Impact of vegetation feedback on the temperature a

Authors: SuJong Jeong ChangHoi Ho TaeWon Park Jinwon Kim Samuel Levis

Publish Date: 2010/05/04

Volume: 37, Issue: 3-4, Pages: 821-833

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Abstract

This study examines the potential impact of vegetation feedback on the changes in the diurnal temperature range DTR due to the doubling of atmospheric CO2 concentrations during summer over the Northern Hemisphere using a global climate model equipped with a dynamic vegetation model Results show that CO2 doubling induces significant increases in the daily mean temperature and decreases in DTR regardless of the presence of the vegetation feedback effect In the presence of vegetation feedback increase in vegetation productivity related to warm and humid climate lead to 1 an increase in vegetation greenness in the midlatitude and 2 a greening and the expansion of grasslands and boreal forests into the tundra region in the high latitudes The greening via vegetation feedback induces contrasting effects on the temperature fields between the mid and highlatitude regions In the midlatitudes the greening further limits the increase in T max more than T min resulting in further decreases in DTR because the greening amplifies evapotranspiration and thus cools daytime temperature The greening in highlatitudes however it reinforces the warming by increasing T max more than T min to result in a further increase in DTR from the values obtained without vegetation feedback This effect on T max and DTR in the high latitude is mainly attributed to the reduction in surface albedo and the subsequent increase in the absorbed insolation Present study indicates that vegetation feedback can alter the response of the temperature field to increases in CO2 mainly by affecting the T max and that its effect varies with the regional climate characteristics as a function of latitudesDaily mean temperatures over the Northern Hemisphere land surface have increased drastically during the recent several decades and the warming trend is likely to continue into the future due to the continued increase in the atmospheric concentrations of anthropogenic greenhouse gases GHGs especially CO2 Solomon et al 2007 While the increase in the daily mean temperature is regarded as one of the most definite indicators of global warming changes in the daily temperature maximum T max and minimum T min provide more detailed information than the mean temperature T mean alone because changes in T mean are attributed to changes in either T max or T min or both Historical records show that the increase in T min is larger than that in T max Karl et al 1993 Easterling et al 1997 and that the asymmetric changes in them result in decrease in the diurnal temperature range DTR The decrease in DTR is also regarded as a fingerprint for identifying the anthropogenic causes of global warming Stone and Weaver 2003 Thus future climate projections must evaluate not only mean temperatures but also the daily temperature extremes T max and T min and DTRThe future DTR changes projected by various climate models have been examined in the Intergovernmental Panel on Climate Change IPCC the Fourth Assessment Report AR4 Meehl et al 2007 General consensus among these models is that higher GHG concentrations will result in a decrease in DTR continuation of this trend was already found in historical records Solomon et al 2007 The changes in DTR result from complex feedbacks among GHG concentrations radiative transfer atmospheric and oceanic circulations clouds and precipitation For example DTR decreases have been attributed to enhanced nocturnal temperatures due to increased longwave forcing the changes in shortwave radiation and cloud amounts are also attributed to the projected DTR decreases For instance increases in cloudiness lessen surface insolation to reduce T max and DTR Stone and Weaver 2003 Nevertheless there still exist large uncertainties regarding the response of daily temperature field to the increase in GHGs because the response of DTR to global climate conditions induced by the increase in GHGs are also controlled by a number of additional climate factors such as landsurface vegetation and moisture availability Dai et al 1999 Stone and Weaver 2003 Zhou et al 2009Previous studies reported that land–atmosphere interactions are important in determining DTR in addition to the changes in radiation and clouds especially in boreal summers Zhou et al 2007 Zhang et al 2009 For example with sufficient surface moisture availability increases in radiative energy input will mostly increase evapotranspiration to limit the increase in sensible heat flux and surface temperatures By contrast the lack of surface moisture limits evaporation and ground heat flux Kim et al 2002 thus the increased radiative energy input is primarily balanced by the increase in sensible heat flux Thus surface temperature increases are larger over drier surfaces than wet ones for the same amount of radiative forcing The changes in surface energy budget indicate attenuation of daytime temperature increases over wet surfaces In addition sufficient moisture from the land can lead to an increase in precipitation which further reduces incident shortwave radiation and T max Fischer et al 2007 Considering these landsurface processes vegetation is one of the important factors that regulate moisture availability and temperatures Bonan 2001 Bonan et al 2003 However the role of vegetation feedback in shaping DTR variations remains to be quantifiedSatellitederived leaf area index and/or stationobserved vegetation phenology data show earlier emergence and enhancement of vegetation activity over most Northern Hemisphere eg Myneni et al 1997 Ho et al 2006 Schwartz et al 2006 Climatic consequences of the increase in vegetation activity have been investigated in previous studies eg Bounoua et al 2000 Cowling et al 2009 Jeong et al 2009a b Jeong et al 2009a b reported that the increase in vegetation greenness has reduced spring warming via a cooling effect of vegetation–evapotranspiration feedback over East Asia This process mainly affects T max with minimal impact on T min As more leaves emerge and flourish evapotranspiration increases given sufficient moisture So the vegetation–evapotranspiration feedback can result in asymmetric responses between T max and T min and alter DTR changes associated with future CO2 increasesIn this study we examine the potential impact of vegetation feedback on the DTR changes associated with an increase in CO2 during boreal summer The impact of CO2 doubling on climate is obtained from a centurylong global model run with and without the coupling of a dynamic global vegetation model DGVM that is employed to represent the effect of vegetation feedback in the global climate system Bounoua et al 1999 Levis et al 1999 2000 Notaro et al 2007 O’ishi and AbeOuchi 2009 The global climate model GCM used in this study is the National Center for Atmospheric Research NCAR Community Atmospheric Model version 3 CAM3 that incorporates the most recent dynamics scheme and parameterized physics The model used in this study has been configures with spectral T42 approximately 2875° × 2875° horizontally and 26 hybridsigma levels in the vertical Detailed information on the model is documented in Collins et al 2004 2006 and will not be repeated here Landsurface processes in CAM3 are calculated by the Community Land Model version 3 CLM3 Oleson et al 2004 that calculates the heat moisture and momentum fluxes between land surfaces and atmosphere as well as the thermal and hydrologic processes at the surface and the interior of nearsurface soil layer Bonan et al 2002 Oleson et al 2004 Dickinson et al 2006 A comprehensive discussion on CLM and the surface flux calculations have been provided in Oleson et al 2004Coupled with CAM3 at a T42 horizontal resolution CLM3 is comprised of 3799 grid points each a collection of subgrid elements of four primary land cover types glacier lake wetland and vegetation The vegetated portion of the grid cell is represented by the fractional coverage of plant functional types PFTs The model uses seven primary PFTs namely needleleaf evergreen or deciduous trees broadleaf evergreen or deciduous tree shrub grass and crop These seven primary PFTs are further refined to tropical temperate and boreal deciduous or evergreen trees C3 and C4 grasses and evergreen and deciduous shrubs by bioclimatic rules Oleson et al 2004 In each PFT leaf phenology in the CLM3 is prescribed and the seasonal course of leaf area index LAI for each PFT is derived through interpolating the monthly PFTspecific LAI from National Oceanic and Atmospheric Administration NOAAAdvanced Very HighResolution Radiometer AVHRR data as described by Bonan et al 2002

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