Hansen
We, and young people, need help from people who understand the essence of climate science. See
Forest versus Trees
https://www.columbia.edu/~jeh1/mailings/2025/ForestTrees.06August2025.pdfor its abbreviation:
Seeing the Forest for the Treeshttps://mailchi.mp/caa/seeing-the-forest-for-the-trees also available on my Substack:
Seeing the Forest for the Trees - by James Hansenhttps://jimehansen.substack.com/p/seeing-the-forest-for-the-treesSummary: seeing the forest for the trees
Climate change depends on climate sensitivity and the strength of the forcing that drives change. Of the main sources of information – paleoclimate, modern observations, and GCMs – the first two are least ambiguous, but all three are consistent with climate sensitivity 4.5°C ± 1°C (2σ, 95% confidence) for doubled CO2, which excludes IPCC’s best estimate of climate sensitivity (3°C for doubled CO2). IPCC also underestimates the strength of the aerosol climate forcing.
In the real world, climate sensitivity and aerosol forcing are independent, but they are joined at the hip in climate assessments that focus on the ability of GCMs to reproduce observed global warming. It is reasonable that climate modelers use observed global temperature change to help constrain the GCMs. The complication is that there are two major unknowns: climate sensitivity (mainly because the cloud feedback is uncertain) and the climate forcing (because the aerosol forcing is unmeasured), while there is only one hard constraint (the observed global warming rate). As a result, if climate sensitivity turns out to be high, greater aerosol forcing (i.e., greater aerosol cooling) is required for agreement with observed global temperature.
Independent sources of information, from paleoclimate on climate sensitivity and from satellite data on the cloud feedback, show that, in reality, climate sensitivity is high. Thus, aerosol forcing (and the aerosol cooling effect) have also been underestimated by IPCC. In addition, aerosol cooling has weakened since 2005, mainly because of reduced emissions from China and ships.
Those are the principal conclusions of our two papers (“Global warming in the pipeline” and “Global warming has accelerated”) that address the fundamental issues of climate sensitivity and the human-made climate forcing. These issues are a large part of the “forest” of climate science.
Within that part of the climate science forest, many uncertainties remain. For example, how does the cloud feedback work? Tselioudis et al.[3] suggest that it is mainly from a poleward shifting of climate zones, as opposed to an effect of global warming on cloud microphysics. It is important to understand such issues, as the correct explanation may affect the continuing climate change.
Another example: we argue that reduction of ship aerosols has more effect on global temperature than reduction of aerosols from China, even if the mass reduction of Chinese emissions is larger. Ships emissions are more efficient in affecting clouds because they are injected into relatively pristine ocean air at altitudes that have greatest effect on cloud formation. Observed global distributions of albedo and temperature change are consistent with a large role for ship emissions, although alternative explanations for those distributions may be possible. Temporal changes of albedo and temperature also match better with the 2015 and 2020 changes of ship emissions, rather than with the decrease of emissions from China, which began in 2006.
The forest of climate science includes other areas – besides climate sensitivity and climate forcings – that are also important. For example, potential impacts of climate change include shutdown of the overturning ocean circulation and large sea level rise,[4] which may be the most important of all the climate issues. These climate impacts depend on the magnitude of global warming, which is a reason to first consider climate sensitivity and climate forcings.