Young people's burden: requirement of negative CO 2 emissions

The period since 1970 is the time with high growth rate of GHG climate forcing, maintained at approximately +0.4 W m-2 decade-1.

“The period since 1970 is the time with high growth rate of GHG climate forcing, which has been maintained at approximately +0.4 W m -2 decade -1”

The growth rate of greenhouse gas climate forcing has accelerated markedly in the past decade.

“we show that the growth rate of greenhouse gas climate forcing has accelerated markedly in the past decade”

Today, global fossil fuel emissions continue at rates that make these targets increasingly improbable.

“Today, global fossil fuel emissions continue at rates that make these targets increasingly improbable”

The present global warming rate, based on a linear fit for 1970-present, is +0.18°C per decade.

“The present global warming rate, based on a linear fit for 1970-present (dashed line in Fig. 2b) is +0.18 • C per decade.”

Recent warming is sufficient to remove the illusion of a hiatus of global surface warming after the 1997-1998 El Niño.

“the recent warming is sufficient to remove the illusion of a hiatus of global surface warming after the 1997-1998 El Niño”

Earth is substantially out of energy balance.

“causing Earth to be substantially out of energy balance (Cheng et al., 2017)”

• cites Cheng et al., 2017Not checked yet

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Global temperature in 2014-2016 reached successive record high levels for the period of instrumental data.

“Global temperature in 2014-2016 reached successive record high levels for the period of instrumental data”

Global temperature excluding short-term variability now exceeds +1°C relative to the 1880-1920 mean.

“global temperature excluding short-term variability now exceeds +1 • C relative to the 1880-1920 mean”

Global temperature has risen well out of the Holocene range.

“global temperature has risen well out of the Holocene range”

Global temperature has risen above the centennially smoothed Holocene range.

“global temperature has risen above the centennially smoothed Holocene range.”

The modern trend line of global temperature crossed the early Holocene smoothed temperature maximum of +0.5°C in about 1985.

“the modern trend line of global temperature crossed the early Holocene (smoothed) temperature maximum (+0.5 • C) in about 1985.”

• AssumptionThe authors state this as an assumption: The 1880-1920 mean temperature is 0.1°C warmer than the Little Ice Age minimum.

Our GHG forcings yield a CO2 forcing 6.7% larger than the central IPCC estimate for the CO2 change from 1750 to 2011.

“Our GHG forcings, calculated with formulae of Hansen et al. (2000), yield a CO 2 forcing 6.7 % larger than the central IPCC estimate (Table 8.2 of Myhre et al., 2013) for the CO 2 change from 1750 to 2011.”

• cites Hansen et al., 2000Not checked yet

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• cites Myhre et al., 2013Not checked yet

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Addition of the smaller terms raises the imbalance to +0.75 ± 0.25 W m-2 averaged over the solar cycle.

“Addition of the smaller terms raises the imbalance to +0.75 ± 0.25 W m -2 averaged over the solar cycle”

The updated analysis with data for 2005-2015 finds a decade-average 0.7 W m-2 heat uptake in the upper 2000 m of the ocean.

“Here we update the von Schuckmann and Le Traon (2011) analysis with data for 2005-2015 (Fig. 5) finding a decade-average 0.7 W m -2 heat uptake in the upper 2000 m of the ocean.”

• cites von Schuckmann and Le Traon, 2011Not checked yet

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Five additional cases including CO2 removal achieve atmospheric CO2 targets of either 350 or 450 ppm in 2100.

“Five additional cases including CO 2 removal (Fig. 10b) achieve atmospheric CO 2 targets of either 350 or 450 ppm in 2100”

Atmospheric CH4 growth resumed in the past decade.

“but growth resumed in the past decade (Fig. 7).”

Growth of atmospheric CO2 is about half of fossil fuel CO2 emissions.

“Growth of atmospheric CO 2 is about half of fossil fuel CO 2 emissions as discussed in Appendix A8 and illustrated in Fig. A8.”

CH4 growth averaged 10 ppb yr-1 in 2014-2016, almost as fast as in the 1980s.

“CH 4 growth averaged 10 ppb yr -1 in 2014-2016, almost as fast as in the 1980s.”

The amount of CO2 that must be extracted from the system exceeds the difference between the atmospheric amount without extraction and the target amount.

“The amount of CO 2 that must be extracted from the system exceeds the difference between the atmospheric amount without extraction and the target amount”

Atmospheric CH4 stopped growing between 1998 and 2006, indicating that its sources were nearly in balance with the atmospheric oxidation sink.

“Atmospheric CH 4 stopped growing between 1998 and 2006, indicating that its sources were nearly in balance with the atmospheric oxidation sink”

Closing the gap (0.01 W m-2) between actual growth of GHG climate forcing in 2015 and RCP2.6, with CO2 alone, would require extraction from the atmosphere of more than 0.7 ppm of CO2 or 1.5 PgC due to the emissions gap of a single year (2015).

“Closing the gap (0.01 W m -2 ) between actual growth of GHG climate forcing in 2015 and RCP2.6 (Fig. 8b), with CO 2 alone, would require extraction from the atmosphere of more than 0.7 ppm of CO 2 or 1.5 PgC due to the emissions gap of a single year (2015).”

Annual increase in atmospheric CO2, averaged over a few years, grew from less than 1 ppm yr-1 50 years ago to more than 2 ppm yr-1 today.

“Annual increase in atmospheric CO 2 , averaged over a few years, grew from less than 1 ppm yr -1 50 years ago to more than 2 ppm yr -1 today (Fig. 6)”

Already in 2015 there is a huge gap between reality and RCP2.6.

“Already in 2015 there is a huge gap between reality and RCP2.6.”

Continued high emissions, between constant emissions and +2 % yr-1, would require additional extraction of 595-1530 PgC at a cost of USD 89-535 trillion or 1.1-6.7 trillion per year over 80 years.

“In contrast, continued high emissions, between constant emissions and +2 % yr -1 , would require additional extraction of 595-1530 PgC (Fig. 10b) at a cost of USD 89-535 trillion or 1.1-6.7 trillion per year over 80 years.”

• AssumptionThe authors state this as an assumption: 100 PgC can be stored via improved agricultural and forestry practices.

Reducing global emissions at a rate of 3 % yr-1 (or more steeply) maintains global warming at less than 1.5°C above preindustrial temperature.

“Note that reducing global emissions at a rate of 3 % yr -1 (or more steeply) maintains global warming at less than 1.5 • C above preindustrial temperature.”

Even the cases with annual emission reductions -6 and -3 % yr-1 required small extractions to compensate for back-flux of CO2 from the ocean that accumulated there historically.

“Even the cases with annual emission reductions -6 and -3 % yr -1 required small extractions to compensate for back-flux of CO 2 from the ocean that accumulated there historically.”

Scenarios ranging from constant emissions to +2 % yr-1 emissions growth can be made to yield 450 ppm in 2100 via extraction of 339-1270 PgC from the atmosphere.

“Scenarios ranging from constant emissions to +2 % yr -1 emissions growth can be made to yield 450 ppm in 2100 via extraction of 339-1270 PgC from the atmosphere (Fig. 10b).”

If emissions grow 2 % yr-1, warming reaches ∼4°C by 2100.

“If emissions grow 2 % yr -1 , modestly slower than the 2.6 % yr -1 growth of 2000-2015, warming reaches ∼ 4 • C by 2100.”

Taking 50 years as the average time for tropical, temperate and boreal trees to experience sink saturation yields 55 PgC as the potential storage in forests this century.

“Taking 50 years as the average time for tropical, temperate and boreal trees to experience sink saturation yields 55 PgC as the potential storage in forests this century.”

Warming is about 2°C if emissions are constant until 2100.

“Warming is about 2 • C if emissions are constant until 2100.”

Recent warming removes the illusion of a hiatus of global warming since the 1997-1998 El Niño.

“Recent warming removes the illusion of a hiatus of global warming since the 1997-1998 El Niño (Fig. 2).”

Rapid emission reductions substantially reduce the temperature rise in 2100.

“As expected, the rapid emission reductions substantially reduce the temperature rise in 2100.”

20% of current global energy use exceeds the sum of present hydropower (6.8%), nuclear (4.4%), wind (1.4%), solar (0.4%), and other renewable energies (0.9%).

“Note that 20 % of current global energy use is a huge amount (Fig. 13), exceeding the sum of present hydropower (6.8 %), nuclear (4.4 %), wind (1.4 %), solar (0.4 %), and other renewable energies (0.9 %).”

Fossil fuel emission reduction of 3 % yr-1 beginning in 2021 yields maximum global warming ∼1.5°C for climate sensitivity 3°C for 2×CO2, with neither CO2 removal nor geoengineering.

“Fig.12 shows that fossil fuel emission reduction of 3 % yr -1 beginning in 2021 yields maximum global warming ∼ 1.5 • C for climate sensitivity 3 • C for 2 × CO 2 , with neither CO 2 removal nor geoengineering.”

Resurgence of CH4 growth is partly responsible for growth of GHG climate forcing.

“Resurgence of CH 4 growth is partly responsible, but CO 2 is by far the largest contributor to growth of GHG climate forcing (Fig. 8).”

Growth of airborne CO2 is about half of fossil fuel CO2 emissions, with the remaining portion being net uptake by the ocean and biosphere.

“Growth of airborne CO 2 is about half of fossil fuel CO 2 emissions (Fig. A8), the remaining portion of emissions being the net uptake by the ocean and biosphere (Ciais et al., 2013).”

• cites Ciais et al., 2013Not checked yet

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Even as fossil fuel emissions increased by a factor of 4 over the past half century, the ocean and biosphere have continued to take up about half of the emissions.

“The data reveal that, even as fossil fuel emissions have increased by a factor of 4 over the past half century, the ocean and biosphere have continued to take up about half of the emissions (Fig. A8, right-hand scale).”

Required CO2 extraction is at least about 150 PgC in the updated scenarios.

“Required extraction is at least ∼ 150 PgC in our updated scenarios, which incorporate growth of emissions in the past 4 years and assume that emissions will continue at approximately current levels until a global program of emission reductions begins in 4 years (in 2021 relative to 2020; see Figs. 9 and 10 for reduction rates).”

• AssumptionThe authors state this as an assumption: Emissions will continue at approximately current levels until a global program of emission reductions begins in 4 years (in 2021 relative to 2020).

Using USD 2500 per kilowatt as reactor cost and assuming 85% capacity factor yields a cost of USD 10 trillion to produce 20% of present global energy use (12.9 Gtoe).

“Using USD 2500 per kilowatt as reactor cost and assuming 85 % capacity factor (percent uptime for reactors) yields a cost of USD 10 trillion to produce 20 % of present global energy use (12.9 Gtoe).”

Earth is now out of energy balance.

“Also, Earth is now out of energy balance, implying that more warming will occur, even if atmospheric GHG amounts are stabilized at today's level.”

For cases with rapidly declining emissions of -3% and -6% yr-1, TCRE is 1.31 and 1.25°C per 1000 PgC at 2100, respectively.

“Our two cases with rapidly declining emissions never achieve 1000 PgC emissions, but TCRE can still be computed using the IPCC formulae, yielding TCRE = 1.31 and 1.25 • C per 1000 PgC at 2100 for the cases of -3 and -6 % yr -1 respective emission reductions.”

CO2 is by far the largest contributor to growth of GHG climate forcing.

“CO 2 is by far the largest contributor to growth of GHG climate forcing (Fig. 8).”

The rapid warming of the past century has brought temperature above the range in the prior 11,700 years of the Holocene.

“As we will show, the rapid warming of the past century has brought temperature above the range in the prior 11 700 years of the Holocene.”

Growth of international ship and air emissions largely offset the emission reductions from the Kyoto Protocol and the Soviet collapse.

“However, growth of international ship and air emissions (Fig. 1b) largely offset these reductions and the growth rate of global emissions actually accelerated from 1.5 % yr -1 in 1973-2000 to ∼ 2.5 % yr -1 after 2000 (Fig. A1).”

Ocean heat uptake in the upper 2 km of ocean during 2005-2015 was 0.5-0.7 W m-2 averaged over the global (ocean + land) area.

“Ocean heat uptake in the upper 2 km of ocean during 11 years from 2005 to 2015 using analysis method of von Schuckmann and LeTraon (2011).Heat uptake in W m -2 (0.5 and 0.7) refers to global (ocean + land) area -i.e., it is the contribution of the upper ocean to the heat uptake averaged over the entire planet.”

• cites von Schuckmann and LeTraon (2011)Not checked yet

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Observed atmospheric CH4 amount is diverging on the high side of the optimistic scenarios.

“Observed atmospheric CH 4 amount (Fig. A13a) is diverging on the high side of these optimistic scenarios.”

Methane is the largest climate forcing other than CO2.

“Methane (CH 4 ) is the largest climate forcing other than CO 2 (Fig. 4).”

CO2 growth rate is highly correlated with global temperature, with the CO2 change lagging global temperature change by 7-8 months.

“(b) CO 2 growth rate is highly correlated with global temperature, with the CO 2 change lagging global temperature change by 7-8 months.”

As yet there is little evidence for a substantial increase in CH4 emissions from hydrates or permafrost either now or over the last 1,000,000 years.

“There is also concern that global warming will lead to a massive increase in CH 4 emissions from methane hydrates and permafrost (O'Connor et al., 2010), but as yet there is little evidence for a substantial increase in emissions from hydrates or permafrost either now or over the last 1 000 000 years (Berchet et al., 2016;Warwick et al., 2016;Quiquet et al., 2015).”

• cites Berchet et al., 2016Not checked yet

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• cites Warwick et al., 2016Not checked yet

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• cites Quiquet et al., 2015Not checked yet

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The growth rate of global emissions accelerated from 1.5 % yr-1 in 1973-2000 to approximately 2.5 % yr-1 after 2000.

“However, growth of international ship and air emissions (Fig. 1b) largely offset these reductions and the growth rate of global emissions actually accelerated from 1.5 % yr -1 in 1973-2000 to ∼ 2.5 % yr -1 after 2000 (Fig. A1).”

Observed N2O growth is exceeding all scenarios.

“Observed N 2 O growth is exceeding all scenarios (Fig. A13b).”

The simple climate model used has a transient climate sensitivity near the midpoint of the sensitivity of models illustrated in Fig. 10.20a of IPCC (2013).

“we show (Appendix A2) has a transient climate sensitivity near the midpoint of the sensitivity of models illustrated in Fig. 10.20a of IPCC (2013)”

• cites IPCC, 2013Not checked yet

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The Eemian, which lasted from about 130,000 to 115,000 years ago, was moderately warmer than the Holocene.

“The Eemian, which lasted from about 130 000 to 115 000 years ago, was moderately warmer than the Holocene”

The temperature in an anomalous single year such as 2016 is not an appropriate comparison to the Holocene.

“the temperature in an anomalous single year such as 2016 is not an appropriate comparison.”

Holocene climate stability allowed sea level to be stable for the past several millennia as civilizations developed.

“Holocene climate stability allowed sea level to be stable for the past several millennia (Kopp et al., 2016) as civilizations developed.”

• cites Kopp et al., 2016Not checked yet

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The 350 ppm CO2 target is moderately stricter than the 1.5°C warming target.

“The 350 ppm CO 2 target is moderately stricter than the 1.5 • C warming target.”

Targets for limiting global warming now require negative emissions, i.e., extraction of CO2 from the air.

“Such targets now require”

The current GISTEMP analysis can be used for temperature change since 1880.

“we can use the current GISTEMP analysis”

Earth is now as warm as it was during the prior Eemian interglacial period.

“Earth is now as warm as it was during the prior (Eemian) interglacial period”

The early Holocene maximum in the Marcott et al. data reaches +0.5°C relative to the 1880-1920 mean of modern data.

“The early Holocene maximum in the Marcott et al. (2013) data thus reaches +0.5 • C relative to the 1880-1920 mean of modern data.”

• cites Marcott et al., 2013Not checked yet

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• AssumptionThe authors state this as an assumption: The 1880-1920 mean temperature is 0.1°C warmer than the Little Ice Age minimum.

There is increasing evidence that some slow feedbacks can be triggered within decades.

“There is increasing evidence that some slow feedbacks can be triggered within decades”

Several analyses of temperature change since 1880 are in close agreement.

“The several analyses of temperature change since 1880 are in close agreement (Hartmann et al., 2013).”

• cites Hartmann et al., 2013Not checked yet

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The goal to keep global warming less than 1.5°C likely requires negative net CO2 emissions later this century if high global emissions continue in the near term.

“the goal to keep global warming less than 1.5 • C likely requires negative net CO 2 emissions later this century if high global emissions continue in the near term”

• cites Fuss et al., 2014Not checked yet

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• cites Anderson, 2015Not checked yet

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• cites Rogelj et al., 2015Not checked yet

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Limiting the period and magnitude of temperature excursion above the Holocene range is crucial to avoid strong stimulation of slow feedbacks.

“Limiting the period and magnitude of temperature excursion above the Holocene range is crucial to avoid strong stimulation of slow feedbacks.”

Unless and until there is substantial change in the rate of added climate forcing, the underlying warming is expected to continue at a comparable rate.

“unless and until there is substantial change in the rate of added climate forcing we expect the underlying warming to continue at a comparable rate.”

Comparisons of current global temperature with the earlier Holocene must bear in mind the centennial smoothing inherent in the Holocene data.

“comparisons of current global temperature with the earlier Holocene must bear in mind the centennial smoothing inherent in the Holocene data”

• cites Marcott et al., 2013Not checked yet

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Sea level reached 6-9 m higher than today during the Eemian.

“when sea level reached 6-9 m higher than today”

• Also stated in: § 2 Global temperature change · plain

  “The Eemian, which lasted from about 130 000 to 115 000 years ago, was moderately warmer than the Holocene and experienced sea level rise to heights 6-9 m (20-30 ft) greater than today.”

• Also stated in: § 2.2 Temperature during current and prior interglacial periods · plain

  “Sea level reached 6-9 m higher than today during the Eemian (Dutton et al., 2015)”

• cites Dutton et al., 2015Not checked yet

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Correction for seasonality bias in proxy temperature reconstruction will tend to push early Holocene temperatures lower, increasing the gap between today's temperature and early Holocene temperature.

“Correction for this bias will tend to push early Holocene temperatures lower, increasing the gap between today's temperature and early Holocene temperature”

• cites Liu et al., 2014Not checked yet

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• cites Marcott and Shakun, 2015Not checked yet

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The Marcott et al. Holocene record shows a decline of 0.6°C from early Holocene maximum temperature to a Little Ice Age minimum in the early 1800s.

“This record shows a decline of 0.6 • C from early Holocene maximum temperature to a”

• cites Marcott et al., 2013Not checked yet

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Continued high fossil fuel emissions today place a burden on young people to undertake massive technological CO2 extraction if they are to limit climate change and its consequences.

“continued high fossil fuel emissions today place a burden on young people to undertake massive technological CO 2 extraction if they are to limit climate change and its consequences.”

There is now a danger that temperature rises so far above the Holocene range that slow feedbacks are activated to a degree that continuing climate change will be out of humanity's control.

“there is now a danger that temperature rises so far above the Holocene range that slow feedbacks are activated to a degree that continuing climate change will be out of humanity's control.”

The near-planetary energy balance anticipated at 350 ppm CO2 implies a global temperature close to recent values, about +1°C relative to preindustrial.

“The near-planetary energy balance anticipated at 350 ppm CO 2 implies a global temperature close to recent values, i.e., about +1 • C relative to preindustrial.”

GHGs absorb Earth's infrared radiation, warming Earth's surface by reducing heat radiation to space.

“GHGs absorb Earth's infrared (heat) radiation, thus serving as a”

The most threatening slow feedback likely is ice sheet melt and consequent significant sea level rise, as occurred in the Eemian.

“The most threatening slow feedback likely is ice sheet melt and consequent significant sea level rise, as occurred in the Eemian”

The ratio of land SAT change to SST change is reduced only to about 1.8 after 1000 years in climate models.

“The ratio of land SAT change to SST change is reduced only to ∼ 1.8 after 1000 years in climate models”

• cites Fig. A6, Appendix A6Not checked yet

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The temperature in 2016 based on the 1970-present linear trend, at least 1°C relative to 1880-1920, provides a meaningful comparison.

“the temperature in 2016 based on the 1970-present linear trend (at least 1 • C relative to the 1880-1920 mean) does provide a meaningful comparison.”

The Northern Hemisphere did not become cool enough to reestablish ice sheets on North America or Eurasia.

“the Northern Hemisphere did not become cool enough to reestablish ice sheets on North America or Eurasia.”

Earth's energy imbalance assures that this temperature will continue to rise unless and until the global climate forcing begins to decline.

“Earth's energy imbalance assures that this temperature will continue to rise unless and until the global climate forcing begins to decline.”

Dutton et al. conclude that the best estimate for Eemian temperature is +1°C relative to preindustrial.

“Dutton et al. (2015) conclude that the best estimate for Eemian temperature is +1 • C relative to preindustrial.”

• cites Dutton et al., 2015Not checked yet

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A danger of 1.5 or 2°C targets is that they are far above the Holocene temperature range.

“A danger of 1.5 or 2 • C targets is that they are far above the Holocene temperature range.”

The CH4 forcing includes its indirect effects, as increasing atmospheric CH4 causes tropospheric ozone and stratospheric water vapor to increase.

“The CH 4 forcing includes its indirect effects, as increasing atmospheric CH 4 causes tropospheric ozone (O 3 ) and stratospheric water vapor to increase”

• cites Myhre et al., 2013Not checked yet

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If 1.5 or 2°C temperature levels are allowed to long exist they will spur slow amplifying feedbacks, which have potential to run out of humanity's control.

“If such temperature levels are allowed to long exist they will spur”

• cites Hansen et al., 2013bNot checked yet

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• cites Rohling et al., 2013Not checked yet

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• cites Masson-Delmotte et al., 2013Not checked yet

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Mean temperature over the next several decades will not be lower than 1°C.

“we know that mean temperature over the next several decades will not be lower than 1 • C.”

The McKay et al. and Hoffman et al. data are equivalent to a global Eemian temperature of just under +1°C relative to the Holocene.

“the McKay et al. (2011) and Hoffman et al. (2017) data are equivalent to a global Eemian temperature of just under +1 • C relative to the Holocene.”

Models give more confidence to the lower part of the McKay et al. Eemian SST range.

“models, as described by Masson-Delmotte et al. (2013), give more confidence to the lower part of that range.”

• cites Masson-Delmotte et al., 2013Not checked yet

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The dominant human-caused drivers of climate change are changes of atmospheric GHGs and aerosols.

“The dominant human-caused drivers of climate change are changes of atmospheric GHGs and aerosols”

Because land covers about 30% of the globe, SST warmings should be multiplied by 1.24-1.3 to estimate global temperature change.

“because land covers ∼ 30 % of the globe, SST warmings should be multiplied by 1.24-1.3 to estimate global temperature change.”

The conclusion that the modern trend line crossed the early Holocene maximum is supported by the accelerating rate of sea level rise, which approached 3 mm yr-1 at about that date.

“This conclusion is supported by the accelerating rate of sea level rise, which approached 3 mm yr -1 at about that date”

• cites Hansen et al., 2016Not checked yet

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There is widespread agreement that 2°C warming would commit the world to multi-meter sea level rise.

“There is widespread agreement that 2 • C warming would commit the world to multi-meter sea level rise”

• cites Levermann et al., 2013Not checked yet

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• cites Dutton et al., 2015Not checked yet

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• cites Clark et al., 2016Not checked yet

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Hemispheric solar insolation anomalies in the latter half of the Holocene favored ice sheet growth in the Northern Hemisphere and ice sheet decay in Antarctica.

“Hemispheric solar insolation anomalies in the latter half of the Holocene favored ice sheet growth in the Northern Hemisphere and ice sheet decay in Antarctica”

• cites Hansen et al., 2016Not checked yet

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The response of surface air temperature over land is twice as large as the SST response to climate forcings in 21st century simulations with models.

“The response of surface air temperature (SAT) over land is twice as large as the SST response to climate forcings in 21st century simulations with models”

• cites Collins et al., 2013Not checked yet

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Amplifying feedbacks lead to large climate change in response to even weak climate forcings such as ice age cycles caused by small perturbations of Earth's orbit.

“Amplifying feedbacks lead to large climate change in response to even weak climate forcings such as ice age cycles caused by small perturbations of Earth's orbit”

• cites Zachos et al., 2001Not checked yet

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• cites Royer et al., 2012Not checked yet

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• cites Franks et al., 2014Not checked yet

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The central value found in a wide range of modern climate models remains 3°C for 2 × CO2.

“The central value found in a wide range of modern climate models (Flato et al., 2013) remains 3 • C for 2 × CO 2 .”

• cites Flato et al., 2013Not checked yet

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The aerosol forcing uncertainty is of order 50%.

“The aerosol forcing uncertainty, described by a probability distribution function (Boucher et al., 2013), is of order 50 %.”

• cites Boucher et al., 2013Not checked yet

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The ice sheet feedback works mainly via the albedo effect.

“The ice sheet feedback works mainly via the albedo (reflectivity) effect.”

Global warming can activate slow feedbacks, so the time for the system to reach a fully equilibrated state is even longer.

“global warming can activate”

Because of the ocean's thermal inertia, full atmosphere-ocean response to forcing requires a long time: models suggest even after 100 years only 60-75% of surface warming for a given forcing has occurred, with 25-40% still in the pipeline.

“because of the ocean's great thermal inertia (heat capacity), full atmosphere-ocean response to the forcing requires a long time: atmosphere-ocean models suggest that even after 100 years only 60-75 % of the surface warming for a given forcing has occurred, the remaining 25-40 % still being”

• cites Hansen et al., 2011Not checked yet

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• cites Collins et al., 2013Not checked yet

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Global warming of about 5°C between the last glacial maximum and the Holocene is accounted for almost entirely by radiative forcing caused by decrease in ice sheet area and increase in GHGs.

“Global warming of ∼ 5 • C between the last glacial maximum and the Holocene (Masson-Delmotte et al., 2013) is accounted for almost entirely by radiative forcing caused by decrease in ice sheet area and increase in GHGs”

• cites Masson-Delmotte et al., 2013Not checked yet

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• cites Lorius et al., 1990Not checked yet

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• cites Hansen et al., 2007Not checked yet

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The ocean has great heat capacity, so it takes decades to centuries for Earth's surface temperature to achieve most of its fast-feedback response to a change in climate forcing.

“The ocean has great heat capacity (thermal inertia), so it takes decades to centuries for Earth's surface temperature to achieve most of its fast-feedback response to a change in climate forcing”

• cites Hansen et al., 1985Not checked yet

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Climate models and empirical data concur that amplifying feedbacks dominate on both short and long timescales.

“climate models and empirical data concur that amplifying feedbacks dominate on both short and long timescales”

The Charney analysis was based on climate models in which ice sheets and all long-lived GHGs except the specified CO2 doubling were fixed.

“The Charney analysis was based on climate models in which ice sheets and all long-lived GHGs (except for the specified CO 2 doubling) were fixed.”

Earth's energy imbalance defines additional global warming that will occur without further change in forcings.

“This imbalance thus defines additional global warming that will occur without further change in forcings.”

Paleoclimate data allow narrowing of the uncertainty in fast-feedback climate sensitivity.

“paleoclimate data allow narrowing of the uncertainty.”

• cites Rohling et al., 2012aNot checked yet

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Taking climate sensitivity as 0.75°C (W m-2)-1 forcing, global warming of 1.15°C implies that 1.55 W m-2 of the total 2.5 W m-2 forcing has been used up to cause observed warming.

“Taking climate sensitivity as 0.75 • C (W m -2 ) -1 forcing, global warming of 1.15 • C implies that 1.55 W m -2 of the total 2.5 W m -2 forcing has been”

Such large soil carbon loss could wreak havoc with efforts to achieve the net soil and biospheric carbon storage that is likely necessary for climate stabilization.

“such large soil carbon loss could wreak havoc with efforts to achieve the net soil and biospheric carbon storage that is likely necessary for climate stabilization”

The ocean and land release more of these gases as the planet becomes warmer.

“the ocean and land release more of these gases as the planet becomes warmer.”

Continued high fossil fuel emissions might drive 2°C soil warming and a 55 PgC soil carbon loss by 2050.

“suggesting that continued high fossil fuel emissions might drive 2 • C soil warming and a 55 PgC soil carbon loss by 2050.”

• cites Crowther et al. (2016)Not checked yet

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Efforts to limit non-CO2 forcings could keep their growth small or even slightly negative.

“we conclude that efforts to limit non-CO 2 forcings could keep their growth small or even slightly negative”

The timescale for large ice sheet melt decreases dramatically as the magnitude of warming increases.

“Ice sheet modeling of Applegate et al. (2015) explicitly shows that the timescale for large ice sheet melt decreases dramatically as the magnitude of warming increases.”

• cites Applegate et al. (2015)Not checked yet

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This limitation to fast feedbacks is not a limitation for the past (1850-present) because measured GHG changes include any GHG change due to slow feedbacks.

“This is not a limitation for the past, i.e., for the period 1850-present, because we employ measured GHG changes, which include any GHG change due to slow feedbacks.”

The reason the required extraction exceeds the difference is that ocean outgassing increases and vegetation productivity and ocean CO2 uptake decrease with decreasing atmospheric CO2.

“The well-known reason (Cao and Caldeira, 2010) is that ocean outgassing increases and vegetation productivity and ocean CO 2 uptake decrease with decreasing atmospheric CO 2 , as explored in a wide range of Earth system models (Jones et al., 2016).”

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The ice albedo effect is supplemented by a change in surface albedo in ice-free regions due to vegetation changes.

“The ice albedo effect is supplemented by a change in surface albedo in ice-free regions due to vegetation changes.”

The GHG feedback plays a leading role in determining the magnitude of paleoclimate change.

“The GHG feedback plays a leading role in determining the magnitude of paleoclimate change”

Sea level change lags temperature change by 1-4 centuries.

“Grant et al. (2012) find that sea level change lags temperature change by 1-4 centuries.”

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The equilibrium fast-feedback climate sensitivity used (0.75°C (W m-2)-1, 3°C for 2×CO2) is consistent with climate models and paleoclimate evidence.

“We use equilibrium fast-feedback climate sensitivity 0.75 • C (W m -2 ) -1 (3 • C for 2 × CO 2 ).This is consistent with climate models (Collins et al., 2013: Flato et al., 2013) and paleoclimate evidence (Rohling et al., 2012a;Masson-Delmotte et al., 2013;Bindoff and Stott, 2013).”

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• cites Rohling et al., 2012aNot checked yet

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Human-made climate forcing projected this century with continued high fossil fuel emissions, equivalent to doubling atmospheric CO2, would likely lead to substantial ice sheet collapse and multi-meter sea level rise on the timescale of a century.

“Hansen (2005Hansen ( , 2007) ) argues on heuristic grounds that the much faster and stronger human-made climate forcing pro- jected this century with continued high fossil fuel emissions, equivalent to doubling atmospheric CO 2 , would likely lead to substantial ice sheet collapse and multi-meter sea level rise on the timescale of a century.”

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Continued high GHG emissions would likely cause multi-meter sea level rise within 50-150 years.

“Hansen et al. (2016), based on a combination of climate modeling, paleo-data, and modern observations, conclude that continued high GHG emissions would likely cause multimeter sea level rise within 50-150 years.”

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The decline in GHG climate forcing growth during the first decade of the 21st century was due to the Montreal Protocol, which initiated phase-out of production of ozone-destroying gases, primarily CFCs.

“(3) the Montreal Protocol, which initiated phase-out of the production of gases that destroy stratospheric ozone, primarily chlorofluorocarbons (CFCs).”

Ice sheet size, judged from sea level, varies almost synchronously with temperature for the temporal resolution available in paleoclimate records.

“Ice sheet size, judged from sea level, varies almost synchronously with temperature for the temporal resolution available in paleoclimate records”

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There is reason to suspect that the GHG feedback may already be important in modern climate.

“there is reason to suspect that it may already be important in modern climate.”

Every 1°C global mean soil surface warming can cause a 30 PgC soil carbon loss.

“Crowther et al. (2016) synthesize results of 49 field experiments across North America, Europe and Asia, inferring that every 1 • C global mean soil surface warming can cause a 30 PgC soil carbon loss”

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The decline in GHG climate forcing growth during the first decade of the 21st century was due to decline of the airborne fraction of CO2 emissions.

“The decline was due to (1) decline of the airborne fraction of CO 2 emissions (Fig. A8)”

The vegetation albedo effect provides a significant amplification of warming as Earth's temperature increases from its present climate state, because dark forests tend to replace tundra or sparse low-level vegetation in large areas of Eurasia and North America.

“This vegetation albedo effect provides a significant amplification of warming as Earth's temperature increases from its present climate state, because dark forests tend to replace tundra or sparse low-level vegetation in large areas of Eurasia and North America”

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Use of biochar to improve soil fertility provides additional carbon storage of up to 0.7-1.8 PgC yr-1.

“Use of biochar to improve soil fertility provides additional carbon storage of up to 0.7-1.8PgC yr -1 (Woolf et al., 2010;Smith, 2016).”

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This scenario keeps the temperature excursion above the Holocene level small enough and brief enough that it has the best chance of avoiding ice sheet instabilities and multi-meter sea level rise.

“This scenario keeps the temperature excursion above the Holocene level small enough and brief enough that it has the best chance of avoiding ice sheet instabilities and multi-meter sea level rise (Hansen et al., 2016).”

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Energy, land and water requirements of these technologies impose economic and biophysical limits on CO2 extraction.

“However, energy, land and water requirements of these technologies impose economic and biophysical limits on CO 2 extraction (Smith et al., 2016).”

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Soil carbon sequestration has potential to store 0.7 PgC yr-1.

“Smith (2016) estimates that soil carbon sequestration has potential to store 0.7 PgC yr -1 .”

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Total CO2 extraction costs will be high due to the remaining requirements for technological extraction.

“it is not necessary for reaching the conclusion that total CO 2 extraction costs will be high due to the remaining requirements for technological extraction.”

100 PgC is an appropriate ambitious estimate for potential carbon extraction via a concerted global-scale effort to improve agricultural and forestry practices with carbon drawdown as a prime objective.

“We conclude that 100 PgC is an appropriate ambitious estimate for potential carbon extraction via a concerted global-scale effort to improve agricultural and forestry practices with carbon drawdown as a prime objective.”

Reforestation and afforestation together have carbon storage potential of about 1.1 PgC yr-1.

“Smith et al. (2016) estimate that reforestation and afforestation together have carbon storage potential of about 1.1 PgC yr -1 .”

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Costs estimates are ∼USD 150-350 (tC)-1 for crop-based BECCS.

“Costs estimates are ∼ USD 150-350 (tC) -1 for crop-based BECCS (Smith et al., 2016).”

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The intended nationally determined contributions (INDCs) submitted by 189 countries include carbon drawdown through land use plans with aggregate removal rate of ∼2 PgCO2 yr-1 (∼0.55 PgC yr-1) after 2020.

“The intended nationally determined contributions (IN-DCs) submitted by 189 countries include carbon drawdown through land use plans (United Nations, 2016) with aggregate removal rate of ∼ 2 PgCO 2 yr -1 (∼ 0.55 PgC yr -1 ) after 2020.”

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Soil carbon sequestration and soil amendment with biochar compare favorably with other negative emission technologies with less impact on land use, water use, nutrients, surface albedo, and energy requirements.

“Smith (2016) shows that soil carbon sequestration and soil amendment with biochar compare favorably with other negative emission technologies with less impact on land use, water use, nutrients, surface albedo, and energy requirements”

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Extraction of CO2 from the air is required if large, long-term excursion of global temperature above its Holocene range is to be averted.

“Extraction of CO 2 from the air, also called negative emissions or carbon dioxide removal (CDR), is required if large, long-term excursion of global temperature above its Holocene range is to be averted, as shown above.”

Such extraordinary cost, along with land area, fertilizer and water requirements, suggests that continued high emissions would likely force humanity to live with climate change running out of control.

“Such extraordinary cost, along with the land area, fertilizer and water requirements (Smith et al., 2016) suggest that, rather than the world being able to buy its way out of climate change, continued high emissions would likely force humanity to live with climate change running out of control with all the consequences that would entail.”

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If rapid emission reductions are initiated soon, it is still possible that at least a large fraction of required CO2 extraction can be achieved via relatively natural agricultural and forestry practices with other benefits.

“Nevertheless, if rapid emission reductions are initiated soon, it is still possible that at least a large fraction of required CO 2 extraction can be achieved via relatively natural agricultural and forestry practices with other benefits.”

If large fossil fuel emissions are allowed to continue, the scale and cost of industrial CO2 extraction may become unmanageable.

“On the other hand, if large fossil fuel emissions are allowed to continue, the scale and cost of industrial CO 2 extraction, occurring in conjunction with a deteriorating climate and costly dislocations, may become unmanageable.”

Recent costs of Chinese and South Korean light water reactors are in the range USD 2000-3000 per kilowatt.

“Recent costs of Chinese and South Korean light water reactors are in the range USD 2000-3000 per kilowatt (Chinese Academy of Engineering, 2015; Lovering et al., 2016).”

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Cooling by aerosols or other methods that alter the amount of sunlight absorbed by Earth could be effective more quickly than removing CO2 from the air.

“Despite these issues, it is apparent that cooling by aerosols, or other methods that alter the amount of sunlight absorbed by Earth, could be effective more quickly than the difficult process of removing CO 2 from the air.”

An economic study of the carbon-fee-and-dividend policy in the US supports the conclusion that GDP increases as the fee rises steadily.

“An economic study of this carbon-fee-and-dividend policy in the US (Nystrom and Luckow, 2014) supports the conclusion that GDP increases as the fee rises steadily.”

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The estimate of +0.75 ± 0.25 W m-2 for the current Earth energy imbalance averaged over the solar cycle is still valid.

“We conclude that the estimate of +0.75 ± 0.25 W m -2 for the current Earth energy imbalance averaged over the solar cycle is still valid.”

Pathways to rapid emission reductions are feasible.

“Our conclusion that the world has overshot appropriate targets is sufficiently grim to compel us to point out that pathways to rapid emission reductions are feasible.”

A net decrease in climate forcing by non-CO2 GHGs of about -0.25 W m-2 relative to today is plausible.

“We conclude, as discussed in Appendix A13, that a net decrease in climate forcing by non-CO 2 GHGs of perhaps -0.25 W m -2 relative to today is plausible, but we must note that this is a dramatic change from the growing abundances, indeed accelerating growth, of these gases today.”

Data adjustments and uncertainties are small in comparison with the long-term warming.

“However, the important point is that these data adjustments and uncertainties are small in comparison with the long-term warming.”

Coal provided almost 45% of global fossil fuel CO2 emissions.

“almost 45 % of global fossil fuel CO 2 emissions (BP, 2016).”

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The possibility of geoengineering can hardly be seen as alleviating the overall burden being placed on young people by continued high fossil fuel emissions.

“In summary, although research on climate interventions is warranted, the possibility of geoengineering can hardly be seen as alleviating the overall burden being placed on young people by continued high fossil fuel emissions.”

More warming will occur even if atmospheric GHG amounts are stabilized at today's level.

“Also, Earth is now out of energy balance, implying that more warming will occur, even if atmospheric GHG amounts are stabilized at today's level.”

Achieving net negative forcing of -0.25 W m-2 for non-CO2 gases would allow CO2 to be 365 ppm instead of 350 ppm while yielding the same total GHG forcing.

“Achieving this net negative forcing of -0.25 W m -2 for non-CO 2 gases would allow CO 2 to be 365 ppm, rather than 350 ppm, while yielding the same total GHG forcing.”

These studies refute the common argument that environmental protection is damaging to economic prosperity.

“These studies refute the common argument that environmental protection is damaging to economic prosperity.”

Success of both CO2 extraction and non-CO2 GHG controls requires a major role for developing countries.

“Success of both CO 2 extraction and non-CO 2 GHG controls requires a major role for developing countries, given that they have been a large source of recent deforestation (IPCC, 2013) and have a large potential for reduced emissions.”

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The world has already overshot appropriate targets for GHG amount and global temperature.

“We conclude that the world has already overshot appropriate targets for GHG amount and global temperature”

Human-made aerosols cause a negative climate forcing of order -1 W m-2 with uncertainty of at least 0.5 W m-2.

“Human-made aerosols today are believed to cause a large, albeit poorly measured, negative climate forcing (Fig. 4) of the order of -1 W m -2 with uncertainty of at least 0.5 W m -2 (Fig. 7.19, Boucher et al., 2013).”

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The difficulty of stabilizing climate was markedly increased by a delay in emission reductions of 8 years, from 2013 to 2021.

“The difficulty of stabilizing climate was thus markedly increased by a delay in emission reductions of 8 years, from 2013 to 2021.”

The aerosol cooling effect is not anticipated to be largely reduced even if fossil fuel use declines.

“Given that human population continues to grow, and that human-caused climate effects such as increased desertification can lead to increased aerosols, we do not anticipate a large reduction in the aerosol cooling effect, even if fossil fuel use declines.”

• AssumptionRests on an assumption: Human population continues to grow.
• AssumptionRests on an assumption: Human-caused climate effects such as increased desertification can lead to increased aerosols.

Mitigation costs are not large in comparison to costs of extracting CO2 from the air.

“Rather we use this calculation to show that mitigation costs are not large in comparison to costs of extracting CO 2 from the air.”

Climate interventions are no substitute for the reduction of carbon dioxide emissions needed to stabilize climate and avoid deleterious consequences of rapid climate change.

“We concur with the assessment (NAS, 2015b) that such climate interventions are no substitute for the reduction of carbon dioxide emissions needed to stabilize climate and avoid deleterious consequences of rapid climate change.”

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The Montreal Protocol has stifled and even reversed growth of specific trace gases that destroy stratospheric ozone and cause global warming.

“The Montreal Protocol has stifled and even reversed growth of specific trace gases that destroy stratospheric ozone and cause global warming (Prather et al., 1996;Newman et al., 2009).”

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CO2 is the dominant forcing in future climate scenarios.

“CO 2 is the dominant forcing in future climate scenarios.”

The recent Montreal Protocol agreement to phase down hydrofluorocarbons should cause annually added forcing of Montreal Protocol trace gases plus other trace gases to become near zero or slightly negative, thus at least partially offsetting growth of other non-CO2 GHGs, especially N2O.

“Indeed, recent agreement to use the Montreal Protocol (2016) to phase down production of minor trace gases, the hydrofluorocarbons (HFCs), should cause annually added forcing of Montreal Protocol trace gases (MPTGs) + other trace gases (OTGs) (red region in Fig. 8) to become near zero or slightly negative, thus at least partially off-setting growth of other non-CO 2 GHGs, especially N 2 O.”

Without the protocol, the potential for growth of forcings of HFCs and other gases added by protocol amendments is possibly as large as +0.2 W m-2.

“Protocol amendments that add other gases such as HFCs are important; forcings of these gases are small today, but without the protocol their potential for growth is possibly as large as +0.2 W m -2 (Prather et al., 2013).”

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A net negative forcing of -0.25 W m-2 for non-CO2 gases would allow CO2 to be 365 ppm rather than 350 ppm while yielding the same total GHG forcing.

“A net negative forcing of -0.25 W m -2 for non-CO 2 gases would allow CO 2 to be 365 ppm, rather than 350 ppm, while yielding the same total GHG forcing.”

The anticipated benefit of the Montreal Protocol over the 21st century is a drop in climate forcing of -0.23 W m-2.

“The anticipated benefit over the 21st century is a drop in climate forcing of -0.23 W m -2 (Prather et al., 2013).”

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Given the current imbalance and the long N2O atmospheric lifetime, it is nearly inevitable that N2O will continue to increase this century, even if emissions growth is checked.

“Given the current imbalance (emissions exceeding atmospheric losses by about 30 %; Prather et al., 2012) and the long N 2 O atmospheric lifetime (116 ± 9 years; Prather et al., 2015) it is nearly inevitable that N 2 O will continue to increase this century, even if emissions growth is checked.”

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There can be no expectation of an N2O decline that offsets the need to reduce CO2.

“There can be no expectation of an N 2 O decline that offsets the need to reduce CO 2 .”

Forcings of HFCs and other gases added by protocol amendments are small today.

“Protocol amendments that add other gases such as HFCs are important; forcings of these gases are small today, but without the protocol their potential for growth is possibly as large as +0.2 W m -2 (Prather et al., 2013).”

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