The Potential of Carbon Sinks - Part 1

The Oxford Dictionary describes Carbon Sinks as being: ‘A forest, ocean, or other natural environment viewed in terms of its ability to absorb carbon dioxide from the atmosphere’ through the process of carbon sequestration. This process is a vital and key aspect of our natural world, the natural lungs of the plant which is essentially being marred by the industrious actions over the centuries of human action. Carbon sinks are important because they take in the harmful carbon dioxide that has been released and store it – this action of removal is vital, as without it the effect of climate change would be unprecedented.
 

Figure 1. Carbon Sequestration and Input Cycle Source: Nasa

Figure 1 shows the 2 key carbon sinks on our planet: plants and soil by proxy of land and the ocean. As of far, 48% of the carbon produced through fossil fuel combustion has been sequestered and stored by the ocean (Nasa). The oceans over 70% of the earth’s surface, making it a vital and necessary consideration in the context of carbon sinks. Water, cold water especially is superior in absorbing carbon than warm water is. The thermohaline circulation process becomes vital here, distributing the carbon away from cooler waters and into warmer ones which can create issues of ocean acidification (explored in later posts).

Understanding the natural carbon sinks are vital, especially so where the efficiency of the land and oceans to absorb excess carbon dioxide in the system is vital to reduce the radiative forcing that is key in climate change this creates a feedback system between land/ocean and climate. Climate models, for example he carbon-climate model NCAR carbon-CSM1.4, are showing that, where carbon levels are rising the capacity of land and ocean acting as carbon sinks decrease (Fung et al 2005).

Forest and plants are also an important purveyor of carbon sequestration, as demonstrated by the intake of 1.65 x 10¹⁵g of carbon in the Chinas forests that cover 14% of the country. ‘China's… net carbon removal constituted an estimated 28-37% of CO2 emissions in the 1980s and 1990s.’ This figure shows just how important carbon sinks are for the removal of carbon in the atmosphere. But human actions of deforestation and a rise in fossil fuel burning is a critical threshold and like finite fossil fuels there is a finite ability of the land and oceans to sequester carbon.

Ocean Carbon Sinks & Ocean Acidification:

It seems simple so far. Release carbon and the ocean will breathe it in. Unfortunately, it is not that simple. Oceans are a sink for 25% of anthropogenic emissions annually (Heinze et al 2015) through two methods:

-       1. ‘abiotic inorganic cycling of carbon that involves CO2 air–sea gas exchange’ (Heinze et al 2015: 329 and Nightingale et al 2000)

-       2. ‘hydration to carbonic acid, dissociation of carbonic acid’ which considers the biological capacity of the carbon cycling (Heinze et al 2015: 329), eventually creating 2 Bicarbonate Ions through actions of photosynthesis (IPCC) that are difficult to release back in the atmosphere (figure 3). This leads to the build-up of dissolved organic carbon in the oceans – a reservoir of stored carbon.  

Time Scales & the actual and potential ability for carbon to be stored:

Heinze et al (2015) estimate that this carbon sequestration in ocean carbon sinks skirts on the time frame of 10 000 years. Carbon ‘breathed in’ by the oceans aren’t stored indefinitely by the oceans, there is the potential that stored carbon will be released where they are part of the surface waters. Here, the Thermohaline Circulation is important – figure 2 shows the major ocean circulation patterns and the dominant carbon sinks. Where the waters become ‘deep water’ they are transported away due to density and salinity controls – but more importantly the transported water at some stage can become ‘surface waters’. This allows the ability of the carbon to be released back out.

Figure 2. Thermohaline Circulation and Carbon Sinks of the Oceans (Source: Nasa Earth Observatory)

Figure 3.Carbon Ocean Acidification: Biological Carbon Cycling (Source: NOAA)