Way back in nineteen sixty-seven, a couple of smart cookies called Suki
Manabe and Richard Wetherald were working on a project for the United States Weather Service,
to build a numerical model that could be used to study the climate.
In those days of course, the world’s most powerful mainframe supercomputers had far less
computing power than the mobile phone you have in your pocket today, so Manabe and Wetherald
were trying to make the physics as simple as possible so that the valve-driven contraptions
of their time wouldn’t overheat and cause a major fire in the basement of the building.
To achieve that simplification the two scientists wanted to know the minimum number of discrete
levels in the atmosphere, and which greenhouse gases it was essential to include in the model
so that it would adequately reflect the way temperatures vary with altitude. They were kind
of focussed on water vapour initially because that is the main greenhouse gas by far, but when they
included the other main gases, they created a data table representing the first ever robust estimate
of how much the world would warm if carbon dioxide concentrations doubled. That estimate
was two-point three six degrees Celsius, not far off today’s super computer model estimates of
three degrees Celsius. What their model also found though, was that while increased levels of carbon
dioxide caused a warming effect in the lower part of the atmosphere that we call the troposphere,
it actually had a cooling effect higher up, in the region known as the stratosphere. Now you
might think, ‘well that’s handy, the tropospheric warming that we humans have apparently caused
by our profligate carbon dioxide emissions will surely be offset by this stratospheric cooling,
right? Which means we can all calm down and stop panicking’. And that would be nice, wouldn’t it?
If it were even remotely true, which it isn’t. A slew of recent research papers has found that,
far from being an advantageous temperature offset, cooling in the upper layers of our
atmosphere could have consequences that we could well do without. It may be endangering the safety
of the satellites that run our daily lives, and even more worryingly, there’s now evidence that
it’s opening up another hole in the ozone layer. This time it’s not above Antarctica though,
it’s at the other end of the world above the Arctic – which is just what we need isn’t it!?
Hello and welcome to just have a think, I’ll come onto that rather daunting Ozone
layer hole a bit later in the video, but first of all, it’s worth having a
quick look at that original work by Manabe and Wetherald, because it’s apparently a bit of an
iconic thing in the climate modelling world. So, here’s a very basic representation of how
the earth’s atmosphere is composed. Down here near the surface you’ve got the troposphere,
which goes up to about 11 kilometres. Then there’s the stratosphere which reaches up
to about 50 kilometres. After that you’ve got the mesosphere, which is where meteors tend
to provide us with their impressive light shows. Then finally there’s the thermosphere which goes
right out to about seven hundred kilometres above the earth’s surface. That’s where
low earth orbit stuff like the space station and most commercial satellites can be found.
As we go upwards, you might instinctively expect the air to get progressively colder,
and for the most part you’d be right, certainly until you reach the stratosphere anyway. Then
some weird stuff happens that actually causes air temperature to start rising again. To
explain the intricate detail of why that happens, I recommend jumping over to my YouTube buddy Simon
Clark’s channel. Simon has a PhD in theoretical atmospheric physics, and in twenty-twenty-two he
made this brilliantly entertaining explainer video all about the upper levels of our atmosphere.
You can jump straight to that video by clicking up there somewhere or by following the link that I’ve
left in the description section. By the way, Simon also recently published this book,
called Firmament, which explains in language that you and I can understand exactly how all
the earth’s atmospheric systems interact, and I have to say, it’s pretty mind-blowing stuff!
At the very basic level that I can get my head around, greenhouse gases in the lower
atmosphere absorb the long wave length infra-red radiation leaving the surface of the planet,
which causes it to warm up. By the time you get to the stratosphere, most of this available thermal
energy has already been captured, so thing get colder. But the other atmospheric molecule we’ve
all heard about is Ozone. Ozone mainly rattles around at between about fifteen and thirty
kilometres up, and it’s very good at absorbing the SHORT wave length light coming INTO the TOP of the
atmosphere from the sun. That light warms up the Ozone molecules, and the heating that occurs as a
result is enough to overcome the cooling effect of being further away from the Earth’s surface. Weird
though it is, that temperature swing from warm to cold to warmer again is a completely normal
function of our planet’s atmosphere. What Manabe and Wetherald did in their pioneering work more
than fifty years ago was to simulate atmospheric temperatures changes based on three levels of CO2
concentrations –a hundred and fifty parts per million, three hundred parts per million, and
six hundred parts per million. Down here at the surface we can see the two-point three-six degrees
Celsius rise that the model predicted when CO2 concentrations doubled from three hundred to
six hundred parts per million. What Manabe and Wetherald’s models showed, was that a doubling
of atmospheric CO2 concentrations would lead to a significant decrease in stratospheric rewarming,
starting at somewhere around twelve kilometres up and becoming more and more pronounced the further
up they looked. By the early 2000s, real-world measurements of multidecadal changes in the
thermal structure of the atmosphere were available from weather balloon networks, satellite-based
microwave sounders, and reanalyses of conditions from the near surface up to the lower part of the
stratosphere, at between twenty and twenty-five kilometres up. What those measurements revealed
was significant anthropogenic influence, not only as a result of well-mixed greenhouse gases, but
also from the depletion of stratospheric ozone and it’s subsequent recovery thanks to the landmark
Montreal Protocol of nineteen eighty seven, AND changes in particulate pollution as industry and
transport systems got larger and dirtier, and then more recently started getting cleaner.
This latest paper, published in May twenty-twenty-three by a team at the
Woods Hole Oceanographic Institution, is the first to use modern satellite technology to continue
those stratospheric measurements from twenty-five kilometres all the way up to fifty kilometres,
where measurements are less affected by pollution and changes in stratospheric
ozone and where the temperature signal of CO2 increase is expected to be considerably larger.
The researchers compared and compiled data from several different satellite sources
and drilled into quite a considerable amount of detail to produce their graphs
and charts , which had enough sensitivity to detect atmospheric changes from events
like the Mount Pinatubo volcanic eruption in nineteen-ninety-one.
What they found was that taking measurements right up to fifty kilometres improved the detectability
of a human fingerprint by a factor of five. The additional cooling that they found up there as a
result of increased CO2 concentrations pretty much completely dismisses the theory that our
warming planet is simply a result of changes in solar activity. If that was the case then
the upper layers of the atmosphere would be warming, not cooling. So that’s one myth busted.
But, what about those other, slightly worrying consequences that I mentioned earlier?
Well, satellite data provides incontrovertible evidence that higher concentrations of carbon
dioxide molecules have permeated right up through all the levels of our atmosphere,
not just the stratosphere. This paper, published in October twenty-twenty-two,
found that the mesosphere and lower thermosphere cooled by one point seven degrees Celsius between
two-thousand-and-two and twenty-nineteen, and that the expected doubling of CO2 levels
towards the end of this century will further cool these zones by about seven point five degrees
Celsius. According to analysis carried out by NASA, that cooling is a causing
an additional contraction of our atmosphere that can’t be explained by natural variation
like the recent slow-down in solar activity. Those changes might sound pretty negligible,
but believe it or not they’re enough to reduce the amount of drag experienced by orbiting satellites.
That’s actually beneficial for the satellites themselves because if they don’t have to fight
so hard against drag then they’ll be able to stay in orbit for longer. But as science writer
Fred Pearce points out in this recent article for three-sixty Yale, that effect also holds
true for the increasingly large amount of space debris that is also hurtling around our planet.
There are apparently more than thirty thousand bits of orbiting detritus that are more than ten
centimetres in size. A decrease in atmospheric drag means that this debris will ALSO now stick
around for much longer before falling back towards earth, and that increases the risk of catastrophic
collisions with the space hardware that we now rely on for so much of our modern technology.
Now I don’t want to get too hysterical here, but it’s fair to say that more collisions will
cause more debris, which increases the risk of yet more collisions, and so on and so on,
and if we start to lose significant numbers of these things then it could disrupt stuff
we now take for granted, like mobile telephony, television broadcasting,
GPS guidance systems, internet access, weather forecasting, and environmental monitoring.
What really caught my attention though was this paper, published in June
twenty-twenty-one explaining how fragile our ozone layer is, and how susceptible
it appears to be to the additional high level stratospheric cooling we’ve just looked at.
Most folks assume we fixed the Ozone layer when the world banned the use of chlorofluorocarbons,
or CFCs, back in nineteen-eighty-seven. That was what the Montreal protocol was all about wasn’t
it? We certainly stopped the main human cause of the problem, but it was always going to take
decades for nature to restore ozone levels back to where they were. That process was initially
predicted to take until about the middle of this century. We looked at what drives ozone depletion
in a video last year, which you can jump back to by clicking up there somewhere. It’s mostly caused
by very cold clouds that form in the stratosphere during the winter seasons over the poles.
Thankfully, Antarctica appears to be continuing on its path to Ozone recovery,
but the additional stratospheric cooling caused by increased human emissions of CO2 now seems to
be opening up a new hole above the Arctic, as this twenty-twenty NASA satellite image shows.
The authors of the paper are not sure what’s causing the difference in temperature influence
between the two poles, but they do make the very important observation that, unlike the vast
uninhabited frozen wilderness at the southern end of our planet, the regions immediately surrounding
the Arctic include some of the most densely populated areas on the earth, and given that ozone
depletion is closely linked with rises in cases of skin cancer as a result of over exposure to
harmful ultra violet sunlight, it might be worth adding this consequence to the ever increasing
list of reasons to start taking far more urgent action, not only to eradicate human-induced carbon
dioxide emissions into our atmosphere but also to warn people in more northern climes about
the dangers of over exposure to sunlight so that we can start adapting our behaviour accordingly.
Just a thought! And as always feel free to leave yours down in the comments section below.
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