There are a lot of different flavors of people who profess
or demonstrate that they don’t understand what science knows about climate
change: those that don’t think global
warming is happening, those that take the more malevolent view that it’s not
happening and there’s a global scientific conspiracy to conceal the truth,
those that just think it’s happening some but it isn’t a problem, or those see
it as a concern but also think that it isn’t going to be a bigger problem in
the near future.
As someone who has spent time trying to learn about the science,
as various aspects have been important to my professional career, and as someone
who tries to keep up with the science as more is learned, it pains me (to put
it mildly) that there are people who believe that it isn’t happening, or worse,
that it’s a “hoax”.
So I have tried to explain, in simple terms, sometimes
indignantly, that there is scientific data that shows conclusively that global
warming is happening right now, and the cause of it Is increasing concentrations
of CO2 in the atmosphere, and the cause of that is human activities, primarily
and predominantly the burning of fossil fuels.
And one of the key elements of trying to explain this is the data that
shows the middle stratosphere is cooling.
Now, I wrote a blog post a couple of years ago, by request,
as to why science knows that CO2 is causing this. That post is here: https://tugpullpushstop.blogspot.com/2017/03/my-explanatinon-of-why-we-know-its-co2.html
Point Number 2 is entitled: If CO2 is absorbing longwave
radiation, there must be a directly observable effect.
Well, there is a directly observable effect, and that’s the
cooling of the middle stratosphere. If
you are wondering why I emphasize the middle stratosphere and not the whole
stratosphere, it’s because there are two factors that cool the entire
stratosphere – one is ozone depletion, and the other is the increasing
concentration of CO2 in the troposphere.
As ozone depletion has leveled off (thank you, Montreal Protocol), the
cooling rate of the entire stratosphere has slowed. More on this later.
However, the effects of ozone depletion and CO2 are somewhat
separated in location in the stratosphere.
The effects of ozone depletion happen in the lower and middle stratosphere,
but the effect of increasing tropospheric CO2 is mainly in the middle stratosphere. That’s shown here:
So now let’s get briefly into why increasing tropospheric
CO2 causes cooling in the middle stratosphere.
First of all, the troposphere warms and cools convectively; as the earth’s surfaces warm up or cool down,
the overlying atmosphere changes temperature accordingly, and warm air rises
and cool air descends, making cells of circulation around the globe, that get
moved around and stirred together by the circulation of the atmosphere (which
is partly affected by the rotation of the Earth). If you want more than that, study
meteorology. But the stratosphere warms
and cools radiatively – it is warmed by infrared radiation rising from the
troposphere, and it is cooled as heat radiates into space. If you wonder about the difference between
convective and radiative warming, turn on your stove and hold your hand over
the burner (not too close). The heat
you feel, especially if the burner is red (speaking of an electric stove element
here), is infrared radiation. Now, if
you put a pot of water on the burner, the bottom of the pan heats up, that heat
gets transferred to the water, the water circulates as it gets warmer – that’s
convection.
So, the simple and KEY fact is this: as tropospheric CO2 increases, it traps more heat
aka longwave IR in the troposphere, so less longwave IR is reaching the middle
stratosphere over time to warm it. And since
the middle stratosphere is getting less longwave IR, and it’s still radiating
to space, it cools off. I’ll return to
this point in my conclusions.
The reason I’m writing this now is that when I recently
brought up the phenomenon of middle stratospheric cooling on Twitter in response
to a tweet, I was challenged by a non-believer (putting it mildly) with a
couple of resources from oppositional Web sites. What was provided was not something I was
unfamiliar with – in searching previously on this subject, I’ve encountered it,
and I’ve also seen explanations of some of the problem embedded in it. It relates
back to the problem of deconvoluting the effects of ozone depletion and increasing
tropospheric CO2.
One of the problems that I’ve encountered is that there isn’t
a lot of literature about this phenomenon, and sometimes what I’m trying to
find isn’t the main point of a scientific paper. But I decided now is the time to consolidate
the most up-to-date papers I can find.
So I went out and found some. And
the bottom line is – it’s still happening.
So let’s get started.
One of the key figures I’ve noted and posted comes from Remote Sensing
Systems (RSS). This figure shows data
from the middle stratospheric channel C13 of the Advanced Microwave Sounding
Unit (AMSU). There are AMSUs on several satellites, and some of them are still
operating. So what I don’t know is why
there is no AMSU data for the middle stratosphere at RSS after mid-2013. Maybe someone who knows will read this and
understand it.
Below are the results of my reference searching. I’m providing a link to the paper page (even
though only the abstract may be available unless you're a subscriber), the title of the paper, and the
relevant quotes.
Postmillennium changes in stratospheric temperature
consistently resolved by GPS radio occultation and AMSU observations (2017)
“This study exploits two independent sets of satellite observations by Advanced Microwave Sounding Unit (AMSU) and GPS radio occultation (RO).”
“The observations indicate a statistically significant global cooling in the middle stratosphere since 2001 at a mean rate of −0.14 to −0.36 K/decade and insignificant change in the lower stratosphere.”
Regional and seasonal stratospheric temperature trends in
the last decade (2002–2014) from AMSU observations (2015)
“SSU operated on NOAA platforms from 1979 to 2006, i.e., covering more than 25 years. It provided invaluable global‐scale stratospheric temperature data; however, SSU data require special processing before it can be used in climate applications.”
“They found that over the more than three decades coverage period the stratosphere cooled with a rate of −0.1 to −0.2 K/decade at the lower stratosphere to ∼−0.5 to −0.6 K/decade in the middle and upper stratosphere, but the cooling slowed down in the period of 1998–2015 compared to the earlier one (1979–1997).”
Towards a physical understanding of stratospheric cooling
under global warming through a process-based decomposition method (2016)
“... this study demonstrates that the changes of radiative radiation due to CO2, ozone and water vapor are the main drivers of stratospheric cooling in both winter and summer. They contribute to the cooling changes by reducing the net radiative energy (mainly downward radiation) received by the stratospheric layer.”
Attributing the forced components of observed stratospheric
temperature variability to external drivers (2015)
“Finally, the anthropogenic response in the upper stratosphere gives rise to a cooling of ∼2–3 K over the 27‐year period, with two thirds of this attributed to GHGs, and one third to ozone depletion.”
Stratospheric Temperature Trends Inferred from the Merged
SSU and AMSU-A Observations (2017)
“The total decreases in global mean temperatures found from the merged dataset were from 1.8K in the middle stratosphere to 2.4K in the upper stratosphere during 1979-2015.”
The Stratospheric Changes Inferred from 10 Years of AIRS and
AMSU-A Radiances (2017)
“The results show a cooling rate of 0.65 ± 0.11 (2σ) K decade−1 in the upper stratosphere above 6 hPa, approximately 0.46 ± 0.24 K decade−1 in two midstratospheric layers between 6 and 30 hPa, and 0.39 ± 0.32 K decade−1 in the lower stratosphere (30–60 hPa).”
Stratospheric Temperature Trends over 1979–2015 Derived from
Combined SSU, MLS, and SABER Satellite Observations (2016)
“Linear trends over 1979–2015 show that cooling increases with altitude from the lower stratosphere (from ~−0.1 to −0.2 K decade−1) to the middle and upper stratosphere (from ~−0.5 to −0.6 K decade−1).”
So, as can be seen, through 2015 there is a clear
mid-stratospheric cooling trend in different data sets, using different data instruments,
using different methods, and consistent with what was previously known about
how stratospheric cooling should occur and what causes it. There some variability in the actual numbers, but that's to be expected with the different instruments/measurements/reporting periods.
But why is there a discrepancy in some of the papers that
have been repeatedly cited on oppositional Web sites?
The basic problem with those papers is not in the science,
it’s in the instruments used. The SSU
was imperfect, as one of my references notes, and the methods used to look at SSU
data had to integrate over most of the stratosphere. So they couldn’t distinguish well between the
lower and mid-stratosphere, so when they report the cooling of the whole
stratosphere, the interpretation includes the effects of both ozone depletion
and increasing tropospheric CO2. So
using these instruments and data, over the time period available, there’s a
strong cooling trend caused by both ozone depletion and increasing CO2, which slows
down near the end of the data set because of the cessation of ozone
depletion. Combining that trend with
the imprecision of the SSU (plus the fact that there were multiple SSUs and merging
their data was challenging) means that it’s possible to say that the cooling signal
integrated over the entire stratosphere, into the period when ozone depletion
had leveled off, became much smaller in the SSU data.
But let’s sum up to this point, before my conclusions, with a quote
from the RSS Web site:
“The plot shows the that middle stratosphere cooled during
the most recent 15 years, even as the lower stratosphere ceased cooling.” See?
It’s simple.
CONCLUSIONS: Now
that it’s been pretty well established that the middle stratosphere continues
to cool, here’s an evaluation of the implications. This is a case where Occam’s Razor cuts
pretty sharply. The CO2 molecule absorbs
and re-radiates IR radiation (heat); that’s basic physics (actually physical
chemistry). The stratosphere warms and
cools radiatively; that’s basic (atmospheric)
physics. The effect of increasing CO2 in
the troposphere is to trap more IR in the troposphere, causing less to reach
the stratosphere; again, basic physics
and atmospheric dynamics. And because
less IR reaches the stratosphere, especially the mid-stratosphere, it will cool. Basic physics again. So to insist that this isn’t happening is to
not be in opposition to “global warming”, it’s to be in opposition to basic
physics.
The implications of this are in my previously posted article
(linked above), but here it is in a nutshell.
Since mid-stratosphere cooling is happening, then the effects of increasing
CO2 in the atmosphere are also happening.
That means that the troposphere should be warming, and the additional
heat will be transferred into the Earth climate system; melting ice, warming the oceans, changing
atmospheric circulation patterns, etc.
That also means that paleoclimate events make sense, so an abrupt increase
in Earth’s temperature should accompany the massive and rapid addition of CO2
to the atmosphere during the Paleocene-Eocene Thermal Maximum; increasing CO2 should drive the temperature
change at the end of a glacial period, as Shakun et al. 2012 showed; decreasing atmospheric CO2 should cause the
temperature of the Earth to decrease, as happened in the late Ordovician. Essentially, as Dr. Richard Alley said, “But
an increasing body of science indicates that CO2 has been the most important
controller of global average climate of the Earth."
So, I’m not going to try to affect your opinion about what
might happen in the future as Earth’s climate warms. But there is very little doubt that
increasing CO2 in the atmosphere (the troposphere, really) is causing Earth’s
climate to warm, and mid-stratospheric cooling is a key observation showing
that it is. In fact, if it could
somehow possibly be shown that middle stratospheric cooling ISN’T happening,
that would be an almost fatal blow to the entire scientific framework of CO2-influenced
climate change.
I’m not losing any sleep over that possibility.
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