{"id":266280,"date":"2023-07-09T13:15:04","date_gmt":"2023-07-09T11:15:04","guid":{"rendered":"https:\/\/climatescience.press\/?p=266280"},"modified":"2023-07-09T13:15:08","modified_gmt":"2023-07-09T11:15:08","slug":"how-much-warming-can-we-expect-in-the-21st-century","status":"publish","type":"post","link":"https:\/\/climatescience.press\/?p=266280","title":{"rendered":"How much warming can we expect in the 21st century?"},"content":{"rendered":"\n
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From Climate Etc.<\/a><\/p>\n\n\n\n

by Hakon Karlsen<\/p>\n\n\n\n

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A comprehensive explainer of climate sensitivity to CO2<\/sub><\/p>\n\n\n\n

Short summary<\/strong><\/p>\n\n\n\n

According to the Intergovernmental Panel on Climate Change (IPCC), the atmosphere\u2019s\u00a0climate sensitivity<\/em>\u00a0to CO2<\/sub>is likely between 2.5 and 4.0\u00b0C. Simply put, this means that (in the very long term) Earth\u2019s temperature will rise between 2.5 and 4.0\u00b0C when the amount of CO2<\/sub>\u00a0in the atmosphere doubles.<\/p>\n\n\n\n

A 2020 study (Sherwood20<\/a>) greatly influenced how the IPCC calculated the climate sensitivity. Sherwood20 has been \u201cextremely influential, including in informing the assessment of equilibrium climate sensitivity (ECS) in the 2021 IPCC Sixth Assessment Scientific Report (AR6); it was cited over twenty times in the relevant AR6 chapter<\/em>\u201c,\u00a0according to Nic Lewis<\/a>. A\u00a0Comment in Nature<\/a>\u00a0confirmed this view.1)<\/sup><\/a><\/p>\n\n\n\n

Nic Lewis took a closer look at this study, and in September 2022, he published his own study (Lewis22<\/a>) that criticizes Sherwood20. By correcting errors and using more recent data, including from AR6, Lewis22 found that the climate sensitivity may be about 30% lower than what Sherwood20 had found.<\/p>\n\n\n\n

If we know what the climate sensitivity is, and if we also know approximately the amount of greenhouse gases that will be emitted going forward, then the amount of future warming that\u2019s caused by greenhouse gases can also be estimated.<\/p>\n\n\n\n

In terms of future emissions, a 2022 study (Pielke22<\/a>) found that something called\u00a0RCP3.4<\/em>\u00a0is the most\u00a0plausible<\/em>\u00a0emissions scenario. Traditionally, another scenario (RCP8.5), has been used as a\u00a0business-as-usual<\/em>\u00a0scenario, but this is now widely regarded as an extremely unlikely scenario, with unrealistically high emissions.<\/p>\n\n\n\n

Assuming that the climate sensitivity from Lewis22 is correct and that RCP3.4 is the most appropriate emissions scenario, then we find that global temperatures will rise by less than 1\u00b0C from 2023 to 2100 (not accounting for natural variability).<\/p>\n\n\n\n

How much the Earth\u2019s surface air temperature will rise this century depends, among other things, on how\u00a0sensitive<\/em>\u00a0the atmosphere is to greenhouse gases such as CO2<\/sub>, the amount of greenhouse gases that are emitted, and natural variations. It\u2019s hard to predict natural variations, so the focus here will be on climate sensitivity and greenhouse gas emissions (in particular CO2<\/sub>).<\/p>\n\n\n\n

Climate sensitivity<\/strong><\/p>\n\n\n\n

Climate sensitivity is the amount of warming that can be expected in the Earth\u2019s surface air temperature if the amount of CO2 in the atmosphere doubles. So if the climate sensitivity is 3\u00b0C, and the amount of CO2<\/sub>\u00a0in the atmosphere quickly doubles and stays at that level, then the Earth\u2019s surface air temperature will \u2013 in the long term \u2013 rise by 3\u00b0C.2)<\/sup><\/a>\u00a0In the long term<\/em>, in this case, is more than 1000 years, but most of the temperature increase happens relatively fast.<\/p>\n\n\n\n

The exact value for the climate sensitivity isn\u2019t known, and the uncertainty range has traditionally been\u00a0very<\/em>\u00a0large. In 1979, the so-called Charney report found the climate sensitivity to be between 1.5 and 4.5\u00b0C. 34 years later, in 2013, the IPCC reached the exact same conclusion \u2013 that it\u2019s\u00a0likely<\/em>\u00a0(66% probability) that the climate sensitivity is between 1.5 and 4.5\u00b0C. However,\u00a0the uncertainty in the Charney report may have been underestimated<\/a>. So even though the official climate sensitivity estimate didn\u2019t change, it wouldn\u2019t be correct to say that no progress was made during those 34 years.<\/p>\n\n\n\n

In climate science, there are several different\u00a0types<\/em>\u00a0of climate sensitivity. I won\u2019t go into detail about the various types just yet, but I\u2019ll have something to say about some of them later in the article \u2013 when it becomes relevant. The type of climate sensitivity referred to above \u2013 in the Charney report and by the IPCC \u2013 is called\u00a0equilibrium climate sensitivity<\/em>\u00a0(ECS).<\/p>\n\n\n\n

Why so much uncertainty? (Feedback effects)<\/strong><\/p>\n\n\n\n

There\u2019s broad agreement that without so-called\u00a0feedback effects<\/em>, the equilibrium climate sensitivity (ECS) would be close to 1.2\u00b0C\u00a03)<\/sup><\/a>, which is quite low and not particularly dangerous. The reason for the great uncertainty comes from how\u00a0feedback effects<\/em>\u00a0affect the temperature.<\/p>\n\n\n\n

A feedback effect can be either positive or negative. A positive feedback effect amplifies warming, contributing to a higher climate sensitivity. A negative feedback dampens warming and contributes to a lower climate sensitivity.<\/p>\n\n\n\n

The strengths of feedback effects can vary based on the atmosphere\u2019s temperature and composition, and how much of the Earth is covered by ice and vegetation, among other things. Earth\u2019s climate sensitivity is thus not a constant. And for this reason, the equilibrium climate sensitivity, ECS, has been defined as the long-term increase in temperature as a result of a doubling of CO2<\/sub>\u00a0from pre-industrial levels<\/em>\u00a0(which was about 284 parts per million (ppm)).<\/p>\n\n\n\n

Atmospheric CO2<\/sub>\u00a0concentration currently stands at approximately 420 ppm, which means there\u2019s been a near 50% increase since the second half of the 19th century.4)<\/sup><\/a>\u00a0Since the concentration of CO2<\/sub>\u00a0hasn\u2019t yet doubled (and also since\u00a0the long term<\/em>\u00a0is a long way away), the temperature has risen less than than the magnitude of the equilibrium climate sensitivity. To be more precise, the temperature increase has been approximately 1.2\u00b0C over the past 150 years.<\/p>\n\n\n\n

Types of feedback mechanisms<\/strong><\/p>\n\n\n\n

There are several different feedback mechanisms<\/a>. Here are some of the most important ones:<\/p>\n\n\n\n