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Sinks and sources

Stephanie Robinson
Monday, September 30, 2013

Carbon sequestration! If you have no idea what this is, I wouldn't blame you — it sounds complicated ... and boring. But simply put, it means the ability to take carbon-dioxide — a colourless, odourless gas — out of the atmosphere and hold it in storage.

Carbon can be found in some form or another in all living things — in the soil, plants and animals, water and in the air. It moves between these hosts in what is known as the carbon cycle.

In this cycle, places which release carbon into the atmosphere are called carbon "sources" and those which "sequester" carbon are called carbon "sinks". There are a number of 'sources' and "sinks" but to keep things simple, we will just concentrate on the role that plants play in the carbon story.

Forests have sometimes been called the "lungs of the planet". If you'd paid any attention in Basic Science class, you would have learnt that plants "breathe in" carbon dioxide (CO2 ) from the air and "breathe out" oxygen.

It's a little more complicated than that but the general idea is that plants use the CO2 to help produce the energy they need to survive. Some of this CO2 is stored in the plant itself but in some cases, a lot more of it is transferred to the soil beneath it. Over time, soil sediments build up; the mass becomes heavy and compacts. Fast forward a couple of millennia and the decomposed matter in the soil will eventually form deposits of fossil fuels like coal, petroleum and natural gas.

Wetlands (mangroves, sea grasses and marshes) are recognised as some of the most effective plant carbon sinks — some studies quote they suck up to 10 times more carbon dioxide from the atmosphere than the average rainforest.

There's a reason for this. While most tropical forests store carbon above the ground, wetlands also move a lot of their carbon into the soil.

Mangroves are a key wetland ecosystem in the tropics. They trap sediment and organic matter driven through their roots by water currents to form thick layers of mangrove mud. Falling leaves and other decomposing plant and animal material are also trapped within the suffocating mud substrate adding to the high amount of carbon in mangrove soil.

So we've established that wetlands are great carbon sinks. That's cool but why is a little extra carbon in our atmosphere a problem because of global warming and climate change?

Like a blanket, our atmosphere wraps around our planet, protecting us from the sun's harmful ultra-violet (UV) rays. This layer of gasses also traps heat and provides a controlled temperature range for plant life to flourish on Earth and from it, every animal and human further on up the food chain. Most temperate countries compare this to a greenhouse — hence the popular phrase "Green House Effect".

But the composition of our atmosphere is changing largely due to the accumulation of certain green house gases (GHGs) like CO2 which thicken the atmospheric layer. More heat is trapped on the Earth's surface and in turn, increases global temperatures.

It makes sense. Using the blanket example, the thicker the blanket, the warmer you'll be underneath. Heat is good but anyone who's been to the Sahara Desert will tell you that too much heat can be downright uncomfortable, if not fatal.

Temperature is a major component of climate and is important for life on Earth. The plants and animals we have in today's world, have all survived because they have adapted to climatic conditions. It dictates their location, seasonal fruiting, migration, mating etc. Those that didn't adapt have all died out.

One of the main causes of this recent warming is the increasing amount of CO2 in the atmosphere over the last century.

In the past, CO2 was added naturally through volcanic eruptions but now a lot more carbon is being directly added through the burning of fossil fuels like coal and petroleum and with the explosion of human population consumption of natural resources is bigger than ever.

For example land use for urban development and agriculture removed a lot of the forests which absorb CO2.

Pollution is also affecting the health and ability of our natural resources to function properly.

In short, we're putting more carbon in the atmosphere, getting rid of our carbon sinks and making our planet weaker. If forests are the lungs of the planet, then we're poking holes all over it!

But perhaps our lack of alarm is because global warming is not a new phenomenon. Throughout recorded history, the Earth has undergone a natural cycle of warming and cooling periods. Some people will argue that we are just in the middle of another natural warming period. But natural or not, scientists do agree our planet is warming faster than any other period and this is affecting the way in which our climate systems works.

We might not be able to fix climate change but what we can do is fix the things we can and this includes reducing our impact on our natural resources.

A lot of wetlands (mangroves included) are being removed because they smell, they're unsightly, presumably a waste of flat land, block easy access to the ocean and in some cases, simply because we really need that sea view to drive up our property value.

Development is essential but shouldn't be pursued blindly without weighing its consequences. Natural resources once gone are difficult to replace and even if we can, it may take decades to get them fully functioning again.

Carbon sequestration is hardly a topic to get the heart racing but there's a chance that the more we know about the benefits our natural environment offers, the more value we will attribute to it and the more measures we will take to ensure that our natural resources stick around for the long-term.

Small steps can often lead to big changes.

* This is the second article in a series supporting the National Mangrove Awareness Campaign in Fiji. Stephanie Robinson is the co-ordinator of the AusAID Building Resilience to Climate Change Program at WWF South Pacific.





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