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How do carbon sinks lower CO2 emissions?

Over the last two months, anthropogenic behaviour has significantly slowed down due to a worldwide lockdown. As we remain inside, the natural world continues to move on and many environmental benefits have been seen. This includes; improvements to air quality and visibility, clearer waters in Venice and liberated wildlife. However, one particular outcome has caught the attention of many. Since human activity is the sole driver of CO2 emissions, this crisis has caused emissions to dip to the lowest they’ve been since WWII. Although, there has been speculation as to whether this will remain or be completely offset when countries loosen restrictions in the coming months. 

In an attempt to curtail this pandemic virus, restrictions show clear evidence that when we slow down Earth bounces back. Carbon sinks are one way the earth tries to counterbalance our impact on CO2 emissions. 

What are carbon sinks? 

Carbon sinks are natural systems that absorb and store more carbon dioxide from the atmosphere than they release. There’s a total of 43,500 billion tonnes of carbon stored in these natural systems. Oceans and vegetation are the two largest forms of carbon sinks, less than half remains of the carbon emitted into the atmosphere from anthropogenic activity. Oceans can absorb up to a ⅓ of global emissions and vegetation can store as much as 45% of all land carbon and absorb pollutants such as ozone.

How do the two largest carbon sinks lower carbon emissions?

Terrestrial carbon sinks such as vegetation naturally obtain carbon by photosynthesis. This process turns carbon dioxide into sugars which can then be stored in aboveground biomass such as leaves and trunks, as well as in soils. The world’s vegetation has a colossal capacity of about 450 billion tonnes of carbon. This equates to nearly the same amount of carbon that humans would pump into the atmosphere over 50 years at current emission rates.  Furthermore, vegetation can improve air quality via dry deposition. This is the deposition of pollutants, gases and particulate matter, onto plant matter where they are then actively absorbed by plant tissue. Subsequently lowering CO2 emissions further as air pollution impacts climate change. Therefore, this tremendous ecosystem is a vital tool in combating climate change. 

Another critical tool is oceans. As oceans cover 71% of the earth’s surface, they are the world’s largest carbon sink. The amount of carbon stored in the oceanic sink (760 gigatons) exceeds the quantity in the atmosphere. There are two ways in which oceans take in CO2 : via photosynthesis and diffusion. These processes form what is well known as  the ‘ocean carbon pump’, which is split into the biological and physical pumps. CO2 enters the biological pump by photosynthesis, enters the food web and is eventually stored in the seabed. However, the physical pump is reliant on ocean circulation. After CO2 is dissolved in the surface water by diffusion, ocean currents and mixing processes transport it from the surface deep into the ocean’s interior, where it accumulates over time. In the deep waters, CO2 can be stored for as long as 1,000 years due to thermohaline circulation. A study by ETH Zurich states that “without the oceanic sink the concentration of CO2 in our atmosphere and the extent of anthropogenic climate change would be considerably higher.” 

The future carbon sinks

It’s clear that both of these carbon sinks are monumental in buffering our impact and maintaining the climate, however, they are both fragile systems. We could reach the tipping point if we push them to their limits. The impact of the ocean absorbing over 2 trillion tons of carbon each year is taking its toll already. A National Oceanic and Atmospheric Administration (NOAA) climate scientist, Jeremy Mathis says “At some point the ability of the ocean to absorb carbon will start to diminish”. In addition, we are reducing the ability for vegetation to store carbon, annually 18.7 million acres of forest is lost. It needs to be understood that biomass is a scarce resource and if it becomes a bigger source than sink it has the potential to add large amounts of carbon to the atmosphere and oceans.

COVID19 has presented us with a rare opportunity to reevaluate and change our ways to create a sustainable future. Society can collectively reduce CO2 emission by decreasing unnecessary travel, food and goods. Consequently, this can benefit our carbon sinks thus further reducing CO2 concentration. Just like everything else in nature we have to work in unison. 

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