7.3.4.5 Summary of Marine Carbon Cycle Climate Couplings
Couplings between the marine carbon cycle and climate are summarised in Table 7.3 and below.
Table 7.3. Couplings between climate change (increased atmospheric pCO2, warming) and ocean carbon cycle processes. The response in terms of direct radiative forcing is considered (furthering or counteracting uptake of anthropogenic CO2 from the atmosphere). The two quantitatively most important marine processes for neutralization of anthropogenic CO2 work on long time scales only and are virtually certain to be in effect.
| Marine Carbon Cycle Process | Major Forcing Factors | Response | Start | Re-equilibration Time Scale (kyr) | Likelihood | Comment |
|---|
| + = positive feedback |
|---|
| – = negative feedback and |
|---|
| Quantitative Potential |
|---|
| Biological export production of organic carbon and changes in organic carbon cycling | Warming, ocean circulation, nutrient supply, radiation, atmospheric CO2, pH value | (Sum of effects not clear) +/– medium | immediate | 0.001–10 | Likely | Complex feedback chain, reactions can be fast for surface ocean, nutrient supply from land works on longer time scales, patterns of biodiversity and ecosystem functioning may be affected |
| Biological export production of calcium carbonate | Warming, atmospheric CO2, pH value | (Sum of effects not clear) +/– small | immediate | 0.001–1 | Likely | Complex feedback chain, extinction of species likely, patterns of biodiversity and ecosystem functioning may be affected |
| Seawater buffering | Atmospheric CO2, ocean circulation | – high | immediate | 5–10 | Virtually certain | System response, leads to ocean acidification |
| Changes in inorganic carbon chemistry (solubility, dissociation, buffer factor) | Warming, atmospheric CO2, ocean circulation | + medium | immediate | 5–10 | Virtually certain | Positive feedback dependent on ‘bottleneck’ ocean mixing |
| Dissolution of calcium carbonate sediments | pH value, ocean circulation | – high | immediate | 40 | Virtually certain | Patterns of biodiversity and ecosystem functioning in deep sea may be affected |
| Weathering of silicate carbonates | Atmospheric CO2, warming | – medium | immediate | 100 | Likely | Very long-term negative feedback |
7.3.4.5.1 Robust findings
• A potential slowing down of the ocean circulation and the decrease of seawater buffering with rising CO2 concentration will suppress oceanic uptake of anthropogenic CO2.
• Ocean CO2 uptake has lowered the average ocean pH (increased acidity) by approximately 0.1 since 1750. Ocean acidification will continue and is directly and inescapably coupled to the uptake of anthropogenic CO2 by the ocean.
• Inorganic chemical buffering and dissolution of marine CaCO3 sediments are the main oceanic processes for neutralizing anthropogenic CO2. These processes cannot prevent a temporary buildup of a large atmospheric CO2 pool because of the slow large-scale overturning circulation.
7.3.4.5.2 Key uncertainties
• Future changes in ocean circulation and density stratification are still highly uncertain. Both the physical uptake of CO2 by the ocean and changes in the biological cycling of carbon depend on these factors.
• The overall reaction of marine biological carbon cycling (including processes such as nutrient cycling as well as ecosystem changes including the role of bacteria and viruses) to a warm and high-CO2 world is not yet well understood. Several small feedback mechanisms may add up to a significant one.
• The response of marine biota to ocean acidification is not yet clear, both for the physiology of individual organisms and for ecosystem functioning as a whole. Potential impacts are expected especially for organisms that build CaCO3 shell material (‘bio-calcification’). Extinction thresholds will likely be crossed for some organisms in some regions in the coming century.