We compare linear trends based on minimization of absolute deviations and minimization of squared deviations. Differences are generally less than 0.05°C/decade over 1959-96. Over 1979-93, they exceed 0.10°C/decade for lower tropospheric time series and 0.15°C/decade for the lower stratosphere. Trend fitting by minimization of absolute deviations degrades the previously reported global lower tropospheric trend agreement of 0.03°C/decade between the MSU and radiosonde data.
To assess trend significance, we employ two methods to account for temporal autocorrelation effects. When our preferred method is applied, virtually none of the individual 1979-93 trends in deep-layer temperatures are significantly different from zero. We also test the significance of a trend in d(t), the time series of differences between a pair of datasets. Use of d(t) removes variability common to the time series being compared and facilitates identification of small but meaningful trend differences. Roughly 30% of the dataset comparisons show significant differences in lower tropospheric trends, primarily related to differences in measurement system. Significant trends in d(t) are also found between different versions of the MSU channel 4 data, and between channel 4 time series bases on globally-complete MSU data and on MSU data subsampled with the incomplete rediosonde coverage.
We also compute the 95% confidence intervals for individual trends and show that these overlap for almost all datasets considered. For lower tropospheric trends over 1979-93, the adjusted 95% confidence intervals for all datasets encompass both zero and the model-projected trends due to anthropogenic effects.