Ecosystems have the potential to act as Natural Climate Solutions by taking carbon dioxide out of the atmosphere and storing it in the form of plant stems, roots, and soil organic matter. The more popular forms of Natural Climate Solutions include planting trees and restoring endemic, deep-rooted, perennial grasslands. To be effective, these Natural Climate Solutions must sequester carbon dioxide from the atmosphere, and then store this carbon in plants and soils for longer periods than CO2’s lifetime in the atmosphere, which can exceed many hundreds of years. Unfortunately, the time carbon may be stored in terrestrial ecosystems is often less than the lifetime of carbon in the atmosphere. A chronic factor that shortens the lifetime of carbon in an ecosystem is the rapid decomposition of the annual influx of dead plant matter (leaves and stems). Other carbon losses from terrestrial ecosystems are episodic. They occur by fire, harvesting, and natural mortality as plants age or they suffer from insect infestation, disease, and drought.
What else can we do? In California, wetlands have much potential to be effective and long-term carbon sinks. Most of the carbon stored in wetland is stored out of sight in underlying soils rather than in the form of highly visible trees, as is the case of our iconic redwoods. Because wetlands are flooded, little oxygen reaches their soils. Therefore, microbial respiration, which causes the decomposition of dead organic material, is greatly slowed. Consequently, organic peat soil forms and it can bury atmospheric carbon for thousands of years.
Where can we find wetlands? The San Francisco Bay Estuary and Sacramento-San Joaquin Delta were once ringed by expansive native wetlands, with an area exceeding 2200 km2. Unfortunately, over 90% of these wetlands have been lost due to drainage for agriculture and urban/suburban development since the Gold Rush. On the other hand, there is great potential and opportunity to restore these wetlands. This action would stop the current loss of soil carbon and land subsidence, as well as renew their capacity to sequester carbon for extremely, long time periods.
California’s wetlands have special abilities to be effective and long-term carbon sinks. First, they have long growing seasons. Second, the dominant plants, e.g., tules and cattails, grown in freshwater can reach to several meters in height. Their height and density enable them to capture much sunlight and be among the most productive ecosystems in the world meaning they can take a lot of carbon out of the atmosphere. And third, they can store the dead organic plant matter as peat for centuries and millennia because flooding inhibits their decay by microbes.
Our long-term measurements of the net carbon uptake of a network of restored wetlands in the Delta testify to this effect. Our data show that restored wetlands can take up and store on average 300 gC m-2 every year. For perspective, 300 gC m-2 y-1 is similar to the mass of 4 layers of newspaper, one meter square, piled upon one another year by year. We also have found peat to grow at a rate between 1 to 2 cm per year, which can approach a foot per decade.
Is this rate of carbon sequestration big in terms of acting as a Natural Climate Solution? In comparison, a typical ecosystem may take up one-half this amount.
Another lesson we have learned is that all wetlands are not created equal, nor function the same. First, their productivity can vary with time since establishment. Second, their productivity will vary across the landscape due to salinity gradients that are established across the Bay estuary. Third, their productivity may vary year by year due to the amount of runoff from the Sierra Nevada mountains during droughts. And fourth, their productivity can be modulated by the vast amount of standing, dead biomass produced left standing. This biomass can have legacy effects that inhibit growth and photosynthesis the following year. In fact, the Indigenous people, living in the region before European settlers arrived, are known to have burned these wetlands periodically; their signature is noted in layers of charcoal we find in the peat cores that are dated back 7000 years. Finally, each wetland forms as a distinct and complex mosaic of vegetation and water, with varying sizes and shapes of vegetation patches (and their edges with the water), as water and sediments course through them.
As we choose to restore wetlands in the Bay Area, we must remember there will be a number of positive and negative consequences in their establishment that should be recognized. Let’s start with the positive attributes. Restoring wetlands builds peat soil that helps protect adjacent land from sea-level rise, King tides, and storm surges rise without the need for expensive construction and maintenance of levees and sea walls. Restoring wetlands on Delta islands can stop and reverse soil subsidence; many of these drained islands have elevations 5 to 10 m below sea-level because their peat soil has been exposed to oxygen and it has been decomposed by microbial respiration. Stopping soil subsidence in the Delta helps protect the very vulnerable Delta water transfer system, that serves over 25 million Californians, from collapse due to pressure on the levees or from future earthquakes. Tidal wetlands are also wonderful nurseries for native fish (e.g., salmon and Delta smelt) and game. They are a rich habitat for a diverse array of animals (river otters, racoons, coyotes, bears), as well as coastal, local, or migratory birds (ducks, geese, Sandhill cranes) and birds of prey (hawks). This rich and diverse ecosystem is nurtured by the complex food web of invertebrates that are associated with these wetlands. And there are cultural and recreational benefits. These beautiful wetlands can be explored by wildlife watchers, outdoor enthusiasts, and walkers alike. They also cool the local climate bringing much needed respite during the hotter months.
On the flip side, restoring wetlands in the semi-arid climate of California leads to high rates of water loss through evaporation in our water-scarce state; up to 1000 mm of water is evaporated each year, which is about the same amount of water used by irrigated almond orchards in the San Joaquin Valley. Mosquitoes thrive in wetlands and can be carriers of West Nile virus. Also, these wetlands are known to be large sources of methane, another greenhouse gas many times stronger than CO2, but with a shorter lifetime. If the wetlands continue to take up lots of carbon dioxide, it can counteract the effect of the methane emissions, but only if the wetlands remain for the next 100 years at least.
At this point, we find an analogy between restored wetlands and the families in Leo Tolstoy’s Anna Karenina
‘Happy families are all alike; every unhappy family is unhappy in its own way’
Each restored wetland will have its own peculiarity. So, we cannot apply a ‘one size fits all’ strategy to establishing and maintaining them. In practice, it will be best to manage these wetlands for their multiple ecological services. And, once we restore wetlands, we must be ready to maintain and manage them beyond the end of this century; otherwise, the carbon that was painstakingly stored will be returned to the atmosphere.
What is our takeaway messages? In a changing world, the positive attributes of these restored wetlands easily outweigh their negative attributes as they provide a simple easy and cost-effective method to help address multiple problems at once. Furthermore, wetlands take up carbon at much greater rates and store them for much longer times than terrestrial ecosystems. But we have less area to act upon compared to the area of terrestrial ecosystems. In the end, restoring wetlands can be viewed as an effective arrow in our quiver of Natural Climate Solutions. What is more effective is stopping carbon emissions, tout suite. If we don’t put carbon into the atmosphere, we won’t need to take it out.