One word answer : productivism.
The ideological roots of both parties are similar (marxism). They both acknowledge that the climate should be preserved. However, they disagree on the extent to which it should be taken into account. LO wants to keep productivism, NPA does not.
LO is a trotskist party. It is not opposed to producing a ton of material goods for the well being of the masses. It has an environmental agenda, but blames most climatic disasters (for instance fukushima) on the capitalist gestion of the production. The production of wealth itself should be kept.
NPA does not claim to be a communist party. It wants to establish a "21st century ecological socialism ". It wants a control on the level of production, as well as on the technologies used to produce goods, in order to preserve the environment. Furthermore, controlling the level of production would enable to increase leasure time. This approach is similar to that of the degrowth theory, although the NPA does not avocate for reducing the growth, only choosing the proper level.
This leads to differences in their respective political agendas for the 2017 election. NPA is opposed to nuclear energy and GMOs, LO is not.