On Long‐Term SABER CO₂ Trends and Effects Due to Nonuniform Space and Time Sampling

Abstract

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the TIMED satellite has been continuously operating for more than 16 years, since 2002, monitoring the CO2 concentration on nearly a global scale in the middle and upper atmosphere (from 65 km up to 110 km). A recent reanalysis (Qian et al., 2017, https://doi.org/10.1002/2016JA023825) concluded that different deseasonalizing methodologies may have a strong impact on long‐term trend analysis, ultimately yielding different altitude profiles of the global mean CO2 trend. In this work, we aim to understand how the nonuniform spatial and temporal sampling inherent in the SABER CO2 data set affects the determination of the long‐term trends. In addition, our goal is to disentangle reported differences in SABER CO2 trends due to different time averaging windows and methodologies used for trend estimation. The Whole Atmosphere Community Climate Model is used for synthetic studies of the time series. We demonstrate that, due to the time varying data gaps and nonuniform sampling of local times, different time binning of the SABER CO2 data may indeed bias the long‐term trend estimation. We show and discuss how the 60‐day averaging reduces the bias in relative trends. We also conclude that different deseasonalizing methodologies (averaged over the same temporal bins) yield negligible differences on the trend determination. Taking this into account the global mean CO2 relative trend does not deviate statistically from the tropospheric value below 1 × 10−3 mb (90 km). Above about 90 km, there is a positive slope in the global CO2 trend profile, but with substantially reduced magnitude for 60‐day binned data.

Role Of the Sun and the Middle atmosphere/thermosphere/ionosphere In Climate (ROSMIC): a retrospective and prospective view

“…The main driver of long-term trends in the MLT region is CO 2 . The apparent discrepancy between the observed (ACE/FTS and TIMED/SABER) and modeled long-term trend of the CO 2 volume mixing ratio has been resolved Rezac et al 2018). The reason for the apparent difference was found to be the neglect of some influences on observational data during the derivation of concentrations from observations.…”

Middle Atmosphere Temperature Trends in the Twentieth and Twenty‐First Centuries Simulated With the Whole Atmosphere Community Climate Model (WACCM)

“…WACCM CO 2 has been compared extensively against observations other than SABER (Garcia et al, ) and shown to produce vertical profiles consistent with these independent observations and no trend beyond that ascribable to the growth of anthropogenic emissions (Garcia et al, ). In addition, Rezac et al () have recently presented a study of trends in CO 2 using a technique that retrieves simultaneously and consistently both temperature and CO 2 . These retrievals, which are completely independent from WACCM CO 2 data, show profiles and trends of CO 2 that agree with those calculated with the model, but they are not available above 95 km.…”

Radiometric Stability of the SABER Instrument

“…The length and consistency of the SABER data set make it ideal for examining trends in the middle atmosphere. Recent studies have analyzed trends in temperature (Garcia et al, ), carbon dioxide (Yue et al, ; Rezac et al, ), and water vapor (Yue et al, ) using SABER data. A tacit but central assumption of these analyses is that the SABER instrument has remained stable; that is, its absolute radiometric calibration is unchanged, over the life of the instrument; or, that any changes with time are significantly smaller than the observed trends.…”