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Carbon cycle (changes)

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The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the the biosphere, soils, rocks, oceans, and the atmosphere of the Earth. One important aspect is the production and consumption of CO2 contained in the atmosphere, which is directly linked with Global warming.


How do plants affect the carbon dioxide concentration?

This is the answer that Nathan Urban gave over at the the Azimuth blog here:

Short answer: natural forests haven’t added much to the industrial increase in atmospheric CO2. Humans burning forests and cutting trees down have added some CO2, but it’s still much smaller than the amount of CO2 added by fossil fuel burning.

Longer answer:

Plants emit CO2 during the day and night (autotrophic respiration). During the day they also absorb CO2 during photosynthesis, so they are net carbon sinks. At night, there is no photosynthesis, and so they are net carbon sources.

Growing plants absorb far more CO2 during photosynthesis than they respire. This is what makes them grow. (Most of the carbon in a tree trunk, for example, comes from the air, not from the soil.)

Over the long term (decades to centuries), forests are mostly carbon neutral, if you follow them through their whole life cycle into death: when they die and decay, they emit carbon back to the atmosphere. (Over shorter periods, forests can alter atmospheric CO2 depending on whether they’re mostly growing, dying, being burnt in fires, etc.)


As carbon cycle data always is at a given time and space we give some useful below. The pre-industrial global carbon cycle approximate sizes of reservoirs (flows in bold):

ReservoirPre-industrial(Gt carbon and flow Gt/year)
Land (plants,soil)2300
Fossil fuel3700
Land uptake60
Air-land exchange59.6
Ocean uptake70
Air-ocean exchange70.6
Ocean burial0.2

The source is Sarmiento and Gruber, see the References.


  1. It has been claimed that much higher concentrations of CO 2CO_2 than today have naturally occurred in earth’s history. When was that and what impact did it have?

  2. How will a higher concentration of CO 2CO_2 in the atmosphere affect plant life?

Partial answer to first equation: CO2 concentrations have probably been higher for most of the history of the Earth than they are now. Temperatures were correspondingly been much higher. Here is a rough graph of temperatures over the last 4.6 billion years:

Here “Ma” means “million years ago”. This chart was drawn from many sources, but it was copied from:

  • Barry Saltzman, Dynamical Paleoclimatology: Generalized Theory of Global Climate Change, Academic Press, New York, 2002, fig. 1-3.

As you can see, the last 30 million years have been marked by low temperatures. So the problem is not that CO2 levels are higher than they ever were, or that the Earth is becoming hotter than it ever was: the problem is that these changes are happening very fast.

Over the last 400,000 years we can measure CO2 concentrations directly, using bubbles in ice cores:

You can see several ice ages here — or technically, ‘glacial periods’. Carbon dioxide concentration and temperature go hand in hand, probably due to some feedback mechanisms that make each influence the other. But the scary part is the vertical line on the right where the carbon dioxide shoots up from 290 to 390 parts per million — instantaneously from a geological point of view, and to levels not seen for a long time. Species can adapt to slow climate changes, but not so well to fast ones.



Start with this video:

  • James Hansen, speech at the University of Oregon, video, explaining the last 65 million years.

A short introduction focused on the climate:

  • David Archer: The Global Carbon Cycle (Princeton Primers in Climate), Princeton University Press, 1st edition, 2010

For more detail, try these books:

  • Michael C. Jacobson, Robert J. Charlson, Henning Rodhe, and Gordon H. Orians, eds., Earth System Science: From Biogeochemical Cycles to Global Change, Academic Press, 2000.

  • Lee R. Kump, Prentice Hall, 2010 The Earth System 3e

  • William H. Schlesinger, Biogeochemistry: An Analysis of Global Change, Academic Press, 1991.

  • Skinner, Brian J., and Stephen C. Porter, The Dynamic Earth, John Wiley and Sons, 2009.

Advanced, Current Research

To dig deeper, hit the journals:

• Le Quéré et al., Trends in the sources and sinks of carbon dioxide, Nature Geoscience, 2009.

Efforts to control climate change require the stabilization of atmospheric CO2 concentrations. This can only be achieved through a drastic reduction of global CO2 emissions. Yet fossil fuel emissions increased by 29% between 2000 and 2008, in conjunction with increased contributions from emerging economies, from the production and international trade of goods and services, and from the use of coal as a fuel source. In contrast, emissions from land-use changes were nearly constant. Between 1959 and 2008, 43% of each year’s CO2 emissions remained in the atmosphere on average; the rest was absorbed by carbon sinks on land and in the oceans. In the past 50 years, the fraction of CO2 emissions that remains in the atmosphere each year has likely increased, from about 40% to 45%, and models suggest that this trend was caused by a decrease in the uptake of CO2 by the carbon sinks in response to climate change and variability. Changes in the CO2 sinks are highly uncertain, but they could have a significant influence on future atmospheric CO2 levels. It is therefore crucial to reduce the uncertainties.

There are some nice graphs in the above paper, including figure a, which shows the rate of increase of CO2 concentration. There’s a lot of good information here, but they note that it would be good to have more:

Progress has been made in monitoring the trends in the carbon cycle and understanding their drivers. However, major gaps remain, particularly in our ability to link anthropogenic CO2 emissions to atmospheric CO2 concentration on a year-to-year basis; this creates a multi-year delay and adds uncertainty to our capacity to quantify the effectiveness of climate mitigation policies. To fill this gap, the residual CO2 flux from the sum of all known components of the global CO2 budget needs to be reduced, from its current range of ±2.1 Pg C yr−1, to below the uncertainty in global CO2 emissions, ±0.9 Pg C yr−1. If this can be achieved with improvements in models and observing systems, geophysical data could provide constraints on global CO2 emissions estimates.

It is interesting to compare the above paper to Le Quéré et al’s earlier work, which seems to be much more optimistic about increasing drawdown of carbon by oceans and land:

The abstract:

Atmospheric CO2 has increased at a nearly identical average rate of 3.3 and 3.2 Pg C yr−1 for the decades of the 1980s and the 1990s, in spite of a large increase in fossil fuel emissions from 5.4 to 6.3 Pg C yr−1. Thus, the sum of the ocean and land CO2 sinks was 1 Pg C yr−1 larger in the 1990s than in the 1980s. Here we quantify the ocean and land sinks for these two decades using recent atmospheric inversions and ocean models. The ocean and land sinks are estimated to be, respectively, 0.3 (0.1 to 0.6) and 0.7 (0.4 to 0.9) Pg C yr−1 larger in the 1990s than in the 1980s. When variability less than 5 yr is removed, all estimates show a global oceanic sink more or less steadily increasing with time, and a large anomaly in the land sink during 1990–1994. For year-to-year variability, all estimates show 1/3 to 1/2 less variability in the ocean than on land, but the amplitude and phase of the oceanic variability remain poorly determined. A mean oceanic sink of 1.9 Pg C yr−1 for the 1990s based on O2 observations corrected for ocean outgassing is supported by these estimates, but an uncertainty on the mean value of the order of ±0.7 Pg C yr−1 remains. The difference between the two decades appears to be more robust than the absolute value of either of the two decades.

category: carbon, earth science