eSeasonal oscillation of atmospheric CO2 found on Earth mainly due to activity of land plants.
Seasonal oscillation of atmospheric CO2 found on Earth mainly due to activity of land plants.
Researchers recorded monthly atmospheric CO2 level in many stations around the world from 1957 to 1960, and they found the data collected from the northern hemisphere showed seasonal variations. They attributed this signal to the local combustion of fuels associate with agricultural and industrial activities and the photosynthesis of land plants. Both these can indicate the presence of life. Thus, the seasonal variations of CO2 is a candidate for biosignature.
Keeling, Charles D.
A systematic variation with season and latitude in the concentration and isotopic abundance of atmospheric carbon dioxide has been found in the northern hemisphere. In Antarctica, however, a small but persistent increase in concentration has been found. Possible causes for these variations are discussed.
eCO2 level in Mars’ atmosphere changes seasonally, primarily due to the exchange between atmosphere and Mars’s seasonal polar cap.
CO2 level in Mars’ atmosphere changes seasonally, primarily due to the exchange between atmosphere and Mars’s seasonal polar cap.
Viking landers measured an annual fluctuation in CO2 pressure on Mars believed to be driven abiotic process. Mars has polar caps mainly consisting of frozen CO2 frost. Their seasonal evaporation and condensation are the main reasons for the detected CO2 variation.
eSome abiotic processes can result in regional CO2 variation on time scales shorter than one year.
Some abiotic processes can result in regional CO2 variation on time scales shorter than one year.
Researchers found in karst systems where carbonate rock is the dominant rock type, when rain water infiltrates down through the soil, it will dissolve carbonate minerals and form bicarbonate due to the CO2-enrichment and carbonate undersaturation of the water. When this infiltrating water reaches underground air-filled caves, degassing occurs to maintain the equilibrium of CO2 pressure in the cavity. This whole process removes CO2 from atmosphere to the underground environment and is controlled by precipitation, thus its magnitude varies seasonally.
Serrano-Ortiz, Penélope and Roland, Marilyn and Sanchez-Moral, Sergio and Janssens, Ivan A. and Domingo, Francisco and Goddéris, Yves and Kowalski, Andrew S.
This review article analyzes different abiotic processes that could contribute to the global carbon cycle on short time scales, beginning with high rates of net CO2 release or uptake measured over ecosystems by the FLUXNET community. The two main abiotic interpretations for these ‘‘anomalous’’ measurements are weathering processes and subterranean cavity ventilation. After analyzing their mechanisms and drivers, we evaluate their possible relevance and contributions in the studies mentioned above. Analyzing weathering (calcite dissolution and precipitation) chemistry and using a geochemical model, we conclude that CO2 dissolution processes could explain the measured CO2 release following dry season rain events, but their contribution is far from sufﬁcient to explain large magnitudes of daytime CO2 emissions or annual CO2 uptake measured in some desert ecosystems. In this context, we hypothesize and evaluate a further abiotic mechanism: the role of subterranean cavities as a temporal depot of CO2, along with their seasonal ventilation. A ﬁrst approximation estimates that the subterranean CO2 pool (and its potential ventilation) could represent more than half of the total CO2 content of the atmosphere. Therefore, the non-negligible potential contribution to the net ecosystem carbon balance requires further investigation towards a better understanding of its drivers.
eCO2 oscillation caused by land plants might be offset by orbital effect, which is possible to be a false negative.
CO2 oscillation caused by land plants might be offset by orbital effect, which is possible to be a false negative.
The seasonal variation of CO2 in Earth’s atmosphere mainly due to the growth and decay of land plants which mostly controlled by temperature. On Earth, most continents are in the Northern hemisphere while the Southern hemisphere is covered largely by ocean. Earth reaches its perihelion in December and aphelion in June, making the Southern hemisphere has warmer summer and colder winter. If there were a planet with only slightly difference in continental distribution and the continental area in its two hemisphere doesn’t differ very much, the orbital effect might be offset and produce an annual homogenous CO2 level, which leads to a false negative, i.e. life do exist but not detectable.
eAtmospheric molecular oxygen level changes seasonally in both hemispheres, due to the biological activity.
Atmospheric molecular oxygen level changes seasonally in both hemispheres, due to the biological activity.
The dominant processes that constrain the molecular oxygen levels in the atmosphere are the creation and destruction of organic matter, i.e. photosynthesis, respiration, and combustion. This paper reported that molecular oxygen levels in the Earth’s atmosphere changes seasonally and shows a strong negative correlation with atmospheric CO2 whose seasonal variation is also mainly controlled by terrestrial biosphere. In addition, dissolved molecular oxygen is chemical neutral while CO2 can react with water and form carbonic acid, so the amplitude of O2 variation is larger than that of CO2.
eSeasonal variations in atmospheric O2/N2 ratio that is not related to biological activity has been reported in both hemispheres on Earth.
Seasonal variations in atmospheric O2/N2 ratio that is not related to biological activity has been reported in both hemispheres on Earth.
Researchers observed seasonal variations in O2/N2 ratio at several sites in the Northern and Southern hemisphere. Concurrent CO2 variations were used to remove the effect of land biosphere so that the corrected ratio represents the exchange between the atmosphere and the ocean. Besides marine photosynthesis, the ocean also contribute to this ratio by abiotic process such as variation in water temperature and salinity.
Keeling, Ralph F. and Stephens, Britton B. and Najjar, Raymond G. and Doney, Scott C. and Archer, David and Heimann, Martin
Observationsof seasonavl ariationsin the atmosphericO2/N2ratioarereported at ninebaselinesitesin the northernandsouthernhemispheres.ConcurrentCO2 measurementasreusedto correctfor the effectsof landbioticexchangesof 02 onthe o2/m2cyclesthusallowingtheresidualcomponenot f thecyclesdueto oceanicexchanges of 02 andN 2to be calculated.The residualoceaniccyclesin thenorthernhemisphereare nearlydiametricallyoutof phasewiththecyclesin thesouthernhemisphereT. hemaxima in bothhemisphereosccurin summer.In bothhemispheresth, emiddle-latitudesealevel stationsshowthecycleswith largestamplitudesandearliestphasing.Somewhatsmaller amplitudesareobservedatthehigh-latitudestationsa, ndmuchsmalleramplitudesare observedat thetropicalstations.A modelfor simulatingtheoceaniccomponenot f the atmospheriOc 2/N2cyclesispresentedconsistingof theTM2 atmospheritcracertransport model[Heimann,1995]drivenatthelowerboundaryby 02 fluxesderivedfromobserved 02 saturationanomaliesin surfacewatersandbyN2fluxesderivedfromthenetair-sea heatflux. The modelis optimizedto fit theobservedatmospheriOc 2/N2cyclesby adjustingtheair-seagas-exchangveelocity,whichrelates02 anomalyto 02 flux. The optimumfit correspondtosspatiallyandtemporallyaveragedexchangveelocitiesof 24ñ6 cm/hr for the oceans north of 31 øN and 29ñ 12 cm/hr for the oceans south of 31 oS. These velocitiesagreeto withintheuncertaintiewsiththegas-exchangveelocitiesexpectedfrom the Wanninkhof1formulationof theair-seagas-exchangveelocitycombinedwith EuropeanCentreforMedium-RangWe eatherForecastwsinds[Gibsonetal., 1997]but arelargerthantheexchangveelocitiesexpectedfromtheLissandMerlivatrelation usingthesamewinds.Theresultsimplythatthegas-exchangvelocityfor02, likethatof CO2,maybeenhancedin theopenoceanbyprocessethsatwerenotsystematically accountedfor in theexperimentussedto derivetheLissandMerlivatrelation.
eOzone varies significantly between wet seasons and dry seasons in Amazonia because of rainforest activity.
Ozone varies significantly between wet seasons and dry seasons in Amazonia because of rainforest activity.
Researchers found in the Amazon the main daily course of ozone uptake velocity and the canopy resistance shows clear seasonal variation. Ozone uptake by rainforest is largely controlled by stomatal aperture of plants as in the more humid environment, the total size of open stomatal aperture would be larger and more ozone is stored in the plants. Thus, ozone levels are much higher in the dry season than in the wet season, due to the reduced stoma when the air humidity is relatively low. Besides the seasonal effect, ozone deposition also exhibits a daily pattern, also due to the difference in humidity. Humidity in daytime is lower than to nighttime, so O3 levels are highest in the morning and lowest in early evening.
Rummel, U and Ammann, C and Kirkman, G A and Moura, M A L and Foken, T and Andreae, M O and Meixner, F X
Within the project EUropean Studies on Trace gases and Atmospheric CHemistry as a contribution to Large-scale Biosphere–atmosphere experiment in Amazonia (LBA- EUSTACH), we performed tower-based eddy covariance measurements of O3 flux
5 above an Amazonian primary rain forest at the end of the wet and dry seasons. Ozone
deposition revealed distinct seasonal differences in the magnitude and diel variation.
In the wet season, the rain forest was an effective O3 sink with a mean daytime
(midday) maximum deposition velocity of 2.3cms−1, and a corresponding O3 flux of −2 −1
–11nmolm s . At the end of the dry season, the ozone mixing ratio was about
10 four times higher (up to maximum values of 80 ppb) than in the wet season, as a con-
sequence of strong regional biomass burning activity. However, the typical maximum
daytime deposition flux was very similar to the wet season. This results from a strong
limitation of daytime O3 deposition due to reduced plant stomatal aperture as a re-
sponse to large values of the specific humidity deficit. As a result, the average midday deposition velocity in the dry burning season was only 0.5 cm s-1.
eSeasonal variations of CH4 can be an indicator of temporal changes in biological methanogenesis.
Seasonal variations of CH4 can be an indicator of temporal changes in biological methanogenesis.
Researchers found that methane level measured in Northern hemisphere varies seasonally. The highest levels occurred in late fall (October) while the minimum comes in summer (July). The author suggested that biological methanogenesis contributes to this seasonal cycle, related to variations of the rate of organic matter decay.
eAtmospheric CH4 level is mainly controlled by its reaction with OH which increases in summer, due to higher surface temperatures.
Atmospheric CH4 level is mainly controlled by its reaction with OH which increases in summer, due to higher surface temperatures.
Seasonal changes of CH4 are not totally driven by biological processes. The primary sink of atmospheric CH4 on the Earth is reaction with atmospheric OH, and researchers reported that CH4 variation is largely consistent with the variation of OH. OH is mainly produced from water vapor which increases in summer. Therefore, CH4 seasonal cycles can be attributed to changes in surface temperature.
eEvidence: Researchers have detected CH4 and CO2 in the atmosphere of an exoplanet.
Evidence: Researchers have detected CH4 and CO2 in the atmosphere of an exoplanet.
Using NICMOS instrument on Hubble Space Telescope, the researchers measured the dayside spectrum of an exoplanet HD 209458b transiting a star between 1.5 and 2.5μm. They applied an iterative forward model for spectrum retrieval. The results suggested the presence of methane, water, and a small amount of carbon dioxide. Further improvement of wavelength coverage and resolution would permit more accurate constraints on the composition of exoplanet atmospheres.
Swain, M. R. and Tinetti, G. and Vasisht, G. and Deroo, P. and Griffith, C. and Bouwman, J. and Chen, Pin and Yung, Y. and Burrows, A. and Brown, L. R. and Matthews, J. and Rowe, J. F. and Kuschnig, R. and Angerhausen, D.
Biosignature gases are Earth’s atmosphere is highly effected by biological activity, thus is a great starting point for examining biosignatures in exoplanet atmospheres. One of the proposed biosignatures is the seasonal oscillation in some spectral gases, such as CO2, O2, O3, and CH4. The growth and decay of land plants is the primary driving force of seasonal variation in CO2 and O2. More specifically, because most of land plants are in the northern hemisphere, atmospheric CO2 reaches the lowest level in early fall and highest in early spring. However, there is also debate on the hypothesis that the seasonal oscillation in these gases can be a robust biosignature. It has been found that many abiotic processes, like changing in ocean-atmosphere exchange due to thermal variations, also contributes to the observed seasonal cycles of these gases.