1. Introduction

Climate Change is one of the greatest societal challenges of the 21 st century. The dominant source of Global Warming is the increase of anthropogenic greenhouse gases (GHGs) in the Earth`s atmosphere. The two most important of those species are carbon dioxide (CO 2 ) and methane (CH 4 ). Together they account for ~81% of the anthropogenic radiative forcing (WMO, 2018).

Further increase in the atmospheric abundance of these gases is predicted to result in a warmer climate (IPCC, 2013). However, uncertainties in our knowledge of the budgets of these gases, which are determined by their sources, sinks, and inadequately understood feedback mechanisms, limit the accuracy of current climate change projections from the local to the global scale. In order to reliably predict the climate of our planet, and to guide political conventions on greenhouse gas avoidance, adequate knowledge of the sources and sinks of these greenhouse gases, their feedbacks, and the quantification of natural versus anthropogenic sources is mandatory. In spite of the recognized importance of this issue, our current understanding of sources and sinks of the gases CO 2 and CH 4 is still poor and not
sufficient to address the needs of science and policymakers (IPCC, 2013).

The HALO Circle mission intends to address these deficiencies with a multi-disciplinary approach combining aircraft measurements, models, and satellite data with a focus on the Arctic.

There are manifold reasons to focus on the Arctic region and perform measurements of CO 2 and CH 4 using the unique CoMet payload. The Arctic is warming twice as fast as the global average, making climate change’s polar effects more intense than anywhere else in the world (AMAP, 2015). The Arctic accounts for nearly 50% of all organic carbon stored in the Planet`s soil, but rising temperatures and thawing permafrost threatens its stability. Wetland emissions of methane constitute the largest single source of methane to the atmosphere, even when considering all anthropogenic emissions, and are the most uncertain part of the budget (Kirschke et al., 2013; Saunois et al., 2019). After the tropics, the largest distribution of wetlands is in the Arctic. Fires in boreal forests or tundra peatlands (as seen in Greenland in 2017) are both sources of methane and CO 2 and also accelerate the thawing of permafrost which leads to the release of carbon. There is increasing, but divergent, evidence that changing climate in the modern period has shifted these ecosystems from sinks into net carbon sources (IPCC 2019).

Another important consequence of global warming is the reduced sea-ice extent which effectively „unlocks“ the Arctic Ocean and leads to an expansion of human activities such as oil and gas production. Current emission estimates from major oil and gas fields show substantial differences in estimated CH 4 emissions and their distribution (Scarpelli et al. 2019). These oil and gas fields overlap with the natural sources making the quantification of both natural and anthropogenic sources particularly difficult. Flying around the Arctic cycle will unravel interhemispheric (east-west) differences of GHG fluxes.

In general, passive remote sensors have difficulties accurately measuring GHGs at high latitudes due to the low solar zenith angle and low surface reflectance which compromises their signal-to noise ratio. Nevertheless, new passive remote sensors like TROPOMI on Sentinel-5 Precursor deliver XCH 4 in the Arctic data during summer (Schneising et al., 2019). Active remote sensors using lidar such as the upcoming joint French-German MERLIN mission (Ehret et al., 2017), currently scheduled for launch in 2024, promise to provide high quality methane data at all latitudes and seasons. Therefore, validation of MERLIN in this area of the world, for which the CoMet payload provides unrivalled capacities, are of high importance for this mission. Together, highly-resolved aircraft data, lower-resolution MERLIN data, all passive satellites available during the campaign period, and models will provide a unique combination to constrain the Arctic carbon cycle.