If you thought that Hummer drivers were bad, think again. According to a study by the University of California Berkeley, emissions from motorcycles dwarf that of SUVs. In fact, a sport bike can be responsible for 1.5 times the total greenhouse gas emissions of a typical SUV over the lifetime of the vehicles. The fact that Hummer drivers were given such grief by the environmental community and motorcycle drivers are getting off relatively scott free is surprising.
The following chart shows the amount of greenhouse gas emissions per passenger mile traveled (PMT) for seven classes of vehicles including sedans, SUVs, pickup trucks, diesel buses, and motorcycles (2-cyl, 4-cyl, and sport bikes).
Lifecycle GHG Emissions by Vehicle Class
Adapted From: UC Berkeley Center for Future of Urban Transport Report: Life-cycle Energy and Emissions Inventories for Motorcycles, Diesel Automobiles, School Buses, Electric Buses, Chicago Rail, and New York City Rail
Federal regulations help to control motorcycle emissions, but the regulations don't do enough to make them emissions competitive with a hybrid car, which, if it were represented in this chart above, would easily prevail as the most efficient vehicle per PMT. Looking up the exact amount of emissions that are generated from a specific model of motorcycle is quite difficult because manufacturers aren't required to test and register the fuel economy with the Environmental Protection Agency. Auto manufactures, on the other hand, have to test every make and model and report the fuel economy.
The Los Angeles Times recently covered the topic that motorcycles mileage and emissions are not reported in the government fuel economy guides in the article The Measure of Motorcycles' MPG.
Yes. The Federal Government has been trying to get a handle on motorcycle emissions for years because they are also responsible for high levels of other nasty "criteria air pollutants" like Volatile Organic Compounds (VOCs), PM10, SOx, and NOx. You can find the current EPA regulations here: http://www.epa.gov/otaq/roadbike.htm
Milligrams of VOCs Emitted per Passenger Mile Traveled
Adapted From: UC Berkeley Center for Future of Urban Transport Report referenced in previous figure.
In short, yes; especially sport bikes. The amount of environmental impact and greenhouse gas emissions that motorcycles emit over the course of their lifetimes is very large compared to alternate modes of transportation. If you want to reduce your impact, then you should take the bus during peak hours (i.e. rush hour) because, according to the information published by UC Berkeley, that is the least harmful way to cover ground in a motor vehicle.