Global warming - Carbon Dioxide, Climate Change, Greenhouse Gases

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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/global-warming">global warming</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/global-warming">Introduction & Top Questions</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref274818"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/global-warming/Climatic-variation-since-the-last-glaciation">Climatic variation since the last glaciation</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref274819"><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/global-warming/Causes-of-global-warming">Causes of global warming</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref274820"><a class="w-100 link-gray-900" href="/science/global-warming/Causes-of-global-warming#ref274820">The greenhouse effect</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274821"><a class="w-100 link-gray-900" href="/science/global-warming/Radiative-forcing">Radiative forcing</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274822"><a class="w-100 link-gray-900" href="/science/global-warming/Radiative-forcing#ref274822">The influences of human activity on climate</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref274823" class="has-children"><a class="w-100 link-gray-900" href="/science/global-warming/Radiative-forcing#ref274823">Greenhouse gases</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref274824"><a class="w-100 link-gray-900" href="/science/global-warming/Radiative-forcing#ref274824">Water vapour</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref274825"><a class="w-100 link-gray-900" href="/science/global-warming/Carbon-dioxide">Carbon dioxide</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref274826"><a class="w-100 link-gray-900" href="/science/global-warming/Carbon-dioxide#ref274826">Methane</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref274827"><a class="w-100 link-gray-900" href="/science/global-warming/Surface-level-ozone-and-other-compounds">Surface-level ozone and other compounds</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref274828"><a class="w-100 link-gray-900" href="/science/global-warming/Surface-level-ozone-and-other-compounds#ref274828">Nitrous oxides and fluorinated gases</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274829"><a class="w-100 link-gray-900" href="/science/global-warming/Surface-level-ozone-and-other-compounds#ref274829">Aerosols</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274830"><a class="w-100 link-gray-900" href="/science/global-warming/Land-use-change">Land-use change</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274831"><a class="w-100 link-gray-900" href="/science/global-warming/Land-use-change#ref274831">Stratospheric ozone depletion</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274832"><a class="w-100 link-gray-900" href="/science/global-warming/Land-use-change#ref274832">Natural influences on climate</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref274833"><a class="w-100 link-gray-900" href="/science/global-warming/Volcanic-aerosols">Volcanic aerosols</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274834"><a class="w-100 link-gray-900" href="/science/global-warming/Volcanic-aerosols#ref274834">Variations in solar output</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274835"><a class="w-100 link-gray-900" href="/science/global-warming/Variations-in-Earths-orbit">Variations in Earth’s orbit</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274836"><a class="w-100 link-gray-900" href="/science/global-warming/Feedback-mechanisms-and-climate-sensitivity">Feedback mechanisms and climate sensitivity</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref274837"><a class="w-100 link-gray-900" href="/science/global-warming/Feedback-mechanisms-and-climate-sensitivity#ref274837">Water vapour feedback</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274838"><a class="w-100 link-gray-900" href="/science/global-warming/Feedback-mechanisms-and-climate-sensitivity#ref274838">Cloud feedbacks</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274839"><a class="w-100 link-gray-900" href="/science/global-warming/Feedback-mechanisms-and-climate-sensitivity#ref274839">Ice albedo feedback</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274840"><a class="w-100 link-gray-900" href="/science/global-warming/Feedback-mechanisms-and-climate-sensitivity#ref274840">Carbon cycle feedbacks</a></li></ul></li></ul></div></li><li data-target="#ref274841"><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/global-warming/Climate-research">Climate research</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref274842"><a class="w-100 link-gray-900" href="/science/global-warming/Climate-research#ref274842">Modern observations</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274843"><a class="w-100 link-gray-900" href="/science/global-warming/Climate-research#ref274843">Prehistorical climate records</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274844"><a class="w-100 link-gray-900" href="/science/global-warming/Theoretical-climate-models">Theoretical climate models</a></li></ul></div></li><li data-target="#ref274845"><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/global-warming/Potential-effects-of-global-warming">Potential effects of global warming</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref274846"><a class="w-100 link-gray-900" href="/science/global-warming/Potential-effects-of-global-warming#ref274846">Simulations of future climate change</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref274847"><a class="w-100 link-gray-900" href="/science/global-warming/Potential-effects-of-global-warming#ref274847">Patterns of warming</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274848"><a class="w-100 link-gray-900" href="/science/global-warming/Potential-effects-of-global-warming#ref274848">Precipitation patterns</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274849"><a class="w-100 link-gray-900" href="/science/global-warming/Potential-effects-of-global-warming#ref274849">Regional predictions</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274850"><a class="w-100 link-gray-900" href="/science/global-warming/Ice-melt-and-sea-level-rise">Ice melt and sea level rise</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274851"><a class="w-100 link-gray-900" href="/science/global-warming/Ice-melt-and-sea-level-rise#ref274851">Ocean circulation changes</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref274852"><a class="w-100 link-gray-900" href="/science/global-warming/Ice-melt-and-sea-level-rise#ref274852">Tropical cyclones</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274853"><a class="w-100 link-gray-900" href="/science/global-warming/Environmental-consequences-of-global-warming">Environmental consequences of global warming</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref274854"><a class="w-100 link-gray-900" href="/science/global-warming/Socioeconomic-consequences-of-global-warming">Socioeconomic consequences of global warming</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/global-warming/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/global-warming">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/science/global-warming/images-videos">Images, Videos & Interactives</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex overflow-hidden text-nowrap ml-15"> <a href="https://cdn.britannica.com/50/112550-050-D7D94E18/Grinnell-Glacier-series-photographs-summit-Mount-Gould-2006.jpg" data-href="/media/1/235402/109643" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/50/112550-004-7B056C78/Grinnell-Glacier-series-photographs-summit-Mount-Gould-2006.jpg" 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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/global-warming/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="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/global-warming"> See all related content </a> </div> </div> </div> </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="/news/235402/4b34246d5ca798154af08560fd94f7b9" rel="nofollow">EPA head urges Trump to reconsider scientific finding that underpins climate action, AP sources say</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 26, 2025, 5:45 PM ET (AP) </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="4" id="ref274825">  <span class="marker PREMOD1 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="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/1/235402/19381"><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><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/1/235402/19381">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><p class="topic-paragraph">Of the greenhouse gases, <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. Natural sources of atmospheric CO<sub>2</sub> include <span id="ref1274806"></span><a href="https://www.britannica.com/science/degassing" class="md-crosslink ">outgassing</a> 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 (oxygen-using) organisms. These sources are balanced, on average, by a set of physical, chemical, or biological processes, called “<span id="ref979233"></span><a href="https://www.britannica.com/science/sink-atmospheric-science" class="md-crosslink ">sinks</a>,” that tend to remove CO<sub>2</sub> from the <a href="https://www.britannica.com/science/atmosphere" class="md-crosslink autoxref " data-show-preview="true">atmosphere</a>. Significant natural sinks include terrestrial vegetation, which takes up CO<sub>2</sub> during the process of <a href="https://www.britannica.com/science/photosynthesis" class="md-crosslink " data-show-preview="true">photosynthesis</a>.</p><span class="marker MOD1 mod-inline"></span> <span class="marker PREMOD2 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="153055" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="143200"><a data-id="143200" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/organisms-carbon-cycling-oxygen-environment/-153055"><img src="https://cdn.britannica.com/00/143200-138-E499F3A5/organisms-carbon-cycling-oxygen-environment.jpg?w=800&h=450&c=crop" alt="Understand the carbon and oxygen cycles in the atmosphere" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/M4iYHCQ0.m3u8" } ], "image": "https://cdn.britannica.com/00/143200-138-E499F3A5/organisms-carbon-cycling-oxygen-environment.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/UUkS6Yew", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/d83hRBLi.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="143200">Understand the carbon and oxygen cycles in the atmosphere</span><span>Living organisms influence the cycling of carbon and oxygen through the environment.</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/global-warming/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">A number of oceanic processes also act as <span id="ref1274808"></span><a href="https://www.britannica.com/science/carbon-sink" class="md-crosslink ">carbon sinks</a>. One such process, called the “solubility pump,” involves the <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="descent" href="https://www.britannica.com/dictionary/descent" data-type="EB">descent</a> of surface seawater containing dissolved CO<sub>2</sub>. Another process, the “<span id="ref979234"></span>biological pump,” involves the uptake of dissolved 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 <a href="https://www.britannica.com/science/skeleton" class="md-crosslink " data-show-preview="true">skeletons</a> and other structures made of <span id="ref1274807"></span><a href="https://www.britannica.com/science/calcium-carbonate" class="md-crosslink " data-show-preview="true">calcium carbonate</a> (CaCO<sub>3</sub>). As these organisms expire and fall to the ocean floor, the <a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink " data-show-preview="true">carbon</a> they contain 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 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="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/1/235402/100353"><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/1/235402/100353">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, <a href="https://www.britannica.com/technology/heating-process-or-system" class="md-crosslink " data-show-preview="true">heating</a>, and the generation of <a href="https://www.britannica.com/technology/electric-power" class="md-crosslink " data-show-preview="true">electrical power</a>—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. <span id="ref1254402"></span><a href="https://www.britannica.com/science/anthropogenic-climate-change" class="md-crosslink ">Anthropogenic emissions</a> 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><span class="marker MOD3 mod-inline"></span> <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="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/1/235402/109623"><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/1/235402/109623">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> 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 ppm 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 (<em>see</em> <a href="https://www.britannica.com/science/global-warming/Feedback-mechanisms-and-climate-sensitivity#ref274840" class="md-crosslink " data-show-preview="true">Carbon cycle feedbacks</a>). This may lead to a situation in which the concentration of atmospheric CO<sub>2</sub> builds at an exponential rate (that is, its rate of increase is also increasing).</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">The natural background level of carbon dioxide varies on timescales of millions of years because of 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> (145 million to 66 million years ago), CO<sub>2</sub> concentrations appear to have been several times higher than they are 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 <a href="https://www.britannica.com/place/Earth" class="md-crosslink autoxref " data-show-preview="true">Earth</a> orbital effects linked to the coming and going of the <a href="https://www.britannica.com/science/Pleistocene-Epoch" class="md-crosslink " data-show-preview="true">Pleistocene</a> ice ages (<em>see below</em> <a href="https://www.britannica.com/science/global-warming/Land-use-change#ref274832" class="md-crosslink " data-show-preview="true">Natural influences on climate</a>). By the early 21st century CO<sub>2</sub> levels had 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 autoxref " data-show-preview="true">Industrial Revolution</a>. Atmospheric CO<sub>2</sub> levels continued to increase, and by 2022 they had reached 419 ppm. Such levels are believed to be the highest in at least 800,000 years according to <a href="https://www.britannica.com/science/ice-core" class="md-crosslink " data-show-preview="true">ice core</a> measurements and may be the highest in at least 5 million years according to other lines of evidence.</p><div class="module-spacing"> </div><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph"><span id="ref979239"></span><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 logarithmic 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 wavelengths 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 fossil fuel 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 (<em>see</em> <a href="https://www.britannica.com/science/global-warming/Feedback-mechanisms-and-climate-sensitivity#ref274836" class="md-crosslink " data-show-preview="true">Feedback mechanisms and climate sensitivity</a>). 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 MOD6 mod-inline"></span> </section> <section data-level="4" id="ref274826"> <h2 class="h4"><span id="ref887454"></span><a href="https://www.britannica.com/science/methane" class="md-crosslink " data-show-preview="true">Methane</a></h2> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">Methane (CH<sub>4</sub>) is the second most important <a href="https://www.britannica.com/science/greenhouse-gas" class="md-crosslink autoxref " data-show-preview="true">greenhouse gas</a>. CH<sub>4</sub> is more potent than CO<sub>2</sub> because the radiative forcing produced per molecule is greater. In addition, the infrared window is less saturated in the range of wavelengths of radiation absorbed by CH<sub>4</sub>, so more molecules may fill in the region. However, CH<sub>4</sub> exists in far lower concentrations than CO<sub>2</sub> in the atmosphere, and its concentrations by volume in the atmosphere are generally measured in parts per billion (ppb) rather than ppm. CH<sub>4</sub> also has a considerably shorter residence time in the atmosphere than CO<sub>2</sub> (the residence time for CH<sub>4</sub> is roughly 10 years, compared with hundreds of years for CO<sub>2</sub>).</p><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="141533" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/75/135075-050-032E5335/Methane-cycle-diagram.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/235402/141533"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/75/135075-050-032E5335/Methane-cycle-diagram.jpg"><img src="https://cdn.britannica.com/75/135075-050-032E5335/Methane-cycle-diagram.jpg?w=300" alt="methane cycle" data-width="2000" data-height="1375" 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/75/135075-050-032E5335/Methane-cycle-diagram.jpg" data-href="/media/1/235402/141533">methane cycle</a><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">Natural sources of methane include tropical and northern <a href="https://www.britannica.com/science/wetland" class="md-crosslink " data-show-preview="true">wetlands</a>, methane-oxidizing <a href="https://www.britannica.com/science/bacteria" class="md-crosslink " data-show-preview="true">bacteria</a> that feed on organic material <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="consumed" href="https://www.britannica.com/dictionary/consumed" data-type="EB">consumed</a> by <a href="https://www.britannica.com/animal/termite" class="md-crosslink " data-show-preview="true">termites</a>, volcanoes, seepage vents of the seafloor in regions rich with organic sediment, and methane hydrates trapped along the continental shelves of the oceans and in polar <a href="https://www.britannica.com/science/permafrost" class="md-crosslink " data-show-preview="true">permafrost</a>. The primary natural sink for methane is the atmosphere itself, as methane reacts readily with the <a href="https://www.britannica.com/science/hydroxyl-group" class="md-crosslink autoxref " data-show-preview="true">hydroxyl radical</a> (∙OH) within the <a href="https://www.britannica.com/science/troposphere" class="md-crosslink " data-show-preview="true">troposphere</a> to form CO<sub>2</sub> and water vapour (H<sub>2</sub>O). When CH<sub>4</sub> reaches the <a href="https://www.britannica.com/science/stratosphere" class="md-crosslink " data-show-preview="true">stratosphere</a>, it is destroyed. Another natural sink is <a href="https://www.britannica.com/science/soil" class="md-crosslink " data-show-preview="true">soil</a>, where methane is <a href="https://www.britannica.com/science/oxidation-reduction-reaction" class="md-crosslink " data-show-preview="true">oxidized</a> by bacteria.</p><span class="marker MOD8 mod-inline"></span> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">As with CO<sub>2</sub>, human activity is increasing the CH<sub>4</sub> concentration faster than it can be offset by natural sinks. Anthropogenic sources currently account for approximately 70 percent of total annual emissions, leading to substantial increases in concentration over time. The major anthropogenic sources of atmospheric CH<sub>4</sub> are <a href="https://www.britannica.com/plant/rice" class="md-crosslink " data-show-preview="true">rice</a> cultivation, <a href="https://www.britannica.com/topic/livestock-farming" class="md-crosslink " data-show-preview="true">livestock farming</a>, the burning of <a href="https://www.britannica.com/science/coal-fossil-fuel" class="md-crosslink " data-show-preview="true">coal</a> and <a href="https://www.britannica.com/science/natural-gas" class="md-crosslink " data-show-preview="true">natural gas</a>, the combustion of <a href="https://www.britannica.com/science/biomass" class="md-crosslink " data-show-preview="true">biomass</a>, and the decomposition of organic matter in landfills. Future <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="trends" href="https://www.britannica.com/dictionary/trends" data-type="EB">trends</a> are particularly difficult to anticipate. This is in part due to an incomplete understanding of the climate feedbacks associated with CH<sub>4</sub> emissions. In addition it is difficult to predict how, as human populations grow, possible changes in livestock raising, rice cultivation, and energy utilization will influence CH<sub>4</sub> emissions.</p><span class="marker MOD9 mod-inline"></span> <span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph">It is believed that a sudden increase in the concentration of methane in the atmosphere was responsible for a warming event that raised average global temperatures by 4–8 °C (7.2–14.4 °F) over a few thousand years during the so-called <span id="ref979243"></span><a href="https://www.britannica.com/science/Paleocene-Eocene-Thermal-Maximum" class="md-crosslink " data-show-preview="true">Paleocene-Eocene Thermal Maximum</a>, or PETM. This episode took place roughly 55 million years ago, and the rise in CH<sub>4</sub> appears to have been related to a massive <a href="https://www.britannica.com/science/volcanic-eruption" class="md-crosslink autoxref " data-show-preview="true">volcanic eruption</a> that interacted with methane-containing flood deposits. As a result, large amounts of gaseous CH<sub>4</sub> were injected into the atmosphere. It is difficult to know precisely how high these concentrations were or how long they persisted. At very high concentrations, residence times of CH<sub>4</sub> in the atmosphere can become much greater than the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="nominal" href="https://www.merriam-webster.com/dictionary/nominal" data-type="MW">nominal</a> 10-year residence time that applies today. Nevertheless, it is likely that these concentrations reached several ppm during the PETM.</p><span class="marker MOD10 mod-inline"></span> <span class="marker PREMOD11 mod-inline"></span><p class="topic-paragraph">Methane concentrations have also varied over a smaller range (between roughly 350 and 800 ppb) in association with the Pleistocene <a href="https://www.britannica.com/science/ice-age-geology" class="md-crosslink autoxref " data-show-preview="true">ice age</a> cycles (<em>see</em> <a href="https://www.britannica.com/science/global-warming/Land-use-change#ref274832" class="md-crosslink " data-show-preview="true">Natural influences on climate</a>). Preindustrial levels of CH<sub>4</sub> in the atmosphere were approximately 700 ppb, whereas levels exceeded 1,915 ppb in the <a href="https://www.britannica.com/science/autumn-season" class="md-crosslink autoxref " data-show-preview="true">fall</a> of 2022. (These concentrations are well above the natural levels observed for at least the past 650,000 years.) The net radiative forcing by anthropogenic CH<sub>4</sub> emissions is approximately 0.5 watt per square metre—or roughly one-third the radiative forcing of CO<sub>2</sub>.</p><span class="marker MOD11 mod-inline"></span> </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":5,"pagesTotal":16,"pageId":235402,"pageLength":1383,"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":"D","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":5,"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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Global warming - Carbon Dioxide, Climate Change, Greenhouse Gases | Britannica