New observational evidence for global warming from satellite
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DICKINSON: NEW OBSERVATIONAL EVIDENCE FOR GLOBAL WARMING FROM SATELLITE
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GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 10.1029/2001GL013833, 2002
New observational evidence for global warming from satellite
Menglin Jin1 and Robert E. Dickinson2
Received 26 July 2001; revised 18 October 2001; accepted 2 November 2001; published 23 May 2002.
[1] Accurate measurements of surface radiative temperature, i.e. skin temperature, would be more directly interpretable in terms of the surface response to increase of greenhouse gases than the more conventional screen temperatures. Such measurements have not previously been attempted because of the difficulties of converting existing observations into a meaningful measurement. We have developed procedures for removing the effects of changing satellite orbits and cloud contamination from skin temperatures estimated from AVHRR channels 4 and 5, and so provide a first estimate of the trends of land surface skin temperature over the last two decades. The estimated land temperature increase is not only much greater than that for the atmosphere but also apparently somewhat larger than the estimates of surface air temperature increase from in situ measurement. INDEX TERMS: 1640 Global Change: Remote sensing; 1620 Global Change: Climate dynamics (3309); 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions
1. Introduction
[2] Analyses of surface air temperature (SAT) over the globe have shown over the past century a global increase of about 0.4 to 0.8°C, and in the past two decades of about 0.2°C over the oceans and about 0.3 °C over land [National Research Council (NRC), 2000]. The increase over land has been larger than that over the oceans consistent with the smaller heat capacity of land and the observed warming of the surface ocean [Levitus et al., 2000]. If convection rapidly mixes the troposphere in a vertical column, the surface and tropospheric temperatures should change together [Schneider and Dickinson, 1974; Ramanathan, 1981; Manabe and Stouffer, 1980]. However, a smaller increase of temperature of the lower to mid-troposphere of about 0.0 – 0.2°C over the past two decades is reported from recent analyses of satellite and radiosonde data [NRC, 2000]. This relatively small increase of tropospheric temperature, if real, suggests a need to obtain more observational information related to the variation of temperature change with altitude to be able to better interpret and model the observed change. [3] This paper analyzes satellite surface skin temperature (Ts) and compares it with ‘‘Stephenson screen’’ surface air temperatures (SAT) [Jin and Goetz, 2001; Jones et al., 1999]. This thermodynamic temperature, measured by thermometers sheltered by a wooden box, located 1.5 – 2 m above short grass and waterpermeable, has provided the conventional observations used to assess the occurrence of global climate change. The radiometric ‘‘skin’’ temperature, is derived from the thermal emission of the Earth’s surface. A brightness temperature is calculated from spectral radiances observed by satellite thermal infrared sensors and after removing surface emissivity and atmospheric effects, used to infer skin temperature [Jin and Goetz, 2001 and reference
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therein]. Such data can provide more uniform and denser coverage than that from traditional SAT measurements, which are not designed, sited, or maintained to provide reliable climatic records [Karl et al., 1994]. [4] The spectral radiances from channels 4 and 5 of the NOAA Advanced Very High Resolution Radiometer (AVHRR) estimate skin temperature [Agbu and James, 1994; Wan and Dozier, 1996] at a spatial resolution of 1 Km at nadir, sampled twice daily, for two decades. However, various problems such as orbit drift, cloud contamination, inadequate sampling of the diurnal variation, may degrade the usefulness of the skin temperature measurements for study of climate change. Corrections for these problems have been developed to reduce such errors [Jin and Dickinson, 1999, 2000; Jin, 2000; Jin and Treadon, 2001] and to produce a diurnal cycle of land surface skin temperature. The resulting data set, referred to as ‘‘LSTD’’, is inferred from the thermal emission of terrestrial surfaces that have a line of sight to the overlying atmosphere, i.e. some combination of vegetation canopies and soils. The data include the 24-hour average, maximum and minimum values of diurnal cycle, and are tabulated at monthly intervals and 8 km resolution. The individual monthly average values are estimated to be uncertain by about 2°C and may include spatially varying biases because of variations in surface emissivity and other such factors. However, such biases are expected to largely cancel in considering only anomalies from the time averaged values.
2. Results
[5] This paper examines the trend implied by this land skin temperature for the period 1982 – 1998 using July and January data. The SAT observations are sited irregularly and have been remapped into 5 by 5 degrees. We likewise scale up skin temperature data from 8 km resolution into 5 by 5 degree using quality control filters in which obviously biased 8 km pixel values are removed. Figure 1a shows the anomalies of the global mean for this data from 1982 to 1998. The trend obtained by linear regression is about 0.43°C/decade. This value varies between 0.4 – 0.5°C per decade depending on how global average and annual mean are calculated. A Monte Carlo error analysis [Wilks, 1995] indicates that the probability of the measured trend differing from the actual one by more than 0.2°C/decade due to sampling is less than 10% (Figure 1b). Departures from the trend result from climate variability, e.g., El Nino, La Nina, and volcanic cooling, that are also seen in the ˜ ˜ SAT [NRC, 2000]. For example, the eruption of Mt. Pinatubo reduce global land temperatures over the period 1992 – 1995 [NRC, 2000], and the 97 – 98 El Nino elevated land temperatures ˜ in 1998. Some uncorrected artifacts in the skin temperature record of the effects of volcanic aerosols and of the piecing together of the different AVHRR instruments and records undoubtedly remain. [6] Figure 2a shows that the annual averaged diurnal range, i.e., maximum temperature (Tmax) minus the minimum temperature (Tmin), decreases by about 0.16°C/decade. Figure 2b is the time series of annual global Tmax and Tmin. This decrease of diurnal range is consistent with estimates that the diurnal range of SAT decreased by about 0.1°C over the period but the latter estimate is
Meteorology Department, University of Maryland, College Park, USA. Earth and Atmospheric Sciences, Georgia Institute of Technology, USA. Copyright 2002 by the American Geophysical Union. 0094-8276/02/2001GL013833$05.00
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