The climate is changing and a big reason why it is changing is due to a rise in carbon dioxide (CO2) levels in the atmosphere caused by human activities. To prevent temperatures rising to dangerous levels we need to lock this CO2 away and one of the best ways to do this is to look to nature for help. But just how much carbon does each type of vegetation store? In this article I’m going to try and find answers for how much carbon is sequestered by forests, grassland and even algae in the oceans as we try and find nature-based solutions to climate change.

 

CO2 levels are rising

 

It is becoming common knowledge that CO2 a greenhouse’ gas. Given this name because in a similar way to the windows it traps in heat from the sun. This is vital to some extent, without it temperatures would be average 18 degrees below freezing (NOAA).

There are a variety of greenhouse gases, CO2 is not the only one, some like methane (CH4) are actually even better at trapping heat. But CO2 is the one we hear about the most because it accounts for the highest percentage of emissions by a long way, with 81% of greenhouse gas emissions in the USA being from CO2 (USEPA 2018).  It also hangs around the longest period of time in the atmosphere adding to the impact.

CO2 in the atmosphere has risen dramatically in the last hundred years, since the industrial revolution led to large scale burning of fossil fuels such as coal, oil and natural gas. In 2013 CO2 levels rose to 400 parts per million (ppm) for the first time in recorded history.

As we work to reduce these emissions from human activities, we also need to find a way to store some of this gas to slow down dangerous warming. But before we reach for ambitious carbon capture technology, we have a lot of nature-based solutions to storing carbon but which ones do it best?

 

How is this carbon stored by plants and trees?

 

All plants and trees capture and store atmospheric carbon dioxide (CO2).

Without turning this into a high school biology lesson, this occurs as they absorb the gas and combined with water and light from the sun, produce sugars in the process of photosynthesis. Some of the carbon is stored in the leaves, stems and roots of the plants and some ends up in the soils where it can stay locked away for thousands of years.

 

Some common jargon

 

Before I delve into the data around how much carbon is stored by various types of vegetation, I thought I would quickly give some definitions around common technical terms.

Carbon sequestration – this refers to the process of capturing CO2 from the atmosphere and storing it securely as carbon.

Carbon sink or carbon reservoir – this could be anything from an individual plant to soils and even the ocean. It is anything that locks carbon away, preventing it from entering the atmosphere.

Carbon source – the opposite of a carbon sink, this results in the release of trapped carbon into the atmosphere. This could be from the burning of fossil fuels, the chopping down of trees or the disturbance of soils.

 

Trees

 

How much carbon does a tree sequester each year?

 

Of all the different types of vegetation, trees are the best carbon sinks due to their larger size and therefore increased area of ‘biomass’ (leaves, stems, roots etc) within which carbon is stored.

As a rough guide, a tree of 10 years of age can absorb up to 48 pounds of CO2 per year. (1) That is approximately 1/757th of the average US citizens emissions in a year.

The amount of carbon a tree sequesters varies depending on species, and the rate at which the carbon is sequestered varies over time too.

A fast-growing tree species will sequester more carbon in the first years as it grows quickly, but these trees tend not to live as long as some slower growing trees. Slow growing trees, which do not sequester as much carbon over the initial years, eventually grow to a bigger size and tend to live a much longer time.

Ideally, you want a fast-growing tree that lives a long time, but that is pretty much impossible because to gain the stability a tree must accumulate mass slowly.

 

How many trees to offset 1 ton of co2?

 

Very roughly, it would take approximately 42 trees to offset 1 imperial ton of CO2 in a year. Based on the broad figure that a tree of 10 years old can sequester 48 pounds of CO2 per year. 

Offset providers such as carbonify.com adopt a rule of 5 new trees planted to offset a ton of CO2 which assumes that at least one of those trees will reach 40 years of age or more.

These figures are all fairly arbitrary because there are so many factors to consider which I will explain in the following sections.

 

How much carbon is currently stored in forests?

 
 

In US forests, the carbon stored in live and dead trees themselves averages 5.5kg of carbon per square meter, or 49 thousand pounds per acre. 51% of which is in the live tree sections, 24% of which is other above-ground wood, 17% of which is in the roots, 6% in standing deadwood and 3 % in the leaves. (2)

 

It’s not just about the trees

 

To simply look at the trees themselves as the carbon sinks in forests is to oversimplify things massively. At a global scale approximately 69% of forest carbon is stored in the soil, with just 31% stored in the biomass of the trees (IPCC 2000). If you take just tropical forests the balance is around 50/50. Nevertheless, soil carbon is a very important carbon sink in any forest.

To put into context how important soils are as a carbon sink. There is twice the amount of stored carbon in soils as there is in the atmosphere currently. (3)

For this reason, climate change might actually impact the amount of soil stored in soils as they dry out in some regions.

 

How should we be planting trees to mitigate climate change?

 

This is where things start to get complicated. We have to be careful not just to plant fast-growing species anywhere because that might even have negative impacts.

As mentioned above, the soil is a very important carbon sink. Some soils are even bigger sinks than others such as peat soils. These soils cover just 3% of the earth’s surface but store twice as much carbon as all the world’s forests (UN).  If we were to plant trees on these soils, the trees would take up moisture from the peat and cause carbon emissions to increase dramatically. So ideally, trees should be planted on soils that are fertile enough to support growth, but which contain low levels of carbon to make sure the benefits from the trees are fully felt.

There is also the question of whether we let forests mature naturally, ending up with complex structures with many different species or if we plant fast-growing species, harvest the wood and then plant more.

There are pros and cons to both these models. The first option mimics the natural processes of the earth and therefore has benefits for wildlife too. A recent study also showed that a more complex forest structure would increase the carbon sequestration. (4) Other factors also come into play such as increased numbers of fungi and microorganisms which can increase carbon storage within the forest soils.

The second option allows carbon to be rapidly sequestered by fast growing trees, which sequester the majority of carbon in the initial years of growth. Once they reach full size, the rate of sequestration slows and the trees are felled. But the carbon is not lost, it is potentially locked away in wood products and building materials and then another tree can be grown in it’s place. Of course, we would have to factor in the emissions for making those products, but they could replace materials with higher emissions such as concrete. The wood could also be used as biomass fuel, which would reduce the reliance on fossil fuels. But critics point to this as an unsustainable solution.

 

Grassland

 
 

Grasslands of one type or another are one of the most common habitats on earth, covering 15 million km2 in tropical areas and 9 million km2 across the rest of the planet. That is around 26% of ice-free regions.  

 

How much carbon does grass sequester?

 

Perhaps unsurprisingly grasslands do not store anywhere near as much carbon in their biomass as trees, due to much smaller size above and below ground. However, soils in grassland habitats are very important carbon sinks.

In total, grasslands store 343 gigatons of carbon in the vegetation and top one metre of soil. Sequestering an average of 0.5 gigatons per year. (5)

As with forests, the potential of a grassland to store carbon varies. In general the amount of carbon a grassland can store increases when there is a greater mix of different species. (6)

The majority of grasslands are used for grazing livestock such as cows or sheep (20 million km2). The intensity at which this grazing is carried out affects how much carbon is stored in the soils. Lowering the amount of livestock on a grassland has been found to increase the amount of carbon sequestered.

Condition of the grasslands is also important, if grasslands become degraded they can start to lose carbon. In the past 30 years approximately 3.02 gigatons of carbon has been lost from grassland soils, either through degradation or land use change. (6)

The ability of a grassland soil to absorb carbon also depends on the microbial activity. Higher microbial activity leads to more carbon being absorbed. It can take a long time to restore this balance in the soil when converting other habitats such as arable cropland to grassland. (7)

This has led some authors to question the merit of converting croplands to grassland as a way of storing carbon and tackling climate change. (8) Studies have shown that this only alters the top section of the soils in the short to medium term. Especially if the new grassland is grazed with animals such as cattle which have other negative impacts on the environment such as methane emissions and fertilizer use.

 

Wetlands

 

There are many types of habitat that are emerged in water either all or part of the time. These include swamps, marshes, lakes and many others.

 

How much CO2 do wetlands absorb?

 
 

Despite only making up 3% of total land area, wetlands sequester 30% of all soil carbon. (9) Prairie wetlands alone sequester 7.5 tons of carbon per acre.

Similar to grasslands, the majority of the carbon is stored in the sediment which turns into soils as vegetation breaks down. This soil, which forms in these waterlogged acidic conditions, is known as ‘peat’, which forms slowly over time but can store huge amounts of carbon. In Scotland the amount of carbon stored in peat soils is 140 times that produced by the country in CO2 emissions in one year.

As with other habitats, the ability of a particular wetland to absorb carbon depends on many factors from water availability to temperature. Cooler regions, in particular, store large amounts of carbon. In the USA, Upper Midwest and Eastern Mountain regions account for nearly half the wetland carbon storage in the entire country.

The draining and altering of various wetlands across the world (usually to make them suitable for farming) has led to a dramatic reduction in the ability of these habitats to store carbon. Creating and restoring wetland habitats can therefore have a big impact in the fight against climate change. Estimates show doubling the amount of carbon stored in the world’s wetlands would be the equivalent of removing one million cars from the road.

 

Algae and other sea vegetation

 
 

How much CO2 do algae absorb?

 

Algae (or phytoplankton) absorb approximately 45 to 50 gigatons of carbon per year into their cells. (10)

These algae (and carbon) is either eaten and passed up the food chain, or decomposes and ends up in ocean sediments. In combination with other organisms from the ‘sunlit layer’ they send 15% of the organic material they produce to the deep oceans. Once more vitally locking carbon away and preventing it from entering the atmosphere. (11)

The amount of algae in the oceans is being affected by global warming. The warmer top section is no longer mixing as readily with the cooler ocean below and nutrients are not becoming available for the algae to thrive. (12)

It was thought that potentially by stimulating the growth of algae in oceans, carbon dioxide could be captured from the atmosphere and stored in the oceans instead. Scientists tested this theory by spreading iron fertilizer in sections of oceans to stimulate more algae growth. Unfortunately, the results were not that promising and they found that 100 times less carbon was absorbed than some scientists had predicted. Even more worryingly they found other unwanted side effects occurred such as the emission of methyl bromide which can break down the ozone layer.

Even though algae acts as a carbon sink, much more carbon is stored in coastal areas, where more substantial plants such as seaweeds grow. Approximately 50 to 70% of the total carbon stored in marine ecosystems is in these coastal areas. (13) So in terms of restoring and creating habitats for carbon storage our efforts are likely better spent in these coastal areas on salt marshes and wetlands.

 

The problems with offsetting emissions

 

Offsetting emissions buys us some time in the fight against climate change. However, there are a number of reasons why we can’t rely on this as a solution.

Firstly, it simply won’t be enough. With a growing global population estimated to reach 9 billion by 2050, simply restoring habitats and planting trees won’t be enough to offset the increase in emissions. Therefore, alongside storing more carbon, we must work to reduce our emissions quite dramatically at the same time to keep global temperature rises within non-dangerous limits.

The psychology of offsetting emissions is also troubling. It allows us to feel like it is ok to carry on ‘business as usual’ because trees and other plants with clean up the mess after us. This is a dangerous mindset and why many people are critical of the concept of offsetting.

As explained in the article, climate plays a vital role in the amount of carbon that can be stored by vegetation. As temperatures rise, it brings into play a lot of uncertainty as soils begin to dry out and soils can no longer hold as much carbon. There is no point in creating lots of habitat such as new forests, if in a few decades time they are unable to survive in the extreme conditions we have created.

   

References

  1. Figure is taken from http://urbanforestrynetwork.org/benefits/air%20quality.htm
  2. Birdsey, Richard A. “Carbon storage and accumulation in United States forest ecosystems.” Gen. Tech. Rep. WO-59. Washington DC: US Department of Agriculture, Forest Service, Washington Office. 51p. 59 (1992)
  3. Genxu, Wang, et al. “Soil organic carbon pool of grassland soils on the Qinghai-Tibetan Plateau and its global implication.” Science of the Total Environment 291.1-3 (2002): 207-217.
  4. Gough, Christopher M., et al. “High rates of primary production in structurally complex forests.” Ecology (2019)
  5. Part of Springer Nature 2018 175 K. Lorenz and R. Lal, Carbon Sequestration in Agricultural Ecosystems, https://doi.org/10.1007/978-3-319-92318-5_4 
  6. Chen, Shiping, et al. “Plant diversity enhances productivity and soil carbon storage.” Proceedings of the National Academy of Sciences 115.16 (2018): 4027-4032.
  7. Lange, Markus, et al. “Plant diversity increases soil microbial activity and soil carbon storage.” Nature communications 6 (2015): 6707.
  8. Gosling, Paul, Christopher van der Gast, and Gary D. Bending. “Converting highly productive arable cropland in Europe to grassland:–a poor candidate for carbon sequestration.” Scientific reports 7.1 (2017): 10493
  9. https://phys.org/news/2018-11-wetland-experts-role-vital-carbon.html
  10. Falkowski, Paul. “The power of plankton.” Nature 483.Suppl 7387 (2012): S17.
  11. Laws, Edward A., et al. “Temperature effects on export production in the open ocean.” Global Biogeochemical Cycles 14.4 (2000): 1231-1246. Algae make good carbon sinks because they are short lived, being replaced on average every two to six days.
  12. Behrenfeld, Michael J., et al. “Climate-driven trends in contemporary ocean productivity.” Nature 444.7120 (2006): 752.
  13. Duarte, Carlos M., Jack J. Middelburg, and Nina Caraco. “Major role of marine vegetation on the oceanic carbon cycle.” Biogeosciences discussions 1.1 (2004): 659-679.

Rob Wreglesworth

Rob is the head writer and podcast producer for The Disruptive Environmentalist. He is on a mission to build a community of people that are passionate about solving environmental problems.
Rob Wreglesworth
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