ALBERT

Description: Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present 1-3 . Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity 3,4 . The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming 5-8 . Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30-298 Tg C yr -1 during the 2000-2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177-623 Tg C yr -1 in western North America could disappear by the end of the century. © 2012 Macmillan Publishers Limited. All rights reserved.

Description: A limitation of the Met Office operational data assimilation scheme is that surface-sensitive infrared satellite sounding channels cannot be used during daytime periods where Numerical Weather Prediction (NWP) model background land surface temperature (LST) biases are greater than 2 K. The Met Office Unified Model (UM) has a significant cold LST bias in semi-arid regions when compared with satellite observations. UM LST biases were evaluated at global resolution and in a Limited Area Models (LAM) at 2.2 km resolution over the SALSTICE (Semi-Arid Land Surface Temperature and IASI Calibration Experiment) experimental domain in southeastern Arizona. This validation is in conjunction with eddy-covariance flux tower measurements. LST biases in the Global Atmosphere/Land 3.1 (GA/L3.1) configuration were largest in the mid-morning with respect to Terra (−13.6 ± 2.8 K at the Kendall Grassland site). The diurnal cycle of LST in Global Atmosphere/Land 6.1 (GA/L6.1) showed a significant improvement relative to GA/L3.1. The higher resolution LAM showed added value over the global configurations. The spatial distribution of the LST biases relative to Moderate Resolution Imaging Spectroradiometer (MODIS) and the modelled bare soil cover fraction were found to be moderately correlated (0.61 ± 0.08) during the daytime, which suggests that regions of cold LST bias are associated with low bare soil cover fraction. Coefficients of correlation with the shrub surface fractions followed the same trend as the bare soil cover fraction although with a less significant correlation (0.36 ± 0.09), and indicate that the sparse vegetation canopies in southeastern Arizona are not well represented in UM ancillary datasets. The x-component of the orographic slope was positively correlated with the LST bias (0.35 ± 0.06) and identified that regions of cold model LST bias are found on easterly slopes and regions of warm model LST bias are found on westerly slopes. An overestimate in the modelled turbulent heat and moisture fluxes at the eddy-covariance flux sites was found to be coincident with an underestimate in the ground heat flux.