Penn StateNASA

Module Summary and Final Tasks


End of Module Recap:

  • Carbon flows through the Earth system by a combination of biological, chemical, and physical mechanisms.
  • The global carbon cycle is a critical part of the climate system since it controls the concentration of CO2 in the atmosphere, and CO2 is, because of its long residence time in the atmosphere, the main greenhouse gas "driver" of climate change.
  • The carbon cycle is complicated by the fact that it has fast and slow parts to it, with the deep ocean exerting a lot of inertia that limits rapid changes; it also has a variety of positive and negative feedback mechanisms in which the flow rates depend on temperature. Finally, it has a number of important human components to it — burning fossil fuels, deforestation, and soil disruption.
  • The global carbon cycle is probably never in a steady state, but it has negative feedback mechanisms that drive it toward a steady state. The response time of the system is very long, measured in tens of thousands of years due to the vast, sluggish deep ocean part of the system. Some parts of the system can respond (partially) to changes very quickly, as evidenced by the seasonal variations in atmospheric CO2 seen in the Mauna Loa record.
  • The human alterations to the carbon cycle are significant — we now add more than 10 times as much carbon to the atmosphere as volcanoes. Roughly half of this fossil fuel carbon has remained in the atmosphere, with the rest being taken up by the oceans and land plants.
  • Land plants respond to higher CO2 concentrations in the atmosphere by increasing their efficiency, which enables them to take up increased amounts of CO2 from the atmosphere, but this effect has a saturation point — an atmospheric CO2 level where the uptake of CO2 does not increase any more.
  • The oceans are also an important part of the uptake of carbon, aided by chemical reactions that convert CO2 gas into carbonate ions in solutions that are then exported to the deep ocean through the sinking of calcium carbonate shells, a process called the biological pump.
  • The chemical reactions associated with the uptake of CO2 by the oceans also influence the acidity of the oceans. If the oceans take up too much atmospheric CO2, the acidity will increase — this acidification of the oceans is made worse if the rate of carbon emissions goes up. If the oceans become too acidic, it may have a detrimental impact on biological productivity in the oceans and thus the biological pump may be weakened, which will limit the amount of carbon the oceans can absorb.
  • The various IPCC emissions scenarios all lead to increased temperatures in the next century — even if we reduce the rate of emissions from current levels, the atmospheric CO2 concentration will increase, and thus the temperature will increase.
  • There are some important unknowns in the carbon cycle, including how soil respiration will respond to temperature — if it is very sensitive, then at higher temperatures the soil reservoir will become a source of carbon rather than a sink. Furthermore, warming in the high latitudes of the northern hemisphere may lead to important emissions of carbon from permafrost. Permafrost is in fact currently melting, and the soil trapped in these frozen soils could increase the global warming of the next century by an additional degree C.


You should have completed the lab exercise on the carbon cycle model. If you have not done so already, please do so before moving onto the next module. Incomplete assignments will negatively impact your final grade.


  • Lab 6: Carbon Cycle Modeling