Our two recent papers (Global warming in the pipeline[1] and Global warming has accelerated[2])
[hereafter Paper 1 and Paper 2] were long – due to our research
approach and our intent to raise numerous issues. Thus, we summarize the
most important conclusions here.
Principal objectives of research in climate change are to evaluate
climate sensitivity and the forcings that are driving climate change.
Our analysis approach places comparable emphasis on each of three
sources of information: (1) paleoclimate data, i.e., the long history of
climate change, (2) modern observations of ongoing climate change, and
(3) global climate modeling. Full exploitation of all three research
tools allows conclusions to be reached with a higher degree of
confidence than otherwise would be possible.
We summarize these three analyses, each in a page or at maximum two
pages. These summaries are intended for people with some scientific
bent. If we do not get such people to appreciate the science, the clique
(see below) will continue to obfuscate reality. However, these
summaries still make for a long document. Here we skip to the Summary.
4. Summary: 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
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.
5. Communication of the climate situation
The Secretary General of the United Nations asserts that the goal of
keeping global warming under 1.5°C is still reachable if nations
increase their ambitions for future emission reductions. In reality, the
1.5°C goal has long been deader than a doornail. This raises the
question: are we, the scientific community, doing an adequate job of
informing governments and the public?
In our present communication, we criticize IPCC’s science analysis.
However, despite the flaws that we note, IPCC is doing what they were
asked to do. Their reports contain authoritative information
painstakingly written by experts in their fields. The reports are useful
references, but governments and the public need more to properly inform
their decision-making.
When we presented our most recent paper,[2] responses in the media by
other scientists consisted of ad hominem attacks on the first author,
e.g., “Hansen exaggerates,” “Hansen makes lots of mistakes,” “Hansen is
not collegial,” and comments that our analysis was “too simple” and our
conclusions were “outside the mainstream.” None of the comments
addressed the climate science in our paper, which we have summarized
here. Yet these few articles in the media, appearing on the day that our
paper came out, were sufficient to shut down public discussion of our
paper.
Issues raised in our paper are relevant to understanding the course of
climate change. So, how is it that a small (all-male)[5] clique is able
to control the climate research conversation? At least they spurred the
first author to move back to Columbia University (see End of an Era),[6] where it may be possible to work more with young people, and hopefully communicate more effectively.
We are grateful to the people who continue to support CSAS. This year,
our long-time friend, colleague, and senior scientist Makiko Sato
retired. Her unique combination of scientific and artistic abilities is
irreplaceable; her dedication as our climate data expert will be sorely
missed. The consolation is that we can now support two entry-level
positions at Columbia University: one specializing in climate data and
one as program coordinator for CSAS, a position that has been vacant for
the past several years. Gen Z is coming of age; we hope to find new
graduates ready to realize their potential to help shape the future.
6. International Court of Justice
Let’s end on a bright, scintillating, note: the recent ruling by the
International Court of Justice in the Hague on global climate change,
which deserves far more attention than it has received. It is the first
time that the ICJ has taken up climate change. In a unanimous decision
the Court determined that:
". . . customary international law sets
forth obligations for States to ensure the protection of the climate
system and other parts of the environment from anthropogenic greenhouse
gas emissions. These obligations include the following:
(a) States have a duty to prevent significant harm to the environment by
acting with due diligence and to use all means at their disposal to
prevent activities carried out within their jurisdiction or control from
causing significant harm to the climate system and other parts of the
environment, in accordance with their common but differentiated
responsibilities and respective capabilities;
(b) States have a duty to co-operate with each other in good faith to
prevent significant harm to the climate system and other parts of the
environment, which requires sustained and continuous forms of
co-operation by States when taking measures to prevent such harm. . . ."
Philippe Sands,
legal scholar, author, and leader in getting the case before the Court
and arguing the case, was thrilled that the Court’s ruling was even
stronger than he had hoped. Over time, this ruling surely will be used
extensively and affect courts globally, even within the United States,
despite the fact that such ICJ decisions are advisory.
My long-time attorney Dan Galpern and I went to the Hague during the
trial, even though we knew that I would not be able to deliver testimony
before the Court that I had prepared at the request of the government
of Mauritius (because that government had since been turned out of
office in an election). Instead, I presented my testimony[7] in
a press briefing organized with the help of Eelco Rohling, which
included discussions by Rohling and his Utrecht University colleagues
Appy Sluijs (Prof. of Paleo-oceanography) and Ingrid Robeyns (Prof. of
Ethics of Institutions) and by Dan Galpern.
Philippe Sands notes that at least some of the judges read my testimony,
it was mentioned during their proceedings, and Sands believes that it
affected their ruling. I mention this because CSAS donors have been
supporting our legal efforts for many years. The legal approach can be
slow and often ends in disappointment, but it is an essential part of
actions to preserve climate, and thus we want to emphasize the
successes.
Fireflies. Lastly, I note that Lightning bugs
are making a comeback this summer. We will miss the displays we saw on
our farm in Bucks County, but I will see if there are any in Riverside
Park. Longterm, insects[8] will depend on whether insecticides and herbicides are controlled.
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