Final answer:
The decrease of global marine NPP during the 1980s and 1990s was likely driven by increased atmospheric CO₂ levels affecting climate and marine ecosystems, ocean circulation pattern changes, and the impacts of overfishing.
Step-by-step explanation:
Decline of Global Marine NPP during the 1980s and 1990s:
The decline in global marine Net Primary Production (NPP) by 6% during the 1980s and 1990s can be attributed to several factors. One of the main drivers is likely the increase in atmospheric CO₂ levels due to industrialization, which has been shown to impact global climate and could affect marine ecosystems. Additionally, changes in ocean circulation patterns, possibly from the noted shift in Earth's orbit known as the Milankovitch cycles, and a history of overfishing, may have played roles in the decline of marine NPP by impacting nutrient availability and altering the balance of marine species.
An increase in greenhouse gas emissions has led to ocean temperature rises that can trigger shifts in the population distribution and health of marine species, affecting their productivity. Overfishing has also contributed to the decline by depleting fish stocks, leading to changes in the food web dynamics and potentially decreasing NPP. Ultimately, this complex interplay of climate change and human activity has led to a notable decrease in the marine productivity that forms the basis of oceanic food webs.