How can mountaintop CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; observations be used to constrain regional carbon fluxes?

John C. Lin,Britton B. Stephens,Derek V. Mallia,Dien Wu

doi:10.5194/acp-17-5561-2017

Abstract

Abstract. Despite the need for researchers to understand terrestrial biospheric carbon fluxes to account for carbon cycle feedbacks and predict future CO2 concentrations, knowledge of these fluxes at the regional scale remains poor. This is particularly true in mountainous areas, where complex meteorology and lack of observations lead to large uncertainties in carbon fluxes. Yet mountainous regions are often where significant forest cover and biomass are found – i.e., areas that have the potential to serve as carbon sinks. As CO2 observations are carried out in mountainous areas, it is imperative that they are properly interpreted to yield information about carbon fluxes. In this paper, we present CO2 observations at three sites in the mountains of the western US, along with atmospheric simulations that attempt to extract information about biospheric carbon fluxes from the CO2 observations, with emphasis on the observed and simulated diurnal cycles of CO2. We show that atmospheric models can systematically simulate the wrong diurnal cycle and significantly misinterpret the CO2 observations, due to erroneous atmospheric flows as a result of terrain that is misrepresented in the model. This problem depends on the selected vertical level in the model and is exacerbated as the spatial resolution is degraded, and our results indicate that a fine grid spacing of ∼ 4 km or less may be needed to simulate a realistic diurnal cycle of CO2 for sites on top of the steep mountains examined here in the American Rockies. In the absence of higher resolution models, we recommend coarse-scale models to focus on assimilating afternoon CO2 observations on mountaintop sites over the continent to avoid misrepresentations of nocturnal transport and influence.

Highlights

Scientific consensus among climate scientists points to carbon dioxide (CO2) as the main greenhouse gas leading to climate change (IPCC, 2014)
This study investigated whether the three mountaintop sites could detect signals of ecosystem disturbance, Brooks et al (2016) did not examine issues related to erroneous atmospheric transport in complex terrain or compare modeled CO2 against observed values
Given the large extent of the Earth’s surface covered by hills and mountains and the large amount of biomass and potential for carbon storage in complex terrain (Fig. 1), we call for expanded efforts in observing and modeling CO2 and other tracers on mountaintop sites

Summary

Introduction

Scientific consensus among climate scientists points to carbon dioxide (CO2) as the main greenhouse gas leading to climate change (IPCC, 2014). A strong need exists to quantify and understand global carbon fluxes, among which the terrestrial biospheric component is the most dynamic, potentially even reversing signs on an annual basis from year to year (Le Quéré et al, 2015; Sarmiento et al, 2010). Because mountains cover approximately a quarter of the Earth’s land surface (Blyth et al, 2002), it is imperative to quantify and understand carbon fluxes over “complex terrain”. Much of the biomass and potential for terrestrial carbon storage in other parts of the world are found in hills or mountains, partly due to the fact that historical deforestation and biomass removal have been most pronounced in easier-to-access, flat regions (Ramankutty and Foley, 1999)

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