My teacher was talking about how if we added nitrogen and phosphorus fertilizers in the ocean, we would help reduce carbon dioxide. I don't understand. How does that work?

Here are some articles which discuss that.

http://scholar.google.com/scholar?q=the+effect+of+nitrogen+and+phosphorus+fertilizers+on+the+ocean&hl=en&um=1&ie=UTF-8&oi=scholart

nitrogen and phosphrous are fertilizers, and that promotes growth of sea plant life (algae), which uses carbon dioxide to manufacture more plant tissue.

However, you teacher is probably wrong in the longer run. The algae will eventually die, and the carbon in it has to go somewhere. It is not clear what the long term consequences are.
There is a better way of getting algae to grow, that is seeding with iron, as that is the limiting factor in algae growth in the ocean. Several experiments have been done, but the long term consequences are still out.
http://hubpages.com/hub/What-is-the-role-of-accumulating-ocean-algae-in-global-warming_1

Thanks alot everyone! That really helped.

The idea behind adding nitrogen and phosphorus fertilizers in the ocean to reduce carbon dioxide is based on a process called "ocean fertilization" or "iron fertilization." Here's how it works:

1. Ocean fertilization aims to increase the growth of phytoplankton in the ocean. Phytoplankton are microscopic plants that perform photosynthesis, absorbing carbon dioxide from the atmosphere during their growth.

2. Nitrogen and phosphorus are essential nutrients for phytoplankton growth. By adding these fertilizers to the ocean, it is believed that the growth and abundance of phytoplankton can be increased.

3. As phytoplankton grow, they consume carbon dioxide through photosynthesis, effectively removing it from the atmosphere. This process is similar to how land plants absorb carbon dioxide during their growth.

4. Additionally, when phytoplankton die, they sink to the ocean floor, carrying the carbon they have absorbed with them. Some portion of this carbon can be eventually stored in the deep ocean, potentially contributing to long-term carbon sequestration.

However, it is important to note that ocean fertilization is still a subject of much debate and research. While it has the potential to remove carbon dioxide from the atmosphere, it also carries risks and uncertainties. The environmental impacts and effectiveness of this approach are not yet fully understood. It is critical to consider the potential side effects, such as changes in the marine ecosystem and possible release of additional greenhouse gases, before implementing large-scale ocean fertilization projects.