Based on evidence of past changes , we know that coastal wetlands can be surprisingly adaptable to changes in sea level. Rising sea levels can actually cause higher rates of sediment deposition in many types of estuaries, so that the floor of the wetland increases in elevation along with the rising sea. This process helps explain the persistence of these unique and highly productive ecosystems through times of much higher and much lower sea levels than today. But despite this inherent adaptability, a number of important coastal marshes, including those in Chesapeake Bay and parts of the Mississippi River Delta, are currently experiencing submersion and erosion and are expected to be heavily impacted by future sea-level rise .
A recent article in the journal Geophysical Research Letters looks more closely at these feedbacks and what they mean for coastal wetlands in the future. The authors found that the response of coastal marshes to sea-level rise depended both on the rate of rise and the amount of sediment found in the marsh water. Their model showed that marshes with very little suspended sediment could not keep up with even a very slow rise in sea-level, while those with more sediment could adapt to a rise of several centimeters per year. (Tidal range, the difference between high and low tide, also affected the response.) At the higher rates of sea-level rise projected by more recent studies, however, only marshes with very high sediment concentrations and very large tidal ranges could be expected to survive beyond the end of the 21st century. Others would fail to keep up and would eventually be inundated with water.
This article is an interesting example of the importance of feedbacks and ecosystem responses in modeling climate change impacts. More than that, though, it offers at least two compelling lessons for those of us interested in managing ecosystems for climate change adaptation.
First, we can see that a lot of the impacts we’ve had on natural systems over the past few centuries will greatly limit their ability to adapt to climate change. While some coastal marshes are naturally low in sediment, others have been so affected by flood control and other changes that they are submerging even under current sea levels. In some cases, we can restore some of the adaptive capacity of these systems by reversing past damage.
But the case of the coastal marshes also highlights the importance of reducing greenhouse gas emissions as quickly and as dramatically as possible. Many ecosystems can adapt to climate change up to a certain point, and we may be able to give them an additional boost by improving management practices. However, for many ecological systems abrupt changes may occur once thresholds or tipping point are reached–beyond which the system can no longer absorb change and shifts to a new state. In the case of coastal wetlands such a threshold would occur when the rate of sea level rise outpaces the ability of the system to generate new marsh. Once these wetlands are finally submerged and converted to open water it is very unlikely that they will be able to return to their former state. Unless we greatly limit the rate and magnitude of climate change, there will be very little we can do to help ecosystems adapt to the rapid, extreme changes in climate to which we are now committing ourselves.
Kirwan, M.L. et al. Limits on the adaptability of coastal marshes to rising sea level. Geophysical Research Letters 37, L23401 (2010).