Tim Garrett, an associate professor of atmospheric sciences at the University of Utah, makes the provocative claim that global warming is unstoppable. It's a very interesting argument and is reminiscent of Jevon's Paradox. I'll have to do more thinking before I post on the crux of Dr. Garrett's main argument, this post is about a minor point in the article linked above.
Garrett opines that conservation and efficiency are useless in the long term with respect to minimizing humanity's primary energy conversion but yet he bicycles to work, line dries his clothes, and uses a push lawnmower. Ah, there's something into which I can sink my meager teeth. Is using a push lawnmower more Earth friendly than an electric or gasoline mower? I'm going to make a guess and then see if I can confirm or refute the guess with numerical estimates. You'll have to trust that the guess is prior to the calculation. My guess is that the electric lawnmower is most Earth friendly, followed by the push mower with the gas mower bringing up the rear.
First, I'll have to state my definition of Earth friendly. I'm going to go with primary energy consumption for all inputs, i.e., for the electric mower it will be the electrical energy used by the mower plus the energy required to generate and transmit the electricity. The gasoline will be the gas used by the lawnmower plus all energy used in extracting, refining, and distributing the gasoline. I won't include gas for the car to go to the gas station since that trip will be added to a fuel trip for the car. For the push mower, it will be the energy used by the person pushing plus the energy used in planting, fertilizing, harvesting (or slaughtering, etc.), and distributing the food providing the energy. I won't include the embedded energy in the lawnmowers, though that may hurt the push mower. Needless to say, there's ample room for error in this analysis but damn the torpedoes, full speed ahead.
Let's set the lawn as follows: 1,500 meters^2 (a little over 16,000 feet^2 or about 0.37 acres). Starting with the gas mower, I'll look at the Honda HRX217K2HMA. It has a cutting width of 21" but I'll use 20", there must be some overlap. Its maximum self-propelled speed is 4 m.p.h., I'll go with 3 as an average. Cutting 1500 meters at 20" requires 1.84 miles of mowing, at 3 m.p.h., that will take 0.613 hours. Sounds about right. The mower uses the Honda GCV190 engine, which uses 1.21 quarts/hour of gasoline. Therefore, it will use about 0.742 quarts or about 0.186 gallons to mow the lawn.
Using the estimates from my previous post, the 0.186 gallons will require a total on the order of 2.84*10^7 joules of primary energy from an oil well. To this I must add the energy required to walk 1.84 miles and using other figures from the same post, this will require 181 kilocalories or 6.4*10^6 joules of primary energy to produce. This is yields a total of 3.48*10^7 joules (Big hat tip to Chris for pointing out the factor of 100 error in my original post).
For the electric mower, I'll use a rechargeable, the Earthwise 20 in Cordless Electric Lawn Mower. I'll reduce the cutting width by an inch as for the gas mower and find that I'll travel 1.93 miles. I'll figure that I'm going a little more slowly, say 2 m.p.h., since it's not self-propelled. Thus, I'll need 0.96 hours or 58 minutes. The specification says that a charge is good for 45 minutes (give or take) so I'll need 1.29 charges. This is for a 24 volt, 17.2 ampere-hour battery that I'll assume we recharge when it's 80% discharged. Thus, a charge uses 1.19*10^6 joules and the lawn takes 1.54*10^6 joules of electricity from the wall socket. Again using figures from my earlier post linked above and assuming some fossil fuel (coal, natural gas) is used, this quantity of energy will require about 5.27*10^6 joules of primary energy to provide the battery charge.
Since the electric isn't self-propelled, I'll be working harder to push it, so I'll estimate about half again the caloric input as for the gas mower, or 9.6*10^6 joules of fossil fuel input to push it for a total of 1.49*10^7 joules total. Note that, by my estimate, it takes more fossil fuel to have me push the lawn mower than to power the swirling blades.
Finally, let's look at the manual, reel, or push mower. I'll use the Brill Razorcut 38 Push Reel Lawn Mower. The cutting width of this mower is 15.2", I'll reduce it by an inch as above and find that I have to walk 2.58 miles. Since I'm doing all the work, I'll assume that I can move at about 1.75 m.p.h. and thus I'll mow for 1.47 hours. This handy page says that, at 180 pounds, I'll burn 447 kilocalories/hour for a total of 659 kilocalories. Again going back to the earlier post linked above, producing the food to power me through this walk will take about 2.34*10^7 joules of fossil fuel energy.
It would seem that my instinct was correct. In increasing order of energetic impact, it's the electric, followed by the manual, and finally, bringing up the rear, the gas powered mower. My original analysis was off by a factor of 100 with respect to the energy contained in the gas mower's fuel burn (blush) so it isn't as bad as I'd first calculated.
Finally, take a look at what is, in my opinion, the best solution. Here's the Epic Cordless Electric Solar Mower Model EP21H. It features 45 minutes on a charge, has a 21" wide cut, and the optional solar panel will recharge it in about three sunny days. This will leave the 9.6*10^6 joules of food energy as the only fossil fuel consumption to mow your lawn.