Introduction

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 instantaneous heat retention or instantaneous heating influence. We present here a measure that summarizes the increase of the instantaneous 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) 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.

Instantaneous 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. We prefer to use “instantaneous radiative forcing” 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.

Observations

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.

Figure 1. Global map of sampling sites

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 2021, 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 two (2010-2021). The CO₂ increase during the year from 1 Jan 2021 through 31 Dec 2021 is 2.5 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 next decade we are in started out with two successive record increases, 15.0 ppb during 2020 and 18.3 ppb in 2021 (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 ppb/yr, from 2010 to 2009 it was 0.97 ppb/yr, while in 2020-2021 it was 1.34 and 1.28 ppb/yr respectively (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.

Radiative Forcing Calculations

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 279.4 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 (716.6 ppb for CH₄, 269.4 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.

Figure 2. Radiative climate forcing by greenhouse gases during the last two millennia. Forcing by CO₂ averaged over the years 1 to 1800 CE is zero. Likewise, for CH₄ and N₂O the averaged forcing over the years 1000-1800 is zero. The red line shows the sum of climate forcing by 29 industrial gases . Click on image to view full size figure.

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] for the industrial gases.

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 abundances of the long-lived greenhouse gases are much smaller (<0.1% for CO₂, and CH₄ <0.5%).

When we 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 [2016]. ERFs are more strongly model-dependent and therefore add additional uncertainty.

2021 Results

Figure 3. Radiative climate forcing by all GHGs since pre-industrial times through 2021. The horizontal arrow shows where climate forcing surpassed 1% of Earth’s weather and climate engine.

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, 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 3.39 Watt/m², the value for 2021 that corresponds to 1.43% of absorbed solar radiation, can do.

Download the table data

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 (removing CO₂ from the atmosphere) 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 64% of the climate forcing by all greenhouse gases during 2021, its rate of increase during the last five years accounts for 78% 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 loss of 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.

After the pause in the CH₄ growth rate from 1999 to 2006, climate forcing by CH₄ has increased by 0.052 W/m² in 15 years. Over the last five years, the increase of instantaneous climate forcing by CH₄ has been 15% as large as the increase caused by CO₂. 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. 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 31% of the current climate forcing that is due to methane, but the increase of forcing by N₂O over the last five years has been 70% as large as the increase caused by methane. 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.

The 5-year average rate of increase of forcing by all industrial gases comprises only 3.2% of the increase of forcing by all gases because the abundances of several important gases controlled by the original Montreal Protocol decreased.

Figure 4. . In order to bring out multi-year trends, the annual increases have been smoothed as a moving average with an approximate width of 10 years. The last five years of the smoother are preliminary and are not shown. Instead, direct averages of the last five years are shown as open diamond symbols.

How much energy are we talking about?

When we multiply the 2021 average heating intensity of 3.387 Watt m^-2 by the surface area of the Earth we have 1728 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 2021 equals the electrical output of 1.73 million large power plants. Global electricity production from all power plants in 2021 was 3.06 TW [BP Statistical Review, 2021]

Let’s also compare 1728 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 20.3 TW in 2021 [BP. Stat. rev. 2021]. Therefore, the excess heat retention by greenhouse gases in 2021 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 1728 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, 5.2% of the Greenland ice cap, which would raise global sea level by 39 cm, or 15”. 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 0.36 °C (0.65 °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 14 months.

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 1.4 °C (2.5 °F), and the ocean surface by 0.70 °C (1.3 °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 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.

Earth transitioned from the last ice age in about 6000 years to the warm interglacial period (the Holocene) we experience today, between 17500 yr BP (years before present) and 11500 yr BP, the start of the Holocene. The increase in GHG forcing was 2.5 W/m² and global average temperature increased by 2.5 °C (4.0 °F) between these dates, with GHG forcing leading temperature [Shakun et al., 2012]. What will today’s 3.4 W/m² do? A striking difference between today’s increase and the last de-glacial transition is the speed, which averaged 0.41 W/m² per decade most recently, whereas it averaged 0.0042 W/m² per decade during the deglacial. Seen from a geological perspective, the current increase is an explosion.

We are committing Earth, and ourselves, to climate chaos for thousands of years

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. [2009] 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 if 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.

Concluding comments

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? 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 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 subsidies [IMF, 2019] for the global fossil fuel industry, which 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, instead of damage, our environment (also a public health benefit) and 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 salaries 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.

References

• Archer, D. et al. (2009),Atmospheric life time of fossil fuel carbon dioxide, Ann. Rev. Earth Planet. Sci. 37, 117-134, doi: 10.1146/annurev.earth.031208.100206
• BP Statistical Review of World Energy, 68th edition, (2019)
• Dlugokencky, E. J., K. A. Masarie, P. M. Lang, and P. P. Tans, (1998) Continuing decline in the growth rate of the atmospheric methane burden, Nature, 393, 447-450.
• Dlugokencky, E. J., S. Houweling, L. Bruhwiler, K. A. Masarie, P. M. Lang, J. B. Miller, and P. P. Tans, (2003), Atmospheric methane levels off: Temporary pause or a new steady-state?, Geophys. Res. Lett., 19, doi:10.1029/2003GL018126.
• Dlugokencky, E.J., L. Bruhwiler, J.W.C. White, L.K. Emmons, P.C. Novelli, S.A. Montzka, K.A. Masarie, P.M. Lang, A.M. Crotwell1, J.B. Miller, and L.V. Gatti, (2009), Observational constraints on recent increases in the atmospheric CH4 burden, Geophys. Res. Lett., 36, L18803, doi:10.1029/2009GL039780
• Etheridge, D.M., L.P. Steele, R.L. Langenfelds, and R.J. Francey, (1996), Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn, J. Geophys. Res. 101, 4115–4128.
• Etminan, M., G. Myhre, E.J. Highwood, and K.P. Shine, (2016), Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing, Geophys. Res. Lett 43, 12,614-12,623, doi:10.1002/2016GL071930.
• Forster et al. (2021), The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. Chapter 7 in Climate Change 2021: The Physical Science Basis, Cambridge Univ. Press, Cambridge UK and New York, NY USA
• Friedlingstein, P. et al. (2019), Global carbon budget 2019, Earth Sys. Sci. Data 11, 1783-1838, doi: 10.5194/essd-11-1783-2019
• The Guardian Weekly. D. Carrington and M. Taylor, What lies beneath, The vast fossil-fuel projects that could wreck international efforts to limit global heating, 20 May 2022.
• IMF, International Monetary Fund. D. Coady, I. Parry, N.-P. Le, and B. Shang, Global Fossil Fuel Subsidies Remain Large: An Update Based on Country-Level Estimates, IMF Working Paper WP/19/89, 2019.
• IPCC5 (2014), Climate Change 2013: The Physical Science Basis. Cambridge Univ. Press, Cambridge UK and New York, NY USA.
• Kopp, G. and J. Lean (2011), A new, lower value for total solar irradiance: Evidence and climate significance, Geophys. Res. Lett. 38, L01706
• Lacis, A., G.Schmidt, D. Rind, and R. Ruedy (2010), Atmospheric CO2: Principal control knob governing Earth temperature, Science 330, 356-359. doi: 10.1126/science.1190653
• Lacis, A. A., Hansen, J. E., Russell, G. L., Oinas, V. & Jonas, J. (2013). The role of long-lived greenhouse gases as principal LW control knob that governs the global surface temperature for past and future climate change. Tellus B, 65, 19734, doi:10.3402/tellusb.v65i0.19734
• Lan, X., S. Basu, S. Schwietzke, L. Bruhwiler, E. Dlugokencky, S. Michel, O.A. Sherwood, P. Tans, K. Thoning, G. Etiope, Q. Zhuang, L. Liu, Y. Oh, J.B. Miller, G. Petron, B. Vaughn, and M. Crippa, 2021, Improved constraints on global methane emissions and sinks using δ13C-CH4, Global Biogeochemical Cycles 35, e2021GB007000, doi.org/2021GB007000.
• Lan, X., Nisbet, E. G., Dlugokencky, E. J., & Michel, S. E. (2021). What do we know about the global methane budget? Results from four decades of atmospheric CH4 observations and the way forward . Philosophical Transactions of the Royal Society A, 379(2210), 20200440. https://doi.org/10.1098/rsta.2020.0440.
• Langenfelds, R. L., P. J. Fraser, R. J. Francey, L. P. Steele, L. W. Porter and C. E. Allison, (1996), The Cape Grim Air Archive: The First Seventeen Years, 1978 – 1995, Baseline Atmospheric Program (Australia) 1994-95, pp. 53-70.
• MacFarling Meure,C., D.Etheridge, C.Trudinger, P. Steele, R. Langenfelds, T. van Ommen, A. Smith, and J. Elkins (2006), Law Dome CO₂, CH₄, and N₂O ice core records extended to 2000 years BP, Geophys. Res. Lett. 33, L14810, doi: 10.1029/2006GL026152.
• Masarie, K.A. and P.P. Tans, Extension and integration of atmospheric carbon dioxide data into a globally consistent measurement record, J. Geophys. Res. 100, 11593-11610, 1995.
• Montzka, S. A., et al., (2018), A persistent and unexpected increase in global emissions of ozone-depleting CFC-11 Nature, 557, 413-417. https://doi.org/10.1038/s41586-018-0106-2
• Nisbet, E. G., Manning, M. R., Dlugokencky, E. J., Fisher, R. E., Lowry, D., Michel, S. E., et al. (2019) Very strong atmospheric methane growth in the 4 years 2014–2017: Implications for the Paris Agreement. Global Biogeochemical Cycles, 33, 318–342. https://doi.org/10.1029/2018GB006009.
• Ramaswamy et al., (2001), Radiative Forcing of Climate Change, Chapter 6 in Climate Change 2001: The Scientific Basis. Cambridge Univ. Press, Cambridge UK and New York, NY USA.
• Schaefer, H., et al. (2016), A 21st century shift from fossil-fuel to biogenic methane emissions indicated by ¹³CH₄, Science, 352, 80–84.
• Schwietzke, Stefan, Owen Sherwood, Lori Bruhwiler, John B. Miller, Giuseppe Etiope, Ed Dlugokencky, Sylvia Englund, Victoria Arling, Bruce Vaughn, James W.C. White, and Pieter Tans (2016), Upward revision of global fossil fuel methane emissions based on isotopic data, Nature 538, 88-91, doi: 10.1038/nature19797
• Shakun, J.D., et al. (2012), Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Nature 484, 49-54.
• Smith et al. (2021), The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity Supplementary Material, Table 7.SM.7 in Climate Change 2021: The Physical Science Basis, Cambridge Univ. Press, Cambridge UK and New York, NY USA
Steffensen, J.P., et al. (2008), High-Resolution Greenland ice core data show abrupt climate change happens in few years, Science 321, 680-684.
• Tans, P. (2009), An accounting of the observed increase in oceanic and atmospheric CO2 and an outlook for the future, Oceanography 22, no. 4, 26-35.
• UNEP, United Nations Environment Programme and Climate and Clean Air Coalition (2021). Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions.
• Williams, D., Earth Fact Sheet, NASA, accessed Aug. 2019.