Drylands cover 41% of Earth’s surface and are the largest source of interannual variability in the global carbon sink. Drylands are projected to experience accelerated expansion over the next century, but the implications of this expansion on variability in gross primary production (GPP) remain elusive. Here we show that by 2100 total dryland GPP will increase by 12 ± 3% relative to the 2000–2014 baseline. Because drylands will largely expand into formerly productive ecosystems, this increase in dryland GPP may not increase global GPP. Further, GPP per unit dryland area will decrease as degradation of historical drylands outpaces the higher GPP of expanded drylands. Dryland expansion and climate-induced conversions among sub-humid, semi-arid, arid, and hyper-arid subtypes will lead to substantial changes in regional and subtype contributions to global dryland GPP variability. Our results highlight the vulnerability of dryland subtypes to more frequent and severe climate extremes and suggest that regional variations will require different mitigation strategies.

There are no direct methods to evaluate calculated soil heat flux (SHF) at the surface (G₀). Instead, validation and cross evaluation of methods for calculating G₀ usually rely on the conventional calorimetric method or the degree of the surface energy balance closure. However, there is uncertainty in the calorimetric method itself, and factors apart from G₀ also contribute to nonclosure of the surface energy balance. Here we used a novel approach to evaluate nine different methods for calculating SHF, including the calorimetric method and methods based on analytical solutions of the heat diffusion equation. The SHF (G_z) measured by a self‐calibrating SHF plate at a depth of z = 5 cm below the surface (hereafter G_{m_5cm}) was deployed as a reference. Each SHF calculation method was assessed by comparing the calculated G_z at the same depth (hereafter G_{c_5cm}) with G_{m_5cm}. The calorimetric method and simple measurement method performed best in determining G_{c_5cm} but still underestimated G_{m_5cm} by 19% during the daytime. Possible causes for this underestimation include errors and uncertainties in SHF measurements and soil thermal properties, as well as the phase lag between G_{c_5cm} and G_{m_5cm}. Our results indicate that the calorimetric method achieves the most accurate SHF estimates if self‐calibrating SHF plates are deployed at two depths (e.g., 5 cm and 10 cm), soil temperature and water content measurements are made in a few depths between the two plates, and soil thermal properties are accurately quantified.

The effects of large eddies on turbulence structures and flux transport were studied using data collected over a flat cotton field during the Energy Balance Experiment 2000 in the San Joaquin Valley of California in August 2000. Flux convergence (FC; larger fluxes at 8.7 m than 2.7 m) and divergence (FD) in latent heat flux (LE) were observed in a disturbed, unstable atmospheric surface layer, and their magnitudes largely departed from the prediction of Monin‐Obukhov similarity theory. From our wavelet analysis, it was identified that large eddies affected turbulence structures, scalar distribution, and flux transport differently at 8.7 m and 2.7 m under the FC and FD conditions. Using the ensemble empirical mode decomposition, time series data were decomposed into large eddies and small‐scale background turbulence, the time‐domain characteristics of large eddies were examined, and the flux contribution by large eddies was also determined quantitatively. The results suggest that large eddies over the frequency range of 0.002 Hz < f < 0.02 Hz (predominantly 300–400 m) enhanced the vertical velocity spectra more significantly at 8.7 m than 2.7 m, leading to an increased magnitude of the cospectra and thus LE at 8.7 m. In the FD case, however, these large eddies were not present and even suppressed in the vertical velocity spectra at 8.7 m. Consequently, the cospectra divergence over the low‐frequency ranges primarily caused the LE divergence. This work implies that large eddies may either improve or degrade the surface energy balance closure by increasing or decreasing turbulent fluxes, respectively.