It should not come as a shock to any of us that climate change as a result of human action, or perhaps at this point inaction, is a scientifically accepted fact. Additionally, to reach a goal of limiting average global warming to 1.5 degrees Celsius, scientists  tell us we need to find ways to pull CO2 out of the atmosphere, not just stop emitting it. 

Why? Well, it’d certainly be easier if we could pretend past emissions don’t exist. But, today, about half of all the CO2 emitted since 1850 is still in our atmosphere contributing to climate change. So, how on Earth do we capture, and store, large amounts of carbon safely? 

It turns out there’s some good news. While we work on building large-scale technologies that draw down carbon from the atmosphere, nature has provided a backup plan. Wetlands are the most productive landscape for capturing and storing carbon. Although they occupy only 7% of the earth’s surface, wetlands are currently storing about 225 billion metric tons of carbon. That’s a little more than 6 times the world’s annual CO2 emissions. 

But even as wetlands have potential to help fight climate change, a warming climate threatens their ability to capture carbon. A new study carried out by a team of scientists at the Yellow River Delta in China’s Shandong Province shows that the warmer coastal wetlands get, the less carbon dioxide they can pull from the atmosphere. 

The Yellow River Delta study found that as salt levels in the wetland’s soil increased, carbon storage capacity decreased. Let’s unpack this a bit. When salty water evaporates from wetlands, it leaves salt behind.  Evaporation, caused by increasing temperatures, makes for saltier soil. 

Okay, so what if the soil is saltier? Well, for one, saltiness determines which plants can survive in wetlands. As we might expect, salty soil leads to salt-loving plants taking over wetland landscapes. The Yellow River Delta study found that these salt-tolerant plants tend to be smaller than the plants they’re replacing. And, since plants store carbon through the process of photosynthesis, smaller plants mean less photosynthesis, and less carbon storage. 

Yellow River, Shandong Province China, Source: CGTN

As temperatures rise, wetlands are storing less carbon in the hot summer months than in the cooler autumn months. This marks a departure from plants’ normal carbon storing process. Typically, plants sequester carbon dioxide in the spring and summer when they are leafed out, storing more CO2 for the short term. Then, in the winter and fall, plants store less CO2.

The Yellow River Delta Study shows that as wetlands warm, seasonal variation reverses. Summer months will be much less productive for storing carbon than fall months. Seasonal carbon storage is short-term, but this disruption of normal carbon variations shows how deeply humans are impacting the environment.  Human-caused climate change is threatening not only seasonal carbon cycles in wetlands, but also decreasing their carbon storage capacity overall. 

As we release carbon into the atmosphere, contributing to global warming, we can expect to see saltier coastal wetlands. And as this study establishes, changes in salt levels alter the types of plants present in the wetland, affecting both seasonal carbon dioxide variations as well as the overall ability for wetlands to store carbon on a long-term scale. 

This should raise warning flags. Although they cover just 7% of our planet’s surface, humanity needs wetlands to store as much carbon as they possibly can. But the more carbon we release into the atmosphere, the less ability wetlands will have to effectively store it. We cannot depend on our ecosystems to repair the consequences of our actions. 

This is nature’s final warning to us. As we watch the most effective carbon-capturing landscape lose its potential to help us through this crisis, humanity must take broad and ambitious action to combat climate change, now.