CO2: Difference between revisions

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<p><b>However, progress is too slow, to keep below two degrees, we will need to dramatically reduce current emissions and simultaneously remove 10-15 gigatons of CO<sub>2</sub>/yr from the atmosphere by 2050 and scale that to about 20+ gigatons annually by 2100. Depending on how quickly we reduce emissions, the amount we need to remove from the atmosphere scales proportionally.</b></p><p>Greenhouse gas emissions are described in units of tons. It’s hard to think about how much “a ton of gas” really is -- this is how big, at surface temperature and pressure. Here’s an animated video visualizing a bunch of these one-ton balls in New York City.</p>

<HTML><iframe width="100%" height="315" src="https://www.youtube.com/embed/DtqSIplGXOA" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></HTML><p>You may also hear people talk about “''400 parts per million CO<sub>2</sub>''” or similar. This maps directly to the amount of gas emitted: when emissions mix into the atmosphere, we reference concentrations of the emitted gas as a portion of the atmosphere. This is like stirring sugar into a cup of coffee. Climate modeling is based on these concentrations. For a bit more on how this measurement works, see <i>Basic intro to units and measurement</i>.</p>xxx<p>For context (thanks to <a href="https://cnce.engineering.asu.edu/klaus-lackner/">Klaus Lackner</a> for the following distillation), <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html">1 ppm</a> is worth about 7.5 Gigaton of CO<sub>2</sub>. But roughly half of it mixes into the surface ocean and some goes into the biosphere. So the net result is that it takes about 15 Gigaton of CO<sub>2</sub> to raise the atmospheric concentration by 1 ppm. We <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/gr.html">raise it by about 2.5 ppm</a> per year.</p><hr><h1 id="basic-intro-to-units-and-measurement">Basic intro to units and measurement</h1><h2 id="measuring-greenhouse-gases">Measuring greenhouse gases</h2><p>Greenhouse gases are released by burning stuff as well as the product of other chemical reactions in industry. Generally, we’re talking about mostly carbon dioxide (<b>CO<sub>2</sub></b>), methane (natural gas) (<b>CH<sub>4</sub></b>), and nitrous oxide (<b>N<sub>2</sub>O</b>). N<sub>2</sub>O and CH<sub>4</sub> are <a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)">more potent greenhouse gases</a>, but they occur in an order of magnitude less quantity than CO<sub>2</sub>, so removing them is generally much harder. Methane also has a much shorter half life in the atmosphere than CO<sub>2</sub>.</p><h2 id="here-are-the-units-you-ll-usually-see-">Here are the units you’ll usually see:</h2><h3 id="tons-of-co2">Tons of CO<sub>2</sub></h3><p>Literally just a ton of CO<sub>2</sub> by mass. This is the most common unit. It is often unclear whether someone means metric or imperial tons, which is annoying.</p><h3 id="tons-of-co2e">Tons of CO<sub>2</sub>E</h3><p>The E means “equivalent”, you’ll often see this when reading about offsets or macro-scale emissions comparisons. “Equivalent” means another greenhouse gas,<a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)"> normalized to the warming potential of CO<sub>2</sub></a>. As calculated, methane is about <a href="https://www.epa.gov/ghgemissions/understanding-global-warming-potentials">28-36x equivalent (GWP 100)</a>, so 1 ton of methane would count as 28-36 tons CO<sub>2</sub>E.</p><p><i>(Update March 2: my original methane GWP value was incorrect, fixed now, thanks <a href="https://snarfed.org">Ryan Barrett</a> for flagging)</i></p><h3 id="tons-of-c">Tons of C</h3><p>This is mostly used when talking about sinking carbon in plants and soils, and it just means we only measure the C in the CO<sub>2</sub>, which maps 3:11 by molar mass. (Because the two Oxygen atoms each have a mass of 16)</p><h3 id="-mega-tons-giga-tons-of-any-of-these">(Mega)tons, (giga)tons of any of these</h3><p>Megaton = one million tons; Gigaton = one billion tons.</p><h2 id="measuring-greenhouse-gases-when-they-re-mixed-into-the-sky">Measuring greenhouse gases when they’re mixed into the sky</h2><p>The atmosphere is made of a mixture of gases:</p><figure class="kg-card kg-image-card kg-card-hascaption"><img src="https://lh3.googleusercontent.com/1pH_QSrDc-I9ljY0oV0g7P7ULBFYTSmb2MeFOaCgsZT5Gt2OTwqCPfanli7t7Ima7BX1SJ3J1zQINuDN_wu4DkVa5sHQupmWU0FchMvlfSorns5b-TsrRP5Y_N4UiqS-wQdq7pHJ" class="kg-image"><figcaption>Figure source: <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Earth">Wikipedia Atmosphere of Earth</a></figcaption></figure><p>Greenhouse gases are a <b>super small part of it</b>, so instead of describing tiny absolute percentages, we use these units to describe how much:</p><h3 id="parts-per-million-ppm-">Parts per million (PPM)</h3><ul><li>Used mostly for CO2</li></ul><h3 id="parts-per-billion-ppb-">Parts per billion (PPB)</h3><ul><li>Used mostly for CH<sub>4</sub>, CFCs, and other trace gases.</li></ul><p>This is what we mean by methane being far more dilute than CO<sub>2</sub> (and therefore unrealistic to capture) -- we need to measure it in parts per <i>billion</i>.</p><p>This sense of scale helps explain why removing CO<sub>2</sub> is an expensive proposition, thermodynamically and hence monetarily: it’s an extremely dilute (parts per million!) gas in solution — we’re talking about separating a tiny fraction of the air from the rest of the air.</p><p>Finally, taking a step back: the sensitivity of this system is really incredible -- we’re talking about changing the lives of people around the world by <b>adding less than a basis point to the absolute concentration of a trace gas</b>. It’s <i>wild</i>.</p><hr><h1 id="do-we-really-need-to-remove-co2">Do we really need to remove CO<sub>2</sub>?</h1><p>In the 90s, negative emissions were <a href="https://en.wikipedia.org/wiki/Overton_window">not mainstream</a> apart from early research by <a href="https://en.wikipedia.org/wiki/David_Keith_(scientist)">David Keith</a>, <a href="https://en.wikipedia.org/wiki/Klaus_Lackner">Klaus Lackne</a>r, and others.</p><p>There was some combination of optimism that the problem wasn’t so bad, that the world would decarbonize at a sufficient rate that negative emissions wouldn’t be necessary, and that a policy framework would force action (large scale carbon price/cap and trade, which still doesn’t exist worldwide). This was all combined with fears that negative emissions present a moral hazard by giving ourselves an “out” for crucial emissions reduction work. A good summary of the history is <a href="https://www.amazon.com/Carbon-Capture-Press-Essential-Knowledge/dp/0262535750">here</a>.</p><p>In an attempt to hit a 2 degree warming target, the <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement">Paris agreement</a> calls on countries to set “Nationally Determined Contributions”, or <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/nationally-determined-contributions-ndcs">NDCs</a> -- commitments to a specific amount of emissions reduction over the coming decades. <b>But these commitments are not even close to enough:</b></p>

<p>If you’re in the US, you’re responsible for emitting about 19 tons of greenhouse gases a year. <a href="http://projectwren.com/">Wren</a> has a nice calculator that asks you about your commuting habits, flights, and diet to estimate your total GHG emissions. <a href="https://erikareinhardt.com/personal-climate-action">Erika Reinhardt wrote a detailed guide</a> on how you can reduce your emissions. With that in mind, see emissions per capita:</p>

~~<iframe src="https://ourworldindata.org/grapher/co-emissions-per-capita" style="width: 100%; height: 600px; border: 0px none;"></iframe>~~

~~<iframe src="https://ourworldindata.org/grapher/co2-concentration-long-term" style="width: 100%; height: 600px; border: 0px none;"></iframe>~~<p>For context (thanks to <a href="https://cnce.engineering.asu.edu/klaus-lackner/">Klaus Lackner</a> for the following distillation), <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html">1 ppm</a> is worth about 7.5 Gigaton of CO<sub>2</sub>. But roughly half of it mixes into the surface ocean and some goes into the biosphere. So the net result is that it takes about 15 Gigaton of CO<sub>2</sub> to raise the atmospheric concentration by 1 ppm. We <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/gr.html">raise it by about 2.5 ppm</a> per year.</p><hr><h1 id="basic-intro-to-units-and-measurement">Basic intro to units and measurement</h1><h2 id="measuring-greenhouse-gases">Measuring greenhouse gases</h2><p>Greenhouse gases are released by burning stuff as well as the product of other chemical reactions in industry. Generally, we’re talking about mostly carbon dioxide (<b>CO<sub>2</sub></b>), methane (natural gas) (<b>CH<sub>4</sub></b>), and nitrous oxide (<b>N<sub>2</sub>O</b>). N<sub>2</sub>O and CH<sub>4</sub> are <a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)">more potent greenhouse gases</a>, but they occur in an order of magnitude less quantity than CO<sub>2</sub>, so removing them is generally much harder. Methane also has a much shorter half life in the atmosphere than CO<sub>2</sub>.</p><h2 id="here-are-the-units-you-ll-usually-see-">Here are the units you’ll usually see:</h2><h3 id="tons-of-co2">Tons of CO<sub>2</sub></h3><p>Literally just a ton of CO<sub>2</sub> by mass. This is the most common unit. It is often unclear whether someone means metric or imperial tons, which is annoying.</p><h3 id="tons-of-co2e">Tons of CO<sub>2</sub>E</h3><p>The E means “equivalent”, you’ll often see this when reading about offsets or macro-scale emissions comparisons. “Equivalent” means another greenhouse gas,<a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)"> normalized to the warming potential of CO<sub>2</sub></a>. As calculated, methane is about <a href="https://www.epa.gov/ghgemissions/understanding-global-warming-potentials">28-36x equivalent (GWP 100)</a>, so 1 ton of methane would count as 28-36 tons CO<sub>2</sub>E.</p><p><i>(Update March 2: my original methane GWP value was incorrect, fixed now, thanks <a href="https://snarfed.org">Ryan Barrett</a> for flagging)</i></p><h3 id="tons-of-c">Tons of C</h3><p>This is mostly used when talking about sinking carbon in plants and soils, and it just means we only measure the C in the CO<sub>2</sub>, which maps 3:11 by molar mass. (Because the two Oxygen atoms each have a mass of 16)</p><h3 id="-mega-tons-giga-tons-of-any-of-these">(Mega)tons, (giga)tons of any of these</h3><p>Megaton = one million tons; Gigaton = one billion tons.</p><h2 id="measuring-greenhouse-gases-when-they-re-mixed-into-the-sky">Measuring greenhouse gases when they’re mixed into the sky</h2><p>The atmosphere is made of a mixture of gases:</p><figure class="kg-card kg-image-card kg-card-hascaption"><img src="https://lh3.googleusercontent.com/1pH_QSrDc-I9ljY0oV0g7P7ULBFYTSmb2MeFOaCgsZT5Gt2OTwqCPfanli7t7Ima7BX1SJ3J1zQINuDN_wu4DkVa5sHQupmWU0FchMvlfSorns5b-TsrRP5Y_N4UiqS-wQdq7pHJ" class="kg-image"><figcaption>Figure source: <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Earth">Wikipedia Atmosphere of Earth</a></figcaption></figure><p>Greenhouse gases are a <b>super small part of it</b>, so instead of describing tiny absolute percentages, we use these units to describe how much:</p><h3 id="parts-per-million-ppm-">Parts per million (PPM)</h3><ul><li>Used mostly for CO2</li></ul><h3 id="parts-per-billion-ppb-">Parts per billion (PPB)</h3><ul><li>Used mostly for CH<sub>4</sub>, CFCs, and other trace gases.</li></ul><p>This is what we mean by methane being far more dilute than CO<sub>2</sub> (and therefore unrealistic to capture) -- we need to measure it in parts per <i>billion</i>.</p><p>This sense of scale helps explain why removing CO<sub>2</sub> is an expensive proposition, thermodynamically and hence monetarily: it’s an extremely dilute (parts per million!) gas in solution — we’re talking about separating a tiny fraction of the air from the rest of the air.</p><p>Finally, taking a step back: the sensitivity of this system is really incredible -- we’re talking about changing the lives of people around the world by <b>adding less than a basis point to the absolute concentration of a trace gas</b>. It’s <i>wild</i>.</p><hr><h1 id="do-we-really-need-to-remove-co2">Do we really need to remove CO<sub>2</sub>?</h1><p>In the 90s, negative emissions were <a href="https://en.wikipedia.org/wiki/Overton_window">not mainstream</a> apart from early research by <a href="https://en.wikipedia.org/wiki/David_Keith_(scientist)">David Keith</a>, <a href="https://en.wikipedia.org/wiki/Klaus_Lackner">Klaus Lackne</a>r, and others.</p><p>There was some combination of optimism that the problem wasn’t so bad, that the world would decarbonize at a sufficient rate that negative emissions wouldn’t be necessary, and that a policy framework would force action (large scale carbon price/cap and trade, which still doesn’t exist worldwide). This was all combined with fears that negative emissions present a moral hazard by giving ourselves an “out” for crucial emissions reduction work. A good summary of the history is <a href="https://www.amazon.com/Carbon-Capture-Press-Essential-Knowledge/dp/0262535750">here</a>.</p><p>In an attempt to hit a 2 degree warming target, the <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement">Paris agreement</a> calls on countries to set “Nationally Determined Contributions”, or <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/nationally-determined-contributions-ndcs">NDCs</a> -- commitments to a specific amount of emissions reduction over the coming decades. <b>But these commitments are not even close to enough:</b></p>

@@ Line 11: / Line 11: @@
 <p><b>However, progress is too slow, to keep below two degrees, we will need to dramatically reduce current emissions and simultaneously remove 10-15 gigatons of CO<sub>2</sub>/yr from the atmosphere by 2050 and scale that to about 20+ gigatons annually by 2100. Depending on how quickly we reduce emissions, the amount we need to remove from the atmosphere scales proportionally.</b></p><p>Greenhouse gas emissions are described in units of tons. It’s hard to think about how much “a ton of gas” really is -- this is how big, at surface temperature and pressure. Here’s an animated video visualizing a bunch of these one-ton balls in New York City.</p>
-<HTML><iframe width="100%" height="315" src="https://www.youtube.com/embed/DtqSIplGXOA" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></HTML>
+<HTML><iframe width="100%" height="315" src="https://www.youtube.com/embed/DtqSIplGXOA" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></HTML><p>You may also hear people talk about “''400 parts per million CO<sub>2</sub>''” or similar. This maps directly to the amount of gas emitted: when emissions mix into the atmosphere, we reference concentrations of the emitted gas as a portion of the atmosphere. This is like stirring sugar into a cup of coffee. Climate modeling is based on these concentrations. For a bit more on how this measurement works, see <i>Basic intro to units and measurement</i>.</p>xxx<p>For context (thanks to <a href="https://cnce.engineering.asu.edu/klaus-lackner/">Klaus Lackner</a> for the following distillation), <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html">1 ppm</a> is worth about 7.5 Gigaton of CO<sub>2</sub>. But roughly half of it mixes into the surface ocean and some goes into the biosphere. So the net result is that it takes about 15 Gigaton of CO<sub>2</sub> to raise the atmospheric concentration by 1 ppm. We <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/gr.html">raise it by about 2.5 ppm</a> per year.</p><hr><h1 id="basic-intro-to-units-and-measurement">Basic intro to units and measurement</h1><h2 id="measuring-greenhouse-gases">Measuring greenhouse gases</h2><p>Greenhouse gases are released by burning stuff as well as the product of other chemical reactions in industry. Generally, we’re talking about mostly carbon dioxide (<b>CO<sub>2</sub></b>), methane (natural gas) (<b>CH<sub>4</sub></b>), and nitrous oxide (<b>N<sub>2</sub>O</b>). N<sub>2</sub>O and CH<sub>4</sub> are <a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)">more potent greenhouse gases</a>, but they occur in an order of magnitude less quantity than CO<sub>2</sub>, so removing them is generally much harder. Methane also has a much shorter half life in the atmosphere than CO<sub>2</sub>.</p><h2 id="here-are-the-units-you-ll-usually-see-">Here are the units you’ll usually see:</h2><h3 id="tons-of-co2">Tons of CO<sub>2</sub></h3><p>Literally just a ton of CO<sub>2</sub> by mass. This is the most common unit. It is often unclear whether someone means metric or imperial tons, which is annoying.</p><h3 id="tons-of-co2e">Tons of CO<sub>2</sub>E</h3><p>The E means “equivalent”, you’ll often see this when reading about offsets or macro-scale emissions comparisons. “Equivalent” means another greenhouse gas,<a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)"> normalized to the warming potential of CO<sub>2</sub></a>. As calculated, methane is about <a href="https://www.epa.gov/ghgemissions/understanding-global-warming-potentials">28-36x equivalent (GWP 100)</a>, so 1 ton of methane would count as 28-36 tons CO<sub>2</sub>E.</p><p><i>(Update March 2: my original methane GWP value was incorrect, fixed now, thanks <a href="https://snarfed.org">Ryan Barrett</a> for flagging)</i></p><h3 id="tons-of-c">Tons of C</h3><p>This is mostly used when talking about sinking carbon in plants and soils, and it just means we only measure the C in the CO<sub>2</sub>, which maps 3:11 by molar mass. (Because the two Oxygen atoms each have a mass of 16)</p><h3 id="-mega-tons-giga-tons-of-any-of-these">(Mega)tons, (giga)tons of any of these</h3><p>Megaton = one million tons; Gigaton = one billion tons.</p><h2 id="measuring-greenhouse-gases-when-they-re-mixed-into-the-sky">Measuring greenhouse gases when they’re mixed into the sky</h2><p>The atmosphere is made of a mixture of gases:</p><figure class="kg-card kg-image-card kg-card-hascaption"><img src="https://lh3.googleusercontent.com/1pH_QSrDc-I9ljY0oV0g7P7ULBFYTSmb2MeFOaCgsZT5Gt2OTwqCPfanli7t7Ima7BX1SJ3J1zQINuDN_wu4DkVa5sHQupmWU0FchMvlfSorns5b-TsrRP5Y_N4UiqS-wQdq7pHJ" class="kg-image"><figcaption>Figure source: <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Earth">Wikipedia Atmosphere of Earth</a></figcaption></figure><p>Greenhouse gases are a <b>super small part of it</b>, so instead of describing tiny absolute percentages, we use these units to describe how much:</p><h3 id="parts-per-million-ppm-">Parts per million (PPM)</h3><ul><li>Used mostly for CO2</li></ul><h3 id="parts-per-billion-ppb-">Parts per billion (PPB)</h3><ul><li>Used mostly for CH<sub>4</sub>, CFCs, and other trace gases.</li></ul><p>This is what we mean by methane being far more dilute than CO<sub>2</sub> (and therefore unrealistic to capture) -- we need to measure it in parts per <i>billion</i>.</p><p>This sense of scale helps explain why removing CO<sub>2</sub> is an expensive proposition, thermodynamically and hence monetarily: it’s an extremely dilute (parts per million!) gas in solution — we’re talking about separating a tiny fraction of the air from the rest of the air.</p><p>Finally, taking a step back: the sensitivity of this system is really incredible -- we’re talking about changing the lives of people around the world by <b>adding less than a basis point to the absolute concentration of a trace gas</b>. It’s <i>wild</i>.</p><hr><h1 id="do-we-really-need-to-remove-co2">Do we really need to remove CO<sub>2</sub>?</h1><p>In the 90s, negative emissions were <a href="https://en.wikipedia.org/wiki/Overton_window">not mainstream</a> apart from early research by <a href="https://en.wikipedia.org/wiki/David_Keith_(scientist)">David Keith</a>, <a href="https://en.wikipedia.org/wiki/Klaus_Lackner">Klaus Lackne</a>r, and others.</p><p>There was some combination of optimism that the problem wasn’t so bad, that the world would decarbonize at a sufficient rate that negative emissions wouldn’t be necessary, and that a policy framework would force action (large scale carbon price/cap and trade, which still doesn’t exist worldwide). This was all combined with fears that negative emissions present a moral hazard by giving ourselves an “out” for crucial emissions reduction work. A good summary of the history is <a href="https://www.amazon.com/Carbon-Capture-Press-Essential-Knowledge/dp/0262535750">here</a>.</p><p>In an attempt to hit a 2 degree warming target, the <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement">Paris agreement</a> calls on countries to set “Nationally Determined Contributions”, or <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/nationally-determined-contributions-ndcs">NDCs</a> -- commitments to a specific amount of emissions reduction over the coming decades. <b>But these commitments are not even close to enough:</b></p><!--kg-card-begin: html-->
-<p>If you’re in the US, you’re responsible for emitting about 19 tons of greenhouse gases a year. <a href="http://projectwren.com/">Wren</a> has a nice calculator that asks you about your commuting habits, flights, and diet to estimate your total GHG emissions. <a href="https://erikareinhardt.com/personal-climate-action">Erika Reinhardt wrote a detailed guide</a> on how you can reduce your emissions. With that in mind, see emissions per capita:</p><!--kg-card-begin: html-->
-<iframe src="https://ourworldindata.org/grapher/co-emissions-per-capita" style="width: 100%; height: 600px; border: 0px none;"></iframe><!--kg-card-end: html--><p>You may also hear people talk about “400 parts per million CO<sub>2</sub>” or similar. This maps directly to the amount of gas emitted: when emissionsmix into the atmosphere, we reference concentrationsof the emitted gas as a portion of the atmosphere. This is like stirring sugar into a cup of coffee. Climate modeling is based on these concentrations. For a bit more on how this measurement works, see <i>Basic intro to units and measurement</i>.</p><!--kg-card-begin: html-->
-<iframe src="https://ourworldindata.org/grapher/co2-concentration-long-term" style="width: 100%; height: 600px; border: 0px none;"></iframe><!--kg-card-end: html--><p>For context (thanks to <a href="https://cnce.engineering.asu.edu/klaus-lackner/">Klaus Lackner</a> for the following distillation), <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html">1 ppm</a> is worth about 7.5 Gigaton of CO<sub>2</sub>. But roughly half of it mixes into the surface ocean and some goes into the biosphere. So the net result is that it takes about 15 Gigaton of CO<sub>2</sub> to raise the atmospheric concentration by 1 ppm. We <a href="https://www.esrl.noaa.gov/gmd/ccgg/trends/gr.html">raise it by about 2.5 ppm</a> per year.</p><hr><h1 id="basic-intro-to-units-and-measurement">Basic intro to units and measurement</h1><h2 id="measuring-greenhouse-gases">Measuring greenhouse gases</h2><p>Greenhouse gases are released by burning stuff as well as the product of other chemical reactions in industry. Generally, we’re talking about mostly carbon dioxide (<b>CO<sub>2</sub></b>), methane (natural gas) (<b>CH<sub>4</sub></b>), and nitrous oxide (<b>N<sub>2</sub>O</b>). N<sub>2</sub>O and CH<sub>4</sub> are <a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)">more potent greenhouse gases</a>, but they occur in an order of magnitude less quantity than CO<sub>2</sub>, so removing them is generally much harder. Methane also has a much shorter half life in the atmosphere than CO<sub>2</sub>.</p><h2 id="here-are-the-units-you-ll-usually-see-">Here are the units you’ll usually see:</h2><h3 id="tons-of-co2">Tons of CO<sub>2</sub></h3><p>Literally just a ton of CO<sub>2</sub> by mass. This is the most common unit. It is often unclear whether someone means metric or imperial tons, which is annoying.</p><h3 id="tons-of-co2e">Tons of CO<sub>2</sub>E</h3><p>The E means “equivalent”, you’ll often see this when reading about offsets or macro-scale emissions comparisons. “Equivalent” means another greenhouse gas,<a href="https://en.wikipedia.org/wiki/Global_warming_potential%23Global_Temperature_change_Potential_(GTP)"> normalized to the warming potential of CO<sub>2</sub></a>. As calculated, methane is about <a href="https://www.epa.gov/ghgemissions/understanding-global-warming-potentials">28-36x equivalent (GWP 100)</a>, so 1 ton of methane would count as 28-36 tons CO<sub>2</sub>E.</p><p><i>(Update March 2: my original methane GWP value was incorrect, fixed now, thanks <a href="https://snarfed.org">Ryan Barrett</a> for flagging)</i></p><h3 id="tons-of-c">Tons of C</h3><p>This is mostly used when talking about sinking carbon in plants and soils, and it just means we only measure the C in the CO<sub>2</sub>, which maps 3:11 by molar mass. (Because the two Oxygen atoms each have a mass of 16)</p><h3 id="-mega-tons-giga-tons-of-any-of-these">(Mega)tons, (giga)tons of any of these</h3><p>Megaton = one million tons; Gigaton = one billion tons.</p><h2 id="measuring-greenhouse-gases-when-they-re-mixed-into-the-sky">Measuring greenhouse gases when they’re mixed into the sky</h2><p>The atmosphere is made of a mixture of gases:</p><figure class="kg-card kg-image-card kg-card-hascaption"><img src="https://lh3.googleusercontent.com/1pH_QSrDc-I9ljY0oV0g7P7ULBFYTSmb2MeFOaCgsZT5Gt2OTwqCPfanli7t7Ima7BX1SJ3J1zQINuDN_wu4DkVa5sHQupmWU0FchMvlfSorns5b-TsrRP5Y_N4UiqS-wQdq7pHJ" class="kg-image"><figcaption>Figure source: <a href="https://en.wikipedia.org/wiki/Atmosphere_of_Earth">Wikipedia Atmosphere of Earth</a></figcaption></figure><p>Greenhouse gases are a <b>super small part of it</b>, so instead of describing tiny absolute percentages, we use these units to describe how much:</p><h3 id="parts-per-million-ppm-">Parts per million (PPM)</h3><ul><li>Used mostly for CO2</li></ul><h3 id="parts-per-billion-ppb-">Parts per billion (PPB)</h3><ul><li>Used mostly for CH<sub>4</sub>, CFCs, and other trace gases.</li></ul><p>This is what we mean by methane being far more dilute than CO<sub>2</sub> (and therefore unrealistic to capture) -- we need to measure it in parts per <i>billion</i>.</p><p>This sense of scale helps explain why removing CO<sub>2</sub> is an expensive proposition, thermodynamically and hence monetarily: it’s an extremely dilute (parts per million!) gas in solution — we’re talking about separating a tiny fraction of the air from the rest of the air.</p><p>Finally, taking a step back: the sensitivity of this system is really incredible -- we’re talking about changing the lives of people around the world by <b>adding less than a basis point to the absolute concentration of a trace gas</b>. It’s <i>wild</i>.</p><hr><h1 id="do-we-really-need-to-remove-co2">Do we really need to remove CO<sub>2</sub>?</h1><p>In the 90s, negative emissions were <a href="https://en.wikipedia.org/wiki/Overton_window">not mainstream</a> apart from early research by <a href="https://en.wikipedia.org/wiki/David_Keith_(scientist)">David Keith</a>, <a href="https://en.wikipedia.org/wiki/Klaus_Lackner">Klaus Lackne</a>r, and others.</p><p>There was some combination of optimism that the problem wasn’t so bad, that the world would decarbonize at a sufficient rate that negative emissions wouldn’t be necessary, and that a policy framework would force action (large scale carbon price/cap and trade, which still doesn’t exist worldwide). This was all combined with fears that negative emissions present a moral hazard by giving ourselves an “out” for crucial emissions reduction work. A good summary of the history is <a href="https://www.amazon.com/Carbon-Capture-Press-Essential-Knowledge/dp/0262535750">here</a>.</p><p>In an attempt to hit a 2 degree warming target, the <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement">Paris agreement</a> calls on countries to set “Nationally Determined Contributions”, or <a href="https://unfccc.int/process-and-meetings/the-paris-agreement/nationally-determined-contributions-ndcs">NDCs</a> -- commitments to a specific amount of emissions reduction over the coming decades. <b>But these commitments are not even close to enough:</b></p><!--kg-card-begin: html-->
 <iframe scrolling="no" frameborder="0" marginheight="0px" marginwidth="0px" style="background: #fff; display: initial; margin: 0 auto;" src="https://cbhighcharts2019.s3.eu-west-2.amazonaws.com/UNEP+Emissions+Gap/emissions_gap.html" width="100%" height="600px"></iframe><!--kg-card-end: html--><h4 id="there-are-two-complementary-scary-things-about-this-chart-">There are two complementary scary things about this chart:</h4><ul><li>All existing Paris commitments (“NDCs” in the above figure) don’t get us even close to a 2 degree trajectory</li><li>Countries are <a href="https://climateactiontracker.org/countries/">not even close to on track to hit even these commitments</a>. This is an incredible collective action problem — each individual country faces minimal/zero “official” consequences for failing to do so.</li></ul><h3 id="here-s-what-the-emissions-gap-looks-like-for-a-1-5-degree-target-source-">Here’s what the <a href="https://www.unenvironment.org/interactive/emissions-gap-report/2019/">emissions gap</a> looks like for a 1.5 degree target (<a href="https://www.carbonbrief.org/unep-1-5c-climate-target-slipping-out-of-reach">source</a>):</h3><!--kg-card-begin: html-->

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