What reduces carbon dioxide in atmosphere?

The planet has a number of huge carbon sinks, removing CO2 from the atmosphere. The largest is the ocean, as CO2 dissolves. This absorbs roughly 80% of all carbon dioxide.

Other sinks are photosynthesis by vegetation and phytoplankton, and the weathering of silicate rock. For more details, read about the carbon cycle.

Currently, sources of carbon must outweigh the sinks because atmospheric CO2 is increasing globally by roughly 2 parts per million by volume per year, as can be seen in the NOAA measurements of atmospheric CO2. CO2 is even higher in cities and industrial areas, in places where fossil fuels are being burnt in quantity.

The easiest way for us to reduce the carbon dioxide levels is to reduce carbon dioxide emissions. This is the scientific consensus view, and is the view held by the IPCC, all major scientific bodies and national academies of science around the world.