Hi Ye
Thanks for these comments. The Hansen et al paper is a prepublication
draft and I know he is reworking it for final publication. Would you
mind if I shared some of your comments with him as it would be hugely
valuable to have him address some of the points you raise, if he is
minded to do that.
One of my problems with this paper is my lack of formation in the
underlying climate science and modelling to be able to undertake a
proper critical analysis. Like most others, I am obliged to assume that
if Hansen is saying it, it must be true. But I'm sure that even Hansen
would recognise that that's not necessarily a wise move!
Regards
Robert
On 27/02/2023 14:17, Ye Tao wrote:
Dear all,
Thank you for a fascinating discussion. I believe there is a bit of
confusion, which arises from the differing definitions for ECS
(Equilibrium Climate Sensitivity, see p4 of article) vs ESS (Earth
System Sensitivity, also p4). Bottom line: we will most likely NOT
have a 6C increase in global average temperature by the end of the
century by _holding constant the current level of GHG_. A highly
probable range is 2.3C-4C. This is explained below.
ECS is an artificial concept defined to enable computer model
development and computational thought experiments. Its assessment
requires holding constant slow-response components (forests and ice
cover). These artificial constraints, which greatly simplify code
and speed up calculations, cannot be strictly true in nature; we have
seen forests burning up (cut down increasing quickly due to feedbacks
in the human system) and we have witness dramatic arctic melting, all
within decadal time scales. Therefore, ECS cannot be used directly to
predict reality on and beyond the time scale on which these processes
occur. Hence, Fig. 4 is not a surrogate for reality.
In spite of its artificialness, projections based on a chosen ECS
value is useful for describing a range of times (year to a few
decades) when the simplifying assumptions of unchanging land surface
types can be taken as approximately acceptable. This is also because
some of these changes, when they do happen, can have mutually
cancelling effects at the level of radiative forcing. For example,
darkening arctic oceans could be partially compensated for by brighter
barren landscape emerging out of clear-cut forests and burnt boreal
forests. Within this short time scale, we can use ECS and the
temperature response function of your (favorite) model to
semi-quantitatively project future temperature trends. See Fig. 4 of
paper for examples based on the GISS models. For all intents and
purposes, you can trust Fig. 4, _at the very most_, to Year = 100 on
the X axis.
Beyond Year = 100, Fig. 4 is no longer useful for future projections.
All we can say is that paleoclimate data _potentially_ provides a
constraint on how bad things could get thereafter. This is done by
Hansen et al in Figure 7. Reality is most likely between Figures 4
and a scaled version (ESS/ECS).
Regarding ESS, there is one caveat to keep in mind. The caveat is
that_the ESS value provided in the paper (10C per doubling of eCO2) is
based on historical data at temperature ranges *cooler* than that of
the present and near-future_. And we are obviously heading to hotter,
unprecedentedly warm territories for which paleoclimate data do not
exist. In other words, ESS is itself a function of the temperature at
which it is evaluated (taking the slope of the tangent point along the
temperature axis, theoretically speaking. Linear fitting to a range
of temperature data in practice.).
There is evidence that ESS is a positive function of temperature:
SNYDER, C. W. Revised estimates of paleoclimate sensitivity over the
past 800,000 years. 2019./Climatic Change/
<https://link.springer.com/article/10.1007/s10584-019-02536-0>. 156.
121–138
<https://link.springer.com/article/10.1007/s10584-019-02536-0>. The
implication would be that going forward, ESS could well be larger than
the 10C per doubling of eCO2 Hansen suggests. Oops!Luckily for us
mortals, or unluckily since you are a curious person, no one on this
list would live long enough, which requires waiting several thousand
years, to see experimental verification of the actual value of ESS
valid for the hotter end of the temperature range.
Practically, in thinking about a real-world trajectory, one can
consider the effective, realized climate sensitivity to morph towards
larger values as time passes, from the small Transient climate
response (TCR = 2C per doubling), to ECS (4C per doubling), and then
to ESS (>10C per doubling). Discussions about what happens by the
end of the century therefore could use values intermediate between ECS
and ESS, i.e. somewhere between 4C per doubling of CO2 and somewhat
larger than 10C per doubling of CO2.
The implication of the discussion above is that your previous
understanding of global warming by the end of the century is still
valid. At the current level of GHG, with a rapidly successful
decarbonization program that eliminated fossil fuel aerosols, we will
likely see warming somewhere between 2.3C (as indicated by Figure 4,
and the fact that a ECS of 3.5 was used in its making, in stead of a
more likely 4) and 6C (scaled by the ESS/ECS=3 ratio). The actual
value is more likely to be on the lower end of this range, i.e.
between 2.3C and 4C.
Note, importantly, that assumptions also include immediate carbon
neutrality and holding constant current GHG levels, which for those of
us who really thought deeply and understood the impossibility of
meaningful CO2 capture, can readily accept as being compatible with a
voluntary or involuntary contraction of the human enterprise,
constrained by primary productivity projections and the inevitably
increasing cost of energy production (declining EROI) going forward.
Cheers,
Ye
On 2/27/2023 8:22 AM, Robert Chris wrote:
Doug and Ron
Here's how I arrived at my conclusions.
From the extracts below, I conclude, given that by 2020 (or
thereabouts) we had already doubled atmospheric GHGs from
pre-industrial including non-CO_2 GHGs, that we will eventually warm
the surface by 10^o C with a climate response e-folding time of ~100
years provided the offsetting cooling from anthropogenic aerosols
continues to decline and is eventually largely eliminated.That means
that by 2050 the warming would be ~2.4^o C less the residual aerosol
cooling of, say 0.4^o C, giving their estimate of 2^o C.
·When all feedbacks, including ice sheets, are allowed to respond
to the climate forcing, the equilibrium response is approximately
doubled, i.e., ESS is ~ 10°C.
·Yet the time required for the [improved] model to achieve 63% of
its equilibrium response remains about 100 years.(See Fig 4b –
note log x-axis.)
·With all trace gases included, the increase of GHG effective
forcing between 1750 and 2021 is 4.09W/m^2 , which is equivalent
to increasing the 1750 CO_2 amount (278 ppm) to 561 ppm (formulae
in Supporting Material). We have already reached the GHG climate
forcing level of doubled CO_2 .[Note that they develop a scaling
factor of 2.4^o C per W/m^2 which corresponds to 10^o C for the
4W/m^2 of current GHG forcing.]
·Declining aerosol amount implies acceleration of global warming
above the 1970-2010 rate.
·Global temperature responds reliably to climate forcing on
decadal time scales, with about 50% of the response in the first
decade, with about 15% more in the next 100 years
·we expect some [aerosol] reduction and a forcing increase of at
least +0.1 W/m^2 per decade [between 2010 and 2050].
·we estimate that the global warming rate in 2010-2040 will be at
least 50% greater than in 1970-2010, i.e., at least 0.27°C per
decade.
·The poster child for warming in the pipeline is Fig. 7, showing
that equilibrium warming for today’s GHG level, including slow
feedbacks, is about 10°C. Today’s level of particulate air
pollution reduces equilibrium warming to about 7°C.
·The 7-10°C global warming is the eventual response *if today’s
level of GHGs is fixed and the aerosol amount is somewhere
between its year 2000 amount and preindustrial amount*. (emphasis
added) [Note that the assumptions here are that ‘today’s level of
GHGs is fixed’, which I take to mean that future emissions are
ignored, and aerosols are currently lower than they were in 2000.]
Here's a simple graph showing the realisation of 10^o C of
warming with an e-folding time of 100 years.Assume it starts in
2020 or thereabouts (when atmospheric CO_2 e reached 556ppmv.).
Regards
Robert
On 27/02/2023 03:50, Douglas Grandt wrote:
Ron and Robert,
Visually, the shape of the curve is something like this … more or
less … as best I can fathom.
This is my interpretation of Hansen's assumptions and conclusions,
but I very well could be wrong …
Perhaps somebody has chart generating software that would be more
precise than my eyeball.
Doug
On Feb 26, 2023, at 8:58 PM, Ron Baiman <[email protected]> wrote:
*6.3 C (63% of 10) by 2020*
On Sun, Feb 26, 2023 at 7:56 PM Ron Baiman <[email protected]> wrote:
Robert,
Do you have a page number or an explanation of how you arrived
at your figures? In the
paper(https://arxiv.org/abs/2212.04474) on p. 31 I'm finding:
"The 7-10 C global warming is the eventual response if today's
level of GHGs is fixed and the aerosol amount is somewhere
between its year 2000 amount and preindustrial amount." but
the key temp Figure 7 on p. 18 doesn't extend beyond 2025. In
the section on Climate response times (p. 32) the paper states
that the in 2020 GISS GCM: "...the time required for the model
to achieve 63% of its equilibrium response remains about 100
years" which would put the expected temp based on forcing
estimated in the paper at 6.3 C (63% or 10) by 2023. Is this
where you're getting your 6.3 C by 2120 from? Unfortunately, I
have not had the time (and probably not the background) to go
through the entire paper and understand it well!
Best,
Ron
On Sun, Feb 26, 2023 at 7:09 PM Ron Baiman <[email protected]>
wrote:
Thanks for the correction Robert!
Sent from my iPhone
On Feb 26, 2023, at 6:41 PM, Robert Chris
<[email protected]> wrote:
Ron
Hansen et al say that the 10degC is based on 'today's GHG
level' and that it has an e-folding time of 100 years.
That implies 6.3degC by 2120 and a bit less by 2100.
Regards
Robert
On 26/02/2023 23:43, Ron Baiman wrote:
Jim Hansen et al (https://arxiv.org/abs/2212.04474 )
believe that existing legacy GHG's have put us in "in the
pipeline" for 10 degrees C warming by 2100!
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