<i>Astroclimes</i> – measuring the abundance of CO2 and CH4 in the Earth’s atmosphere using astronomical observations

ABSTRACT Monitoring the abundance of greenhouse gases is necessary to quantify their impact on global warming and climate change. Carbon dioxide ($\mathrm{CO_{2}}$) and methane ($\mathrm{CH_{4}}$) are the two most important greenhouse gases when it comes to global warming. Although a number of satellites and ground-based networks measure the total column volume mixing ratio (VMR) of these gases, they rely on sunlight, and column measurements at night are comparatively scarce. We present a new algorithm, Astroclimes, that hopes to complement and extend nighttime $\mathrm{CO_{2}}$ and $\mathrm{CH_{4}}$ column measurements. Astroclimes can measure the abundance of greenhouse gases on Earth by generating a model telluric transmission spectra and fitting it to the spectra of telluric standard stars in the near-infrared taken by ground-based telescopes. An extensive data set from the CARMENES spectrograph in the Calar Alto Observatory was compiled, which included all of the publicly available data from 2016 to 2024, as well as new observations carried out alongside a weather balloon launch. A Markov Chain Monte Carlo (MCMC) analysis on this extensive data set showed that Astroclimes was able to recover the long term trend known to be present in the molecular abundances of both $\mathrm{CO_{2}}$ and $\mathrm{CH_{4}}$, but not their seasonal cycles. Using the Copernicus Atmosphere Monitoring Service (CAMS) global greenhouse gas reanalysis model (EGG4) as a benchmark, we identified an overall vertical shift in our data and quantified the long-term scatter in our retrievals. We found that our ground level and column-averaged $\mathrm{CO_{2}}$ dry mole fractions (DMFs) exhibit a scatter of $\pm 5$ ppm and $\pm 9$ ppm, respectively. For $\mathrm{CH_{4}}$, these values were $\pm 31$ ppb and $\pm 42$ ppb, respectively. The vertical shift was 14 ppm and 15 ppm for the $\mathrm{CO_{2}}$ ground level and column-averaged DMFs, respectively, and 42 ppb and 7 ppb for the $\mathrm{CH_{4}}$ ground level and column-averaged DMFs, respectively. The scatter on a 1 h time-scale, however, is much lower, of the order of $\pm 1$ ppm and $\pm 10$ ppb for ground $\mathrm{CO_{2}}$ and $\mathrm{CH_{4}}$, respectively, which is on par with the uncertainties on individual measurements. Although currently the precision of the method is not in line with state-of-the-art techniques using dedicated instrumentation, which achieve sub-ppm precision, it shows promise for further development.