June 2025 Atmospheric CO2 Content 429.61 ppm; June 2024 426.91 ppm; June 2023 423.68 ppm

[hatrack]

https://gml.noaa.gov/ccgg/trends/

https://gml.noaa.gov/ccgg/trends/data.html

Surprised ESRL is still allowed to publish the data . . . .

[OKIsItJustMe]

1. Target Atmospheric CO2: Where Should Humanity Aim? - Hansen, et al.

Abstract: Paleoclimate data show that climate sensitivity is ~3°C for doubled CO₂, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO₂ for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO₂ was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO₂ fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO₂ forcings. An initial 350 ppm CO₂ target may be achievable by phasing out coal use except where CO₂ is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO₂ is not brief, there is a possibility of seeding irreversible catastrophic effects.

Hansen, J., M. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target atmospheric CO₂: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231, doi:10.2174/1874282300802010217.
https://www.giss.nasa.gov/pubs/docs/2008/2008_Hansen_ha00410c.pdf

[hatrack]

2. Yes, a paper from seventeen years ago . . .

And that paper was preceded by Hansen's appearance on Capitol Hill (at which point we knew it was for real) a full 20 years before.

[OKIsItJustMe]

3. Climate impact of increasing atmospheric carbon dioxide. - Hansen et al. 1981

Summary. The global temperature rose by 0.2°C between the middle 1960's and 1980, yielding a warming of 0.4°C in the past century. This temperature increase is consistent with the calculated greenhouse effect due to measured increases of atmospheric carbon dioxide. Variations of volcanic aerosols and possibly solar luminosity appear to be primary causes of observed fluctuations about the mean trend of increasing temperature. It is shown that the anthropogenic carbon dioxide warming should emerge from the noise level of natural climate variability by the end of the century, and there is a high probability of warming in the 1980's. Potential effects on climate in the 21st century include the creation of drought-prone regions in North America and central Asia as part of a shifting of climatic zones, erosion of the West Antarctic ice sheet with a consequent worldwide rise in sea level, and opening of the fabled Northwest Passage.

Hansen et al. 1981
Hansen, J., D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. Russell, 1981: Climate impact of increasing atmospheric carbon dioxide. Science, 213, 957-966, doi:10.1126/science.213.4511.957.
https://www.giss.nasa.gov/pubs/docs/1981/1981_Hansen_ha04600x.pdf

His appearance on Capitol Hill served notice that “anthropogenic carbon dioxide warming (had) emerge(d) from the noise level of natural climate variability.”

[OKIsItJustMe]

4. My key point is that figure of 350 ppm

(Does anyone remember where the name 350.org came from?)

350 ppm was not suggested as an end goal, but as an initial target, the logic being that once we lowered atmospheric CO₂ to that point, we would have developed methods to accomplish it, and we could continue using those methods to lower atmospheric CO₂ to a level determined to be “safe.”