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Earth's global energy budget

Capsule: An update of the Earth's global annual mean energy budget is given in the light of new observations and analyses. Changes over time and contributions from the land and ocean domains are also detailed.

Abstract: An update is provided on the Earth's global annual mean energy budget in the light of new observations and analyses. In 1997 Kiehl and Trenberth provided a review of past such estimates and performed a number of radiative computations to better establish the role of clouds and various greenhouse gases in the overall radiative energy flows, with top-of-atmosphere (TOA) values constrained by Earth Radiation Budget Experiment values from 1985 to 1989, when the TOA values were approximately in balance. The Clouds and the Earth's Radiant Energy System (CERES) measurements from March 2000 to May 2004 are used at TOA but adjusted to an estimated imbalance from the enhanced greenhouse effect of 0.9 W m-2. Revised estimates of surface turbulent fluxes are made based on various sources. The partitioning of solar radiation in the atmosphere is based on the International Satellite Cloud Climatology Project (ISCCP) ISCCP-FD computations that utilize the ISCCP cloud data every 3 hours globally. Surface upwards longwave radiation is adjusted for spatial and temporal variability and the back longwave radiation is computed as a residual to ensure a balance. Values are also presented for the land and ocean domains that include a net transport of energy from ocean to land of 2.2 Petawatts (PW), of which 3.2 PW is from moisture (latent energy) transport while net dry static energy transport is from land to ocean.

Figure. The global annual mean Earth's energy budget for the March 2000 to May 2004 period in W m^-2. The broad arrows indicate the schematic flow of energy in proportion to their importance.

Submitted to Bulletin of American Meteorological Society, 22 January 2008.

Authored by: Kevin E. Trenberth (corresponding author), John T. Fasullo and Jeffrey Kiehl
NCAR/Climate and Global Dynamics Division, Boulder, CO 80307, USA
Email: trenbert@ucar.edu

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Global net primary production predicted from vegetation class, precipitation and temperature

Abstract: Net Primary Production (NPP), the difference between CO2 fixed by photosynthesis and CO2 lost to autotrophic respiration, is one of the most important components of the carbon cycle. Our goal was to develop a simple regression model to estimate global NPP using climate and land cover data. Approximately 5,500 global data points with observed mean annual NPP, land cover class, precipitation and temperature were compiled. Precipitation was better correlated with NPP than temperature and it explained much more of the variability in mean annual NPP for grass or shrub dominated systems (r2=0.68) than tree dominated systems (r2=0.39). For a given precipitation level, tree dominated systems had significantly higher NPP (~100-150 gC m-2 yr-1) than non-tree dominated systems. Consequently, previous empirical models developed to predict NPP based on precipitation and temperature (e.g., the Miami model) tended to overestimate NPP for non-tree dominated systems. Our new NCEAS model predicts NPP for tree dominated systems based on precipitation and temperature, but for non-tree dominated systems NPP is solely a function of precipitation because including a temperature function increased model error for these systems. Lower NPP in non-tree dominated systems is not entirely explained by decreased water or nutrient use efficiency but is related to more frequent fire disturbances and higher nutrient loss rates. Late 20th century above ground and total NPP for potential native vegetation using the NCEAS model is estimated to be ~28 and ~46 Pg C yr-1, respectively. The NCEAS model estimated a ~13% increase in global TNPP for potential vegetation from 1901-2000 based on changing precipitation and temperature patterns.

Accepted: Ecology, 3 January 2008.

Authored by: DelGrosso, S., Parton, W., Stohlgren, T., Zheng, D., Bachelet, D., Prince, S., Hibbard, K., and R. Olson.