Hansen 2025 – review by Bruce Melton, Climate Change Now Initiative
Summary – Hansen’s 350 ppm CO2 and 1 degree C warming target is on full display where he repeatedly asserts that acceleration of warming and effects now implies that engineered cooling solutions are required to avoid untenable futures, and suggest the moral hazard disrespecting engineered cooling and atmospheric greenhouse gas removals is used today by academia and the environmental movement to wrongly frame solar radiation management negatively. Ship’s fuels sulfur regulations are responsible for half of the warming jump and actual aerosol cooling is likely 10x greater than IPCC suggestions. Climate restoration is the only path Hansen can see as further warming allows tipping. Without restoration, he suggests that AMOC collapse is inevitable in 20 to 30 years. The point of no return plays heavily in this paper, but geoengineering can reverse fast feedbacks, though geoengineering can fail at higher greenhouse gas concentrations. The International Maritime Organizations new ship fuels global cooling sulfur regulations have caused half of the 2023-24 temperature jump, where one-third of the warming unmasking is complete after five years; the next third requires a century, and the final third requires millennia. Prominent in this classically verbose yet extremely thorough publishing of Hansen’s is that the “point of no return” of cooling, removal and reduction actions being effective is far ahead of the standard 2100 time frame and rapidly closing. He also suggests carbon removal is too expensive based in an inaccurate interpretation of Keith 2018. All and more are described in the notes below.
Selected quotes and commentary listed by topic:
Global warming in 2023 and 2024 was one third of all warming in the last 200 years… “Global temperature leaped more than 0.4°C (0.7°F) during the past two years, the 12-month average peaking in August 2024 at +1.6°C relative to the temperature at the beginning of last century (the 1880-1920 average). This temperature jump was spurred by one of the periodic tropical El Niño warming events, but many Earth scientists were baffled by the magnitude of the global warming, which was twice as large as expected for the weak 2023-2024 El Niño.“
Temperature response to shipping fuels sulfur regulations… “warming from the 2020 reduction of ship aerosols is one-third complete after five years; the next third requires a century and the final third requires millennia.
Tipping is real… “Tipping points are also real. Some feedbacks can pass a point such that the process accelerates and causes amplifying climate feedback. For example, global warming may melt Arctic permafrost, releasing large amounts of greenhouse gases to the atmosphere. Or heating and drying of the Amazon rainforest may reach a point that the rainforest is not self-sustaining, with fires releasing much of the carbon stored in the forest. Many tipping point processes are reversible if Earth cools, but the recovery time varies and may be long for some feedbacks.”
Restore our climate to the relatively stable Holocene… “Late Eemian climate also featured shutdown of the North Atlantic overturning circulation, as revealed by ocean cores of seafloor sediments. Shutdown of this ocean circulation short-circuits interhemispheric transport of heat by the global ocean conveyor, which normally transports a huge amount of heat – 1,000 trillion watts – from the Southern Hemisphere into the Northern Hemisphere. That heat amounts to 4 W/m2 of energy averaged over the Northern Hemisphere, but it is mostly concentrated in the North Atlantic region, which is thus warmer than expected for its latitude. When the ocean conveyor shut down, that heat stayed in the Southern Ocean, where it may have contributed to collapse of the West Antarctic ice sheet. Meanwhile, in the North Atlantic region, there was evidence of powerful storms. This picture of the Eemian, if filled out in finer detail including the sequencing of events, may help us anticipate where our present climate is headed, if effective actions are not taken to halt and reverse human-made climate change, restoring relatively stable Holocene climate.”
Shutdown of AMOC by 2050… (abstract) “We find that polar ice melt and freshwater injection onto the North Atlantic Ocean exceed prior estimates and, because of accelerated global warming, the melt will increase. As a result, shutdown of the Atlantic Meridional Overturning Circulation (AMOC) is likely within the next 20-30 years, unless actions are taken to reduce global warming – in contradiction to conclusions of IPCC.”
“Our climate simulations led to the staggering conclusion that continued growth of ice melt will cause shutdown of the North Atlantic and Southern Ocean overturning circulations as early as midcentury and “nonlinearly growing sea level rise, reaching several meters in 50-150 years.” (authors emphasis, page 28)
Geoengineering recommended but not … Hansen presents a powerful argument for geoengineering, very specifically stating that geoengineering and only geoengineering can avoid the point of no return. Yet, he purposefully states that in their section on “Purposeful Global Cooling (below), “We do not recommend implementing climate interventions, but we suggest that young people not be prohibited from having knowledge of the potential and limitations of purposeful global cooling in their toolbox.”
Ship’s Fuels Forcing 10X Greater than IPCC… “Review41 of five ship aerosol modeling studies finds a range 0.07 to0.15 W/m2, with mean 0.12 ± 0.03 W/m2. A recent model result42 of 0.2 W/m2refers to ocean area and is thus a globalforcing of 0.14 W/m2. None of these modeled Ship Aerosol Forcings would have much effect on global temperature because GHG forcing currently is increasing 0.4-0.5 W/m2 per decade. However, if the aerosol effect is highly nonlinear (i.e., if aerosols emitted into polluted air have much less effect on clouds than aerosols emitted into a pristine atmosphere), decreased ship emissions may have a large effect on Earth’s albedo (reflectivity). The largest effect should be in the North Pacific and North Atlantic, where ship emissions dominate over natural sulfate aerosols (Sidebar 5).Fortunately, Earth’s albedo has been monitored for almost a quarter of a century by the CERES (Clouds and the Earth’s Radiant Energy System) satellite instrument,44 which reveals a stunning darkening of Earth (Figure 6).45 Earth’s albedo decreased about 0.5%(of 340 W/m2), which is 1.7 W/m2 additional heating of Earth since 2010! Such albedo change is equivalent to an increase of CO2 by 138 ppm, from the 419 ppm actually measured at the begin-ning of 2024 to 557 ppm.”
“How can we reconcile our estimate of 0.5 W/m2 for ship aerosol forcing with the six aerosol modeling studies men-tioned above,41,42 which are in mutual agreement that the global ship aerosol forcing is small, in the range 0.07-0.15W/m2? Let’s first summarize our alterna-tive analysis of the aerosol forcing and then suggest an approach to resolve the large difference. Our initial estimate of the ship aero-sol forcing was based on the precise CERES satellite data, calibrated abso-lutely with Argo float data.44,55 The CERES data show that Earth’s albedo(reflectivity) decreased 0.5% since 2010, corresponding to a 1.7 W/m2 global average increase of Absorbed Solar Radiation. Based on the spatial and temporal coincidence of the increased absorption with regions where the effect of ship aerosols should be largest –the North Pacific and North Atlantic –we infer a Ship Aerosol Forcing of ∼0.5W/m2, an order of magnitude larger than follows from the IPCC aerosol for-mulation.”
Purposeful Global Cooling Section
“Today’s older generations – despite having adequate information – failed to stem climate change or set the planet on a course to avoid growing climate disasters. And they tied one arm of young people behind their back by supporting only renewable energies as an alternative to fossil fuels. Now, as it has become clear that climate is driving hard toward the Point of No Return, there are efforts to tie the other arm of young people behind their back. We refer to efforts to prohibit actions that may be needed to affect Earth’s energy balance, temporarily, while the difficult task of reducing greenhouse gases is pursued as rapidly as practical – namely Solar Radiation Modification (SRM). Purposeful global cooling with such climate interventions is falsely described as “geoengineering,” while, in fact, it is action to reduce geoengineering. Humanmade climate forcings are already geoengineering the planet at an unprecedented, dangerous, rate.
We, the authors – who range in experience from young people just beginning our careers to older scientists who have spent half a century in research aimed at better understanding of Earth’s climate – are concerned about the danger of again “being too late” in informing the public about actions that may be needed to preserve the marvelous world we inherited from our parents. We do not recommend implementing climate interventions, but we suggest that young people not be prohibited from having knowledge of the potential and limitations of purposeful global cooling in their toolbox. We do not subscribe to the opinion that such knowledge will necessarily decrease public desire to slow and reverse growth of atmospheric greenhouse gases; on the contrary, knowledge of such research may increase public pressure to reduce greenhouse gas amounts.”
The Point of No Return… “The Point of No Return – The greatest climate threat is probably the danger of the West Antarctic ice sheet collapsing catastrophically.”
“The problem of West Antarctic ice sheet collapse is complicated because it may be related to – spurred by – shutdown of the North Atlantic overturning circulation, which is part of a global ocean conveyor that normally transports heat from the Southern Ocean into the Northern Hemisphere. When the North Atlantic Overturning circulation shuts down, that heat stays in the Southern Hemisphere where it can contribute to Antarctic ice melt.”
“Point of No Return research deserves greater attention than it has received. There is evidence that global climate models IPCC has relied on do not realistically represent the possibility of shutdown of the North Atlantic Overturning Circulation,1,96 nor do they simulate the rapid sea level changes that occur in the paleoclimate record.49 A more powerful research approach would give emphasis to paleoclimate analysis and to observations of ongoing climate changes at least comparable to global climate modeling.”
Geoengineering to avoid the point of no return… “Sidebar 11. Ice shelves adhered to the Antarctic continent extend down the side of the continent to depths as great as 2 km in the Southern Ocean, where they provide the strongest buttressing force Footnote130 holding the ice sheet in place. Ice shelves are the “cork” that prevents rapid expulsion of Antarctic ice into the Southern Ocean – especially the vulnerable West Antarctic ice, which rests on bedrock below sea level. Footnote131 The rapid Eemian sea level rise likely was preceded by melting of Antarctic ice shelves. Today, ice shelves around Antarctica are again melting, with the melting accelerated by slowdown of the ocean overturning circulation. The overturning is driven by cold, salty water near the Antarctic coast that sinks to the ocean floor, compensated by rising, warmer water; this circulation is an escape valve for deep ocean heat. Global warming today is increasing ice melt around Antarctica, freshening and reducing the density of the upper ocean, thus reducing the overturning circulation Footnote49 and escape of ocean heat to space during the cold Antarctic winter. Based on a conservative estimate Footnote110 of observed ice melt in 2011 and a 10-year doubling time for the melt rate, a global climate model yields a 30% slowdown of the overturning circulation in 2025, Footnote132 consistent with observational data. Footnote133 Thus, today the ocean surface layer around Antarctica is freshening and cooling (Figure 3, Cheng et al.), Footnote134 but the ocean below is warming. Purposeful aerosol cooling recharges this overturning Antarctic circulation, allowing deep ocean heat to escape to the atmosphere and space and cooling the ocean at depth while warming much of the thin surface layer as the upwelling deep-ocean heat melts sea ice (Figure 24).”
IPCC Approach is insufficient… “the United Nations IPCC approach, heavily emphasizing global climate modeling, is insufficient.”
Carbon capture at the giga-ton scale does not exist – Of course it doesn’t, we haven’t built it yet… “Carbon capture at the giga-ton scale does not exist; the estimated annual cost of CO2 extraction is now $2.2-4.5 trillion dollars per year,116”
Hansen’s Cost of Air Capture Error – Carbon Capture Costs Four Times Reality at $2.2-4.5 trillion dollars per year – Reference 116… Hansen uses unrealistically high costs for carbon removal. Hansen’s assumption on removal costs are in error. He assumes the evaluation of ari capture costs of Keith 2018 is nearly four times higher than Keith 2018 says it is based on a simple error. Hansen assumed Kieth’s numbers were for capture of carbon (C). they were not. Keith 2018 appropriately assumed capture of CO2 and his numbers are not nearly 4 times higher than he states as Hansen and Kharecha assume.
Hansen’s Cost of Air Capture Error… “the estimated annual cost of CO2 extraction is now $2.2- 4.5 trillion dollars per year,116” Reference 116 is, “Assuming empirical cost estimates of 451-924 TnUS$/tC, based on a pilot direct-air CO2 capture plant. See J. Hansen, P. Kharecha, “Cost of carbon capture: Can young people bear the burden?” Joule2 (2018): 1405-7”
Here is Hansen and Kharecha’s Errored Explanation…
“Keith et al.6 built a pilot plant capturing CO2, which provides the best basis so far for estimating the cost of CO2 extraction. Their estimated cost range is $94–$232/tCO2, where tCO2 is metric tons of CO2. This cost appears to be much lower than estimates in an earlier study.7 However, it would be a grave misconception to think that the Keith study provides hope for a ‘‘get out of jail free card’’ for the climate problem. First, note that the $94/tCO2 estimate applied only to a case in which CO2 was processed to a point of being ready for use in production of a carbon-based fuel. That use of the CO2 does not result in negative emissions when the fuel is burned. Keith’s cost estimate for cases in which extracted CO2 is prepared for storage is $113–$232/tCO2.
Second, note that Keith does not include the cost of CO2 storage, which has been estimated 7 as $10–$20/ tCO2. Inclusion of storage makes the cost estimate for carbon capture and storage (CCS) $123–$252/tCO2. Finally, note that costs are often discussed in units of $/tC, where tC is tons of carbon. A ton of CO2 is 44/12 times heavier than a ton of C. Thus, the Keith study implies a removal cost of $451–$924/tC.”
Keith 2018 however, does not use cost per ton C, he uses costs per ton CO2…
– From Keith 2018, summary image, “When CO2 is delivered at 15 MPa, the design requires either 8.81 GJ of natural gas, or 5.25 GJ of gas and 366 kWhr of electricity, per ton of CO2 captured. Levelized cost per t-CO2 from atmosphere ranges from 94 to 232 $/t-CO2.”
– From Keith 2018 summary, “When CO2 is delivered at 15 MPa, the design requires either 8.81 GJ of natural gas, or 5.25 GJ of gas and 366 kWhr of electricity, per ton of CO2 captured. Levelized cost per t-CO2 from atmosphere ranges from 94 to 232 $/t-CO2.”
– Table 2 also shows inputs and outputs with costs ranging from $94 to $232 “levelized ($/t-CO2)”
– $94 to $232 ton CO2 is also mentioned in the Comparison with Prior Estimates section, “The most influential prior estimate of DAC costs was provided by a 2011 American Physical Society (APS) study.4 The study estimated the cost of an aqueous Calooping technology like that presented here. The APS ‘‘realistic’’ case had costs of 780 $/t-CO2-avoided and 550 $/t-CO2-captured, where the ‘‘avoided’’ value includes emission from electricity supply outside the plant boundary. Our cost range is 94–232 $/t-CO2 captured, and if we use the financial and gas price assumptions of the APS (CRF = 12% and 6 $/GJ), then our costs would be 107–249 $/t-CO2 for the A and B variants in Table 2.”
Geoengineering reverses fast feedbacks… Modeling understates fast feedback response to geoengineering… “Modeling limitations are why we suggest comparable emphasis on paleoclimate studies, climate modeling, and modern observations of ongoing changes. In the latter category, there is the global, natural experiment of cooling by stratospheric aerosols provided by the 1991 Pinatubo volcanic eruption, which spread aerosols into both hemispheres. The maximum negative forcing was about −3 W/m2, more than enough to offset Earth’s present energy imbalance of 1-1.5 W/m2 and cause global cooling. Such negative forcing, if maintained for years, would cause reversal of fast feedbacks, including regrowth of sea ice area. Major effects of the brief Pinatubo forcing included global cooling in the next two years that peaked at 0.3 °C and a 50% reduction in the growth rate of atmospheric CO2 that lasted about three years. Negative effects included a temporary reduction of stratospheric ozone in the tropics and adverse changes of precipitation patterns.”
Geoengineering fails at high GHG concentrations, but needed to prevent overshoot, and can reduce Antarctic ice loss and sea level rise… “high greenhouse gas scenario such as RCP8.5 creates such great warming and melting that aerosol intervention will almost surely be fruitless in the end. Our Figure 15 is a shocking revelation that real-world greenhouse gases are increasing at nearly the RCP8.5 rate. Policy must focus on reducing actual greenhouse gas emissions to a steeply declining growth rate relative to RCP8.5(Figure 15). Solar Radiation Modification (SRM) – whether via stratospheric aerosols or otherwise – should be considered only as a possibility to address temporary overshoot of safe global temperature while atmospheric greenhouse gases are reduced as rapidly as practical. With that caveat, numerous studies, e.g., 136,137 suggest that stratospheric aerosols have potential to reduce the risks of Antarctic ice loss and sea level rise.”
Greenhouse gas (carbon dioxide) removal indicated in the Purposeful Global Cooling section… “Solar Radiation Modification (SRM) – whether via stratospheric aerosols or otherwise – should be considered only as a possibility to address temporary overshoot of safe global temperature while atmospheric greenhouse gases are reduced as rapidly as practical.”
Purposeful cooling actions… Hansen only says “purposeful cooling actions,” but both geoengineering and carbon removal are purposeful cooling actions. “The gap between reality and the growth rate required to keep global warming less than +2 °C is so great (see Figure 15) that it is now implausible to keep warming under that target without purposeful cooling actions, in addition to reducing greenhouse gas amounts.”
Moral hazard… “whether moral hazard plays out should depend on how SRM is framed, e.g. as a panacea or get-out-of-jail card vs. a complementary measure. SRM must be presented as an auxiliary tool that could help reverse some of the damage already set in motion by the fossil fuel industry and irresponsible politics. The environmental movement and academia have a huge responsibility in steering public debate on SRM, which they have largely shunned to date.”
Risk vs. Risk Framework… “There is a need for analysis that compares the risks and benefits of purposeful global cooling scenarios against scenarios with no such cooling. This comparative risk analysis is typically absent in objections to SRM research; in a similar vein, proponents of SRM research should appreciate valid concerns about the moral hazard hypothesis and deal with it in a comparative “risks vs. risks” framework.”
Hansen et al., Global Warming Has Accelerated: Are the United Nations and the Public Well-Informed?, Environment, Science and Policy for Sustainable Dev, February 3, 2025.
https://www.tandfonline.com/doi/full/10.1080/00139157.2025.2434494
