2e (kg):                  1,263


(combined with 2002/2003)


Academic Year 2002/2003


Air travel (km):            13,140

CO2e (kg):                    5,532


Offsets:  For this year, I used an auto swap strategy to reduce emissions. 


During my sabbatical year when we were overseas (2007/2008), we lent our relatively energy-efficient vehicle (Toyota Prius) to a friend who drives an energy-inefficient vehicle (Toyota Tacoma).  Our friend drove our car about 12,000 miles that year and thus saved 4,400 kg of CO2 going into the atmosphere.  Based on $3.50 per gallon of gas in the Bay Area, our friend also saved about $1,700 in gas costs over the year!  I still owed about 1 ton of CO2e to the planet, which I did through purchasing a carbon offset from Terra Pass