Sure it's winter, and the slushy snow and bitter cold doesn't make the best environment for biking to and from work or the grocery store. But in countries around the world, the biking culture is so vital that many people brave the elements not only for their health, but for the health of the planet.
Cities such as Amsterdam, Portland, OR, and Bogota are well known for their commitment to developing infrastructure that encourages residents to strap on a helmet and bike those thirty minutes to the office instead of sitting in traffic for an equivalent time. The benefits of biking are clear: a healthy source of exercise, fewer carbon emissions than driving, less stressful than traffic jams, and fewer cars on the road (ultimately driving the cycle to diminish dependence on fossil fuels).
However there have been several studies that caution thinking about riding your bike as a zero-emission carbon footprint. The gist behind these arguments are that while you're not burning fossil fuels directly, you are burning fat, and that fat came from food that you ate, some of which likely came from agricultural industries that use fossil fuels. A Harvard graduate student Daniel Thorpe published his findings that cyclists who eat a lot of animal byproducts can be equally as harmful to the environment as individuals who eat plant-based diets and drive low-emission vehicles.
That being said, your eating choices in general also have a huge impact on your carbon footprint, and eating locally produced food substantially lowers your emissions, whether on a bike or in a vehicle!