However, the effects on the atmosphere and overall carbon cycle can be intentionally and/or naturally reversed with reforestation. Arctic methane emissions indirectly caused by anthropogenic global warming also affect the carbon cycle and contribute to further warming. The ocean contains the largest active pool of carbon near the surface of the Earth. It is bordered by the state of Haryana on three sides and by Uttar Pradesh to the east. Teaching about the human impacts on climate is supported by five key concepts: Teaching this principle is supported by five key concepts: a. The World Resources Institute have posted a useful resource: the World GHG Emissions Flow Chart, a visual summary of what's contributing to manmade CO2 (eg - electricity, cars, planes, deforestation, etc). [1][9][10] Restoring balance to this natural system is an international priority, described in both the Paris Climate Agreement and Sustainable Development Goal 13. In other words: When you drive a car, the engine burns fuel which creates a certain amount of CO2, depending on its fuel consumption and the driving distance. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of CO 2, causing concentrations in the atmosphere to rise. [88] Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO2. [26] Organic carbon is a major component of all organisms living on earth. In fact, human emit 26 gigatonnes of CO2 per year while CO2 in the atmosphere is rising by only 15 gigatonnes per year - much of human … De La Rocha C.L. Agriculture. [13] Methane produces a larger greenhouse effect per volume as compared to carbon dioxide, but it exists in much lower concentrations and is more short-lived than carbon dioxide, making carbon dioxide the more important greenhouse gas of the two. Usually used in the context of emissions that are produced as a result of human activities. The marine biological pump is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the deep ocean interior and seafloor sediments. In: Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quere, R.B. CO 2 is absorbed and emitted naturally as part of the carbon cycle, through plant and animal respiration, volcanic eruptions, and ocean-atmosphere exchange. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.[89]. Mountain building processes result in the return of this geologic carbon to the Earth's surface. Both of these gases absorb and retain heat in the atmosphere and are partially responsible for the greenhouse effect. The malaria parasite life cycle involves two hosts. Other geologic carbon returns to the ocean through the hydrothermal emission of calcium ions. Please use this form to let us know about suggested updates to this rebuttal. In CO2 measurements, this feature is apparent in the Keeling curve. This unit will introduce you to the basics of the carbon cycle. Although the presence of carbon in the Earth's core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region. Given that the average depth of the ocean is about four kilometres, it can take over ten years for these cells to reach the ocean floor. The module explains geological and biological components of the cycle. [73], The fast carbon cycle involves relatively short-term biogeochemical processes between the environment and living organisms in the biosphere (see diagram at start of article). [104][105] Fossil carbon extraction that increases atmospheric greenhouse gases is thus described by the IPCC, atmospheric and oceanic scientists as a long-term commitment by society to living in a changing climate and, ultimately, a warmer world.[4][106]. The terrestrial biosphere includes the organic carbon in all land-living organisms, both alive and dead, as well as carbon stored in soils. There is substantial evidence that human activities, especially burning fossil fuels, are leading to increased levels of carbon dioxide and other greenhouse gases in the atmosphere, which in turn amplify the natural greenhouse effect, causing the temperature of the Earth's atmosphere, ocean and land surface to increase. only considering the GHG environmental impact category. This includes volcanoes returning geologic carbon directly to the atmosphere in the form of carbon dioxide. [24] Most carbon in the terrestrial biosphere is organic carbon,[25] while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate. [87], Accordingly, carbon can remain in the lower mantle for long periods of time, but large concentrations of carbon frequently find their way back to the lithosphere. It can also be exported into the ocean through rivers or remain sequestered in soils in the form of inert carbon. However, carbonates descending to the lower mantle encounter other fates in addition to forming diamonds. It includes movements of carbon between the atmosphere and terrestrial and marine ecosystems, as well as soils and seafloor sediments. The overwhelming consensus of scientific studies on climate indicates that most of the observed increase in global average temperatures since the latter part of the 20th century is very likely due to human activities, … The slow or geological cycle can take millions of years to complete, moving carbon through the Earth's crust between rocks, soil, ocean and atmosphere. It takes about a decade for methane (CH 4) emissions to leave the atmosphere (it converts into CO 2) and about a century for nitrous oxide (N 2 O).. After a pulse of CO 2 is emitted into the atmosphere, 40% will remain in the … [16], Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres. [97][103] Nevertheless, ocean uptake also has evolving saturation properties, and a substantial fraction (20-35%, based on coupled models) of the added carbon is projected remain in the atmosphere for centuries to millennia. Concluding on a lighter note, however, the authors mention that actions can and are being taken to help prevent the unwanted evolution of populations. Offline PDF Version  |  The fast cycle operates in the biosphere and the slow cycle operates in rocks. It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation. As an example, preliminary theoretical studies suggest that high pressure causes carbonate melt viscosity to increase; the melts' lower mobility as a result of its increased viscosity causes large deposits of carbon deep into the mantle. UPDATE: Human CO2 emissions in 2008, from fossil fuel burning and cement production, was around 32 gigatoones of CO2 (UEA). [11] The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:[12]:5–6, The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. [29], The ocean can be conceptually divided into a surface layer within which water makes frequent (daily to annual) contact with the atmosphere, and a deep layer below the typical mixed layer depth of a few hundred meters or less, within which the time between consecutive contacts may be centuries. It can then be absorbed by rocks through weathering. However, this is less than one percent of the carbon dioxide put into the atmosphere by burning fossil fuels.[73][74]. The lack of volcanoes pumping out carbon dioxide will cause the carbon cycle to end between 1 billion and 2 billion years into the future. Carbon leaves the terrestrial biosphere in several ways and on different time scales. There the rocks are weathered and carbon is returned to the atmosphere by degassing and to the ocean by rivers. [17], Human activities over the past two centuries have increased the amount of carbon in the atmosphere by nearly 50% as of year 2020, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g., by burning fossil fuels and manufacturing concrete. If the process did not exist, carbon would remain in the atmosphere, where it would accumulate to extremely high levels over long periods of time. The rest remains in the atmosphere, and as a consequence, atmospheric CO2 is at its highest level in 15 to 20 million years (Tripati 2009). Thermohaline circulation returns deep-ocean DIC to the atmosphere on millennial timescales. Major sources and sinks of carbon … [97] Over the past several centuries, direct and indirect human-caused land use and land cover change (LUCC) has led to the loss of biodiversity, which lowers ecosystems' resilience to environmental stresses and decreases their ability to remove carbon from the atmosphere. [117][118], These feedbacks are expected to weaken in the future, amplifying the effect of anthropogenic carbon emissions on climate change. Carbon dioxide is also produced and released during the calcination of limestone for clinker production. Man-made CO2 in the atmosphere has increased by a third since the pre-industrial era, creating an artificial forcing of global temperatures which is warming the planet. A small shift in the balance between oceans and air would cause a CO2 much more severe rise than anything we could produce.” (Jeff Id). (A natural change of 100ppm normally takes 5,000 to 20,000 years. Both of these replacement land cover types store comparatively small amounts of carbon so that the net result of the transition is that more carbon stays in the atmosphere. Comments Policy... You need to be logged in to post a comment. It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the oceans' deeper, more carbon-rich layers as dead soft tissue or in shells as calcium carbonate. Inorganic nutrients and carbon dioxide are fixed during photosynthesis by phytoplankton, which both release dissolved organic matter (DOM) and are consumed by herbivorous zooplankton. The CO2 that nature emits (from the ocean and vegetation) is balanced by natural absorptions (again by the ocean and vegetation). [90] Because the core's composition is believed to be an alloy of crystalline iron and a small amount of nickel, this seismic anomaly indicates the presence of light elements, including carbon, in the core. More directly, it often leads to the release of carbon from terrestrial ecosystems into the atmosphere. The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of climate change. Based on data by the UN Food and Agriculture Organization (FAO), more than … [69] The biological pump is not so much the result of a single process, but rather the sum of a number of processes each of which can influence biological pumping. Partly because its concentration of DIC is about 15% higher[30] but mainly due to its larger volume, the deep ocean contains far more carbon—it is the largest pool of actively cycled carbon in the world, containing 50 times more than the atmosphere[13]—but the timescale to reach equilibrium with the atmosphere is hundreds of years: the exchange of carbon between the two layers, driven by thermohaline circulation, is slow.[13]. Organic carbon stored in the geosphere can remain there for millions of years.[33]. [13] Much of the carbon stored in the earth's mantle was stored there when the earth formed. [115] Also, acid rain and polluted runoff from agriculture and industry change the ocean's chemical composition. [80] Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principle transport mechanism for carbon to Earth's deep interior. Weather is a specific event—like a rain storm or hot day—that happens over a short period of time. The net effect of these processes is to remove carbon in organic form from the surface and return it to DIC at greater depths, maintaining a surface-to-deep ocean gradient of DIC. In fact, studies using diamond anvil cells to replicate the conditions in the Earth's core indicate that iron carbide (Fe7C3) matches the inner core's wave speed and density. [3][4] The increased carbon dioxide has also increased the acidity of the ocean surface by about 30% due to dissolved carbon dioxide, carbonic acid and other compounds, and is fundamentally altering marine chemistry. About 40% of this additional CO2 is absorbed. sfn error: no target: CITEREFBrownlee2010 (, CS1 maint: multiple names: authors list (, Sigman DM & GH Haug. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The exchange between the ocean and atmosphere can take centuries, and the weathering of rocks can take millions of years. [108] A 2019 study indicated that degradation of plastics through sun exposure, releases both carbon dioxide and other greenhouse gases. [1][2] Carbon dioxide in the atmosphere had increased nearly 52% over pre-industrial levels in 2020, forcing greater atmospheric and Earth surface heating by the Sun. CO 2 sticks around. The oceans, land and atpmosphere exchange CO2 continuously so the additional load by humans is incredibly small. [91] Furthermore, another study found that in the pressure and temperature condition of the Earth's inner core, carbon dissolved in iron and formed a stable phase with the same Fe7C3 composition—albeit with a different structure from the one previously mentioned. The biological pump in the past. [71], A single phytoplankton cell has a sinking rate around one metre per day. Carbon sinks in the land and the ocean each currently take up about one-quarter of anthropogenic carbon emissions each year. The natural cycle adds and removes CO2 to keep a balance; humans add extra CO2 without removing any. 19 January 2021 (Carbon … The combustion or respiration of organic carbon releases it rapidly into the atmosphere. Human activities have greatly increased carbon dioxide levels in the atmosphere and nitrogen levels in the biosphere. The embodied carbon and the in-use carbon emissions from the operation of the building (operational carbon) together make up the complete lifecycle carbon footprint of the building. Figure 1: Global carbon cycle. [13] Man-made (synthetic) carbon compounds have been designed and mass-manufactured that will persist for decades to millennia in air, water, and sediments as pollutants. Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". [72], About 1% of the particles leaving the surface ocean reach the seabed and are consumed, respired, or buried in the sediments. Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises … Draw conclusions: In general, how do many human activities influence the carbon cycle? It occurs naturally in Earth's atmosphere as a trace gas.The current concentration is about 0.04% (412 ppm) by volume, having risen from pre-industrial levels of … Carbon principally enters the mantle in the form of carbonate-rich sediments on tectonic plates of ocean crust, which pull the carbon into the mantle upon undergoing subduction. [33], Most of the earth's carbon is stored inertly in the earth's lithosphere. It is generated by natural processes, and absorbed by others. [4][13], In the extremely far future (e.g. Myneni, S. Piao and P. Thornton (2013) "Carbon and Other Biogeochemical Cycles". Epiphytes on electric wires. Login via the left margin or if you're new, register here. These subducted carbonates can interact with lower mantle silicates, eventually forming super-deep diamonds like the one found.[81]. [77] Therefore, by allowing carbon to return to the Earth, the deep carbon cycle plays a critical role in maintaining the terrestrial conditions necessary for life to exist. 2021 SkS Weekly Climate Change & Global Warming Digest #6, 2021 SkS Weekly Climate Change & Global Warming News Roundup #6, Deadlines loom for Capitol Hill action on Trump-era climate issues, Biden's climate executive orders are a mini-Green New Deal, Skeptical Science New Research for Week #5, 2021, Next self-paced run of Denial101x starts on February 9, 2021 SkS Weekly Climate Change & Global Warming Digest #5, 2021 SkS Weekly Climate Change & Global Warming News Roundup #5, Refining the remaining 1.5C ‘carbon budget’, Case study - identifying myths and fallacies in Climate Science (Mis)Information Briefs, Skeptical Science New Research for Week #4, 2021, Study: Accounting for value of nature reinforces Paris climate targets, The SCIARA Project – Interactive Time Travel into the Climate Future, 2021 SkS Weekly Climate Change & Global Warming Digest #4, 2021 SkS Weekly Climate Change & Global Warming News Roundup #4, Analysis: The climate papers most featured in the media in 2020, Denial101x - Making Sense of Climate Science Denial, science and history of C13/C12 measurements. Doing so resulted in the formations of magnesite, siderite, and numerous varieties of graphite. Link to this page. View Archives, Printable Version  |  Various Human Activities That Affect an Ecosystem 1. Most of the carbon dioxide added to the atmosphere by human activities will ultimately be absorbed by the oceans[see graph on page 46]. Oceanic absorption of CO2 is one of the most important forms of carbon sequestering which limit the human-caused rise of carbon dioxide in the atmosphere. . [1] Humans have also continued to shift the natural component functions of the terrestrial biosphere with changes to vegetation and other land use. It also can acidify other surfaces it touches or be washed into the ocean. Magnesium, iron, and other metallic compounds act as buffers throughout the process. Ducklow, H.W., Steinberg, D.K. [28] In 2008, the global total of CO2 released by soil respiration was roughly 98 billion tonnes, about 10 times more carbon than humans are now putting into the atmosphere each year by burning fossil fuel (this does not represent a net transfer of carbon from soil to atmosphere, as the respiration is largely offset by inputs to soil carbon). [111] Chlorofluorocarbons also cause stratospheric ozone depletion. This kind of plant takes both CO2 and water from the atmosphere for living and growing. Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off). Carbon dioxide molecules consist of a carbon atom covalently double bonded to two oxygen atoms. Autotrophs extract it from the air in the form of carbon dioxide, converting it into organic carbon, while heterotrophs receive carbon by consuming other organisms. [85][82] A 2015 study indicates that the lower mantle's high pressure causes carbon bonds to transition from sp2 to sp3 hybridised orbitals, resulting in carbon tetrahedrally bonding to oxygen. The deep ocean gets most of its nutrients from the higher water column when they sink down in the form of marine snow. (free to republish), 2021 SkS Weekly Climate Change & Global Warming News Roundup #8, Guest post: Why avoiding climate change ‘maladaptation’ is vital, Drought-stricken Colorado River Basin could see additional 20% drop in water flow by 2050, Skeptical Science New Research for Week #7, 2021, Coming attraction: IPCC's upcoming major climate assessment, Tips on countering conspiracy theories and misinformation, 2021 SkS Weekly Climate Change & Global Warming Digest #7, 2021 SkS Weekly Climate Change & Global Warming News Roundup #7, Scientists sceptical of new bat study linking climate change to Covid-19 emergence, Investors flee Big Oil as portfolios get drilled, Skeptical Science New Research for Week #6, 2021. Numbers represent flux of carbon dioxide in gigatons (Source: Figure 7.3, IPCC AR4). This is made up of dead or dying animals and microbes, fecal matter, sand and other inorganic material. Nevertheless, the buildup of relatively small concentrations (parts per trillion) of chlorofluorocarbon, hydrofluorocarbon, and perfluorocarbon gases in the atmosphere is responsible for about 10% of the total direct radiative forcing from all long-lived greenhouse gases (year 2019); which includes forcing from the much larger concentrations of carbon dioxide and methane. Human impact on the environment or anthropogenic impact on the environment includes changes to biophysical environments and ecosystems, biodiversity, and natural resources caused directly or indirectly by humans, including global warming, environmental degradation (such as ocean acidification), mass extinction and biodiversity loss, ecological crisis, and ecological … The most important of these is combustion of fossil fuels: coal, oil and natural gas. Carbon dioxide also dissolves directly from the atmosphere into bodies of water (ocean, lakes, etc. [7][8] The largest consequences to the carbon cycle, and to the biosphere which critically enables human civilization, are still set to unfold due to the vast yet limited inertia of the Earth system. While fossil-fuel derived CO2 is a very small component of the global carbon cycle, the extra CO2 is cumulative because the natural carbon exchange cannot absorb all the additional CO2. This process, called carbon outgassing, is the result of carbonated mantle undergoing decompression melting, as well as mantle plumes carrying carbon compounds up towards the crust. Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, a small fraction of which is converted into carbonate. CO 2 remains in the atmosphere longer than the other major heat-trapping gases emitted as a result of human activities. It describes the movement of carbon as it is recycled and reused throughout the biosphere, as well as long-term processes of carbon sequestration to and release from carbon sinks. In a given year between 10 and 100 million tonnes of carbon moves around this slow cycle. CO2 absorption makes water more acidic, which affects ocean biosystems. An embodied carbon or carbon footprint assessment is a subset of most LCA studies, i.e. The dissolved inorganic carbon (DIC) in the surface layer is exchanged rapidly with the atmosphere, maintaining equilibrium. Natural CO2 is not static, however. Deforestation for agricultural purposes removes forests, which hold large amounts of carbon, and replaces them, generally with agricultural or urban areas. In many cases their pathways through the broader carbon cycle are also not yet well-characterized or understood. [18] This will eventually cause most of the carbon dioxide in the atmosphere to be squelched into the Earth's crust as carbonate. [19][20] Once the concentration of carbon dioxide in the atmosphere falls below approximately 50 parts per million (tolerances vary among species), C3 photosynthesis will no longer be possible. 491-528. Today human activities are altering the climate system by increasing concentrations of heat-trapping greenhouse gases in the atmosphere, which raises global temperatures. The deep carbon cycle is intimately connected to the movement of carbon in the Earth's surface and atmosphere. Accordingly, not much is conclusively known regarding the role of carbon in the deep Earth. It can also enter the ocean through rivers as dissolved organic carbon. Current trends in climate change lead to higher ocean temperatures and acidity, thus modifying marine ecosystems. [22][full citation needed]. The largest and one of the fastest growing human impacts on the carbon cycle and biosphere is the extraction and burning of fossil fuels, which directly transfer carbon from the geosphere into the atmosphere. THE ESCALATOR The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments are fairly balanced so that carbon levels would be roughly stable without human influence. With the ever-increasing number of the world’s population, there is similarly a soaring demand for sufficient food.The population growth rate is hence driving the world to clear forests in order to create more room for agriculture. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to make Earth capable of sustaining life. DOM and aggregates exported into the deep water are consumed and respired, thus returning organic carbon into the enormous deep ocean reservoir of DIC. [99][100][101][102] The oceans have been functioning as the larger sink, and are expected to remove half (50%) of the emitted fossil carbon within about a century. Humans have disturbed the biological carbon cycle for many centuries by modifying land use, and moreover with the recent industrial-scale mining of fossil carbon (coal, petroleum and gas extraction, and cement manufacture) from the geosphere. Therefore, the iron carbide model could serve as an evidence that the core holds as much as 67% of the Earth's carbon. You will learn how the carbon cycle, climate and the abiotic and biotic components of the environment influence each other in many ways. Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the element's movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of said layer. It is one of the most important determinants of the amount of carbon in the atmosphere, and thus of global temperatures. DOM is partially consumed by bacteria and respired; the remaining refractory DOM is advected and mixed into the deep sea. Although deep carbon cycling is not as well-understood as carbon movement through the atmosphere, terrestrial biosphere, ocean, and geosphere, it is nonetheless an incredibly important process. The expected increased luminosity of the Sun will likely speed up the rate of surface weathering. Altered biogeochemical cycles combined with climate change increase the vulnerability of biodiversity, food security, human health, and water quality to a changing climate. [3] Oceans are basic (~pH 8.2), hence CO2 acidification shifts the pH of the ocean towards neutral. [clarification needed] Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys. [35] Of the carbon stored in the geosphere, about 80% is limestone and its derivatives, which form from the sedimentation of calcium carbonate stored in the shells of marine organisms. [84] The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle. (Of note, in P. vivax and P. ovale a dormant stage [hypnozoites] can persist in the liver (if untreated) and cause … Polymorphism alters carbonate compounds' stability at different depths within the Earth. Carbon dioxide is released during the metamorphism of carbonate rocks when they are subducted into the earth's mantle. Delhi (English: / ˈ d ɛ l i /; Hindi: [ˈdɪlːiː] Dillī; Punjabi: [ˈdɪlːiː] Dillī; Urdu: [ˈdɛɦliː] Dêhlī), officially known as the National Capital Territory (NCT) of Delhi, is a city and a union territory of India containing New Delhi, the capital of India. [34] It can also be removed by humans through the direct extraction of kerogens in the form of fossil fuels. [13] Enter a term in the search box to find its definition. About 500 gigatons of carbon are stored above ground in plants and other living organisms,[3] while soil holds approximately 1,500 gigatons of carbon. Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial, Last updated on 5 July 2015 by skeptickev. [71], There is a fast and a slow carbon cycle. Use our advanced search page; Browse our curated A-Z index of terms and topics or see our automated list of website topics; Search frequently asked questions or submit a question; Go to the EPA home page [74][75][76], The slow carbon cycle involves medium to long-term geochemical processes belonging to the rock cycle (see diagram on the right). Since the industrial revolution, and especially since the end of WWII, human activity has substantially disturbed the global carbon cycle by redistributing massive amounts of carbon from the geosphere. [83] Consequently, scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments. Carbon dioxide (chemical formula CO 2) is a colorless gas with a density about 53% higher than that of dry air. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle. Carbon, the fourth most abundant element in the universe, moves between the atmosphere, oceans, biosphere, and geosphere in what is called the carbon cycle . This module provides an overview of the global carbon cycle, one of the major biogeochemical cycles. You will learn how carbon moves throughout the different components of the carbon cycle and where carbon is stored in the Earth's system. But consider what happens when more CO2 is released from outside of the natural carbon cycle – by burning fossil fuels. In this unit, examine the science behind global climate change and explore its potential impacts on natural ecosystems and human societies.