American Scientist

To the Editors:

I found the interview with Ariana Sutton-Grier, Sinking Carbon in Coastal Wetlands (November-December, 2020), both interesting and timely, given the recent catastrophes of climate change and sea level rise the world is facing.

As Dr. Sutton-Grier points out, coastal wetlands can be very effective at removing carbon dioxide from the air. But aren’t many of them already full? Most coastal wetlands have developed during the past few thousand years of the Holocene epoch when sea level has been fairly static. In each salt marsh, the vegetative layer has an upper and a lower boundary; the vegetation may continue to enrich the soil beneath it with carbon, but there is a limit to how high the vegetation can grow. So the vegetation doesn’t absorb above-ground carbon the way a forest does unless it actually converts to a forest—which may happen as inorganic sedimentation raises the soil level and the marsh is eventually lost.

On a tectonically stable seacoast, a sea level increase of a few centimeters or even a few meters should have the effect of raising the top of a salt marsh by that same amount, thereby increasing the thickness and volume of that organic layer. The marsh would thus become a small element of a natural self-regulating system for CO₂, thereby mitigating global warming and sea level rise.

As Dr. Sutton-Grier points out, salt marshes are effective carbon sinks only as long as we don’t mess with them. They provide a more permanent solution than reforestation for that purpose. In a world with ever-increasing demand for lumber and agricultural land, many forests unfortunately have to be regarded as repositories for temporary carbon storage rather than as permanent carbon sinks.

David C. Bushnell
Alamo, CA

Dr. Sutton-Grier responds:

This letter brings up an interesting topic: natural carbon sinks and whether they saturate or become “full.” We are still learning about whether even old-growth forest ecosystems are still sequestering carbon or whether they do reach a point where they might be considered full. However, where forests have been destroyed or degraded, we can restore them. There are plenty of opportunities for natural climate mitigation in restored forests.

Coastal wetland ecosystems have been experiencing very slow sea level rise for millennia, which has enabled them to continue to accumulate carbon over long periods of time. Coastal wetlands have been accumulating sediment and carbon to keep pace with sea level rise. Their location at the intersection of land and sea sets coastal wetlands apart from other natural climate mitigation opportunities because they continue to develop deep, organic-rich soils over time and are not a saturating sink.

It is important not to become overly concerned with comparing and contrasting natural climate solutions to one another. We need rapid climate action in order to avoid very serious environmental impacts, which means we need to act on all possible climate solutions, including restoring and protecting all our carbon-absorbing ecosystems (including forests, grasslands, and wetlands) as well as decreasing energy use, increasing energy efficiency, and increasing renewable energy sources. We need immediate action on every solution to help us slow climate change.