Astroclimes -- measuring the abundance of CO$_2$ and CH$_4$ in the Earth's atmosphere using astronomical observations

Abstract

Monitoring the abundance of greenhouse gases (GHGs) such as carbon dioxide (CO$_2$) and methane (CH$_4$) is necessary to quantify their impact on global warming and climate change. 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 CO$_2$ and CH4 column measurements. Astroclimes can measure the abundance of GHGs 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. A Markov Chain Monte Carlo (MCMC) analysis on an extensive dataset from the CARMENES spectrograph showed that Astroclimes was able to recover the long term trend known to be present in the molecular abundances of both CO$_2$ and 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. The scatter on a 1 hour timescale, however, is much lower, and 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, it shows promise for further development.