An important consideration in this study is that we estimated fumigant exposure using proximity to agricultural fumigant applications reported in the PUR data, which is not a direct measure of exposure. However, the PUR data explains a large amount of the variability of measured fumigant concentrations in outdoor air . In conclusion, we did not observe adverse associations between residential proximity to agricultural fumigant use during pregnancy or childhood and respiratory health in the children through 7 years of age. Although we did not observe adverse effects of fumigants on lung function or respiratory symptoms in this analysis, we have seen adverse associations in previous analyses of the CHAMACOS cohort between residential proximity to higher fumigant use and child development. We observed an association between higher methyl bromide use during the second trimester of pregnancy and lower birthweight and restricted fetal growth . We also observed decreases of ~2.5 points in Full-Scale intelligence quotient at 7 years of age for each 10-fold increase in methyl bromide or chloropicrin use within 8 km of the child’s residences from birth to 7 years of age . Future studies are needed in larger and more diverse populations with a greater range of agricultural fumigant use to further explore the relationship with respiratory function and health. The search for bio-signatures seeks to discover evidence of extraterrestrial life through the detection and spectral characterization of exoplanetary atmospheres.

Many possibilities for detectable bio-signatures have been suggested,microgreen fodder system which includes various combinations of CH4, CO2, O2, O3, and H2O based on Earth’s history. Specifically, the concept for searching for a combination of O2 and CH4 gases was first suggested by Love lock as an example of disequilibrium present in Earth’s atmosphere that results from the presence of life. Lovelock observed that the chemical composition of Earth’s present-day atmosphere remained in a state of thermodynamic disequilibrium, whereas the atmospheric constituents of Venus, Mars, and Jupiter were much closer to an equilibrium state. The combined detection of O2 and CH4 would indicate that a planet has a substantial surface flux of both because CH4 is readily oxidized by O2, and on Earth the major sources of both of these gases are biological. But by themselves, neither O2 nor CH4 would be considered a compelling bio-signature . Although this example focuses on bio-signatures of present-day Earth, similar principles can be applied to ancient Earth . Additional disequilibria bio-signatures that have been suggested include N2–O2 and CO2–CH4 pairs . In general, the chemical fluxes and abundances observed in a planet’s atmosphere should be evaluated in the context of stellar and planetary characteristics, to assess the potential of a habitable planet to host life. The search for technosignatures is a continuation of the search for bio-signatures, which includes the idea of looking for spectral evidence of technology in the atmospheres of exoplanets. The term “technosignature” refers generally to any “evidence of technology that modifies its environment in ways that are detectable” , which could include a broad class of astronomically observable phenomena. For an overview of modern prospects in the search for technosignatures, see the reviews by Wright , Socas-Navarro et al. , and Lingam & Loeb . A handful of suggestions have been proposed for detectable atmospheric technosignatures, which focus on a single gaseous species as an indicator of extraterrestrial technology. Molecules such as chlorofluorocarbons and halofluorocarbons are examples of industrial products that can have long atmospheric residence times and could be detectable at mid-infrared wavelengths .

Atmospheric pollution could also indicate planetary-scale technology, such as elevated abundances of NO2 due to combustion that could be detectable in the 0.2–0.7 μm range . CFCs and HFCs are almost entirely produced by industry on Earth, while the major sources of NO2 are also industrial, so the detection of these atmospheric constituents in an exoplanetary atmosphere would provide compelling evidence of technology on another planet.One of the criticisms of these suggestions is that long-lived technological civilizations may be unlikely to accumulate significant amounts of atmospheric pollution. Industrially produced constituents such as CFCs and HFCs would only be observable if there were a regular flux into the atmosphere. One possible scenario could involve the use of industrially produced greenhouse gases in order to terraform a planet like Mars to make it more habitable , but the abundance of such emissions is restricted on Earth today due to a need to prevent undesirable greenhouse warming by these molecules. Detecting NO2 at elevated abundances would be consistent with a planet engaged in widespread combustion, but combustion itself may not be a sustainable practice over long timescales due to the negative impacts of pollution and finite fuel sources. There may be a large number of atmospheric technosignatures that are unique to industry or cities, but any molecules that are only produced for a short time in the history of a planet—and that do not persist for geologic timescales—will be unlikely to be observed. An ideal technosignature would be sustainable for a long time, as such long-lived evidence would be the most likely to actually be detected . The two Laser Geodynamics Satellites, known as LAGEOS, are highly reflective satellites used for geodynamics, with no moving parts, that will remain in stable medium-Earth orbits for more than 8 million years . The LAGEOS satellites are thus an example of a long-duration technosignature, although the detectability of LAGEOS itself around Earth may be challenging at exoplanetary distances. Another possible long-duration technosignature is the use of low-albedo energy collectors, which could be detectable by infrared surface imaging or spectral signatures in reflected light .

Such technosignatures may not be detectable on Earth today, but they represent plausible trajectories for technosignatures in Earth’s future.This Letter suggests that global scale agriculture provides such a technosignature. This is not the first time agriculture has been suggested as a technosignature: Sagan & Lederberg noted that the possibility of agriculture on Mars could be ruled out based on the lack of checkerboard-like patterns from Mariner 9 imagery. In principle, changes in albedo associated with the timing of crop planting and harvesting could also be detectable by conducting observations over multiple epochs that correspond to different planetary seasons , as such changes associated with agriculture can be observed on Earth today . In this Letter, we show that the accumulation of NH3 and N2O from large-scale agriculture is an example of a multi-species and long-lived atmospheric technosignature.The main source of H2 today is natural gas,barley fodder system but other sources of H2 such as biomass or water electrolysis also suffice. The ability to manufacture fertilizer using the atmosphere’s supply of N2 has allowed farmers to enrich their soils with compounds such as ammonium nitrate as a supplement or replacement to urea and manure. These fertilizers release ammonium and/or nitrate ions when dissolved in water, which is then applied to saturate the soil where it can provide a source of nitrogen to plants. Excess fertilizer that is not utilized by plant roots contributes to an increase in nitrogen gas emissions, discussed further below. The Haber–Bosch process revolutionized global agriculture and enabled the production of food surpluses to support a planet populated by billions of people. The expansion of global agriculture has led to an increase in the production of synthetic fertilizers as well as the demand for animal domestication, which leads to an increase in the release of atmospheric nitrogen gases. Indeed, the total anthropogenic fixed nitrogen flux is now equivalent to or greater than nonanthropogenic sources of fixed nitrogen . The most notable nitrogen-based atmospheric constituent due to anthropogenic activity is ammonia . About 81% of the ∼58 Tg yr−1 of nitrogen in total ammonia emissions is anthropogenic, with about 65% from agriculture, 11% from biomass burning, and 5% from other industrial processes . Only 19% of NH3 sources are nonanthropogenic, primarily from the volatilization of NH3 from seawater or undisturbed soil as well as from wild animals. The NH3 from agriculture enters the atmosphere from the volatilization of ammonia in soil as well as from domestic animals, all of which originates from fertilizer production . Atmospheric NH3 due to agriculture and animal husbandry has been observed by the Atmospheric Infrared Sounder on the NASA Aqua satellite over a 14 yr duration and shows rates of emission that have increased by about 2% per year, which correlates with increased fertilizer use in some parts of the world . The residence time of NH3 in the atmosphere is only hours to days, as most NH3 falls back to the surface through wet or dry deposition. If sufficient NH3 remains in the atmosphere, then it can combine with N2O or SO2 to form aerosol particles. The accumulation of detectable and increasing quantities of NH3 on Earth indicates the intensification of agricultural and industrial activities. Another significant atmospheric constituent that arises from anthropogenic activity is nitrous oxide .

Of the ∼16 Tg yr−1 of nitrogen in total N2O emissions, about 40% to 50% is from agriculture and industry, with the most significant nonanthropogenic sources being the oceans and wet tropical soils . Most emissions of N2O are the result of denitrification by microorganisms, which in agriculture is enhanced by nitrates added to soil as fertilizer. Other anthropogenic sources of N2O include irrigation, water degassing, and animal production—much of which is still ultimately connected to the use of fertilizer—as well as biomass burning. The atmospheric residence time of N2O is about 120 yr, with the major sink occurring due to photodissociation in the stratosphere and a smaller but significant sink from reactions with O radicals. Within the troposphere, N2O is relatively uniformly distributed and also acts as an effective greenhouse gas. The presence of N2O on Earth is generally connected with soil microbiology, but anthropogenic activities that are largely connected with agriculture have enhanced such N2O emissions from soil . Other trace gases are also emitted as the result of agriculture and animal domestication. Agriculture contributes NO and NO2 to the atmosphere from biomass burning and soil denitrification, which accounts for about 25% of total NOx emissions —although there are large uncertainties with these estimates . Nevertheless, a much larger fraction of about 65% of present-day NOx emissions is due to fossil fuel combustion; this certainly could serve as a technosignature , but NOx generated from combustion is a separate source from agricultural emissions of NOx that derive from the application of fertilizer. Methane is also emitted from agriculture— notably rice agriculture and ruminant ranching—as well as from biomass burning, landfills, and energy use. About 70% of total CH4 emissions are anthropogenic, with the rate of these emissions continuing to increase . Human civilization continues to expand its use of agriculture, and thereby intensify its use of industrial nitrogen fixation to make fertilizer. But there is no particular reason that agriculture itself depends on growth, and as long as sustainable sources of energy are used, then global-scale agriculture could in principle sustain itself across long timescales based on the use of industrial nitrogen fixation . Whereas processes like combustion may be short-lived due to a finite supply of fossil fuel, the use of industrial nitrogen fixation only requires a planet with a predominantly N2 atmosphere, a supply of H2, and a sustainable source of energy. Thus, the spectral signature of agriculture is well suited as a candidate for a technosignature that could persist for millennial, and perhaps even geological, timescales. There is little imagination required to speculate that extraterrestrial civilizations, if they exist, would find great value in industrial nitrogen fixation . What, then, is the expected spectral signature of an “ExoFarm”? The planetary requirements for agriculture as we know it are a hydrological, carbon, and nitrogen cycle, with an atmospheric reservoir of N2 and abundant O2 for photosynthesis. These requirements themselves are aligned with the disequilibrium bio-signature of the combined detection of O2 and CH4. In the event that such a planet is discovered, then evidence of elevated levels of NH3 combined with N2O would provide evidence of global-scale agriculture. Because of its extremely short lifetime, the observation of NH3 would imply a continuous large-scale source of emissions, which could be sustained for long periods of time through industrial nitrogen fixation.