Updated October 2023
We present a physical measure of the direct climate heating influence of greenhouse gas enhancements. We show how they have increased dramatically since the onset of the industrial revolution. The conclusion that humans are nearly 100% responsible is inescapable. The contributions of different greenhouse gases have evolved over time. The direct heating influence of greenhouse gases is well understood. It is caused by the absorption of infrared light in the atmosphere, a process that is understood in great detail by the same science that gave us lasers, cell phones, and much more. Tangible examples show that the amount of excess heat being retained in the Earth system is easily large enough to force global and regional climate change. It is thus not surprising that we experience the impact of our emissions on the climate.
There are two requirements for a gas to be called a greenhouse gas (GHG): it must absorb infrared radiation (sometimes called heat radiation), and it must have a long residence time in the atmosphere. There has to be enough time for emissions and removals of the gas to mix globally, causing its concentration to be fairly uniform. Climate projections have large model uncertainties due to the complexity of the climate system. However, the uncertainties in GHG measurements themselves are extremely small. Furthermore, the extra heat retention in the atmosphere due to the increased abundance of each gas can be calculated fairly accurately when other factors are kept the same. We call this the direct or instantaneous heat retention or direct heating influence. We present here a measure that summarizes the increase of the direct heating influence since the year 1800 CE (also known as “climate forcing”) supplied from these gases. We also present it as a fraction of the Earth’s weather and climate engine, namely the total amount of solar energy absorbed by Earth and its atmosphere. Because it is based on the observed amounts of GHGs in the atmosphere and on well-known physics of how gases absorb and emit radiation, this measure has little uncertainty.
The Intergovernmental Panel on Climate Change (IPCC) defines climate forcing as “An externally imposed perturbation” of the radiative energy budget of Earth’s climate system [Ramaswamy, 2001]. “Externally” means external to the natural climate system, but the immediate result of human interventions. Specifically, that includes our emissions of greenhouse gases, changes in Earth’s albedo (reflected portion of sun light) when caused directly by human activities such as the replacement of (darker) forests with (more reflective) crop lands and urban areas, human emissions of aerosols and aerosol precursors, irrigation, etc. It does not include changes in solar radiation, clouds, precipitation, snow and ice cover, etc. The latter are determined by the climate system itself, and currently they are a part of the response of the climate system to human intervention. The largest components of climate forcing are the changes in the global atmospheric burdens of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and a number of industrial gases containing bromine, chlorine, or fluorine. The first three are natural greenhouse gases that have been in the atmosphere for at least hundreds of millions of years, but have very sharply increased recently. The other greenhouse gases are, or have been, made by the chemical industry, and were not present in the atmosphere before the mid-20th century. We call them collectively “industrial gases”. The CO2 level is expressed as parts per million (ppm), CH4 and N2O as parts per billion (ppb), and the industrial gases as parts per trillion (ppt).
Although water vapor absorbs and emits infrared radiation, it is not included as a climate forcing agent. Water evaporates from the oceans and land surface (and falls back as rain or snow) in amounts at least 40,000 times larger than direct emissions of vapor due to our combustion of fuels. A change in water vapor is not an “externally imposed” influence on climate. Instead, its abundance is internally generated by the climate system itself, primarily by the temperature of the surface. Furthermore, Lacis and coworkers showed in climate modeling studies [Lacis et al., 2010; 2013] that infrared absorption by CO2, CH4, and N2O is essential to sustain temperatures needed for water vapor to remain in the air in high enough amounts. With their model starting from pre-industrial steady state conditions, when the GHGs were removed from the model, the surface of the Earth turned into an ice ball in three decades, with most of the water vapor frozen out of the atmosphere and the planetary albedo increased. Other spatially heterogeneous and short-lived influences on climate such as aerosols and tropospheric ozone, have uncertain global magnitudes and are also not included here.
Direct climate forcing can be calculated by global climate models by changing only the GHG abundances, while leaving the rest of the modeled climate system such as clouds, surface albedo, temperature, precipitation, etc. unchanged, except for a small immediate temperature adjustment in the stratosphere [Ramaswamy et al., 2001; Etminan et al., 2016]. This creates an imbalance in the Earth’s heat budget, in which less infrared energy leaves the earth, while the incoming solar energy remains the same. The model calculates the imbalance “on the side”, not letting it influence the evolution of its climate. The most important consequence of our definition is that climate forcing by GHGs is well-known and caused by human activities alone. We do not want to entrain the uncertainties of the climate system and uncertain predictions of its response to human influence into our definition of climate forcing. Therefore, we prefer to use direct “Stratosphere Adjusted Radiative Forcing” (called SARF) over the current IPCC choice of using “Effective Radiative Forcings” (“ERF”, Forster et al., 2021). The latter includes responses in the troposphere, effects on clouds and circulation, as well as chemical and biological responses to changing GHGs. In other words, the ERF is more strongly model-dependent and therefore adds additional uncertainty.
The NOAA monitoring program provides high-precision measurements of the global abundance and distribution of GHGs that are used to calculate changes in radiative climate forcing.
Air samples are collected through the NOAA/GML global air sampling network, including a cooperative program for the carbon gases which provides samples from 40-50 global background air sites, including measurements at 5 degree latitude intervals from ship routes (see Figure 1). Beginning in 1980 global averages are calculated for CO2 based on a subset of sampling sites in the marine boundary layer (“MBL”) that give access to well-mixed clean air with minimal influence from local vegetation and from nearby human activities. Weekly data are used from the MBL sites to create a smoothed north-south latitude profile [Masarie and Tans, 1995] from which a global surface average is calculated (See: gml.noaa.gov/ccgg/trends/global.html). We started using the MBL for CH4 in 1983, and for N2O in 2001. Before those dates the air samples came from atmospheric sampling networks with fewer sites and from snow and ice.
For CO2, CH4, and N2O we used the ice and firn data from Law Dome in coastal Antarctica, which have the highest temporal resolution during the last two millennia [Etheridge et al., 1998; MacFarling Meure et al. 2006], and merged them with the MBL data to produce continuous records for global CO2, CH4, and N2O from the year 1 CE through 2022, taking into account time-dependent offsets between the concentrations measured over Antarctica and the global surface mean as observed in the MBL data. Data for the industrial gases is described in https://gml.noaa.gov/hats/flask/camp.html and in https://gml.noaa.gov/hats/US_emissiontracker/emission.html
The rate of CO2 build-up has been accelerating — while it averaged about 1.6 ppm per year (ppm/yr) in the 1980s and 1.5 ppm/yr in the 1990s, the growth rate increased to 1.9 ppm/yr in the decade 2000-2009, and to 2.4 ppm/yr during the last decade plus three (2010-2022). The CO2 increase during the year from 1 Jan 2022 through 31 Dec 2022 is 2.2 ppm (For more information, see https://gml.noaa.gov/ccgg/trends/gl_gr.html).
The growth rate of methane is less monotonic, it slowed from 1983 until 1999, consistent with its concentration approaching steady-state, assuming no trend in CH4 chemical lifetime. Superimposed on this decline is significant inter-annual variability in growth rates [Dlugokencky et al., 1998, 2003]. From 1999 to 2006, the atmospheric CH4 burden was nearly constant, but since 2007 globally averaged CH4 has started increasing again. Causes for the increase during 2007-2008 include warm temperatures in the Arctic in 2007 and increased precipitation in the tropics during 2007 and 2008 [Dlugokencky et al., 2009]. Isotopic measurements argue for a continuing increase of microbial emissions after 2008 (e.g., likely from wetlands, agriculture and waste disposal) [Schaefer et al., 2016; Nisbet et al., 2019; Schwietzke et al. 2016]. Since 2014, the global within-year increase (1 Jan to 31 Dec) in methane has become even larger, averaging 9.2 ± 2.2 ppb/yr through 2019 compared to an average annual increase of 5.7 ± 1.1 ppb/yr in the preceding seven years 2007-2013. The average annual increase for the first three years of this decade, 2020-2022, was 15.8 ppb yr-1, the highest value in our record (https://gml.noaa.gov/ccgg/trends_ch4/). The observed 13C/12C isotopic ratio trend of CH4 suggests that the major sources of these accelerations are microbial [Lan et al., 2021].
The atmospheric global burden of nitrous oxide continues to increase steadily over time, and is accelerating as well. During 2001-2009 the average rate of increase was 0.74 ± 0.15 ppb/yr, for 2010 through 2019 it was 0.92 ± 0.24 ppb yr-1 while during 2020 - 2022 the average annual increase has been 1.30 ppb yr-1 (https://gml.noaa.gov/ccgg/trends_n2o/). Its major source is fertilizers applied for agriculture.
The globally representative values of CO2, CH4, and N2O from 1800 to the present that will be used for the radiative forcing calculations shown below in Table 2 and in Figure 3 are downloadable here.
Radiative forcing from the sum total of observed changes of the industrial gases continues to increase. The abundances of gases included in the original Montreal Protocol on Substances that Deplete the Ozone Layer have declined, but other gases, some of them included in later amendments to the Montreal Protocol, are still increasing. The increased radiative forcing from the sum of the latter group more than compensates for the declines of the original group, so that climate forcing from all industrial gases continues to go up, albeit less rapidly than in the period 1960-1990.
The year 1750 CE is frequently chosen as the start of the industrial era, and as the reference year for gas abundances. The latter is an unfortunate choice because it leads to an anomalously low value for pre-industrial CO2 during the “Little Ice Age” from about 1600 to 1800 (Figure 2). Instead, we pick 1800 CE as the start of the industrial era, and for CO2 we take its average during 1-1800 CE of 279.34 ppm as the pre-industrial value. Both CH4 and N2O tended to be slightly lower during the years 1-1000 than during 1000-1800 (Figure 2), so that we take their averages over the latter period as our estimates for their respective pre-industrial values (717.0 ppb for CH4, 269.1 ppb for N2O). The use of our improved pre-industrial values guarantees that there is no net long-term climate forcing between 1 and 1800 CE, or between 1000 and 1800 CE, consistent with our definition.
In the first release in 2020 of the Power of Greenhouse Gases we used the IPCC [Ramaswamy et al., 2001] recommended expressions, but here we switch to the expressions of Etminan (2016) because the estimated climate forcing of CH4 has been greatly improved by including new spectroscopic information. CH4 has been found to absorb also in the near-infrared, and the inclusion increases the effect of CH4 by ~25%. In addition, Etminan et al. included an improved treatment of partially overlapping absorption bands of CO2 and N2O, but it does not result in noticeable changes in absorption in our range of interest. Also, in this new version of climate forcing we still use our own estimated pre-industrial values to convert from the global abundance changes of CO2, CH4, and N2O to instantaneous radiative forcing (see Table 1). We used the recommended radiative efficiency factors (Watt/m2 per ppb) in the IPCC 6th Assessment [Forster et al., 2021, and Smith et al, 2021] only for the industrial gases because there was an upward revision for CFC-11 and CFC-12. .
The empirical expressions in Table 1 are derived from atmospheric radiative transfer models and generally have an relative uncertainty of about 10% (with a 1 in 20 chance that the true value falls outside the ± 10% bounds). The uncertainties in the global average annual abundances of the three natural long-lived greenhouse gases are much smaller (< 0.04%). However, the uncertainty is larger for the preindustrial abundances of CO2, CH4, and N2O, primarily due to sparse sampling. Those uncertainties are less than 1 %.
|Trace Gas||Radiative Forcing, ΔF in units of (Watt m-2) (simplified expressions)|
|CO2||ΔF = [a1 (C − Co)2 + b1 |C − Co| + c1 (N − No)/2 + 5.36] × ln(C/Co)
a1 = −2.4×10-7 Wm-2ppm-1, b1= 7.2×10-4 Wm-2ppm-1, c1= −2.1×10-4 Wm-2ppm-1
|N2O||ΔF = [a2 (C + Co)/2 +b2 (N + No)/2 + c2 (M + No)/2 + 0.117] × (√N − √No)
a2 = −8.0×10-6 Wm-2ppb-1, b2= 4.2×10-6 Wm-2ppb-1, c2= −4.9×10-6 Wm-2ppb-1
ΔF = [a3 (M+Mo)/2 + b3 (N+No)/2 + 0.043] × (√M − √Mo)
a3 = −1.3×10-6 Wm-2ppb-1, b3= -8.2×10-6 Wm-2ppb-1
|Notes: “ln” is the natural logarithm function (base “e”), C is the CO2 global measured abundance (ppm), M and N are for CH4 and N2O respectively (ppb). The subscript “o” denotes the pre-industrial global abundance.|
If we would adopt the IPCC 6th Assessment [Forster et al., 2021] approach of including in our calculation a subset of climate responses in the troposphere, some effects on clouds and circulation, as well as some chemical and biological responses to changing GHGs, we obtain what is called “effective radiative forcings” (ERF). The ERF for CO2 adds 5%, CH4 forcing is lowered by 14%, N2O forcing is strengthened by 7% respectively compared to Etminan . ERFs are more strongly model-dependent and therefore add additional uncertainty.
Figure 3 portrays how much our emissions of greenhouse gases have changed Earth’s heat budget. We could express it as a fraction of total solar radiation absorbed by the atmosphere and Earth’s surface, which is the energy that runs our entire weather and climate system, driving the winds and ocean currents, evaporating water from the surface, fueling hurricanes and tornadoes, etc. In 1992 the fraction surpassed 1.00%, in 2021 it was 1.43%. The solar constant is taken as 1360.8 W m-2 [Kopp and Lean, 2011]. This is the energy intercepted by Earth’s disk (creating the earth’s shadow). The full area of Earth’s surface is four times larger than a one-sided disk, so that intercepted solar radiation is 1360.8/4 = 340.2 W m-2 averaged over the entire surface of Earth. Of that, 30.6% is reflected back to space (“Bond albedo”, D. Williams, 2019), leaving (1-0.306) times 340.2 = 236.1 W m-2 as the total absorbed solar radiation.
In the section below, “How much energy are we talking about”, we will give some examples of what the globally averaged heat retention of 3.40 Watt/m2, the value for 2022 that corresponds to 1.44% of absorbed solar radiation, can do.
|Enhancements of natural gases (W m-2)||Industrial gases||TOTAL|
|Year||CO2||CH4||N2O||sum of CO2 CH4 N2O||since mid-20th century||W m-2|
Climate forcing by CO2 during the years 1800-1900 was dominated by land use change, such as deforestation as well as the conversion of grass lands to crop lands, accompanied by the loss of organic carbon in soils. These processes were aided by increasing mechanization. Meanwhile, industrial production fueled by burning coal and oil continued to increase, and overtook land use as the main cause of CO2 emissions in the late 19th century, based on a mass balance calculation of emissions and observed changes in the atmosphere and oceans [Tans, 2009]. That same calculation showed that from the mid-20th century on, terrestrial ecosystems (including agriculture) became a net “sink” of carbon (transforming CO2 from the atmosphere to organic materials) when globally averaged, despite ongoing deforestation and fires that convert wood back to CO2. This could be partially due to fertilization of plant growth by higher CO2 and by deposition of reactive nitrogen, which includes fertilizer use. Today, emissions resulting from transportation, manufacturing, and heating/cooling have grown so large that the net global terrestrial sink of ~1 billion metric ton C per year (GtonC/yr) [Friedlingstein et al., 2019] is dwarfed by emissions from fossil fuel burning, at ~10 GtonC/yr. Note that 1 GtonC equals 3.67 Gton CO2. The latter includes the mass of the oxygens, and is often used in emissions statistics.
Table 3a and Figure 4 show that, although CO2 is responsible for 63.5% of the climate forcing by all greenhouse gases during 2022, its rate of increase during the last five years accounts for 76% of the total increase in forcing. Table 3b shows how little the use of ERF would change these proportions. From well-known chemistry of the carbonate system in the oceans we can estimate that, when the atmosphere and oceans are again in chemical equilibrium (after about 1000 years), ~83% of the excess CO2 resides in oceans and ~17% in the atmosphere. In the natural system, very slow calcium carbonate dissolution (which includes the damage to coral reefs) increases the alkalinity of the oceans allowing them to ingest the remaining 17% from the atmosphere, but that is expected to take between 3000 and 7000 years [Archer et al., 2009]. This is how long future generations will likely have to deal with the enhanced atmospheric CO2 unless ways will be found to pull the excess CO2 back out of the atmosphere and out of the oceans. Note that for this purely chemical estimate we assume that other factors, such as ocean circulation and ocean biology do not change, which is unlikely when the climate changes. If we would merely pull excess CO2 out of the atmosphere, the oceans would slowly emit enough of its excess carbon back into the atmosphere to re-establish chemical equilibrium. The same could happen with the terrestrial biosphere, if the main reason for their current global net uptake is fertilization by high CO2.
|2022||5-yr avg. 2018-2022 rate of increase|
|All industrial gases||0.376||0.110||0.0012||0.029|
Table 3b shows how little the use of ERF would change these proportions. The most striking change is that N2O would increase in importance compared to CH4.
|2022||5-yr avg. 2018-2022 rate of increase|
|All industrial gases||0.402||0.116||0.0010||0.023|
After the pause in the CH4 growth rate from 1999 to 2006, climate forcing by CH4 has increased by 0.060 W/m2 in 16 years. Over the last five years, the increase of climate forcing by CH4 has been 17% as large as the increase caused by CO2. CH4 emissions during the pre-industrial era were about 225 million metric ton per year (Mton/yr), whereas today they are ~645 Mton/yr. There have been proposals to remove CH4 from the atmosphere by (yet to be developed) industrial processes, in order to decrease its climate forcing. That would be foolish. Its abundance in the atmosphere is 200 times smaller than CO2. It is much simpler and less energy intensive to decrease its emissions, and let the atmosphere remove the CH4 by photochemical processes, ultimately powered by solar energy, with a time scale of about 9 years. While there are cost-effective ways to reduce CH4 emissions [UNEP, 2021], because today’s CO2 emissions will affect climate for millennia, reduction of CO2 emissions must remain the main focus in mitigating climate change.
Nitrous oxide is responsible for 32% of the current climate forcing that is due to methane, but the increase of forcing by N2O over the last five years has been 65% as large as the increase caused by methane. N2O emissions during the pre-industrial were about 17 Mton/yr, whereas today they are ~32 Mton/yr. Since fertilizer use for agriculture is the largest single cause of the N2O increase it seems likely that N2O’s contribution to climate forcing will continue to accelerate. One could argue that it is more important to decrease N2O emissions than CH4 emissions because N2O has an atmospheric residence time of 121 years compared to CH4’s 9 years. What that means for the full heating influence of emissions before photochemical reactions have removed all of them from the atmosphere is shown in Figure 4. Global heating is a very long-term problem, so that the full heating influence is what matters. It gets larger every year that the emissions are in the atmosphere, but the rate of increase slows down as the remaining amount of the emissions gets smaller. Figure 4 shows that the N2O emissions in 2022 of 32 Mton/year are equivalent to about 1060 Mton of CH4 emissions, which is ~ 64% larger than the current emissions of CH4 itself.
The 5-year average rate of increase of forcing by all industrial gases comprises only 2.9% of the increase of forcing by all gases because the abundances of several important gases controlled by the original Montreal Protocol decreased. Figure 5 shows the success of the Montreal protocol in halting further growth of the ozone hole, and also in turning around the trend of the industrial gases’ contribution to global heating. However, we still need to go further, the growth rate of all GHGs has to become negative, meaning that their abundance is being lowered.
When we multiply the 2022 average heating intensity of 3.404 Watt m-2 by the surface area of the Earth we have 1736 TeraWatt (TW). For comparison, a large electrical power plant produces 1 GigaWatt (GW) of electrical power. One TW equals the output of one thousand of such 1 GW power plants. So the heat retention by greenhouse gases in 2022 equals the electrical output of 1.74 million large power plants.
Global electricity production from all power plants in 2022 was 3.33 TW, which is only a small fraction (0.19%) of the heating produced by greenhouse gases, 1736 TW.
Let’s also compare 1736 TW with all the direct heat produced from all energy uses, the production of electricity including nuclear, transportation, heating/cooling of buildings, industrial processes, biofuels, waste. That total is 20.5 TW in 2022. Therefore, the excess heat retention by greenhouse gases in 2022 was 85 times larger than all the direct heat produced by humanity.
On a personal level, when we burn one gallon of gasoline, and if we would collect the CO2 coming out of tail pipe, it would fill a balloon with a diameter of about 7 feet. That CO2 will be capturing infrared heat in the atmosphere day in day out, for millennia to come. In the first 100 years the total amount of heat captured by that CO2 in the atmosphere (accounting for the portion that leaves the atmosphere by transferring into the oceans) is 120 times larger than the heat released by the original burning of the one gallon. After 500 years it is 350 times larger.
Now we will take a look at what 1736 TW could do in the climate system. If all of that energy were (hypothetically) directed into the Greenland ice cap, in one year it would heat up the ice, and then melt, 5.5% of the Greenland ice cap, which would raise global sea level by 41 cm, or 16”. How much ice is contained in the ice cap? The volume of Greenland’s ice cap could cover the U.S. states of Georgia, Tennessee, Missouri, Iowa, South and North Dakota, and all states to the north and east of that “line” with a 1 km (or 5/8 of one mile) thick layer of ice. When all of that ice melts it would raise global sea level by 7.4 m, or 24 ft.
Alternatively, the energy could go toward heating the upper layers of the oceans. In one year the upper 100 m of all oceans would warm by 0.38 °C (0.68 °F). If all of the energy could be aimed exclusively at the Great Lakes in North America (their water volume is ~22,600 km3), they would completely evaporate in 13 months.
One ZettaJoules (ZJ) of energy (see Figure 4) will heat the upper 1 m of all oceans by 0.7 °C (1.25 °F).
In the above examples, all of the heat was applied to just one purpose, either to melt ice, or to warm sea water, etc. To be sure, we have observed heating of sea water, loss of glaciers and ice sheets, sea level rise, and melting of sea ice plus warming of permafrost soils in the Arctic. So the excess heat gets spread over multiple things, although ~90% goes into the oceans. Furthermore, the Earth’s land surface has warmed from the 1960s and 1970s by 1.5 °C (2.7 °F), and the ocean surface by 0.71 °C (1.1 °F). A warmer surface emits more radiation, so that at the wavelengths where greenhouse gases do not absorb (the so-called “window regions”) a portion of the excess heat is sent to space and is thus not available to heat the surface. In addition, there are additional climate forcings that are directly influenced by human activities. Greatly increased loading of the atmosphere with small particles (aerosols), clearly caused by human activities, is providing a large counterforce. The aerosols scatter light directly, some of it to space, and, on average, enhance the brightness of clouds somewhat. The latter is a fast response of the climate system. It is currently still difficult to quantify the magnitude of these effects. Replacing forests by crop lands increases the reflectivity (albedo) of the surface, so that less sun light is absorbed. At the same time decreasing ice and snow cover (a climate response) leaves the surface darker so that more sun light is absorbed. As already mentioned, there is the known feedback from water vapor; a warmer surface evaporates more, and water vapor absorbs infrared radiation, thus enhancing the effect of all the long-lived greenhouse gases while also injecting more energy (latent heat) in the atmosphere from the condensation of water vapor. This extra energy in the atmosphere could contribute to the increase of extreme weather events that we are experiencing.
Osman et al.  found in a new global temperature reconstruction that the Last Glacial Maximum (LGM) was 7 °C (12.6 °F) colder than today. Earth transitioned from the last ice age to the warm interglacial period we are currently in (the Holocene) over a period of 6000 years, between 17000 yr BP (years Before Present) and 11000 yr BP, the start of the Holocene. Climate forcing from CO2, CH4, and N2O increased by 2.25 W m-2 [Hansen et al., 2023] during the transition. What could today’s 3.4 W m-2 do if it is sustained? The last time CO2 was as high as 420 ppm was during the mid-Pliocene, 4.1 to 4.4 million years ago, with global temperature 4 °C (7.2 °F) warmer than pre-industrial, and sea level 23 m (75 ft) higher (Dumitru, 2019). Today, the world is likely to warm up much faster than the natural transition from the LGM to the Holocene. A striking difference between today’s increase and the last de-glacial transition is the speed of the forcing change which averaged 0.42 W m-2 over the last decade, whereas it averaged 0.0038 W m-2 per decade during the deglacial transition, over a hundred times slower. Seen from a geological perspective, the current increase is an explosion.
Well established chemical oceanography tells us that for every 6 units of CO2 added to the atmosphere, chemical equilibrium between the atmosphere and oceans is restored when about 5 units have entered the oceans, and 1 unit remains in the atmosphere nearly permanently. This is a purely chemical effect because we are assuming that all other factors determining the equilibrium do not change, such as winds, temperature, ocean circulation and ocean biological species and ecosystems. They will very likely change substantially but we cannot make a prediction. Because CO2 is an acid it is expected that coral reefs (made of carbonate skeletons) will weaken or dissolve and that carbonate sediments on the continental slopes of the oceans will partially dissolve. Archer et al.  have estimated that such a process might take 3000-7000 years, after which a new chemical equilibrium is established with all 6 units of the added CO2 present as a component of dissolved ocean salts, and 0 units in the atmosphere when we neglect weathering of silicate rocks on time scales of 100,000 years and longer.
We cannot make a climate prediction for thousands of years of high CO2 but here are some plausible scenarios. Over thousands of years there is time for slow climate feedbacks to take place, such as tens of meters of sea level rise from partial melting of the ice sheets over Greenland and Antarctica. “Slow” does not mean that the process itself will be gradual, it could happen as a sequence of sudden events (for example, see Steffensen et al., 2008). The replacement of ice by bare rock and sea water in the Arctic and Antarctica, and of tundra by forests in the Arctic will darken the polar regions, lowering Earth’s albedo, so that a larger fraction of sunlight is absorbed instead of reflected. There is an enormous amount of organic carbon that accumulated over thousands of years in Arctic permafrost. The latter is now melting, so that the carbon compounds can be metabolized by living organisms leading to emissions of CH4 and CO2 that are out of our control.
There is no question that humanity is collectively responsible for the recent changes, but not in the sense that we have control over how the climate system responds to our interventions. We do not understand the climate system and ecosystems well enough. In recent years we have seen extreme rainfall events, as well as extreme droughts, massively destructive fires, temperatures over 100 °F (38 °C) for several weeks in a large region of northern Siberia as well as prolonged periods with temperatures in Southern Asia so high that they may make some regions uninhabitable. Mass migrations would be one predictable consequence. The frequency and severity of these events were not predicted as little as a decade ago. It does not bode well for how “manageable” global heating might be.
Can we engineer our way out of our global predicament? There have been many different proposals to manipulate Earth’s albedo, or to pull CO2 out of the atmosphere and then bury it underground. We have to remember that when internal combustion engines replaced horse and buggy for transportation (which was real progress!) nobody was aware that CO2 emissions could become a global threat to society. Yet, about 40% of the CO2 emissions of the 1911 Ford Model T are still in the atmosphere today. Are these proposals a manifestation of scientific and engineering hubris? Although perhaps a few of them might work somewhat, the scale of the required technical fixes is so large that unforeseen side effects will almost certainly become apparent. However, the mere promise of such potential fixes diminishes the urgency of the serious reductions of CO2 emissions (and other GHGs) that are required right now. The world is on track for yet more delays facilitated by false illusions such as carbon offsets, net zero emissions (in the future, conveniently), massive tree planting, and on and on. These illusions help enable governments to continue their huge subsidies [IMF, 2019] by not putting any price on environmental and public health costs for the global fossil fuel industry. The industry is actually spending $387 million per day until 2030 to further develop oil and gas fields [The Guardian Weekly, 2022].
Fortunately, it is well-known how we can replace the emissions of CO2 and other gases with renewable energy, and how to improve energy conservation and efficiency. Some “side effects” of aggressive programs to achieve it are quite predictable, such as greatly improved air quality (public health) in both urban and rural areas, a very large number of jobs that improve and preserve, instead of damage, our environment (also a public health benefit) and the jobs that are needed for the complete energy transformation that must happen. Conventional “costs” of such programs are actually the opposite, they are a benefit, namely real wages for real people. The greatest benefits will be for today’s young people, future generations, and the natural environment by avoiding the costliest disasters society’s current course would bestow upon them. It is our moral responsibility to make the right choices.