Modeling the impacts of recent climate change on ecosystem productivity across North America

• Author / Creator

  Mekonnen, Zelalem A

• There is evidence of warming and changes in precipitation over recent decades in most regions of North America (NA) that are affecting ecosystem productivity. The impacts of these changes on land-atmosphere carbon exchange over a wide range of biomes are spatially heterogeneous and uncertain. In this study, a comprehensive mathematical process model, ecosys, was used to estimate the impacts of climate change and major droughts of the last three decades (1979 – 2010) on ecosystem productivity across NA. Uncertainties in model estimates subject to inherent model characteristics and external model drivers such as weather and soil were rigorously tested at selected eddy covariance (EC) flux tower sites over a wide ranges of biomes and climates. In a site scale test of model results, annual gross primary productivity (GPP) modeled for pixels which corresponded to the locations of 20 EC towers in diverse climate zones across NA correlated well (R2 = 0.76) with annual GPP derived from the flux towers in 2005. In a continental-scale test of model results, spatial anomalies in leaf area indices (LAI) from long-term means modeled during major drought events in 1988 and 2002 agreed well with those in Normalized Difference Vegetation Index (NDVI) (geographically weighted regression, R2 = 0.84 in 1988, 0.71 in 2002). GPP modeled in eastern temperate forests and most areas with lower mean annual air temperature (Ta), such as those in northern forests and Taiga, increased due to early spring and late autumn warming, and these eco-regions contributed 92% of the increases in NA GPP of the past 30 years. However, modeled GPP declined in most southwestern regions of NA (accounting >50% of the ecosystems with declining GPP), due to water stress from rising Ta and declining precipitation. Overall, NA modeled GPP increased by 5.8% in the last 30 years, with a positive trend of +0.012 Pg C yr-1 and a range of -1.16 to +0.87 Pg C yr-1 caused by interannual variability of GPP from the long-term (1980 – 2010) mean. NA modeled net ecosystem productivity (NEP) declined by 92% (0.50 Pg C yr-1) and 90% (0.49 Pg C yr-1) from the long-term mean (+0.54 Pg C yr-1), during droughts in 1988 and 2002 respectively. The modeled result in 2002 was corroborated with similar estimate from top-down atmospheric inversion modeling from CarbonTracker that estimated 88% (0.37 Pg C yr-1) declines in 2002 carbon sink from the long-term (2000 - 2010) mean (0.42 Pg C yr-1). Although NA ecosystems in the model remained a much smaller carbon sink during these two drought years, the significant drops in NEP offset 28% of the long-term carbon gains from the long-term mean over the last three decades. The long-term modeled terrestrial carbon sink was estimated to offset ~30% of the fossil fuel emissions of NA, however only 0.03 and 3.2% were offset in 1988 and 2002 leaving almost all fossil fuel emissions to the atmosphere. Interannual variabilities in modeled mid-August LAI and NDVI were the greatest in southwest of US and part of the Great Plains, which could be as a result of frequent El Niño–Southern Oscillation' events that led to major droughts. Although NA terrestrial biosphere has been modeled as a long-term carbon sink, further warming and projected dryness could enhance carbon release hence may reduce net carbon sink of the continent.

• Subjects / Keywords

  • Climate change
  • Ecosys
  • North America carbon cycle
  • North America drought
  • Ecosystem modeling

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3V11VT6B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.