Washing Laundry in Cold Water is the Same As...
How did we get the numbers? According to the Environmental Protection Agency (EPA), the national average of carbon dioxide emissions is 7.78 x 10-4 metric tons CO2 / kWh, or 0.000778 per kWh. 0.000778 metric tons is roughly equal to 1.715 U.S. pounds, so that's what we used for pounds of CO2 per kWh. The EPA used their eGRID (Emissions & Generation Resource Integrated Information Database) nonbaseload subregion and national average emissions rates from 2004 to arrive at that number -- the Energy Information Administration [pdf] reported that number to be 1.34 pounds using data from 1998 - 2000, for example -- so there are different numbers out there; we'll stick with the later version from EPA.
If you have a top-loading machine, and heat your water with gas:
Washing in hot/warm: .178/.089 therms = .267 therms = 3.22 pounds of CO2 per load
Washing in hot/cold: .178 therms = 2.1467 pounds of CO2 per load
Washing in warm/warm: .089/.089 = .178 therms = 2.1467 pounds of CO2 per load
Washing in warm/cold: .089 therm = 2.1467 pounds of CO2
If you have a top-loading machine and an electric water heater:
Washing in hot/warm: 5.24 kWh = 8.9866 pounds of CO2 per load
Washing in hot/cold: 3.58 kWh = 6.1397 pounds of CO2 per load
Washing in warm/warm: 3.58 kWh = 6.1397 pounds of CO2 per load
Washing in warm/cold: 1.92 kWh = 3.2928 pounds of CO2 per load
Washing in cold/cold: 0.24 kWh = 0.412 pounds of CO2 per load
An Environmental Protection Agency Excel spreadsheet calculates the average kWh per load (machine only -- no water heating) in a top-loading machine at 0.21, and the Multi-Housing Laundry Association gives 0.30, so we split the difference at 0.255. Energy use by model can vary, so this ballpark won't apply to all machines.
Check out How Bad Is Taking a Cab to the Airport, Compared to the Flight Itself? for other day-to-day calculations, and stay tuned for more, including the impacts of line-drying your laundry instead of using a dryer.