What is a carbon sink?

A carbon sink is a natural Carbon sequestration process that removes carbon (CO2) from our atmosphere and stores it. Examples of carbon sinks include plants, healthy soil and the ocean. Plants sequester carbon by absorbing carbon dioxide through their leaves and storing it in the form of sugars. Plants then return to the soil as organic matter. In the ocean, phytoplankton (microscopic marine plants) absorb carbon that then returns to be stored on the seafloor.

Carbon sequestration is a process that is vital to Earth’s systems and human life as we know it. As our activities continue to release more and more carbon into the atmosphere, carbon sinks become increasingly important to cultivate and care for.

According to the Guardian, scientists have found that our major carbon sinks of Earth’s trees and land absorbed little CO2 in 2023. This breakdown of these essential systems is tied to rising heat resulting in drought and wildfires. The article explains how the last remaining carbon sink - a tropical rainforest known as the Congo Basin - is the only rainforest remaining that removes more carbon than it releases, due to its biodiversity and resilience (for now) against drought.

This is devastating news to environmentalists everywhere, as well as those who just don’t know it yet. It’s easy to look at the situation and despair. But what can we do to support carbon sequestration as individuals? How can we take the load off of our forests and combat drought, increasing resilience for life on Earth?

Carbon Sequestration at Home

Soil sequesters carbon primarily through the process of plants collecting carbon dioxide from the atmosphere and then returning to the soil as organic matter. Organic matter consists of not only plant material, but microbes such as bacteria and fungi as well as material from once-living organisms. Returning to the soil is a vital process for living things that makes soil carbon sequestration possible.

Soil building practices to sequester carbon are known as “carbon farming.” This includes various ways of soil management that include minimizing the disturbance of the soil through lack of tilling and planting perennial plants (plants that live more than a year). Cover cropping and composting are another way to build the soil for carbon sequestration.

In a Californian urban setting (aka your backyard) you can build the soil by planting cover crops and having a compost method. Cover crops that do well in backyards include Vetch (Common Vetch or Purple Vetch), Crimson Clover, Phacelia (a cool season native), Fava Beans and Field Peas. All of these except Phacelia have the added benefit of adding extra nitrogen to your soil.

Soil building can be done seasonally, during a garden redo or new installation. This will greatly increase soil health, reduce the need for fertilizer, improve water retention, and sequester lots of carbon. Cover crops are often also very cost effective and the seeds can be found in most nurseries.

Ditch the lawn already

Lawns are not sustainable for many reasons. Especially here in California, where we face drought and heat that make lawns expensive and impractical. Not only that, but lawns do not make the best carbon sinks. Yes, grasses can sequester carbon BUT in these cases the soil becomes a limiting factor. Urban lawns do little to build the soil, instead often depleting it as well as can damage soil health through the use of pesticides and fertilizers. Caring for a lawn also releases a lot of emissions through the use of blowers, mowers and weed whackers. Keeping a lawn manicured takes a lot of resources.

Grasslands are one of the biggest carbon sinks on land, but what’s the difference between a prairie and an urban lawn? Turfgrass has an average root depth of about 4” whereas wild prairies and grasslands have deep roots. This increases the amount of carbon they can store, which is also increased by their biodiversity. Native meadows also require less maintenance and the biodiversity supports the soil, making them a better choice for the environment.

I hear people say “ a lawn for their dog or for their kids to play on, but even so there are many alternatives. Carex pansa is a California native sedge that works very well as a lawn substitute here. It needs much less water and foot traffic helps the rhizomes spread so it can grow even more lush. It can even be mowed! However, I think it looks the most attractive as a soft, shaggy meadow that feels much better to lay on than regular grass.

Plant and preserve trees

It should be no surprise that trees sequester a lot of carbon. They absorb carbon dioxide through their leaves and store it within themselves. In a stable climate, forests store more carbon than grasslands but with increasing wildfires, trees can release all their carbon stores as they burn. This is why grasslands are considered more resilient carbon sinks. However, fires are not as much of a concern in urban areas (though this is changing depending on your area) and trees should not be discounted.

Trees provide many environmental services including shade, moisture in the air and habitat for birds as well as mental health benefits. They can also be managed to decrease the risk of fire through pruning and spacing of vegetation, so should still be planted in fire risk areas. The older the tree, the more carbon it sequesters so there is just one more reason why old trees need to be protected. Planting new trees mindfully (so that they have space to grow) and taking care of them as they age is a great way that we, on a community and individual level, can sequester carbon.

The choices we make

There are many ways to help sequester carbon and mitigate climate change. Building soil, switching out your lawn for a native and biodiverse meadow, planting trees, preserving old trees are just some of the ways you might be able to personally contribute by making changes in your garden. Additionally, making sure your landscape gets adequate water is very important to replenish the aquifer and mitigate wildfire risk. Dry, dead soil can’t continue to sequester carbon so making sure you are not letting your yard dry out is vital. Installing a greywater system is also recommended so that you can direct the water from your tap out into the landscape.

We can all make choices to be more environmentally friendly. Our rivers are made of many raindrops and our communities are supported by many choices. Some people have more ability to make an impact, but we can all do something. One thing I plan to do is seed collecting to help native plants and animals regain habitat by spreading the seeds through urban corridors. I collect California poppy seeds (making sure to leave some) along a fence in downtown Oakland. This is totally free and just one example of making a small difference by helping to increase biodiversity in my neighborhood.

Oh, and you bet I’ll be cover cropping this Fall! Berkeley Horticultural Nursery has some cover crops like Crimson Clover for sale and I will be spreading those out in my backyard where I’m currently renting. :)