Anthropogenic Influence on Tropospheric and Surface Temperature Trends: Intercomparison Between Models and Observations
Presiding: A N Maurellis, SRON National Institute for Space Research; M Free, NOAA Air Resources Laboratory
A34A-01 INVITED 15:45h
Observations of Satellite-Based Temperatures of the Troposphere
Bulk atmospheric temperatures from microwave sounding units on polar orbiters (MSUs) and their 1998 replacement, the Advanced MSU or AMSU, require significant adjustment before they may be useful for climate analysis. The University of Alabama in Huntsville (UAH) builds their datasets with the emphasis on reducing uncertainties of the long term trends. UAH has a relatively strict requirement that over the life of co-orbiting satellites, both satellites must agree on the trend in the Earth-viewed temperature during the common overlap. Applying this methodology generates calibration adjustments to the various instruments flown on different spacecraft, adjustments that are somewhat different in a few cases than other similar MSU datasets. From Dec 1978 to Dec 2004, the global lower tropospheric trend is estimated at +0.08 ± 0.05 C/decade. Similar results are found in radiosonde-based datasets. Indeed in site-by-site comparisons with over 150 radiosondes in various groupings, the median trend difference was less than ± 0.02 C/decade. Analysis of MSU retrievals based on linearly combining MSU channel 2 and MSU 4 indicate significant instability in that the linear coefficients of MSU2 and MSU4 designed to reproduce a balloon-based tropospheric target temperature vary widely depending upon the dataset used. The result indicates considerable uncertainty in trends when such retrievals are attempted.
A34A-02 16:00h
Tropospheric Warming As Measured by Radiosondes: RATPAC and Uncertainties
We present results from a new radiosonde temperature dataset, Radiosonde Atmospheric Temperature Products for Assessing Climate ("RATPAC"). The data are based on soundings from 85 stations, adjusted for inhomogeneities through 1997 by John Lanzante, Steve Klein and Dian Seidel, and extended through 2004 using a first-difference procedure. Time series for the globe, tropics and hemispheres show tropospheric warming over recent decades. We compare our results to those from other upper-air temperature datasets and assess uncertainties arising from the limited spatial coverage of the station network and from the methods used to produce the dataset.
A34A-03 INVITED 16:15h
Impact of Surface Changes on US Temperature Trends: Seasonal Cycle
Kalnay and Cai (Nature, 2003) proposed a method to identify the impact of surface changes as the difference between the trends of the surface observations (which reflect all the sources of climate forcing, including surface effects) and the NCEP-NCAR Reanalysis (which only contains forcings that influence the assimilated atmospheric temperature trends). This difference would include not only urbanization effects but also changes in agricultural practices, such as irrigation and deforestation, and any other changes not included in the Reanalysis and should depend on the land type. We slightly correct previous results by excluding West Coast stations. Since the estimated land-use changes impact increases the minimum temperature and decreases the maximum temperature, the impact on the mean temperature is small and geographically similar to the nightlight estimate of Hansen et al (2001), but the impact on the diurnal temperature range is large. The seasonal cycle shows that the impact of the greenhouse gases trends dominates in the winter, whereas the apparent impact of land-use changes is larger in the summer. The impact of the NCDC station corrections on the estimates is also discussed.
A34A-04 16:30h
Are Sea Surface Temperature (SST) Trends Real?
We examine the fundamental physics of SS heating by short-wave (SW) and long-wave (LW) radiation. Solar SW radiation penetrates to some considerable depth, depending on wavelength and turbidity. With an average ocean albedo of 0.09, most of the visible part of the solar spectrum heats the euphotic zone, and through wave action and eddy mixing communicates this energy downward, heating the "mixed layer," conventionally taken as the upper 100 meters. But LW (atmospheric) radiation, typically around 20 microns, cannot penetrate into water; its contribution to SST is minor. Instead, it aids in the evaporation of the ocean "skin" and adds latent heat to the atmospheric boundary layer. As a result, the enhanced (anthropogenic) greenhouse effect from an increase in GH gases makes only a minor contribution to SST. Thus SST should not warm in response to the GH effect. But SST records of the past 25 years (referring to temperature of the mixed layer) do show an increase, comparable to that of land data. This temperature rise has conventionally been ascribed to GH warming. To account for the obvious disparity, we examine more closely the types of measurements that make up the SST. They consist principally of temperature data from engine-cooling water measured at ship inlets (typically around 10 meter depth and below the euphotic zone) and - since 1980 -- an increasing amount of data from drifter buoys in the (warmer) euphotic zone. We hypothesize that the reported SST warming is an artifact of the increasing percentage of (higher-temperature) buoy data and suggest various tests for falsifying the hypothesis. Supporting the hypothesis is the reported disparity between surface and atmospheric temperature trends in the tropics [Douglass et al 2004] and the observed disparity in trends between SST and NMAT (night-time marine air temperatures) [Christy et al 2001]. We suggest that the SST increase observed prior to 1940 is real and not caused by GH warming. D.H. Douglass et al. 2004. Disparity of tropospheric and surface temperature trends: New evidence. GRL 31, L13207, 10.1029/2004GL020212 J.R. Christy et al. 2001. Differential trends in tropical sea surface temperatures since 1979. GRL 28, 186-193.
A34A-05 INVITED 16:45h
Atmospheric Temperature trends: Climate Atmospheric Temperature trends: Climate Models versus Observation
Douglass, Pearson and Singer published a paper [1]: Altitude dependence of atmospheric temperature trends: Climate models versus observation. In that paper they stated: "As a consequence of greenhouse forcing, all state-of the-art general circulation models predict a positive temperature trend that is greater for the troposphere than the surface. This predicted positive trend increases in value with altitude until it reaches a maximum ratio with respect to the surface of as much as 1.5 to 2.0 at about 200-400 hPa. However, the temperature trends from several independent observational data sets of the past 25 years show decreasing as well as mostly negative values. This disparity indicates that the three models examined here fail to account for the effects of greenhouse forcings." It is noted that in the 7 months since publication there has been no public rebuttal by the modeling community. 1. Douglass, Pearson and Singer. GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L13208, doi:10.1029/2004GL020103, 2004