# Influence of Droughs on Mid-Tropospheric CO\({}_{2}\)
## 5 Conclusions  
CO\({}_{2}\) and precipitation play critical roles in global warming and the hydrological cycle, respectively. The long-term CO\({}_{2}\) retrievals from the AIRS aboard the Aqua satellite and the precipitation measurements from the TRMM provide a great opportunity to explore the interaction between CO\({}_{2}\) and precipitation. Our analyses based on new observations suggest that the mid-tropospheric CO\({}_{2}\) concentrations are higher during dry years than wet years over the southwest USA. Results in this paper provide an insight on how droughts can influence CO\({}_{2}\) concentrations in the mid-troposphere. Mid-tropospheric CO\({}_{2}\) concentrations are \(\sim\)1 ppm higher during dry years than wet years. We further use reanalysis meteorology datasets to explore the physics behind the relationship between drought and CO\({}_{2}\). As shown by NCEP2 vertical pressure velocity data, there is stronger rising motion over the southwestern USA during drought conditions, which can bring high CO\({}_{2}\) concentrations from the surface to the mid-troposphere and help increase the CO\({}_{2}\) concentration in the mid-troposphere during dry years. CO\({}_{2}\) surface fluxes (NEE, GPP, respiration, and biomass burning) are used to explore the impact of droughts on the CO\({}_{2}\) surface fluxes. The signals in the CO\({}_{2}\) surface fluxes are a little noisy. During dry years, there is less CO\({}_{2}\) uptake from the biosphere due to decreased photosynthesis in some regions (e.g. northern California and Nevada). As a result, more CO\({}_{2}\) will remain in the atmosphere. There are also positive anomalies of CO\({}_{2}\) emissions associated with respiration over Arizona, Utah, and Nevada. Positive anomalies of CO\({}_{2}\) from biomass burning are also seen in some regions. These results suggest that vertical velocity and biospheric activities can contribute to the positive CO\({}_{2}\) anomaly during droughts. Results obtained from this study suggest that droughts over the southwestern USA lead to more CO\({}_{2}\) in the mid-tropospheric atmosphere during the dry years. Our analyses also suggest that the atmospheric dynamics (e.g., vertical motion) affects the concentration of CO\({}_{2}\) in the mid-troposphere. Finally, the relationship between the CO\({}_{2}\) concentrations and the surface biologic activities explored in this study will help us better understand the interaction between biosphere and atmosphere. The frequencies of extreme weather events (such as droughts and floods) have increased over the last decade, consistent with a warmer and wetter atmosphere driven by radiative imbalance from greenhouse gas warming [52]. This paper reveals that droughts can in turn change the CO\({}_{2}\) concentrations in the atmosphere. Results from this paper can help us better understand CO\({}_{2}\) and precipitation, two important components of the climate system. Observational studies provide constraints that must be incorporated into models, and hence this study will benefit the validation and development of models. Analyses in this study are also applicable for other regions of the world. This study helps us better understand Earth's climate system and provide a quantitatively analysis of the impact of droughs on trace gases. The following are available online at www.mdpi.com/2072-4292/9/8/852/s1, Figure S1: (a) The mean value of Gross Primary Production (GPP) in dry years (JJAS of 2003, 2007, and 2010), (b) The mean value of GPP in wet years (JJAS of 2006, 2008, 2011, and 2012), (c) GPP differences between the dry and wet years. (d) GPP differences within 10% significance level are highlighted in green. Units for GPP are g C m-2 mon-1 in (a)-(c), Figure S2: (a) The mean value of Ecosystem Respiration (Re) in dry years (JAS of 2003, 2007, and 2010), (b) The mean value of Re in wet years (JAS of 2006, 2008, 2011, and 2012), (c) Re differences between the dry and wet years. (d) Re differences within 10% significance level are highlighted in green.