Nitrogen Cycle Included in Climate Models
Wednesday, 14. October 2009, 07:43:08
Prediction is very difficult, especially if it's about the future, Niels Bohr once said. This is certainly true for predictions about climate change. Climate models are important tools for climatologists. They are based upon what we know (and assumptions) about to-days climate and climate change in the past. They rely on the input, which means that climate models don't tell the full story.
To date, climate models ignored the nutrient requirements for new vegetation growth, assuming that all plants on earth had access to as much 'plant food' as they needed. But now climate scientists have successfully incorporated the nitrogen cycle into global simulations for climate change, questioning previous assumptions regarding carbon feedback and potentially helping to refine model forecasts about global warming.
By taking the natural demand for nutrients into account, the authors of a new study have shown that the stimulation of plant growth over the coming century may be two to three times smaller than previously predicted. Since less growth implies less CO2 absorbed by vegetation, the CO2 concentrations in the atmosphere are expected to increase. However, this reduction in growth is partially offset by another effect on the nitrogen cycle: an increase in the availability of nutrients resulting from an accelerated rate of decomposition – the rotting of dead plants and other organic matter – that occurs with a rise in temperature. Combining these two effects, the authors discovered that the increased availability of nutrients from more rapid decomposition did not counterbalance the reduced level of plant growth calculated by natural nutrient limitations; therefore less new growth and higher atmospheric CO¬2 concentrations are expected.
Capture: Schematic illustrating feedback pathways coupling terrestrial carbon and nitrogen cycles. Blue arrows show, in general, the processes represented in previous carbon-only land model components: plant carbon uptake by photosynthesis draws down atmospheric carbon dioxide (Atm CO2); litterfall and plant mortality pass biomass from plant to litter and coarse woody debris (CWD); decomposition of fresh litter generates soil organic matter; respiration by both plants and heterotrophic organisms returns CO2 to the atmosphere. Orange arrows show the additional processes represented in our coupled carbon-nitrogen land model, differentiated here between rapid internal cycling (solid arrows), and slower fluxes between land pools, the atmosphere, and ground water (dashed arrows). The critical feedback pathway connecting heterotrophic respiration with plant growth is highlighted as a thick orange arrow: decomposition of soil organic matter not only releases CO2 to the atmosphere, it also releases nitrogen from the organic matter (mineralization) in forms that can then be taken up by plants (assimilation). Plant nitrogen uptake competes with the demand for mineral nitrogen from heterotrophic organisms decomposing fresh litter (immobilization, abbreviated (i) in the figure.
The inclusion of the nitrogen cycle marks one more step toward a more realistic prediction for the future of the earth’s climate.
Reference:
Thornton et al. (2009)
Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model.
Biogeosciences, 6, 2099-2120
(Open Acces, and thus freely available for download)
Tomorrow is “Blog Action Day ‘09 Climate Change”. It is expected that more than 6000 bloggers from more than 100 different countries will blog about climate change - posts that will be read by more than 10 million readers.
Blog Action Day is an annual event that unites the world's bloggers in posting about the same issue on the same day on their own blogs with the aim of sparking discussion around an issue of global importance - such as “climate change”. Blog Action Day 2009 will be the largest-ever social change event on the web. One day. One issue. Thousands of voices.
You are kindly invited to listen in tomorrow.
To date, climate models ignored the nutrient requirements for new vegetation growth, assuming that all plants on earth had access to as much 'plant food' as they needed. But now climate scientists have successfully incorporated the nitrogen cycle into global simulations for climate change, questioning previous assumptions regarding carbon feedback and potentially helping to refine model forecasts about global warming.
By taking the natural demand for nutrients into account, the authors of a new study have shown that the stimulation of plant growth over the coming century may be two to three times smaller than previously predicted. Since less growth implies less CO2 absorbed by vegetation, the CO2 concentrations in the atmosphere are expected to increase. However, this reduction in growth is partially offset by another effect on the nitrogen cycle: an increase in the availability of nutrients resulting from an accelerated rate of decomposition – the rotting of dead plants and other organic matter – that occurs with a rise in temperature. Combining these two effects, the authors discovered that the increased availability of nutrients from more rapid decomposition did not counterbalance the reduced level of plant growth calculated by natural nutrient limitations; therefore less new growth and higher atmospheric CO¬2 concentrations are expected.
Capture: Schematic illustrating feedback pathways coupling terrestrial carbon and nitrogen cycles. Blue arrows show, in general, the processes represented in previous carbon-only land model components: plant carbon uptake by photosynthesis draws down atmospheric carbon dioxide (Atm CO2); litterfall and plant mortality pass biomass from plant to litter and coarse woody debris (CWD); decomposition of fresh litter generates soil organic matter; respiration by both plants and heterotrophic organisms returns CO2 to the atmosphere. Orange arrows show the additional processes represented in our coupled carbon-nitrogen land model, differentiated here between rapid internal cycling (solid arrows), and slower fluxes between land pools, the atmosphere, and ground water (dashed arrows). The critical feedback pathway connecting heterotrophic respiration with plant growth is highlighted as a thick orange arrow: decomposition of soil organic matter not only releases CO2 to the atmosphere, it also releases nitrogen from the organic matter (mineralization) in forms that can then be taken up by plants (assimilation). Plant nitrogen uptake competes with the demand for mineral nitrogen from heterotrophic organisms decomposing fresh litter (immobilization, abbreviated (i) in the figure.
The inclusion of the nitrogen cycle marks one more step toward a more realistic prediction for the future of the earth’s climate.
Reference:
Thornton et al. (2009)
Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model.
Biogeosciences, 6, 2099-2120
(Open Acces, and thus freely available for download)
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http://www.biogeosciences.net/6/2099/2009/bg-6-2099-2009.html
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http://www.ornl.gov/info/press_releases/get_press_release.cfm?ReleaseNumber=mr20091009-00
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http://www.scientificblogging.com/news_articles/finally_climate_model_incorporates_nitrogen_cycle_and_things_still_look_grim
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http://www.sciencecodex.com/key_new_ingredient_in_climate_model_refines_global_predictions
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http://www.sciencedaily.com/releases/2009/10/091009204032.htm
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http://www.greencarcongress.com/2009/10/nitrogen.html
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http://www.eurekalert.org/pub_releases/2009-10/drnl-kni100909.php
Tomorrow is “Blog Action Day ‘09 Climate Change”. It is expected that more than 6000 bloggers from more than 100 different countries will blog about climate change - posts that will be read by more than 10 million readers.
Blog Action Day is an annual event that unites the world's bloggers in posting about the same issue on the same day on their own blogs with the aim of sparking discussion around an issue of global importance - such as “climate change”. Blog Action Day 2009 will be the largest-ever social change event on the web. One day. One issue. Thousands of voices.
You are kindly invited to listen in tomorrow.
- bon appetit!
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