Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Eleanor J. Burke x
  • User-accessible content x
Clear All Modify Search
Eleanor J. Burke

Abstract

A perturbed physics Hadley Centre climate model ensemble was used to study changes in drought on doubling atmospheric CO2. The drought metrics analyzed were based on 1) precipitation anomalies, 2) soil moisture anomalies, and 3) the Palmer drought severity index (PDSI). Drought was assumed to occur 17% of the time under single CO2. On doubling CO2, in general, PDSI drought occurs more often than soil moisture drought, which occurs more often than precipitation drought. This paper explores the relative sensitivity of each drought metric to changes in the main drivers of drought, namely precipitation and available energy. Drought tends to increase when the mean precipitation decreases, the mean available energy increases, the standard deviation of precipitation increases, and the standard deviation of available energy decreases. Simple linear approximations show that the sensitivity of drought to changes in mean precipitation is similar for the three different metrics. However, the sensitivity of drought to changes in the mean available energy (which is projected to increase under increased atmospheric CO2) is highly dependent on metric: with PDSI drought the most sensitive, soil moisture less sensitive, and precipitation independent of available energy. Drought metrics are only slightly sensitive to changes in the variability of the drivers. An additional driver of drought is the response of plants to increased CO2. This process reduces evapotranspiration and increases soil moisture, and generally causes less soil moisture drought. In contrast, the associated increase in available energy generally causes an increase in PDSI drought. These differing sensitivities need to be taken into consideration when developing adaptation strategies.

Full access
Eleanor J. Burke and Simon J. Brown

Abstract

The uncertainty in the projection of future drought occurrence was explored for four different drought indices using two model ensembles. The first ensemble expresses uncertainty in the parameter space of the third Hadley Centre climate model, and the second is a multimodel ensemble that additionally expresses structural uncertainty in the climate modeling process. The standardized precipitation index (SPI), the precipitation and potential evaporation anomaly (PPEA), the Palmer drought severity index (PDSI), and the soil moisture anomaly (SMA) were derived for both a single CO2 (1×CO2) and a double CO2 (2×CO2) climate. The change in moderate drought, defined by the 20th percentile of the relevant 1×CO2 distribution, was calculated. SPI, based solely on precipitation, shows little change in the proportion of the land surface in drought. All the other indices, which include a measure of the atmospheric demand for moisture, show a significant increase with an additional 5%–45% of the land surface in drought. There are large uncertainties in regional changes in drought. Regions where the precipitation decreases show a reproducible increase in drought across ensemble members and indices. In other regions the sign and magnitude of the change in drought is dependent on index definition and ensemble member, suggesting that the selection of appropriate drought indices is important for impact studies.

Full access
Eleanor J. Burke, Simon J. Brown, and Nikolaos Christidis

Abstract

Meteorological drought in the Hadley Centre global climate model is assessed using the Palmer Drought Severity Index (PDSI), a commonly used drought index. At interannual time scales, for the majority of the land surface, the model captures the observed relationship between the El Niño–Southern Oscillation and regions of relative wetness and dryness represented by high and low values of the PDSI respectively. At decadal time scales, on a global basis, the model reproduces the observed drying trend (decreasing PDSI) since 1952. An optimal detection analysis shows that there is a significant influence of anthropogenic emissions of greenhouse gasses and sulphate aerosols in the production of this drying trend. On a regional basis, the specific regions of wetting and drying are not always accurately simulated. In this paper, present-day drought events are defined as continuous time periods where the PDSI is less than the 20th percentile of the PDSI distribution between 1952 and 1998 (i.e., on average 20% of the land surface is in drought at any one time). Overall, the model predicts slightly less frequent but longer events than are observed. Future projections of drought in the twenty-first century made using the Special Report on Emissions Scenarios (SRES) A2 emission scenario show regions of strong wetting and drying with a net overall global drying trend. For example, the proportion of the land surface in extreme drought is predicted to increase from 1% for the present day to 30% by the end of the twenty-first century.

Full access
Eleanor J. Burke, W. James Shuttleworth, and R. Chawn Harlow

Abstract

MICRO-SWEAT, a soil–vegetation–atmosphere transfer scheme (SWEAT) coupled with a microwave emission model, was used to predict the microwave brightness temperatures (T B) measured at El Reno, Oklahoma, during the Southern Great Plains 1997 (SGP97) field experiment. Comparison with soil-moisture time series measured at four intensively monitored sites revealed the need for a substantially greater soil-saturated hydraulic conductivity than that estimated from soil maps. After revision of the hydraulic conductivity, the modeled and measured time series of soil moisture and surface energy fluxes showed excellent agreement with observations at these 4 sites and with the measurements of the surface soil moisture and T B at the remaining 11 measurement sites. A two-dimensional array of calibrated MICRO-SWEAT models was implemented at 200-m resolution for the El Reno area. There were noticeable differences between the spatial distributions of modeled and measured T B. These differences likely result from imperfect knowledge of the spatial distributions of soil properties, precipitation, and the estimated optical depth of the vegetation used in MICRO-SWEAT. A statistical measure of the usefulness of assimilating the observed soil moisture was explored by assuming the estimation of the optical depth provided the main source of error in the relationship between soil moisture and microwave brightness temperature. Analyses indicated that there is merit in assimilating T B observations for significant portions of the modeled domain, but it is suggested that this would be enhanced if the optical depth of the vegetation were also directly remotely sensed, as proposed in the Soil Moisture and Ocean Salinity (SMOS) mission.

Full access
Eleanor J. Burke, Chris D. Jones, and Charles D. Koven

Abstract

Under climate change, thawing permafrost may cause a release of carbon, which has a positive feedback on the climate. The permafrost-carbon climate response (γPF) is the additional permafrost-carbon made vulnerable to decomposition per degree of global temperature increase. A simple framework was adopted to estimate γPF using the database for phase 5 of the Coupled Model Intercomparison Project (CMIP5). The projected changes in the annual maximum active layer thicknesses (ALTmax) over the twenty-first century were quantified using CMIP5 soil temperatures. These changes were combined with the observed distribution of soil organic carbon and its potential decomposability to give γPF. This estimate of γPF is dependent on the biases in the simulated present-day permafrost. This dependency was reduced by combining a reference estimate of the present-day ALTmax with an estimate of the sensitivity of ALTmax to temperature from the CMIP5 models. In this case, γPF was from −6 to −66 PgC K−1(5th–95th percentile) with a radiative forcing of 0.03–0.29 W m−2 K−1. This range is mainly caused by uncertainties in the amount of soil carbon deeper in the soil profile and whether it thaws over the time scales under consideration. These results suggest that including permafrost-carbon within climate models will lead to an increase in the positive global carbon climate feedback. Under future climate change the northern high-latitude permafrost region is expected to be a small sink of carbon. Adding the permafrost-carbon response is likely to change this region to a source of carbon.

Full access