Overview for the En-ROADS graph “Loss in Ocean Life from Warming.”


Big Messages:

  • As global temperatures rise, ocean ecosystems will face increasing stress, leading to a decline in marine life.
  • Although there is uncertainty on the extent of, and key processes underlying, the loss of marine life due to climate change, studies suggest that warming will reduce marine life globally, especially for larger marine animals.
  • Limiting global temperature rise to 2ºC by 2100 is expected to reduce global loss in ocean life by about 40%. However, complex factors like food web interactions, ocean acidification, and deoxygenation may further impact marine ecosystems, making projections challenging.


Key Dynamics:

  • Plankton are the basis of the ocean food web. These tiny organisms feed everything from small fish to massive whales. If climate change disrupts plankton populations, the entire marine food web is affected.
  • Species higher up in the food chain are more affected. In general, the effects of climate change are worse for organisms higher up the food chain, a phenomenon known as “trophic amplification.” When climate change reduces phytoplankton—the primary producers—this loss is magnified at higher levels, leading to steep population declines for species at the top (Atkinson, 2024). Reasons for this include:
    • Warming oceans become more stratified, which reduces the mixing of nutrients from deeper waters into the surface where most marine life lives. This starves phytoplankton of essential nutrients like nitrogen and phosphorus, reducing their growth and nutritional quality (Kwiatkowski et al., 2018).
    • Warmer temperatures speed up metabolism, making marine animals burn energy faster. Less energy is stored in their bodies, and less is passed up the food chain (Carozza et al., 2018; Guibourd de Luzinais et al., 2023).
    • Warmer waters with fewer nutrients can lead to longer food chains by making phytoplankton smaller. These smaller phytoplankton are eaten by smaller zooplankton, which extends the food chain and limits the energy available to animals higher up (Carozza et al., 2018).
  • Not all regions will experience the same effects. In some areas, particularly temperate regions, organisms higher in the food chain might be less affected than ones lower in the chain (Chust et al., 2014).


Facilitator Tips:

  • Warming affects regions differently. Tropical oceans are expected to see the greatest declines in marine life. Polar regions (e.g., the Arctic) may see temporary increases in ocean biomass as ice melts and new areas become habitable (Lotze et al., 2019). There is still uncertainty about which specific regions will see increases versus decreases in marine life.
  • Climate change may weaken the ocean’s ability to fight climate change. Phytoplankton take in CO₂ during photosynthesis, and when they die, they sink, locking carbon away in the deep ocean for centuries. If phytoplankton populations decline, less CO₂ will be absorbed, creating a feedback loop that worsens climate change and loss of marine life (Buesseler, 2020). This feedback loop is not explicitly included in En-ROADS.


Equity Considerations:

  • The regions projected to lose the most marine life are also home to communities that depend heavily on seafood and fisheries for food and jobs (Lotze et al., 2019). Many of these areas already face food insecurity, economic instability, and other socioeconomic hardships, making them especially vulnerable to further declines in marine resources (Boyce et al., 2020).


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