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Remote sensing is a widely used technique for studying Earth systems, including ecosystem response to global warming. With the recent California drought, remote sensing gives an opportunity to study large-scale CO2 flux changes over time due to insufficient water uptake by plant life. In this study three flux towers were used, to correlate Net Ecosystem Exchange (NEE = PRI * NDVI * PAR), gathered from remote sensing data, and measurements of CO2 flux from in-situ flux towers over an area of 8,149 km in the Sierra Nevada Range. To estimate CO2 from NEE, two linear regressions were used which correlated with the temperature stress response of plants (>23.5 and <23.5 degrees C), with R values of 0.81 and 0.83 respectively. Per-pixel CO2 flux was estimated for flights between 2011 (pre drought) and 2015 (during drought). This study showed a 25% (4.8 g / min) decrease in net carbon fixation between September 2011 and October 2014 over 34 km. Also seen was a decrease in net carbon fixation over 7,737 km of 90.2% (18,863 g / min) between June 2013 to June 2015. Of the 411.6 km affected by the 2013 Rim Fire, this study showed a decrease in net carbon fixation of 85.5% (1,574 g / min) between June 2013 and June 2015. It is estimated that the entire Rim Fire (1,041 km) could have caused a total decrease in net carbon fixation totaling 3,982 grams per minute from 2013 to 2015. Introduction It is hard to be unaware of the current debate on global warming. Some people may just hear about it on the news, and some may be noticing the climate changing right at home. The big contributor to global warming is the accumulation of greenhouse gases. These chemicals trap some of the heat from the sun and allow Earth to be habitable, but an excess in these gasses will increase the global temperature, a process which has already begun. Since the 1880s, the Earth’s temperature has risen by 1.6 degrees Fahrenheit, and it is expected to continue to increase at a faster rate. The ten hottest years in the last 134 have been since 2000, with the exception of 1998, and with 2015 being the hottest year yet (Shaftel). Although greenhouse gases, such as carbon dioxide (CO2), occur naturally, this continued climb has been caused by many human-related processes. The biggest contributor is the burning of fossil fuels like oil, coal and natural gas. Alteration of land use, such as deforestation, and industrial processes like cement and steel production also add to the overload of carbon dioxide (Canadell et al. 304-305). The lifetime of CO2 in the atmosphere has been estimated to be about 200 years, adding to the importance of prevention and recovery (Sonnemann and Grygalashvyly, 1591). The main way to lessen the carbon dioxide in the atmosphere has been through the natural processes our forests provide. It may be considered common knowledge that plant life provides the oxygen necessary for almost all other forms of life on Earth, but it might not be as well-known that our forests take in carbon dioxide. On average, they absorb (or “fix”) almost 122 billion tons of carbon dioxide per year through photosynthesis (Beer et al., 834). In the process of photosynthesis plants take in CO2, water, and energy from the sun, and convert them to sugars and oxygen following the chemical equation: + light 6 CO2 + 6 H2O  C6H12O6 + 6O2 Where H2O is water, C6H12O6 is sugar, and O2 is oxygen. If any one of the materials needed for photosynthesis are lacking, such as water, plants will decrease photosynthesis and may become dormant, therefore decreasing the rate they take in CO2. When plants become water-limited they will close their stomata, or the openings in their leaves in which gas exchange occurs, in order to reduce the evaporation of water. This was seen in a study by Sherrard and Maherali in which plants that were well-watered showed an increase of 303% in stomatal conductance when compared to those that weren’t (Sherrard and Maherali, 2481). To insure survival, plants have to produce energy, so they begin the process of respiration. During respiration plants break down their stored sugars and combine them with oxygen to release carbon dioxide and energy. The plants then release this carbon dioxide into the atmosphere. One large concern is that this decrease in CO2 uptake, as well as its release during respiration, means more carbon dioxide is in the atmosphere, therefore compounding the effects of global warming. A process that can cause significant changes in large-scale carbon dioxide uptake is severe, long-term drought. An example of the detrimental effects drought can have on plant life can be seen in the paper by Huggins et al. in which the species Astragalus jaegerianus was studied from 1999-2010. During this time period the Mojave Desert in Southeastern California was experiencing a severe drought in which the population of A. jaegerianus saw a large decline, and in some areas was almost forced into extinction (Huggins et al. 120). Another study done by Ozolincius, et al. looked at the effect of an artificial drought on a population of a 60-year-old Scots Pine stand in central Lithuania. The authors did this by analyzing indicators such as needle age, litterfall, and crown defoliation (Ozolincius et al. 299). Ozolincius et al. noticed a large decrease in plant structure, in which they saw changes in crown defoliation where needles fell faster and there was a weaker formation of shoots and needles (Ozolincius et al. 305). This method of data acquisition is termed field data collection, and can be highly accurate. One downside to this direct form of data collection is that individuals must spend days at a time in the field gathering data, and must return at regular intervals to repeat the process. Other methods of field data collection have been developed in which there is less need for a research team to spend time out in the field. One technique that has been in use for many years is data collection from in-situ flux towers. These towers assess the amount of a certain variable (water, carbon dioxide, nitrogen, light, etc.) that flows through a unit of area (usually m) per unit time (seconds). These are often very accurate and once in place are relatively inexpensive to operate, but the data is not always readily available and the towers are only useful on a small scale. The issue with collecting field data is it can be very expensive and time consuming, so more advanced techniques have been developed in order to assess forest / ecosystem health. One such technique is airborne and satellite remote sensing, which records wavelengths of light that are reflected off of a surface. Remote sensing data will also contain factors such as latitude / longitude, time, slope of the ground, elevation, and angle of the sun. Many indices have been created to attempt to analyze plant health using ratios between these wavelengths, the efficacy has been heavily studied, and new indices are constantly being developed and improved. One such index is the Normalized Difference Vegetation Index (NDVI), which assesses plant greenness, a representation of plant biodensity. Another important index is PRI, or the Photochemical Reflectance Index. PRI is an indicator of plant stress, and uses the ratio between two wavelengths of light that indicate the presence of the pigment xanthophyll, and lack of chlorophyll, which is degraded when the leaf becomes senescent. Xanthophyll is also seen in leaves with certain nutrient deficiencies (“Xanthophylls”). These indices have been important in the assessment of ecosystem health on both the small and large scale. The recent California drought has been one of the most severe on record, with 62% of the state considered to be in the “extreme” to “exceptional” category (Heim). The drought began in 2012 and has already cost billions of dollars in both agricultural losses and funding for the creation of policies for water conservation. Californians continue to decrease their water consumption (23.9% between June 1 2015 and February 29 2016), but the effects of the drought will be seen for many years after it has ended (Alexander). The area hit the hardest is the Sierra Nevada Range, which covers 39,612 mi of the eastern edge of California. There is a strong need to develop techniques to assess such ecosystems on a large scale in order to determine to what extent their health has declined, as well as what implications that may have for the entire globe. In order to analyze forests on a scale such as that of the Sierra Nevada Range, a combination of previous techniques should be used to insure accuracy, but allow for large-scale analysis. Combining flux tower data with remote sensing allows for such a study, and one of the variables that best determines an ecosystems health is CO2 flux. CO2 flux is the amount of carbon dioxide being taken into, or released from, an ecosystem, and is an indicator of plant photosynthesis and respiration, respectively. CO2 flux is commonly seen in units of micromols of CO2 per m 2 per second. Being able to quantify this CO2 flux using remote sensing would allow an individual to assess how an entire ecosystem may be changing due to a factor such as severe drought. This technique could also determine change over many years with much less cost, time, and effort when compared to the process of collecting field data. In order to estimate CO2 flux with remote sensing, a variable must be compared to a known flux, such as that from a flux tower. This is the technique used for this study, and is similar to the technique Zang et al. used in their research, where they compared the variable NEE (Net Ecosystem Exchange) from remote sensing to flux tower readings to assess seasonal changes in the CO2 flux of crops (Zang et al. 40). Estimating the carbon dioxide exchange between the atmosphere and our forests gives us the opportunity to monitor climate change on a large scale. Assessing how drought affects the ability of plants t

The Impact of Drought on the Sierra Nevada Range: Using Remote Sensing Data to Estimate Large-Scale Carbon Dioxide Flux