Climate Sensitivity is the Number That Could Save—or Doom—Us. If Wrong, All Our Climate Targets Are Wrong! The controversy centers on 1.5 or 4.5. Each wildly different number will radically rewrite your climate change future?
This 1.5 or 4.5 conflict is critical to correctly planning for the survival of humanity. If one of the two new studies below is correct, then the Trump administration, the Heritage Foundation, and all of the climate change deniers can take a victory lap because climate change will not be that bad, using fossil fuel will not be that big a problem, and the urgency for the switch to green energy generation is far overblown.
If the other study is right, humanity is in a deep and immediate global climate change emergency, and the governments of the world need to begin radical fossil fuel reduction to save as much of humanity as possible.
We hope to see more organizations and individuals on both sides of this critical scientific conflict to weigh in. Hopefully, organizations or individuals like Climate Central, Al Gore, Bill McKibben, Michael Mann, the Post Carbon Institute, Katharine Hayhoe, 350.org, Climate Citizens Lobby, Extinction Rebellion, Fridays for the Future, Sunrise Movement, Union of Concerned Scientists, Climate Alliance, The Climate Emergency Forum, and the Climate Action Network will weigh in soon.
On the other hand, there are significantly fewer scientists and organizations that have steadfastly argued that climate change is not the major problem everyone claims. We also hope they will enter into this heated and growing climate change controversy. This climate change controversy is too essential to humanity’s future not to be resolved as quickly as possible.
To understand this climate change controversy, you will need to delve into some of the climate science behind it.
What is Climate Sensitivity and why is it so important?
Climate sensitivity—often expressed as equilibrium climate sensitivity (ECS)—is one of the most consequential parameters in climate science. It describes how much the planet is expected to warm in the long run after atmospheric CO₂ doubles (for example, from ~280 ppm to 560 ppm). Because ECS anchors and defines most climate models and climate risk assessments, getting this number significantly wrong also significantly shifts climate change consequence forecasts, timelines, and the scale of required fossil-fuel reductions to the worse.
A new climate, science controversy, and debate places climate calculation critical ECS values far apart. Hansen et al., affiliated with Columbia University’s climate programs, argue for a higher sensitivity (≈4.5–4.8°C per CO₂ doubling). Another very recent study argues for a lower value (≈1.5°C).
These estimates cannot both be correct under the same definitions and assumptions—and the difference between them implies large and very different risk timelines, policy urgency, and carbon-budget math.
Why this ECS controversy matters now:
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Forecasts & timelines: The climate sensitivity you choose rewrites when and how severe impacts arrive. (There are examples below.)
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Targets & budgets: Required fossil-fuel reduction pathways change dramatically with the wrong ECS sensitivity.
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Planning & risk: Mis-estimation pushes governments, businesses, and communities toward either under-preparation or over-confidence, or potentially into climate chaos.
To help the public and policymakers see clearly, Job One for Humanity is convening an open, professional exchange on the most critical climate change prediction issue of our time. We invite both research teams—and the wider scientific community—to submit technical responses that clarify assumptions, methods, and definitions (ECS vs effective sensitivity vs TCR; feedback treatment; aerosol forcing; paleoclimate constraints). We will publish submissions unedited with sources and host public roundtables to surface areas of agreement and contention.
Further below is a concise summary of each study’s arguments (with links to the full texts). Our goal is simple: resolve key differences quickly so society can plan and act on the best available climate sensitivity number.
The next arguments section will provide a quick summary examination of the key climate science related to each new study’s wildly different climate sensitivity calculations. For almost all of us, the arguments listed below will not be fully understood, yet which climate sensitivity number is correct will still determine the quality of our lives (or rapidly deteriorating quality of life) that you and your children will experience.
Below the two sections covering both studies’ arguments is another section that details the real-world, wildly different climate change timetables and consequences that you and I will experience, depending upon which study’s climate sensitivity number is correct.
For now, it’s only essential to understand that this IS today’s most critical climate change science controversy, which must be resolved quickly for humanity to grasp its accurate current and future climate change condition.
The Scrip.org publication summary of the “CO2 Back-Radiation Sensitivity Studies under Laboratory and Field Conditions.
Here’s what the paper argues—summed up as the main reasons it infers a very low (“~1.5 °C or lower”) climate sensitivity rather than ~4.5 °C. (There is a link to each full study at the end of each argument summary.)
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Strong spectral saturation of CO₂’s 15 µm band. The central 15 µm line is “almost completely” saturated at today’s concentrations, and the band-edge lines add only a tiny increment (the study cites ~0.17% of the full band for weak edges), so doubling CO₂ yields very little extra absorption.
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Small modeled change in total long-wave absorptivity when CO₂ doubles. Using HITRAN-based line data and prior semi-empirical modeling (e.g., Harde; Wijngaarden & Happer), the paper reports only ~1.5% increase in global mean LW absorptivity when going from 400→800 ppm—about 3 W m⁻² extra back-radiation—translating (via Stefan-Boltzmann) to ≈0.5 K warming for a doubling.
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Laboratory measurements show rapid logarithmic saturation. In a bench “Lab Mode,” adding CO₂ to N₂ increased back-radiation in a logarithmic, quickly saturating way consistent with line-by-line expectations; further increases delivered diminishing returns.
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Field measurements found no measurable CO₂ signal up to 5000 ppm. In the “Field Mode” (clear night sky), raising CO₂ from 0→5000 ppm produced no detectable increase in downwelling IR at the sensor’s threshold, whereas a stronger IR absorber (a Freon) was readily detected—used to argue CO₂’s incremental effect is weak under real-air path lengths.
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Path-length/column arguments imply saturation over tens of meters. With current atmospheric CO₂, the effective optical path for the relevant lines is on the order of centimeters to meters; over real atmospheric columns this leads to saturation already at present levels, so further CO₂ increases have little incremental effect.
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Water vapor and clouds dominate back-radiation variability. Modeled and observed downwelling IR vary far more with humidity and cloud cover than with CO₂ changes at today’s levels, so feedbacks keyed to H₂O/clouds are taken to limit the incremental CO₂ forcing.
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Ground IR spectra show saturation signatures. Upward-looking spectra over clear, dry conditions show the 15 µm CO₂ core saturated and edges near saturation, consistent with a small marginal forcing from more CO₂.
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Paleoclimate/feedback argument. Past warm periods (e.g., the Eemian) did not trigger runaway greenhouse states; the paper uses this to question large net positive feedbacks (especially water-vapor) required for high sensitivities like ~4.5 °C per doubling.
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Model-structure skepticism. The authors argue mainstream climate models over-weight positive feedbacks and under-acknowledge non-CO₂ drivers (solar/cosmic-ray/albedo variability), so they see high sensitivity estimates (e.g., ~4.5 °C) as inflated relative to lab/field evidence and semi-empirical calculations above.
Net of these points, the study’s own back-of-the-envelope converts the ~1.5% absorptivity change on doubling CO₂ into ~0.5 °C warming, and it repeatedly frames CO₂-doubling sensitivity as ≲1 °C—i.e., far closer to ~1.5 than to ~4.5 °C as argued by Hansen et al.
Click here for the full copy of the Scrip.org study.
Here is Hansen’s quick summary of his study’s climate sensitivity arguments on why climate sensitivity is 4.5 to 4.8:
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Paleoclimate constraints point to a large fast-feedback sensitivity. From glacial–interglacial and broader Cenozoic data, they infer a fast-feedback sensitivity of ~1.2 °C per W m⁻². Because a CO₂ doubling imposes ~4 W m⁻² of forcing, that maps to ~4.8 °C per doubling—well above 3 °C and incompatible with ~1.5 °C. arXivcsas.earth.columbia.edu
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Earth’s Energy Imbalance (EEI) is large and rising. Direct observations (ocean heat uptake/Argo + satellites) show EEI ~ +1 W m⁻² and increasing. A big, persistent positive imbalance implies unrealized warming “in the pipeline,” which—when fit jointly with historical temperature—requires both high sensitivity and substantial aerosol cooling in past decades. csas.earth.columbia.edu
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Aerosol “masking” was stronger than mainstream estimates—and is now weakening. They argue cooling from human-made sulfate aerosols was more negative through 1970–2010 than typically assumed; recent aerosol declines (e.g., IMO 2020 shipping sulfur rules) have rapidly unmasked GHG warming, helping explain the post-2010—and especially 2023—acceleration. This pairing (stronger past masking + recent aerosol cuts) pushes the inferred sensitivity higher. Columbia Universitycsas.earth.columbia.eduACP
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Observed warming since 2010 demands a higher ECS with realistic aerosol histories. Using their reconstruction, the 1970–2010 warming rate (~0.18 °C/decade) should rise to ≥0.27 °C/decade post-2010 as aerosols decline—consistent, they say, with recent observations—again pointing to ECS >4 °C, not ~3 °C or ~1.5 °C. giss.nasa.gov
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Cloud feedbacks are net positive, not negative. They lean on process studies (e.g., high-cloud height increases; lack of credible evidence for strong negative low-cloud feedback) to argue that cloud feedbacks amplify warming, supporting a larger ECS. csas.earth.columbia.edu
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Model response & diagnostics. By analyzing the temperature and EEI response functions (how fast the system responds to a step forcing), they contend many models mix the ocean too aggressively (too “sluggish” surface response), obscuring the signal; combining EEI with temperature tightens the constraints and again favors higher sensitivity. csas.earth.columbia.edu
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State dependence & slow feedbacks add further risk. While their quoted ECS concerns fast feedbacks, they stress that Earth system sensitivity (ESS)—including ice-sheet and other slow feedbacks—is even larger, reinforcing the case that low ECS values (≈1.5 °C) are incompatible with paleoclimate and today’s energy imbalance. csas.earth.columbia.edu
For contrast, IPCC AR6’s best estimate is 3 °C (likely 2.5–4 °C) and “virtually certain” ECS > 1.5 °C; Hansen et al. argue recent EEI and aerosol trends, plus paleoclimate evidence, push the best estimate closer to ~4.5–4.8 °C. IPCCOxford Academic
Hansen Main sources: Hansen et al., Global Warming in the Pipeline (arXiv/OOCC, 2023) and follow-up analyses in 2024–2025 that apply the same framework to the exceptional recent warming. arXivcsas.earth.columbia.eduColumbia University+1
It is the tale of two climate change futures, very, very different from each other. Climate change researchers worldwide have been losing sleep over this controversy. When you review the wildly different and detailed climate change consequences severity and timetable that each different climate sensitivity number predicts, you will see why this climate change controversy is so important to the future of every person alive today.
Hansen’s Climate Change Future:
Currently, many climate change factors are significantly ahead of the worst-case scenario predictions in the IPCC’s AR6 report. Having those many climate factors worse than the IPCC’s AR6 report does not even include the climate sensitivity error discussed above.
If we plug what we honestly feel at Job One is the far more likely real climate sensitivity amount of 4.5 from the James Hansen study and adjust the AR6 worst-case scenario predictions, the real and likely climate change future looks like the following. (You will not hear this discussed by governments or the media because it would rightly panic the general population, and rightly so.)
Calm yourself. As someone interested in climate change, you may find this controversy unpleasant.
Here is an AI-created transparent, physics-consistent stress-test that substitutes ECS = 4.5 °C (vs. the IPCC’s AR6’s central ECS of 3.0 °C) and shows how temperatures, timelines, and headline consequences would shift under SSP5-8.5.
If Hansen is right, humanity is facing a climate change nightmare, far faster than anyone is saying and far faster than even the wealthiest countries are prepared for. If Hansen is right, this is a very serious climate change consequence and timeline problem for you right now, even without considering what it will do to your children’s or grandchildren’s future.
(Please note that the Un IPCC in AR6 uses 3 for its ECS calculations.)
AI used two complementary scalings:
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Proportional (upper-bound) for late-century/equilibrium-like outcomes: warming scales ~linearly with ECS → multiply by 4.5/3 = 1.5×.
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Transient-aware for near/mid-century (ocean heat uptake dampens the immediate effect): scale by (ECS_new/ECS_AR6)^β with β≈0.7 (near-term) and β≈0.8 (mid-century).
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Timing: if warming is higher at each date, milestones arrive sooner. A simple, conservative rule is ~33% earlier(because 1/1.5 ≈ 0.67) measured from a ~2020 baseline.
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Think of this as a careful what-if overlay on AR6 SSP5-8.5, not an official IPCC output.
| Period (AR6) | AR6 best-estimate warming | Scaled with ECS=4.5 |
|---|---|---|
| 2021–2040 (midpoint ~2030) | ~1.6 °C | ~2.1 °C (transient-aware: 1.6×1.5^0.7 ≈ 2.13) |
| 2041–2060 (midpoint ~2050) | ~2.4 °C | ~3.3 °C (transient-aware: 2.4×1.5^0.8 ≈ 3.32) |
| 2081–2100 (midpoint ~2090) | ~4.4 °C | ~6.6 °C (upper-bound equilibrium: 4.4×1.5 = 6.6) |
For “by YEAR” statements in AR6 SSP5-8.5, bring the year ~33% earlier relative to ~2020:
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2050 → ~2040
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2080 → ~2060
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2090 → ~2067
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2100 → ~2073
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| Topic | AR6 worst-case (baseline) | With ECS=4.5 (temperatures ↑ as above; timing ≈33% earlier) |
|---|---|---|
| Extreme heat, heavy rain, drought | Frequency/severity rises with each +0.5 °C; widespread by mid-century, stronger by late-century. | Hit mid-century-level risks ~2040 (not ~2050) and late-century-level risks ~2060s (not ~2090s). Intensities at a given calendar date are higher because background warming is ~0.5–2.2 °C higher than AR6’s central values. |
| Arctic sea ice (Sept. “practically ice-free”) | At least once before 2050in all scenarios; becomes more frequent at higher warming late-century. | First “practically ice-free” by ~2040 (not “before 2050”), and the frequency increase that AR6 expects late-century arrives mid-to-late 2040s–2050s. |
| Global mean sea level (GMSL) | ~0.23 m by 2050; ~0.77 m by 2100 (likely ranges in AR6). | Timing: the 2100 risk environment arrives ~2073; the 2050 environment arrives ~2040. Amounts: SLR doesn’t scale linearly with ECS; expect higher-end AR6 ranges to be engaged earlier, and tail-risk (ice-sheet) contributions become more salient sooner. Treat ~0.77 m conditions by ~2070s as a plausible stress level, with elevated potential for >1 m by late-century under this hotter, faster pathway. |
| Coastal extremes (historic 1-in-100 yr water levels) | Become at least annual at >½ of sites by 2100. | That threshold is reached ~2070s. More sites cross into “annual extremes” sooner; local defenses reach limits earlier. |
| Ocean warming, acidification, deoxygenation | Substantial, worsening through the century; major ecosystem risks. | High-risk conditions arrive ~33% earlier (many regions by 2040s–2050s). With background warming +0.5 to +2+°C above AR6 at like dates, ecological damages are larger and earlier. |
| Glaciers, ice sheets, permafrost | Continued mass loss; permafrost decline; low-likelihood/high-impactice-sheet instabilities can’t be ruled out. | Earlier exposure to high-loss-rate decades (2040s instead of 2050s); tail risks (e.g., Antarctic instabilities) become relevant earlier in the century. (Note: likelihood labels from AR6 are not mechanically rescaled here.) |
| AMOC (Atlantic overturning) | Very likely to weaken this century; collapse is assessed as unlikely. | Weakening manifests earlier (signals evident sooner than AR6’s timing). No change to AR6’s qualitative “collapse unlikely” judgment purely from this mechanical ECS swap. |
| Human systems & ecosystems (WGII synthesis at high warming) | High/very-high risks by late-century: heat mortality; diseases; food & water risks; biodiversity loss; limits to adaptation, more often exceeded. | These high/very-high risk tiers arrive ~2060s (instead of ~2090s), with higher damages at the same calendar dates due to higher background warming. Some adaptation limits are hit decades earlier. |
How to read these results
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Temperatures: The table provides explicit recalculated warming projections for the near/mid/late-century under an ECS of 4.5.
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Timelines: apply the ~33% earlier rule to AR6’s benchmark years (from a 2020 baseline).
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Impacts: most scale nonlinearly with warming and exposure; the table indicates earlier arrival of AR6’s mid/late-century risk bands plus higher intensity at the exact dates.
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To further illustrate the importance of this science controversy, consider the predictions made using the ECS of 1.5 from the other recent study contesting Hansen’s work.
The predictions are so different that every government in the world could, in one way or another, significantly reduce its fossil fuel reduction programs and drill baby drill for a long time if the second study is correct.
The Other Study’s Climate Change Future :
Here is the same AI transparent, “swap-the-ECS” stress test this time with ECS = 1.5 °C in place of the IPCC’s AR6’s central 3.0 used climate sensitivity number —so you can compare directly to the ECS = 4.5 °C version above. It’s a heuristic overlay on AR6 SSP5-8.5 (not an official re-run of the IPCC models).
How I rescale
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Temperature: scale AR6 best estimates by the ECS ratio. For near/mid-century (ocean heat uptake matters), I use a transient-aware exponent:
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Near-term β≈0.7, mid-century β≈0.8; late-century ~equilibrium → linear.
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Ratio r = ECS_new / ECS_AR6. Here r = 1.5/3.0 = 0.5.
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Timing: shift milestone years measured from ~2020 by a factor 1/r.
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With ECS=1.5 (r=0.5), milestones occur later by ~×2.
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(Earlier, with ECS=4.5, milestones were ~33% earlier because 1/1.5≈0.67.)
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| Period (AR6 bands) | AR6 baseline (ECS=3.0) | ECS=1.5 (r=0.5) | ECS=4.5 (r=1.5) |
|---|---|---|---|
| 2021–2040 (≈2030) | ~1.6 °C | ~0.98 °C (1.6×0.5^0.7 ≈ 0.98) | ~2.13 °C |
| 2041–2060 (≈2050) | ~2.4 °C | ~1.38 °C (2.4×0.5^0.8 ≈ 1.38) | ~3.32 °C |
| 2081–2100 (≈2090) | ~4.4 °C | ~2.2 °C (4.4×0.5) | ~6.6 °C |
Interpretation: With ECS=1.5, warming is substantially lower at each date; with ECS=4.5 it’s higher.
| Benchmark in AR6 | AR6 yr | ECS=1.5 (later by ×2) | ECS=4.5 (earlier by ÷1.5) |
|---|---|---|---|
| “~2030” mid-point | 2030 | 2040 | 2026–2027 |
| “~2050” mid-century | 2050 | 2080 | 2040 |
| “~2090” late-century mid-pt | 2090 | 2160 (beyond 2100) | 2066–2067 |
| “by 2100” statements | 2100 | 2180 (beyond 2100) | 2073–2074 |
(Yes, with ECS=1.5 some late-century AR6 milestones slip beyond 2100.)
| System/topic | AR6 baseline (ECS=3.0) | ECS=1.5 result | ECS=4.5 result (for comparison) |
|---|---|---|---|
| Extremes (heat, heavy rain, drought) | Widespread by mid-century; intensify by late-century. | Later & weaker: AR6’s mid-century risk band shifts to about the 2080s; late-century band may slip beyond 2100. | Earlier & stronger: mid-century band ~2040; late-century band ~2060s. |
| Arctic Sept. “practically ice-free” at least once | Before 2050 in all scenarios. | ~2080 (timing ×2) for first occurrence; frequency ramp delayed toward post-2100. | ~2040 for first occurrence; frequent events migrating into mid-century. |
| Global mean sea level (≈0.23 m by 2050; ≈0.77 m by 2100) | 2050 / 2100 | ~2080 / ~2180; 21st-century rise skewed toward lower end of AR6 ranges. | ~2040 / ~2073; higher-end AR6 ranges engaged earlier, tail-risk salience sooner. |
| Coastal extreme sea levels (~annual at >½ sites by 2100) | By 2100 | ~2180; many sites don’t see annual extremes by 2100. | ~2070s; many sites cross annual-extreme threshold earlier. |
| Ocean warming, acidification, deoxygenation | Strong, worsening through the century. | Slower onset, smaller magnitude by 2100; high-risk states mostly post-2100. | Earlier onset, deeper stress by mid-century; high-risk states well before 2100. |
| Glaciers, ice sheets, permafrost | Continued mass loss; HLHI* tail risks possible. | Reduced 21st-century losses; HLHI tail risks largely pushed beyond 2100. | Accelerated losses; HLHI tail risks become relevant earlier. |
| AMOC | Very likely to weaken; collapse unlikely. | Weaken later/slower; collapse still unlikely. | Weaken earlier/faster; collapse still assessed unlikely (qualitatively). |
| Human & ecosystems (WGII high/very-high risks) | Escalate by late-century; adaptation limits are more often exceeded. | Shift later: many high/very-high risk tiers do not fully materialize by 2100; adaptation limits are less frequently reached this century. | Shift earlier: high/very-high tiers in 2060s; more limits to adaptation crossed earlier. |
HLHI = low-likelihood, high-impact.
Quick takeaways
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With ECS=1.5, SSP5-8.5 still forces hard, but 21st-century realized warming and impacts are markedly lower than AR6’s central case, and many “late-century” AR6 milestones arrive after 2100.
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With ECS=4.5, the opposite holds: higher temperatures sooner and earlier arrival of AR6’s late-century risk bands (often by the 2060s–2070s).
The radical differences between these two ECS calculations should disturb everyone in the climate change science movement due to the significant discrepancies in the outcomes.
The Calls to Action
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“Scientists: send your analysis or rebuttal to ([email protected]) and we’ll publish a curated forum.”
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“Educators/Orgs: co-host a public roundtable on this controversy.”
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“Readers: share this with one climate scientist, one policymaker, and one friend today.”
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More About Climate Sensitivity
Ensuring that the correct climate sensitivity constant is used in climate change computer models is crucial. If you get this single mathematical climate sensitivity constant number wrong, then every other major calculation that uses that incorrect constant will also be wrong!
What this means in the area of climate change is that if this constant is incorrect:
1. All predictions for climate change consequences, including their severity and time frames, would also be wrong.
2. All targets and time frames for the reduction of global fossil fuel use would also be wrong.
3. All risk assessment and climate planning based on this incorrect climate sensitivity constant will also be wrong and potentially dangerous for anyone relying on it. And,
4. The farther the incorrect climate sensitivity constant is from being the correct climate sensitivity constant, the further off-key climate change predictions, targets, and timetables will be. In this climate sensitivity controversy, the calculations could not be farther off; one is about 1.5, and the other is 4.5 to 4.8. One calculation is three times larger than the other.
Other Climate Sensitivity Information
1. We have written extensively over the past decade on the problems of the IPCC and others underestimating the actual climate sensitivity number. Click here to see our summary article loaded with illustrations that show the history of this critical controversy.
2. The immediate political and economic implications are also enormous. If the published study by Scrip.org is correct, the Trump administration, the Heritage Foundation, and the global fossil fuel cartel will be overjoyed. And “drill, baby drill” will become a political and economic slogan you will hear more and more of.
4. As a disclosure, Job One For Humanity has previously used the climate sensitivity calculations of James Hansen. However, after reviewing the new Scrip.org published study, we recognized the urgent need to immediately resolve the scientific challenge posted by Scrip.org to current climate sensitivity calculations as soon as possible.
Are we going to be safer than we think, or is the climate change nightmare coming much faster than anyone is prepared for? Will it kill a major portion of humanity? Those are very possible outcomes for humanity resulting from the conflict between these two opposing studies.
We look forward to climate scientists from all over the world helping to resolve this massive and crucial controversy surrounding the correct climate sensitivity.
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