Poster's note : Important concept suggesting negative feedback on
albedo changes. Applicable, potentially, to all forms of SRM, but
particularly SAI
http://wattsupwiththat.com/2015/07/29/why-volcanoes-dont-matter-much/
Watts Up With That?
Why Volcanoes Don’t Matter Much
Willis Eschenbach / 5 days ago July 29, 2015
The word “forcing” is what is called a “term of art” in climate
science. A term of art means a word that is used in a special or
unusual sense in a particular field of science or other activity. This
unusual meaning for the word may or may not be logical, but each field
has its terms of art, and it’s useless to complain that they don’t
make sense. The IPCC defines “radiative forcing” as follows:
Radiative forcing
Radiative forcing is the change in the net, downward minus upward,
radiative flux (expressed in W m–2) at the tropopause or top of
atmosphere due to a change in an external driver of climate change,
such as, for example, a change in the concentration of carbon dioxide
or the output of the Sun. Sometimes internal drivers are still treated
as forcings even though they result from the alteration in climate,
for example aerosol or greenhouse gas changes in paleoclimates.
Now, the current climate science paradigm says that regarding the
things that affect temperature, everything averages out in the long
run except for any changes in total forcing. The current paradigm
further says that the future evolution of the climate can be forecast
by the simple linear relationship given as:
Change in temperature equals climate sensitivity times change in total forcing.
Me, I think that’s simplistic nonsense, but let’s set my opinion aside
for a bit and compare their forcing claims to the actual observations
of the changes in forcing. As an example, let me use the forcings that
are the result of volcanic eruptions. The larger eruptions blast
aerosols (various molecules and minerals) into the stratosphere,
reducing the incoming sunshine. These forcings have been estimated by
Sato as being of the following amounts:
Figure 1. Volcanic forcings estimated by Sato et al.
http://data.giss.nasa.gov/modelforce/RadF.txt Forcings are negative
because they represent a reduction in available solar energy due to
volcanic aerosols. The large eruption at the far right is Pinatubo in
the Philippines, 1991, and the eruption to its left is El Chichon,
Mexico, in 1982.
You can see that some eruptions, like that of El Chichon, produced a
much larger aerosol cloud in the northern hemisphere (red) than in the
southern (blue), while others like Pinatubo were more equal in the
distribution of the aerosols between the hemispheres. In all cases,
the hemisphere where the volcano is located shows the greatest effect
from the eruption.
As I mentioned above, I think that the idea that the temperature
slavishly follows the changes in forcings to be a fundamental
misunderstanding of how the climate system operates. Instead, I say
that although initially the temperature responds to the forcings, soon
the climate system responds to the resulting changes in temperature by
changing the forcings themselves, often in very non-linear ways. In
particular, I have presented plenty of evidence that the climate
system responds to increasing tropical temperatures by varying the
timing and strength of the daily emergence of the cumulus cloud field.
Part of the climate system response works like this:
On warmer days, the emergence of the tropical cumulus cloud field is
both earlier and stronger. This cuts down on the available solar
energy by reflecting more of it back to space. The high cloud albedo
means that less sunlight reaches the surface, so the surface cools.
And on cooler days, the opposite occurs. The tropical cumulus field
emerges later, and is weaker. As a result, the day warms up more than
it would otherwise, because there is less cloud albedo and thus more
available solar energy.
All that is required to show that this effect exists is to show that
tropical albedo is positively correlated with temperature … as I have
done here, here, and here.
Now, if we assume for the moment that my theory is correct, what kind
of climate response would we expect to find from a volcanic eruption
large enough to put aerosols into the stratosphere and cause some
global cooling? Well, eruptions reduce available solar energy in two
ways—increased reflection from white aerosols, and increased
absorption from dark aerosols.
So the first thing to happen after the eruption would be the reduction
in incoming sunlight from the increased albedo and increased
stratospheric absorption. Then after the decreased sunlight actually
starts to cause widespread cooling, the climate system would respond.
We’d expect the climate system response following such an eruption to
have the following characteristics:
• Right after the eruption, there would be a reduction in available
solar energy, due to the volcanic aerosols in the stratosphere.
• This initial eruption-induced reduction in available solar energy
would be both deeper and sooner after the eruption in the hemisphere
where the eruption occurred than in the opposite hemisphere.
• As a result, the corresponding climate reaction in the eruption
hemisphere would also both be deeper and occur sooner than the climate
reaction in the opposite hemisphere. In other words there will be a
dose-related effect, where a larger reduction is met with a larger
climate reaction.
• The form of the climate reaction will be an albedo reduction, which
will cause increase in available solar energy. The increase in
available energy will be of the same order of magnitude as the
corresponding decrease due to volcanic aerosols.
With those predictions derived from my theory about the nature and
timing of the climate response, we can compare them to what actually
happened when Mount Pinatubo erupted. I’ve taken the albedo records
for the globe and for each hemisphere individually, and analyzed what
happened after the eruption of Pinatubo in June of 1991. This gave me
the anomaly in the amount of solar energy that is actually available
to the climate system. Figure 2 shows three variables for the period
1984-1997, which includes the eruption of Mt. Pinatubo on June 15,
1991.
First, in black, is a closer look at the same dataset shown in Figure
1. Black shows the global average of the Sato volcanic forcing data
for the period 1984-1997.
Second, in violet, is the aforementioned anomaly in the amount of
incoming sunshine, in watts per square metre. This is the “available
energy”, meaning the solar energy that remains after the albedo
reflections.
Third, in gold, is the amount of incoming solar energy that is
absorbed in the stratosphere. Recall that volcanoes affect the
sunshine in two ways—changes in reflection (violet line) and changes
in absorption (gold line). The gold line shows the reductions from
absorption of solar energy by stratospheric aerosols.
Figure 2. Sato estimated volcano forcing (black), available solar
energy anomaly after albedo (violet), and stratospheric absorption
forcing (gold). The observed values (violet and gold) are expressed as
anomalies around the value they had the month before the eruption. See
below for methods and data sources.
Now, the first thing I noticed is that immediately after the eruption,
all three datasets agree with each other—as we would expect, there is
a precipitous drop in downwelling solar radiation. However, after that
they go their separate ways, so it’s hard to tell what the overall
effect of the absorption and the reflection might be.
For that kind of comparison, I use a running post-eruption average.
This is the average forcing over the period from the date of the
eruption to the date in question. So for example, the data point for
January 1996 represents the average forcing from the date of the
eruption until January of 1996. Figure 3 shows that type of
post-eruption average applied to Figure 1, with the actual Figure 1
data shown grayed out in the background for reference.
Figure 3. Post-eruption averages. Total observed eruptive forcing
[reflection (violet) plus absorption (gold)] is shown in yellow. Other
colors as in Figure 1 — black is the Sato estimate of total volcanic
forcing; violet is available solar anomaly after albedo reflections;
gold is stratospheric absorption anomaly. Each point on the graph
represents the average forcing from the eruption until that date.
The important thing to note is that from the eruption to the end of
the record (end of 1997) the Sato forcing estimate (black line) has an
average forcing of about minus one watt per square metre (W/m2).
However, the observed change in total forcing of the period (yellow
line, sum of purple (albedo forcing) and gold (absorption forcing) is
a bit more than plus one watt per square metre.
Also, the speed of the climate response is visible in Figure 3. The
total forcing (yellow line) follows the Sato forcing estimate (black
line) for the first four months or so after the eruption. But after
that, while the Sato calculated forcing continues to become more and
more negative, the observations show that the total observed forcing
does not ever become much more negative than it was at four months
after the eruption. Instead, it runs level for about a year, and then
rapidly increases. By the end of 1993, the observed post-eruption
average forcing has returned to pre-eruption values … while the Sato
theoretical forcing is still at minus two W/m2.
Now, Figures 2 and 3 show the global situation. We also have data for
each hemisphere separately. This will let us observe the difference in
the response of the climate in the two hemispheres. Here are the
observed forcing and the Sato theoretical forcing for the northern and
southern hemisphere.
Figure 4. As in Figure 2 but by individual hemisphere. The two panels
show the Sato estimated forcing (black), the solar absorption forcing
(gold), and the available solar energy after albedo (upper panel, red,
northern hemisphere; lower panel blue, southern hemisphere)
The most notable difference between the hemispheres is the deep drop
in available solar energy in the northern hemisphere (red line, upper
panel) during the months immediately following the eruption. I note
also that following that initial drop, the amount of available energy
in the NH steadily increases in both the absorption (gold) and
reflection (red) datasets.
To conclude this analysis I looked at the post-eruption averages for
the individual hemispheres. Figure 5 shows those results:
Figure 5. As in Figure 3 but by individual hemisphere. These show the
running average starting at the time of the eruption and moving
forwards.
In the northern hemisphere we can see that the initial drop in forcing
was almost as large as the Sato estimate. However, from there, the
climate response kicked in, and the amount of available energy started
to rise rapidly. In the southern hemisphere, on the other hand, the
response was smaller and initially slower.
However, once the SH response began, the available solar energy rose
very quickly. Both hemispheres took about the same amount of time,
about two years, for the average forcing over the post-eruption
interval to return to zero.
And in both hemispheres, the eventual response was nearly
identical—the average change in total available sunshine at the end of
the record is about plus a watt and a half per square metre, compared
to the Sato estimate which has an average change to the end of the
record of minus one watt per square metre.
Conclusions: The main conclusion that I draw from this is that the
central paradigm of modern climate science is wrong—temperature does
not slavishly follow the forcings.
To the contrary, when the tropical temperature changes, the solar
forcing subsequently changes in the opposite direction, negating much
of the effect of the volcanoes.
And in particular, the observations agree with the theoretical
predictions, which were:
• Right after the eruption, there would be a reduction in available
solar energy, due to the volcanic aerosols in the stratosphere.
• This initial eruption-induced reduction in available solar energy
would be both deeper and sooner after the eruption in the hemisphere
where the eruption occurred than in the opposite hemisphere.
• As a result, the corresponding climate reaction in the eruption
hemisphere would also both be deeper and occur sooner than the climate
reaction in the opposite hemisphere. In other words there will be a
dose-related effect, where a larger reduction is met with a larger
climate reaction.
• The form of the climate reaction will be an albedo reduction due to
the temperature reduction, which will cause an increase in available
solar energy. The increase in available energy will be of the same
order of magnitude as the corresponding decrease due to volcanic
aerosols.
These theoretical predictions are all visible in the graphs above, and
they lead back to the title of this piece. The reason volcanoes don’t
matter much is that the climate rapidly responds to re-establish the
status quo ante. Yes, eruptions do put loads of aerosols into the
stratosphere; and yes, these aerosols do cut down available solar
energy; and yes, this does have local effects in space and time … but
because available solar energy in the tropics goes up as the
temperature goes down, the balance is quickly restored. As a result of
this and other restorative phenomena, the climate system has proven to
be surprisingly insensitive to such variations in forcing.
My best regards to everyone,
w.
The Usual Request: If you disagree with someone, please quote the
exact words that you disagree with, so we can all be clear both who
and what you are objecting to.
Methods and Data
Sato Theoretical Forcing: The Sato data is from here. Following Sato,
I have used the aerosol optical depth (AOD) to estimate the forcing.
Sato says that the forcing is estimated as a linear function of the
AOD, which seems reasonable. I have used his formula for the
“instantaneous” forcing (as opposed to the “equilibrium” or other
forcings), since we are discussing the immediate effects of the
eruptions.
Available Solar Energy Anomaly After Albedo: For the albedo data, I
digitized the albedo shown in Figure 5(b) of the most interesting
study, Long-term global distribution of Earth’s shortwave radiation
budget at the top of atmosphere, by Hatzianastassiou et al. I
multiplied the monthly (1 – albedo) by the monthly TOA solar to get
the absolute value of the available solar energy after albedo
reflections. Then I subtracted the “climatology”, which means the
monthly averages, from that dataset to get the anomaly in available
solar energy
Stratospheric Absorption: While researching for this post, I had an
interesting insight about the increase in stratospheric absorption of
solar energy after an eruption. This was that I could use the change
in stratospheric temperature to calculate the amount of additional
sunlight being absorbed, using the Stefan-Boltzmann relationship. For
the stratospheric temperatures, I used the UAH satellite based
estimate of the lower stratosphere, Version 6.0beta2, available here.
Yes, I am aware that this is an uncertain estimate, but it’s accurate
enough for a first-order analysis such as this one.
Sensitivity to Assumed Emissivity: I used the most conservative
assumption, that of a blackbody relationship with emissivity=1. If we
assume a graybody, the change in solar absorption corresponding to a
given temperature difference goes down in proportion to the change in
emissivity. This reduces stratospheric absorption forcing. And this in
turn increases the difference between the observed (yellow line) and
the Sato theoretical forcings (black line) in the period immediately
after the eruption, but makes little difference in the later years
because the stratospheric absorption term is small. For an example of
the change in the early years, using an emissivity of 0.5 reduced the
largest total forcing decrease (reflected plus absorbed) to about
minus one W/m2, rather than the approximately minus 1.75 W/m2 as shown
after the eruption in Figure 3.
Data: One of the bad things about this is that the dataset is so
short. Can’t be helped, because as far as I know there’s no
hemispheric estimate of the albedo during the time of the previous
eruption, El Chichon in 1982. (If you know of such a dataset, please
post a link). But the good side of short data is there’s not much of
it, so it’s easy to move around … for example, I’ve been looking at
one-minute radiation measurements from Mauna Loa, 31 million data
points per year since 1980. That’s hard to download, and too big for
me to put up on something like photobucket.
But here we only have 14 years at 12 months per year = 168 records, so
it’s small enough to put into an Excel spreadsheet, which I’ve done in
.csv format here. The spreadsheet contains the TOA solar values, the
albedo values, the Sato forcing values, the stratospheric temperature
values, and as a special bonus, the hadCRUT4 records for the period
both globally and for individual hemispheres. Enjoy.
