Jym Dyer wrote:

Carbon dioxide is a greenhouse gas. That's what you exhale.

When it comes to greenhouse gases,
quantity is of course as relevant as composition. The CO2
exhaled by the world's fauna is not a relevant quantity.

That's not even really the issue. CO2 from cars and trucks comes from
burning fossil fuels, resulting in a net increase in atmospheric
carbon. CO2 from humans pulling bikes is just recycling the carbon
that came out of the atmosphere in the first place.

RFM