15 Dec 2012
Inside the Carbon Goal
The Carbon Storage
Selecting Carbon Storage as a healthy ocean goal was easy. We have scientific consensus that too much CO2 in the atmosphere is causing Earth to heat up. Citizens, coastal dwellers, farmers, governments, businesses, insurance companies and others are experiencing climate change first-hand in the form of sea level rise, intense storms, floods, droughts, heat waves and other costly effects. Safeguarding any habitats that absorb carbon from the atmosphere and store it away for long periods is clearly a priority.
Water and CO2
All told, ocean waters take up about one-fourth of all the CO2 that humans emit to the atmosphere. This happens in two ways. First, seawater directly absorbs CO2 from the atmosphere, storing it briefly in various chemical forms. This doesn’t qualify as long-term storage, since CO2 in surface waters can easily transfer back into the air. Seawater will absorb less CO2 and release more as it warms and becomes more acidic. Those changes will occur slowly (though continuously) and there isn’t any practical way to affect them directly or quickly. Therefore physical/chemical absorption of carbon into surface seawater is not a useful component of an indicator measuring long-term carbon storage benefit.
Second, microscopic single-cell or colonial plant cells floating in the ocean (plant plankton) use atmospheric CO2 for their carbon source just as land plants do. Most of that carbon enters the food web as the tiny plants are grazed by animal plankton, which are eaten in turn by larger plankton or small fish, then larger fish, etc. But during all that eating and being eaten, most of the carbon is used to fuel respiration for metabolism, locomotion or other energetic processes, releasing CO2 as a result. Since most of that CO2 reenters the atmosphere relatively quickly, the food web does not provide long-term carbon storage. However, some plankton or other organisms die and slowly drop to the bottom. Any carbon contained in these dead cells or carcasses that sinks deep enough or is buried deeply enough in sediments is effectively out of reach of the atmosphere for centuries and does provide long-term storage. Nevertheless, we didn’t assess this ‘deep storage’ process as part of the Carbon Storage, because it changes too slowly to be a useful indicator and because there are no feasible ways to modify its rate.
Long Term Carbon Storage
However, focusing on the ecosystems existing along the oceans shores was more promising, because three coastal ecosystems---mangrove forests, salt marshes and seagrass beds----are remarkably good at storing carbon for centuries or even millennia if they are left undisturbed. In addition, we have the ability to impact these environments positively and negatively. Human developments and environmental pressures of many kinds have caused rapid declines in the extent or condition of coastal habitats, especially during the last 30-40 years; yet people also have the ability to preserve remaining areas, and prevent their disturbance and/or restore areas that were previously harmed. Rapidly changing measures make good components for indicators. Consequently, we focused the Carbon Storage goal on these three systems. We did not include tropical coral reefs, because, for each molecule of carbon incorporated into a coral’s skeleton, a molecule of CO2 is released to the water and ultimately the atmosphere. Thus coral reefs do not store carbon long-term.
So we concentrated on mangroves, salt marshes and seagrasses, powerful partners in reducing the rate of CO2 accumulation in the atmosphere and consequent global warming. Now we needed to find data on their present and historical extent and condition. This was much easier for mangroves, since their extent can be measured relatively easily by satellite imagery and most countries report mangrove extent to the Food and Agriculture Organization every five years. Satellite data cannot separate the 70 or so species of mangroves, and some may fix carbon at greater rates than others, but having a sense of the overall change in extent is a good first start. Salt marshes and seagrasses cannot be as easily measured by satellite imagery, so we had to estimate extent or condition based on data from as many sites which we could find data. All three habitats store about the highest amounts of carbon per square meter observed for any systems on Earth. Since estimates overlap for the three systems, we assumed that they were equally effective at storing carbon. Any differences revealed by future research could be incorporated into the model.
Likely Future Trends
In order to assess the likely near-term (5 year) future of these systems, we looked at their historical (5-year) trends for extent or condition, and also assessed the intensity and importance of a number of pressures that influence their carbon-storage effectiveness. We looked at the following factors as “pressures” that could decrease future carbon storage:
*chemical and nutrient pollution
*the introduction of alien species
*subtidal and intertidal habitat destruction
*destructive artisanal fishing
*changes in sea surface temperature
We also evaluated the things that could reduce pressures and improve carbon storage, including regulatory measures related to pollution reduction, habitat protection and social resilience as indicated by the World Bank’s Worldwide Governance Indicators project.
Data on how we put all of this information together into a
model and calculated the score for Carbon Storage are available at www.oceanhealthindex.org and www.oceanhealthindex.org/About/Methods.
Looking ahead, we hope that expanded research efforts will in time yield better global data on the geographical extent and condition of these three ecosystems that are so important to Carbon Storage. Better data on salt marshes and seagrass ecosystems are particular priorities. Since salt marshes, seagrasses and mangroves are key components of the Coastal Protection and Biodiversity goals and also play important supporting roles for Food Provision (Wild-Caught Fisheries sub-goal), Tourism & Recreation and Sense of Place, preserving them will provide significant and widespread ecological, social and financial benefits.