Greenhouse gas | Definition, Emissions, & Greenhouse Effect

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border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/greenhouse-gas">greenhouse gas</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/greenhouse-gas">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref280533"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/greenhouse-gas#ref280533">Major greenhouse gases</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref280534"><a class="w-100 link-gray-900" href="/science/greenhouse-gas#ref280534">Water vapour</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref280522"><a class="w-100 link-gray-900" href="/science/greenhouse-gas#ref280522">Carbon dioxide</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref280523"><a class="w-100 link-gray-900" href="/science/greenhouse-gas/Methane">Methane</a></li></ul></div></li><li data-target="#ref280535"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/greenhouse-gas/Methane#ref280535">Lesser greenhouse gases</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref280525"><a class="w-100 link-gray-900" href="/science/greenhouse-gas/Methane#ref280525">Surface-level ozone</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref280526"><a class="w-100 link-gray-900" href="/science/greenhouse-gas/Methane#ref280526">Nitrous oxides and fluorinated gases</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" 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Mann</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Associate Professor of Meteorology, Pennsylvania State University, University Park.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Michael E. Mann</span></div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2025-02-26T00:00:00CST" >Feb 26, 2025</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/greenhouse-gas/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm 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class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>On the Web: </dt> <dd><a href="https://web.uvic.ca/~djberg/Chem300A/GroupI_GreenhouseGases_Pfoj1.pdf" target="_blank">University of Victoria - Greenhouse Gases (PDF)</a> (Feb. 12, 2025)</dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/greenhouse-gas"> See all related content </a> </div> </div> </div> </div><div class="ai-top-questions-module module-spacing ai-tq collapsed font-sans-serif"> <div class="font-weight-bold font-14"> Top Questions </div> <ul class="mt-5"> <li class="ai-qa-item mb-5 bg-gray-50 rounded "> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="0"> <div>What are greenhouse gases?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded "> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="1"> <div>How do greenhouse gases affect the Earth's temperature?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded "> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="2"> <div>What are the most common types of greenhouse gases found in our atmosphere?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="3"> <div>How do human activities contribute to the increase of greenhouse gases?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="4"> <div>Why is carbon dioxide considered a significant greenhouse gas?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="5"> <div>What is the greenhouse effect and how does it work?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="6"> <div>How do greenhouse gases and the greenhouse effect relate to climate change?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="7"> <div>What impact do greenhouse gases have on weather patterns and ecosystems?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="8"> <div>What are some methods used to reduce greenhouse gas emissions?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> <li class="ai-qa-item mb-5 bg-gray-50 rounded af3"> <div class="ai-top-question justify-content-between align-items-center font-14 px-10 py-5 rounded " data-qn="9"> <div>How do international agreements aim to address greenhouse gas emissions and climate change?</div> </div> <div class="answer p-10 ui-hidden" data-loaded="false"></div> <div class="ai-loader ui-hidden" /> </li> </ul> <button class="show-more btn btn-unstyled font-14 p-5 pr-10 mb-10 rounded-sm d-none"><em class="material-icons " data-icon="expand_more"></em> Show more</button> <button class="show-less btn btn-unstyled font-14 p-5 pr-10 mb-10 rounded-sm"><em class="material-icons " data-icon="expand_less"></em> Show less</button> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://www.earth.com/news/massive-new-source-of-methane-emissions-discovered-glacial-fracking-arctic/" rel="nofollow">Massive new source of leaking methane gas emissions discovered</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 24, 2025, 5:23 AM ET (Earth.com) </span> </div> </div> <div class="rest-of-recent-news-items"> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref1021129"></span>greenhouse gas</strong>, any <a href="https://www.britannica.com/science/gas-state-of-matter" class="md-crosslink " data-show-preview="true">gas</a> that has the property of absorbing <span id="ref1043564"></span><a href="https://www.britannica.com/science/infrared-radiation" class="md-crosslink " data-show-preview="true">infrared radiation</a> (net <a href="https://www.britannica.com/science/heat" class="md-crosslink " data-show-preview="true">heat energy</a>) <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="emitted" href="https://www.britannica.com/dictionary/emitted" data-type="EB">emitted</a> from Earth’s surface and reradiating it back to Earth’s surface, thus contributing to the <span id="ref1043558"></span><a href="https://www.britannica.com/science/greenhouse-effect" class="md-crosslink " data-show-preview="true">greenhouse effect</a>. <a href="https://www.britannica.com/science/carbon-dioxide" class="md-crosslink " data-show-preview="true">Carbon dioxide</a>, <a href="https://www.britannica.com/science/methane" class="md-crosslink " data-show-preview="true">methane</a>, and <a href="https://www.britannica.com/science/water" class="md-crosslink " data-show-preview="true">water</a> vapour are the most important greenhouse gases. (To a lesser extent, surface-level <a href="https://www.britannica.com/science/ozone" class="md-crosslink " data-show-preview="true">ozone</a>, <a href="https://www.britannica.com/science/nitrous-oxide" class="md-crosslink " data-show-preview="true">nitrous oxides</a>, and fluorinated gases also trap infrared radiation.) Greenhouse gases have a profound effect on the <a href="https://www.britannica.com/science/energy" class="md-crosslink " data-show-preview="true">energy</a> budget of the Earth system despite making up only a fraction of all atmospheric gases. Concentrations of greenhouse gases have varied substantially during Earth’s history, and these variations have driven substantial <span id="ref1043557"></span><a href="https://www.britannica.com/science/climate-change" class="md-crosslink " data-show-preview="true">climate changes</a> at a wide range of timescales. In general, greenhouse gas concentrations have been particularly high during warm periods and low during cold periods.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><div class="assemblies multiple medialist slider js-slider position-relative d-inline-flex align-items-center mw-100" data-type="other"><div class="slider-container js-slider-container overflow-hidden d-flex"><div class="rw-track d-flex align-items-center"><div class="position-relative rw-slide col-100 px-20 "><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="160679" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="151925"><a data-id="151925" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/John-P-Rafferty-carbon-dioxide-conditions-Encyclopaedia/-160679"><img src="https://cdn.britannica.com/25/151925-138-452B0158/John-P-Rafferty-carbon-dioxide-conditions-Encyclopaedia.jpg?w=800&h=450&c=crop" alt="The impact of human activities on climate change" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/3GvPHl8L.m3u8" } ], "image": "https://cdn.britannica.com/25/151925-138-452B0158/John-P-Rafferty-carbon-dioxide-conditions-Encyclopaedia.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/Ph65djgu", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/ZFSNL7CJ.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div><div class="position-absolute top-10 left-10 assembly-slide-tag rounded-lg">1 of 2</div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="151925">The impact of human activities on climate change</span><span>Learn about carbon dioxide and its relationship to warming conditions at Earth's surface, as explained by John P. Rafferty, biological and Earth sciences editor of <em>Encyclopædia Britannica</em>.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/greenhouse-gas/images-videos">See all videos for this article</a></figcaption></figure></div><div class="position-relative rw-slide col-100 px-20 "><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="207483" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="186514"><a data-id="186514" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/emission-methane-greenhouse-gas-wetland-ecosystems-trees/-207483"><img src="https://cdn.britannica.com/14/186514-138-9F8AB590/emission-methane-greenhouse-gas-wetland-ecosystems-trees.jpg?w=800&h=450&c=crop" alt="Understand the processes of production and emission of methane gas in wetlands" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/FZNnL0gn.m3u8" } ], "image": "https://cdn.britannica.com/14/186514-138-9F8AB590/emission-methane-greenhouse-gas-wetland-ecosystems-trees.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/CRwaZuoQ", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/i17ba2Im.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div><div class="position-absolute top-10 left-10 assembly-slide-tag rounded-lg">2 of 2</div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="186514">Understand the processes of production and emission of methane gas in wetlands</span><span>Learn about the emission of methane, a greenhouse gas, by trees in wetland ecosystems.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/greenhouse-gas/images-videos">See all videos for this article</a></figcaption></figure></div></div></div><button disabled="true" class="prev-button js-prev-button position-absolute btn btn-circle shadow btn-lg btn-blue-dark m-20"><span class="material-icons" data-icon="keyboard_arrow_left"></span></button><button disabled="true" class="next-button js-next-button position-absolute btn btn-circle shadow btn-lg btn-blue-dark m-20"><span class="material-icons" data-icon="keyboard_arrow_right"></span></button></div><p class="topic-paragraph">A number of processes influence greenhouse gas concentrations. Some, such as <a href="https://www.britannica.com/science/tectonics" class="md-crosslink " data-show-preview="true">tectonic activities</a>, operate at timescales of millions of years, whereas others, such as vegetation, <a href="https://www.britannica.com/science/soil" class="md-crosslink " data-show-preview="true">soil</a>, <a href="https://www.britannica.com/science/wetland" class="md-crosslink " data-show-preview="true">wetland</a>, and <a href="https://www.britannica.com/science/ocean" class="md-crosslink " data-show-preview="true">ocean</a> sources and sinks, operate at timescales of hundreds to thousands of years. Human activities—especially <a href="https://www.britannica.com/science/fossil-fuel" class="md-crosslink " data-show-preview="true">fossil-fuel</a> combustion since the <span id="ref1043559"></span><a href="https://www.britannica.com/event/Industrial-Revolution" class="md-crosslink " data-show-preview="true">Industrial Revolution</a>—are responsible for steady increases in atmospheric concentrations of various greenhouse gases, especially carbon dioxide, methane, ozone, and <span id="ref1043560"></span><a href="https://www.britannica.com/science/chlorofluorocarbon" class="md-crosslink " data-show-preview="true">chlorofluorocarbons</a> (CFCs).</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="202525" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="185587"><a data-id="185587" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/characteristics-Earth-atmospheric-gas-molecules-properties-greenhouse/-202525"><img src="https://cdn.britannica.com/87/185587-138-18E43D08/characteristics-Earth-atmospheric-gas-molecules-properties-greenhouse.jpg?w=800&h=450&c=crop" alt="Understand how the presence of gas molecules, including greenhouse gases, protect the earth by shielding and trapping infrared radiation" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/MNe7xxOg.m3u8" } ], "image": "https://cdn.britannica.com/87/185587-138-18E43D08/characteristics-Earth-atmospheric-gas-molecules-properties-greenhouse.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/9I0M9Wsr", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/wEJAS5uE.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="185587">Understand how the presence of gas molecules, including greenhouse gases, protect the earth by shielding and trapping infrared radiation</span><span>Learn about the basic physical and chemical characteristics of Earth's various atmospheric gas molecules. Some of those molecules belong to a category of atmospheric gases called greenhouse gases, whose properties help to slow the emission of heat energy, which was absorbed by Earth's surface during the day, back into space at night.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/greenhouse-gas/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">The effect of each greenhouse gas on Earth’s climate depends on its chemical nature and its relative concentration in the <a href="https://www.britannica.com/science/atmosphere" class="md-crosslink " data-show-preview="true">atmosphere</a>. Some gases have a high <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="capacity" href="https://www.britannica.com/dictionary/capacity" data-type="EB">capacity</a> for absorbing infrared radiation or occur in significant quantities, whereas others have considerably lower capacities for absorption or occur only in trace amounts. <span id="ref1043565"></span><a href="https://www.britannica.com/science/radiative-forcing" class="md-crosslink " data-show-preview="true">Radiative forcing</a>, as defined by the <a href="https://www.britannica.com/topic/Intergovernmental-Panel-on-Climate-Change" class="md-crosslink " data-show-preview="true">Intergovernmental Panel on Climate Change</a> (IPCC), is a measure of the influence a given greenhouse gas or other climatic factor (such as solar irradiance or <a href="https://www.britannica.com/science/albedo" class="md-crosslink " data-show-preview="true">albedo</a>) has on the amount of <a href="https://www.britannica.com/science/radiant-energy" class="md-crosslink autoxref " data-show-preview="true">radiant energy</a> impinging upon Earth’s surface. To understand the relative influence of each greenhouse gas, so-called forcing values (given in <a href="https://www.britannica.com/science/watt-unit-of-measurement" class="md-crosslink " data-show-preview="true">watts</a> per square metre) calculated for the time period between 1750 and the present day are given below.</p><span class="marker MOD3 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref280533"><h2 class="h1">Major greenhouse gases</h2> <section data-level="2" id="ref280534"><h2 class="h2"><span id="ref1043556"></span><a href="https://www.britannica.com/science/water-vapor" class="md-crosslink ">Water vapour</a></h2> <span class="marker PREMOD4 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="112177" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/89/62689-050-66A0068F/water-hydrologic-cycle-land-surface-atmosphere-ocean.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/683450/112177"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/89/62689-050-66A0068F/water-hydrologic-cycle-land-surface-atmosphere-ocean.jpg"><img src="https://cdn.britannica.com/89/62689-050-66A0068F/water-hydrologic-cycle-land-surface-atmosphere-ocean.jpg?w=300" alt="hydrologic cycle" data-width="1600" data-height="1160" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/89/62689-050-66A0068F/water-hydrologic-cycle-land-surface-atmosphere-ocean.jpg" data-href="/media/1/683450/112177">hydrologic cycle</a><span>This diagram shows how, in the hydrologic cycle, water is transferred between the land surface, the ocean, and the atmosphere.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph"><a href="https://www.britannica.com/science/water" class="md-crosslink " data-show-preview="true">Water</a> vapour is the most potent greenhouse gas in <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth’s</a> <a href="https://www.britannica.com/science/atmosphere" class="md-crosslink " data-show-preview="true">atmosphere</a>, but its behaviour is fundamentally different from that of the other greenhouse gases. The primary role of water vapour is not as a direct agent of <a href="https://www.britannica.com/science/global-warming/Radiative-forcing#ref274821" class="md-crosslink " data-show-preview="true">radiative forcing</a> but rather as a <a href="https://www.britannica.com/science/climate-meteorology" class="md-crosslink " data-show-preview="true">climate</a> <a href="https://www.britannica.com/science/feedback-biology" class="md-crosslink " data-show-preview="true">feedback</a>—that is, as a response within the climate system that influences the system’s continued activity. This distinction arises because the amount of water vapour in the atmosphere cannot, in general, be directly modified by <a href="https://www.britannica.com/topic/human-behavior" class="md-crosslink autoxref " data-show-preview="true">human behaviour</a> but is instead set by <a href="https://www.britannica.com/science/air" class="md-crosslink autoxref " data-show-preview="true">air</a> temperatures. The warmer the surface, the greater the <a href="https://www.britannica.com/science/evaporation" class="md-crosslink " data-show-preview="true">evaporation</a> rate of water from the surface. As a result, increased evaporation leads to a greater concentration of water vapour in the lower atmosphere capable of absorbing <a href="https://www.britannica.com/science/infrared-radiation" class="md-crosslink " data-show-preview="true">infrared radiation</a> and <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="emitting" href="https://www.britannica.com/dictionary/emitting" data-type="EB">emitting</a> it back to the surface.</p><span class="marker MOD4 mod-inline"></span></section> <section data-level="2" id="ref280522"><h2 class="h2"><span id="ref1043553"></span><a href="https://www.britannica.com/science/carbon-dioxide" class="md-crosslink " data-show-preview="true">Carbon dioxide</a></h2> <span class="marker PREMOD5 mod-inline"></span><div class="assemblies multiple medialist slider js-slider position-relative d-inline-flex align-items-center mw-100" data-type="other"><div class="slider-container js-slider-container overflow-hidden d-flex"><div class="rw-track d-flex align-items-center"><div class="position-relative rw-slide col-100 px-20 "><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="19381" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/20/24020-050-525F29FB/carbon-cycle-Carbon-atmosphere-hydrosphere-forms-formations.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/683450/19381"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/20/24020-050-525F29FB/carbon-cycle-Carbon-atmosphere-hydrosphere-forms-formations.jpg"><img src="https://cdn.britannica.com/20/24020-050-525F29FB/carbon-cycle-Carbon-atmosphere-hydrosphere-forms-formations.jpg?w=300" alt="carbon cycle" data-width="1600" data-height="1000" loading="eager"></picture><div class="position-absolute top-10 left-10 assembly-slide-tag rounded-lg">1 of 2</div><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/20/24020-050-525F29FB/carbon-cycle-Carbon-atmosphere-hydrosphere-forms-formations.jpg" data-href="/media/1/683450/19381">carbon cycle</a><span>Carbon is transported in various forms through the atmosphere, the hydrosphere, and geologic formations. One of the primary pathways for the exchange of carbon dioxide (CO<sub>2</sub>) takes place between the atmosphere and the oceans; there a fraction of the CO<sub>2</sub> combines with water, forming carbonic acid (H<sub>2</sub>CO<sub>3</sub>) that subsequently loses hydrogen ions (H<sup>+</sup>) to form bicarbonate (HCO<sub>3</sub><sup>−</sup>) and carbonate (CO<sub>3</sub><sup>2−</sup>) ions. Mollusk shells or mineral precipitates that form by the reaction of calcium or other metal ions with carbonate may become buried in geologic strata and eventually release CO<sub>2</sub> through volcanic outgassing. Carbon dioxide also exchanges through photosynthesis in plants and through respiration in animals. Dead and decaying organic matter may ferment and release CO<sub>2</sub> or methane (CH<sub>4</sub>) or may be incorporated into sedimentary rock, where it is converted to fossil fuels. Burning of hydrocarbon fuels returns CO<sub>2</sub> and water (H<sub>2</sub>O) to the atmosphere. The biological and anthropogenic pathways are much faster than the geochemical pathways and, consequently, have a greater impact on the composition and temperature of the atmosphere.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div><div class="position-relative rw-slide col-100 px-20 "><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="118423" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/19/114419-050-F6658A82/carbon-cycle.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/683450/118423"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/19/114419-050-F6658A82/carbon-cycle.jpg"><img src="https://cdn.britannica.com/19/114419-050-F6658A82/carbon-cycle.jpg?w=300" alt="carbon cycle" data-width="1600" data-height="925" loading="eager"></picture><div class="position-absolute top-10 left-10 assembly-slide-tag rounded-lg">2 of 2</div><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/19/114419-050-F6658A82/carbon-cycle.jpg" data-href="/media/1/683450/118423">carbon cycle</a><span>The generalized carbon cycle.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div></div><button disabled="true" class="prev-button js-prev-button position-absolute btn btn-circle shadow btn-lg btn-blue-dark m-20"><span class="material-icons" data-icon="keyboard_arrow_left"></span></button><button disabled="true" class="next-button js-next-button position-absolute btn btn-circle shadow btn-lg btn-blue-dark m-20"><span class="material-icons" data-icon="keyboard_arrow_right"></span></button></div><p class="topic-paragraph"><a href="https://www.britannica.com/science/carbon-dioxide" class="md-crosslink " data-show-preview="true">Carbon dioxide</a> (CO<sub>2</sub>) is the most significant greenhouse gas. Natural sources of atmospheric CO<sub>2</sub> include outgassing from <a href="https://www.britannica.com/science/volcano" class="md-crosslink " data-show-preview="true">volcanoes</a>, the <a href="https://www.britannica.com/science/combustion" class="md-crosslink " data-show-preview="true">combustion</a> and natural decay of organic matter, and <a href="https://www.britannica.com/science/respiratory-system" class="md-crosslink " data-show-preview="true">respiration</a> by aerobic (<a href="https://www.britannica.com/science/oxygen" class="md-crosslink " data-show-preview="true">oxygen</a>-using) organisms. These sources are balanced, on average, by a set of physical, chemical, or biological processes, called “sinks,” that tend to remove CO<sub>2</sub> from the <a href="https://www.britannica.com/science/atmosphere" class="md-crosslink " data-show-preview="true">atmosphere</a>. Significant natural sinks include terrestrial vegetation, which takes up CO<sub>2</sub> during <a href="https://www.britannica.com/science/photosynthesis" class="md-crosslink " data-show-preview="true">photosynthesis</a>.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/pop-quiz-18-things-to-know-about-global-warming" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/50/112550-050-D7D94E18/Grinnell-Glacier-series-photographs-summit-Mount-Gould-2006.jpg" alt="Combination shot of Grinnell Glacier taken from the summit of Mount Gould, Glacier National Park, Montana in the years 1938, 1981, 1998 and 2006." class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Pop Quiz: 18 Things to Know About Global Warming</div> </div> </a><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">A number of oceanic processes also act as <a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink " data-show-preview="true">carbon</a> sinks. One such process, the “solubility pump,” involves the descent of surface <a href="https://www.britannica.com/science/seawater" class="md-crosslink " data-show-preview="true">seawater</a> containing dissolved CO<sub>2</sub>. Another process, the “biological pump,” involves the uptake of <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="dissolved" href="https://www.britannica.com/dictionary/dissolved" data-type="EB">dissolved</a> CO<sub>2</sub> by marine vegetation and <a href="https://www.britannica.com/science/phytoplankton" class="md-crosslink " data-show-preview="true">phytoplankton</a> (small, free-floating, photosynthetic organisms) living in the upper ocean or by other marine organisms that use CO<sub>2</sub> to build skeletons and other structures made of <a href="https://www.britannica.com/science/calcium-carbonate" class="md-crosslink autoxref " data-show-preview="true">calcium carbonate</a> (CaCO<sub>3</sub>). As these organisms expire and <a href="https://www.britannica.com/science/autumn-season" class="md-crosslink autoxref " data-show-preview="true">fall</a> to the ocean floor, their carbon is transported downward and eventually buried at depth. A long-term balance between these natural sources and sinks leads to the background, or natural, level of CO<sub>2</sub> in the atmosphere.</p><span class="marker MOD6 mod-inline"></span> <span class="marker PREMOD7 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="100353" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/15/93015-050-5D5910B5/remains-plot-land-Amazon-Rainforest-Brazil-carbon.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/683450/100353"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/15/93015-050-5D5910B5/remains-plot-land-Amazon-Rainforest-Brazil-carbon.jpg?w=300"><img src="https://cdn.britannica.com/15/93015-050-5D5910B5/remains-plot-land-Amazon-Rainforest-Brazil-carbon.jpg?w=300" alt="deforestation" data-width="1182" data-height="800" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/15/93015-050-5D5910B5/remains-plot-land-Amazon-Rainforest-Brazil-carbon.jpg" data-href="/media/1/683450/100353">deforestation</a><span>Smoldering remains of a plot of deforested land in the Amazon Rainforest of Brazil.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">In contrast, human activities increase atmospheric CO<sub>2</sub> levels primarily through the burning of <a href="https://www.britannica.com/science/fossil-fuel" class="md-crosslink " data-show-preview="true">fossil fuels</a> (principally <a href="https://www.britannica.com/science/petroleum" class="md-crosslink " data-show-preview="true">oil</a> and <a href="https://www.britannica.com/science/coal-fossil-fuel" class="md-crosslink " data-show-preview="true">coal</a>, and secondarily <a href="https://www.britannica.com/science/natural-gas" class="md-crosslink " data-show-preview="true">natural gas</a>, for use in transportation, heating, and <a href="https://www.britannica.com/science/electricity" class="md-crosslink " data-show-preview="true">electricity</a> production) and through the production of <a href="https://www.britannica.com/technology/cement-building-material" class="md-crosslink " data-show-preview="true">cement</a>. Other <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="anthropogenic" href="https://www.merriam-webster.com/dictionary/anthropogenic" data-type="MW">anthropogenic</a> sources include the burning of <a href="https://www.britannica.com/science/forest" class="md-crosslink " data-show-preview="true">forests</a> and the clearing of land. Anthropogenic emissions currently account for the annual release of about 7 gigatons (7 billion tons) of carbon into the atmosphere. Anthropogenic emissions are equal to approximately 3 percent of the total emissions of CO<sub>2</sub> by natural sources, and this amplified carbon load from human activities far exceeds the offsetting capacity of natural sinks (by perhaps as much as 2–3 gigatons per year).</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=august-2024">Subscribe</a> </div> </div> </div> </DIV></div><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="109623" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/60/104260-050-AC53F949/Charles-David-Keeling-Curve-American-changes-atmosphere.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/683450/109623"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/60/104260-050-AC53F949/Charles-David-Keeling-Curve-American-changes-atmosphere.jpg"><img src="https://cdn.britannica.com/60/104260-050-AC53F949/Charles-David-Keeling-Curve-American-changes-atmosphere.jpg?w=300" alt="Keeling Curve" data-width="1600" data-height="1120" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/60/104260-050-AC53F949/Charles-David-Keeling-Curve-American-changes-atmosphere.jpg" data-href="/media/1/683450/109623">Keeling Curve</a><span>The Keeling Curve, named after American climate scientist Charles David Keeling, tracks changes in the concentration of carbon dioxide (CO<sub>2</sub>) in Earth's atmosphere at a research station on Mauna Loa in Hawaii. Despite small seasonal fluctuations in CO<sub>2</sub> concentration, the overall trend shows that CO<sub>2</sub> is increasing in the atmosphere.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">CO<sub>2</sub> has consequently <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="accumulated" href="https://www.britannica.com/dictionary/accumulated" data-type="EB">accumulated</a> in the atmosphere at an average rate of 1.4 parts per million (ppm) by volume per year between 1959 and 2006 and roughly 2.0 ppm per year between 2006 and 2018. Overall, this rate of accumulation has been linear (that is, uniform over time). However, certain current sinks, such as the <a href="https://www.britannica.com/science/ocean" class="md-crosslink " data-show-preview="true">oceans</a>, could become sources in the future. This may lead to a situation in which the concentration of atmospheric CO<sub>2</sub> builds at an exponential rate (that is, at a rate of increase that is also increasing over time).</p><span class="marker MOD8 mod-inline"></span> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">The natural background level of carbon dioxide varies on timescales of millions of years due to slow changes in outgassing through <a href="https://www.britannica.com/science/volcanism" class="md-crosslink autoxref " data-show-preview="true">volcanic activity</a>. For example, roughly 100 million years ago, during the <a href="https://www.britannica.com/science/Cretaceous-Period" class="md-crosslink " data-show-preview="true">Cretaceous Period</a>, CO<sub>2</sub> concentrations appear to have been several times higher than today (perhaps close to 2,000 ppm). Over the past 700,000 years, CO<sub>2</sub> concentrations have varied over a far smaller range (between roughly 180 and 300 ppm) in association with the same Earth orbital effects linked to the coming and going of the <a href="https://www.britannica.com/science/ice-age-geology" class="md-crosslink " data-show-preview="true">ice ages</a> of the <a href="https://www.britannica.com/science/Pleistocene-Epoch" class="md-crosslink " data-show-preview="true">Pleistocene epoch</a>. By the early 21st century, CO<sub>2</sub> levels reached 384 ppm, which is approximately 37 percent above the natural background level of roughly 280 ppm that existed at the beginning of the <a href="https://www.britannica.com/event/Industrial-Revolution" class="md-crosslink " data-show-preview="true">Industrial Revolution</a>. Atmospheric CO<sub>2</sub> levels continued to increase, and by 2018 they had reached 410 ppm. According to <a href="https://www.britannica.com/science/ice-core" class="md-crosslink " data-show-preview="true">ice core</a> measurements, such levels are believed to be the highest in at least 800,000 years and, according to other lines of evidence, may be the highest in at least 5,000,000 years.</p><div class="one-good-fact-module"> </div><span class="marker MOD9 mod-inline"></span> <span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph"><a href="https://www.britannica.com/science/radiative-forcing" class="md-crosslink " data-show-preview="true">Radiative forcing</a> caused by carbon dioxide varies in an approximately <a href="https://www.britannica.com/science/logarithm" class="md-crosslink " data-show-preview="true">logarithmic</a> fashion with the concentration of that gas in the atmosphere. The logarithmic relationship occurs as the result of a <a href="https://www.britannica.com/science/saturation-chemistry-and-physics" class="md-crosslink " data-show-preview="true">saturation</a> effect wherein it becomes increasingly difficult, as CO<sub>2</sub> concentrations increase, for additional CO<sub>2</sub> <a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecules</a> to further influence the “infrared window” (a certain narrow band of <a href="https://www.britannica.com/science/wavelength" class="md-crosslink " data-show-preview="true">wavelengths</a> in the infrared region that is not absorbed by atmospheric gases). The logarithmic relationship predicts that the surface warming potential will rise by roughly the same amount for each doubling of CO<sub>2</sub> concentration. At current rates of <a href="https://www.britannica.com/science/fossil-fuel" class="md-crosslink " data-show-preview="true">fossil-fuel</a> use, a doubling of CO<sub>2</sub> concentrations over preindustrial levels is expected to take place by the middle of the 21st century (when CO<sub>2</sub> concentrations are projected to reach 560 ppm). A doubling of CO<sub>2</sub> concentrations would represent an increase of roughly 4 watts per square metre of radiative forcing. Given typical estimates of “climate sensitivity” in the absence of any <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="offsetting" href="https://www.britannica.com/dictionary/offsetting" data-type="EB">offsetting</a> factors, this energy increase would lead to a warming of 2 to 5 °C (3.6 to 9 °F) over preindustrial times. The total radiative forcing by anthropogenic CO<sub>2</sub> emissions since the beginning of the industrial age is approximately 1.66 watts per square metre.</p><span class="marker MOD10 mod-inline"></span></section> </section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-simplify-root"></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":2,"pageId":683450,"pageLength":1254,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.36"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/topic-page.js?v=3.133.36' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":true} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Greenhouse gas | Definition, Emissions, & Greenhouse Effect | Britannica