The Effects of Increasingly Severe Droughts on Carbon Sink Storage in North America

Temperate North America is currently a carbon sink, acting as a reservoir that accumulates and stores carbon. In less than one hundred years, this carbon sink could disappear due to projected changes in drought severity and frequency caused by anthropogenic global climate change. Based on comparisons of the Palmer Drought Severity Index and paleoclimate reconstructions of droughts from tree ring data, the drought that lasted between the years 2000 and 2004 in western North America was the most severe to occur in eight hundred years. According to Schwlam et al. (2012), the carbon dioxide absorption ability of the western North American carbon sink declined from 30 to −298TgCyr−1 in 2000–2004, due to the effects of drought on the ecosystems’ carbon cycling functions. This study suggests that the western North American carbon sink that currently stores 177–623TgCyr−1 could be lost in less than one hundred years, due to the increasing severity of droughts. —Hilary Haskell
Schwalm, CR., 2012. Reduction in Carbon Uptake during Turn of the Century Drought in Western North America. Nature 5: 55156.

            Schwalm et al. quantified drought with precipitation, soil moisture, evaporative fraction and latent heat data. Data from the U.S. Geological Survey for the five main river basins in the western United States during the years 1997–2007 indicated that water availability decreased. The National Agricultural Statistics Survey for 2,383 counties in the western United Sates suggested that cropland productivity declined 5% during the 2004–2005 drought. According to precipitation predictions, the drought in North America between the years 2000–2004 will become a typical occurrence. These hydroclimatic predictions were reached using reliable methods. Summer precipitation and summer PDSI are two separate indices; however, linking the PDSI-based analysis of past drought events with summer precipitation predictions for the future is a well-supported practice. The validity of these assumptions is supported by the Coupled Model Intercomparison Project Phase 5. Precipitation levels and tree-ring PDSI exhibited highly similar frequency distributions between the data sets compared, indicating consistency and accuracy for the PDSI. The similarities between all three drought indicators demonstrate that they are able to reliably predict the frequency and severity of drought. Not only do drought indices indicate increased aridity of western North America, but also trends in snowpack decline. Although regional precipitation is difficult to forecast, climate model predictions generally underestimate drought extent and severity.
            The authors observed carbon and energy fluxes at fifteen eddy-covariance flux tower sites that are part of the global FLUXNET network, using data from the North American Carbon Project Site Synthesis and Ameriflux. This study only considered an eddy-covariance flux tower site in western North America if it provided one year of data for both drought and non-drought conditions in 1997–2007. Across these fifteen sites, despite differences in ecosystems, time period, soils, climates, and other ecosystem disturbances, there was still an obvious decrease in carbon uptake. A decrease in net ecosystem productivity (NEP) of −63gCm2yr1with a 95% confidence interval:−20 to −139gCm2yr1 indicated the resulting decrease in carbon sink storage.
            For grasslands and evergreen needleleaf forests, the drought resulted in reduced gross primary productivity (GPP) that outweighed the effects of ecosystem respiration, creating an overall reduction in carbon dioxide uptake. Respiration is the process by which plants convert the sugars produced during photosynthesis back into energy so that they may be metabolized, resulting in water and carbon dioxide byproducts. Ecosystem respiration corresponds to the sum of all plant respiration in an ecosystem. Conversely, woody savannas demonstrated an increase in carbon dioxide uptake due to reduced ecosystem respiration from the reduced rate of decomposition during droughts.
            The authors also analyzed latent heat flux (LE) as a determinant of drought for three land cover classes (grassland, evergreen needleleaf forests, and woody savannas), and found differing magnitudes of decreased LE. LE is the measure of the exchange of heat between the Earth’s surface and the atmosphere, due to evaporation and transpiration into the troposphere and condensation of water out of the troposphere. LE is used as an indication of drought.  Evergreen forests demonstrated the greatest decrease in LE, followed by woody savannas and needleleaf forests. Sensible heat flux (H), the conductive heat flux between the atmosphere and Earth’s surface (measured by eddy covariance), was less variable than LE. But, there was an increase in H for needleleaf forests and woody savannas that also had a temperature increase of 0.40°C and 0.41°C,respectively, from JuneSeptember. However, H decreased in grasslands due to increased albedo effects from dieback that exposed darker soil and turned bright leaf tissue darker.
            Drought-induced regional deviations for water and carbon balances were seen in precipitation, soil moisture, instrumental era PDSI, Moderate Resolution Imaging Spectoaiometer (MODIS) GPP, MODIS net primary productivity (NPP) and empirically extrapolated FLUXNET data obtained during the years 19972007. Regionally, Montana and Idaho demonstrated the largest changes relative to the baseline period years: 19971999 and 20052007. During these years, area-averaged soil moisture declined 4mm per month, with the largest reduction of 45% occurring during the summer. As expected, area-averaged precipitation decreased 6 mm per month. On the PDSI scale, negative values indicate periods of drought. Decreased precipitation can be seen in the 0.5 baseline period value for the PDSI scale falling to 1.6 for the average value in the region. The largest change of 1.6 to 3.6 on the PDSI scale indicated a shift from slightly wet conditions to a severe drought in the region. During the 20002004 drought, 75% of the sites studied demonstrated mild to severe drought.
            By combining the effects of drought over the region, the authors concluded that there was a considerable reduction in region-wide productivity and carbon uptake. Schwalm et al. quantified the decrease in GPP by −182TgCyr−1 (−38gCm−2yr−1) for  extrapolated FLUXNET of monthly NEP, GPP, and ecosystem respiration; and −234TgCyr−1(−47gCm−2yr−1) for MODIS measures of NPP and GPP.  Both the FLUXNET data and MODIS estimates of NPP and GPP were forced with reanalysis, remote sensing, meteorological, and land cover data. When evaluating the change in the carbon sink strength, only the differences due to the drought that lasted from 20002004 in comparison to the baseline period from 1997−1999 and 20052007 were considered. The three FLUXNET-based estimates of drought anomalies along with the two estimates based on inversions (Jena CO2  Inversion and Carbon Tracker were used as independent estimates of the land sink during the years 19972007)  indicate that the 20042005 western North America drought caused a decline in terrestrial carbon sink strength (NEP)  ranging from −30 to −298TgCyr−1, relative to a baseline sink strength of 177–623TgCyr−1. This reduced the uptake of carbon dioxide by an average of 51%.
            In order to substantiate the severity of the drought in western North America during the years 20042005, the authors considered the paleoclimatic tree ring record to reconstruct the PDSI index for the years 8002006. Doing so revealed that the drought considered in this study had the lowest PDSI value for a five year drought in eight hundred years. In the 1,207 years of paleoclimate data surveyed, there were only two drought events of comparable severity, and both occurred during periods of historical megadroughts that lasted much longer than the drought in this study. On average, these megadroughts reflected less drought severity and were more geographically limited in scale. When only considering single-year summer PDSI values, over the course of the last 2,000 years, tree ring data indicates that only ninety-seven summers were as or more severe than the drought that lasted between 20002004.
            Based on forecasted precipitation patterns and the recent continuation of decreased levels of precipitation, western North America is headed towards a twenty-first century megadrought. This megadrought will further decrease crop productivity, primary production in ecosystems, LE, runoff to water basins, and carbon dioxide uptake, and could possibly cause these patterns to become permanent. The effects of megadrought in the future could greatly diminish the North American carbon sink’s carbon accumulation and storage potential if global climate change patterns continue as projected. 

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