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Carbon & water cycles - Life on Earth

Water, carbon, climate and life on Earth
The key role of the carbon and water stores and cycles in supporting life on Earth with particular reference to climate.

The relationship between the water cycle and carbon cycle in the atmosphere. 
This section has been covered continuously throughout the previous pages.  The section below is simply a summary of much of that information.  The Carbon and Water cycles are linked together in many ways such as;
1. Ocean-atmosphere interchange – physical and biological pumps
2. Volcanic outgassing of both water and carbon transfer them from the lithosphere to the atmosphere
3. Thermohaline circulation – the movement of the Earth’s ocean currents transfers heat energy and organic matter around the globe.  It also allows carbon to be diffused into the water when it cools in the northern hemisphere and be dragged down to the depths of our oceans.
4. Ocean warming – the increasing impact of the enhanced greenhouse effect is forcing the oceans to warm.
5. Permafrost melting – the changes in the carbon cycle are melting parts of the cryosphere, releasing more carbon dioxide and methane from methyl clathrates
6. Photosynthesis and respiration transfer carbon and water between the biosphere and the atmosphere
7. Weathering releases carbon from the lithosphere and transfers it into the atmosphere and oceans
8. Ocean acidification – where ocean water is made less alkaline by rising carbon levels in the water
Therefore, the links between the global water cycle and the global carbon cycle are strong. Both are key ingredients to all life on the planet.

Interrelationships between the water and carbon cycles.
Energy from the sun sets in motion both the carbon and water cycles. Recall that sunlight plus water plus carbon dioxide are combined by photosynthesis in green plants to create carbohydrates.

However, changes to the carbon cycle can have an impact upon the water cycle.  Through enhanced global warming via increasing levels of carbon in the atmosphere the impact has been to super-charge both cycles.

For example, we have seen greater evaporation in parts of the world that creates heavier rainfall in some areas and deeper droughts in others. This shows that the water cycle has been altered by the greater amount of carbon in the air. Similarly, plant growth is accelerated by greater carbon levels in the atmosphere. This accelerated plant growth also contributes more water vapor to the atmosphere during transpiration, which likewise leads to heavier downpours during rain events.

So, both water and carbon are cycling faster and differently as our climate changes. 

Water and carbon cycle links

The role of feedbacks within and between cycles and their link to climate change and implications for life on Earth.

Whilst the burning of fossil fuels, deforestation, and land use changes have altered the carbon budget, this is only the first or PRIMARY stage in the story.  It is being increasingly acknowledged that changes in one part of the carbon and water cycles can cause additional unexpected consequences.  Changes to one part of the climate system can cause additional changes to the way the planet absorbs or reflects energy. These are known as SECONDARY CHANGES and are also called CLIMATE FEEDBACKS.  The worry scientists have is that they could FURTHER increase the amount of warming caused by carbon dioxide alone.

Snow and ice

Snow and ice is melting in the Northern Hemisphere and it has been seen that warming temperatures are already melting a growing percentage of Arctic sea ice. This exposes dark ocean water during the summer. Snow cover on land is also declining in many areas. This changes the ALBEDO of these areas.  The albedo is the proportion of the incident light or radiation that is reflected by a surface, and it is highest for white surfaces and less for darker surfaces (more energy is absorbed).  As snow and ice cover decreases these areas go from having bright, sunlight-reflecting surfaces that cool the planet to having dark, sunlight-absorbing surfaces that bring more energy into the Earth system. This is causing more warming and is hence a positive feedback loop.

Water Vapour

Water vapour is a strong greenhouse gas and according to NASA is the largest feedback factor.1 The relative abundance of water in the atmosphere means it causes about two-thirds of greenhouse warming. As temperatures warm, more water vapour evaporates from the surface into the atmosphere, where it can cause temperatures to climb further.

Clouds
With more water vapour in the atmosphere we are seeing more clouds.  Clouds can both cool the planet (by reflecting visible light from the sun) and warm the planet (by absorbing heat radiation emitted by the surface, see diagram opposite).  According to NASA, in our current climate, clouds have a cooling effect overall, but that could change in a warmer environment. 1

This can also vary by cloud type and how high they are in the atmosphere.   For example, low, warm clouds emit more energy than high, cold clouds. Read more by visiting the NASA website.

Cloud feedback loop

The carbon cycle and the oceans
At present increased atmospheric carbon dioxide concentrations and warming temperatures are causing changes in the Earth’s natural carbon cycle. Much of the carbon emitted by human activity has been absorbed by the oceans, causing the oceans to become less alkaline.  This helps to slows global warming by removing some carbon dioxide from the atmosphere.  However, in the future as warmer ocean waters can hold less dissolved carbon, it will leave more in the atmosphere.

Carbon cycle feedback and the cryosphere
As climate warms, we are seeing the thawing Arctic tundra and permafrost, which can release trapped carbon dioxide or methane to the atmosphere. This is shown below, and is a positive feedback loop.

Permafrost feedback loop

However, extra carbon dioxide can stimulate plant growth and these plants to take additional carbon out of the atmosphere. The limiter to this is when plant growth is limited by water, nitrogen, and temperature. This is a negative feedback loop, as it diminishes the impact of the original change.

Negative_Carbon_Feedback loop

NEXT TOPIC - Human interventions in the carbon cycle

 

Sources
1 – NASA:Earth Observatory, 2010. How Much More Will Earth warm? - https://earthobservatory.nasa.gov/features/GlobalWarming/page5.php

Written by Rob Gamesby

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