Download Carbon Cycle Background

Carbon Cycle Background
Carbon is an essential building block for life on Earth. It is found in the atmosphere (carbon
dioxide), in plants (as part of wood and leaves, for example), in the ocean (for example,
dissolved in water as carbonic acid), and in rock (for example, as fossil fuels such as coal
and oil). Over time, carbon can move from ocean to land to atmosphere and back again,
changing its form along the way. This is known as a ‘carbon cycle’.
Carbon moves through the atmosphere, oceans, plants, soil and the Earth (rocks and
volcanoes) in cycles over time. As it moves between these different ‘pools’, the form of
carbon changes. In the oceans, it might be dissolved, as carbonic acid; in plants carbon is
found in sugars or wood (as we saw in the last activity); in the Earth it may exist as coal, oil
or natural gas; and in the atmosphere, we might find it in the form of the greenhouse gases,
CO2 and CH4 (methane).
As in any cycle, carbon in its various forms has to come from somewhere and go
somewhere else. Where it comes from is called a source; where it goes is called a sink. The
carbon-containing substance doesn’t simply disappear into a sink and stay there.
Eventually, it returns to its starting point and goes through the cycle again, even if that
takes a long time. This means that sinks can be sources, and sources, sinks. If the cycling of
carbon from sources to sinks and back again happens at a constant rate, the cycle is
considered to be in balance. However, if, for example, we transfer carbon from trees and
fossil fuels (sources) and put it into the air (sink) faster than it can be removed, we
disrupt that balance. In this case, the result is an increase in atmospheric levels of CO2.
In some cases, carbon cycles very slowly – it may take as long as millions of years for
carbon to move from racks to the atmosphere or back into deep sediments. In other cases,
carbon can cycle quickly at the rate of months for exchanges between plants and the
atmosphere over the course of a growing season or tens to hundreds of years for the
decomposition process to release carbon back to the atmosphere from organic remains.
The long-term cycle moves carbon into the atmosphere, across the ocean floor, through
fossil fuel deposits and volcanoes, and into rocks and the Earth’s crust. Set against the
human time scale, long-cycle carbon moves so slowly that for all practical purposes it is
locked in storage forever. By contrast, carbon in the short-term cycle flows relatively
briskly through the air, the ocean, the plants and the soil. Key drivers include
photosynthesis and decomposition. During photosynthesis, green plants take in carbon
dioxide and emit oxygen. Conversely, microorganisms that decompose plants consume
oxygen and emit CO2.
In the short run, elemental cycles, such as for carbon tend to remain relatively stable. But
human intervention can change that. When we extract and burn fossil fuels like coal, oil
and natural gas to heat our homes, power our cars and generate electricity, large amounts
of carbon suddenly jump the track from the long-term cycle to the short-term cycle. Most
of that carbon ends up as CO2 and methane (CH4) in the atmosphere. Today, more carbon
travels from the Earth’s surface to the atmosphere than the other way around.
When plants and trees burn or rot, the carbon stored inside of them is also emitted into the
atmosphere. If the sum total of all plants and trees on Earth – its total biomass – decreases,
then the amount of carbon dioxide emitted into the atmosphere will increase. This
reduction in biomass further alters the short-term carbon cycle.
The movement of stored carbon from the long-term cycle into the short-term cycle raises
atmospheric concentrations of CO2 and CH4. Since both carbon dioxide and methane trap
heat, this intensifies the greenhouse effect and raises average global temperature. All
across the globe, higher temperatures affect regional climate and weather patterns, and
cause sea levels to rise. A hotter climate may spark other changes that release even more
carbon dioxide, raising temperature further – and setting off another round of carbon
emission. This is an example of a positive feedback loop that reinforces and magnifies the
risk of climate change.
The more carbon that can be taken out of the atmosphere and stored, the more we can
slow the rate of warming.
Adapted from Climate Change Backpack Handbook, 2008. New England Science Center
Collaborative.