Cronin Timothy W.
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ArticleConsequences of considering carbon–nitrogen interactions on the feedbacks between climate and the terrestrial carbon cycle(American Meteorological Society, 2008-08-01) Sokolov, Andrei P. ; Kicklighter, David W. ; Melillo, Jerry M. ; Felzer, Benjamin S. ; Schlosser, C. Adam ; Cronin, Timothy W.The impact of carbon–nitrogen dynamics in terrestrial ecosystems on the interaction between the carbon cycle and climate is studied using an earth system model of intermediate complexity, the MIT Integrated Global Systems Model (IGSM). Numerical simulations were carried out with two versions of the IGSM’s Terrestrial Ecosystems Model, one with and one without carbon–nitrogen dynamics. Simulations show that consideration of carbon–nitrogen interactions not only limits the effect of CO2 fertilization but also changes the sign of the feedback between the climate and terrestrial carbon cycle. In the absence of carbon–nitrogen interactions, surface warming significantly reduces carbon sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback). If plant carbon uptake, however, is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability stimulating plant growth. The resulting increase in carbon uptake by vegetation exceeds carbon loss from the soil, leading to enhanced carbon sequestration (a negative feedback). Under very strong surface warming, however, terrestrial ecosystems become a carbon source whether or not carbon–nitrogen interactions are considered. Overall, for small or moderate increases in surface temperatures, consideration of carbon–nitrogen interactions result in a larger increase in atmospheric CO2 concentration in the simulations with prescribed carbon emissions. This suggests that models that ignore terrestrial carbon–nitrogen dynamics will underestimate reductions in carbon emissions required to achieve atmospheric CO2 stabilization at a given level. At the same time, compensation between climate-related changes in the terrestrial and oceanic carbon uptakes significantly reduces uncertainty in projected CO2 concentration.
ArticleCorrection to “Importance of carbon-nitrogen interactions and ozone on ecosystem hydrology during the 21st century”(American Geophysical Union, 2009-08-22) Felzer, Benjamin S. ; Cronin, Timothy W. ; Melillo, Jerry M. ; Kicklighter, David W. ; Schlosser, C. Adam
PreprintImpacts of ozone on trees and crops( 2007-07-05) Felzer, Benjamin S. ; Cronin, Timothy W. ; Reilly, John M. ; Melillo, Jerry M. ; Wang, XiaodongIn this review article, we explore how surface-level ozone affects trees and crops with special emphasis on consequences for productivity and carbon sequestration. Vegetation exposure to ozone reduces photosynthesis, growth, and other plant functions. Ozone formation in the atmosphere is a product of NOx that are also a source of nitrogen deposition. Reduced carbon sequestration of temperate forests resulting from ozone is likely offset by increased carbon sequestration from nitrogen fertilization. However, since fertilized croplands are generally not nitrogen-limited, capping ozone-polluting substances in the U.S., Europe, and China can reduce future crop yield loss substantially.
ArticleImportance of carbon-nitrogen interactions and ozone on ecosystem hydrology during the 21st century(American Geophysical Union, 2009-03-18) Felzer, Benjamin S. ; Cronin, Timothy W. ; Melillo, Jerry M. ; Kicklighter, David W. ; Schlosser, C. AdamThere is evidence that increasing CO2 concentrations have reduced evapotranspiration and increased runoff through reductions in stomatal conductance during the twentieth century. While this process will continue to counteract increased evapotranspiration associated with future warming, it is highly dependent upon concurrent changes in photosynthesis, especially due to CO2 fertilization, nitrogen limitation, and ozone exposure. A new version of the Terrestrial Ecosystem Model (TEM-Hydro) was developed to examine the effects of carbon and nitrogen on the water cycle. We used two climate models (NCAR CCSM3 and DOE PCM) and two emissions scenarios (SRES B1 and A2) to examine the effects of climate, elevated CO2, nitrogen limitation, and ozone exposure on the hydrological cycle in the eastern United States. While the direction of future runoff changes is largely dependent upon predicted precipitation changes, the effects of elevated CO2 on ecosystem function (stomatal closure and CO2 fertilization) increase runoff by 3–7%, as compared to the effects of climate alone. Consideration of nitrogen limitation and ozone damage on photosynthesis increases runoff by a further 6–11%. Failure to consider the effects of the interactions among nitrogen, ozone, and elevated CO2 may lead to significant regional underestimates of future runoff.