Astroclimes – 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 (CO2) and methane (CH4) are the two most important greenhouse gases when it comes to global warming, and there are many ground-based networks, such as TCCON, and satellites, such as OCO-2, OCO-3 and GOSAT-2, that are tasked with measuring the total column volume mixing ratio (VMR) of either one or both of these gases. However, these networks all rely on sunlight to carry out their measurements. For column measurements at night, a technique called integrated-path differential absorption (IPDA) has been employed recently using a lidar system. We present a new algorithm, Astroclimes, that hopes to complement and extend nighttime CO2 and CH4 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. We carried out new observations for one night with the CARMENES spectrograph in the Calar Alto Observatory, Spain, as well as a weather balloon launch to measure a local atmospheric profile. After correcting for a small bias in CO2 estimates, we show that our CO2 and CH4 measurements exhibit good agreement with the refereed literature, and our average relative uncertainties for the column-averaged dry air mole fraction of CO2 and CH4 are 0.4 % and 0.5 %, respectively. These uncertainties are precision errors based on the 68 % confidence intervals of our MCMC analysis posterior distribution, they do not include any systematic errors or biases. A historical analysis of archival data from several different instruments will be carried out in future work to further test our algorithm and to identify and quantify potential systematic biases.