most of us will remember exactly where we were and what we were doing when this atrocity
literally exploded onto our television screens on a crisp autumn morning in New York on the 11th of
September 2001 and changed our world forever you may also remember seeing this footage of a US air
traffic control screen showing in real time how the thousands of planes flying over the United
States on that fateful day were systematically landed at the nearest safe destination until by
about midday the skies above the world's largest economy were completely empty for the first time
in many decades a team of scientific researchers from the University of Wisconsin-Whitewater took
atmospheric temperature data from the three days following 9/11 when all aircraft remained
grounded and analyzed something called the diurnal temperature range or DTR over the country DTR is
the difference between peak daytime temperature and minimum nighttime temperature the data showed
that during those first three days that range had increased by about one degree Celsius and
that finding sparked great speculation that it must have been the absence of contrails in the
atmosphere as a result of the grounding of the aircraft that caused the spike in the temperature
range contrails or condensation trails to give them their full name are the white plumes that
we see coming out of the back of aircraft high up in the atmosphere they form as a result of water
vapor in the aircraft's exhaust hitting the very cold air a high altitude to form a stream of ice
crystals the consequential effect is very similar to cirrus clouds reflecting some of the sun's heat
during the day creating a daytime cooling effect and absorbing some of the Earth's heat radiation
during the night slightly reducing nighttime cooling so the theory went that if the diurnal
temperature range had increased by one degree Celsius then the absence of contrails was the
only plausible explanation that hypothesis was subsequently disproved in this paper published
in 2008 in the scientific journal Geophysical Research Letters which showed that a 1 degree
Celsius variance in DTI was quite common over that part of the States during the month of
September and that the 9/11 example was most likely the result of naturally occurring low
cloud cover a later 2010 study conducted by the UK Meteorological Office and a research
team at Leeds University also found that you need about 200 times the quantity of flights
over America to produce any significant effect on the DTR temperature range regardless of those
two scientific papers though the debate about contrails and the multitude of other human-made
aerosol particles in the atmosphere has continued to rumble on in the ensuing decades with some
suggesting that if human industrial activity and transport was ever abruptly halted by some kind
of global scale crisis the immediate drop out of reflected aerosols in the atmosphere would cause a
catastrophic spike in temperature possibly within days or weeks that could potentially lead to the
near-term extinction of our species it's a theory that's become known as the dimming paradox fast
forward to where we are right now in the early months of 2020 facing an unprecedented global
scale crisis and unsurprisingly that debate has once again bubbled up to the surface so perhaps
now is the time to search for some science around the contentious topic of global dimming
hello and welcome to just have a think this animation produced by the European Space
Agency using data from their Copernicus Sentinel satellite shows how the concentration of nitrogen
dioxide above China decreased rapidly in late January 2020 as their nationwide quarantine
program kicked in and then started to build back up again as normal activity began to resume from
the beginning of March a similar reduction can clearly be seen over northern Italy in
late February nitrogen dioxide primarily gets into the atmosphere from the fuel emissions of
cars trucks and buses and it's a pretty nasty air pollutant causing many thousands of illnesses and
deaths every year thanks to an online climate modeling tool called Google Earth null school
we can also see global levels of another air pollutant sulfur dioxide sulfur dioxide is
used in industrial processes like chemical preparation refining pulp making and solvent
extraction it's also a byproduct of fossil fuel burning in power plants with coal-fired power
stations accounting for 50% of annual emissions both these pollutants fall into the category of
atmospheric aerosols on this chart from the AR5 report published by the Intergovernmental
Panel on Climate Change or IPCC the chart shows all of the major contributors to a phenomenon
that scientists call radiative forcing which is essentially the difference between the amount
of solar energy reaching our planet's atmosphere and the amount that subsequently escapes if more
energy is getting out than energy getting in then your planet is cooling and if more energy
is staying in than getting out and your planet is warming anything moving to the right of this line
is causing an increase in radiative forcing by trapping infrared radiation leaving the planet's
surface those things are causing a net increase in atmospheric temperature anything moving to
the left of the line is causing a decrease in radiative forcing generally as a result of their
ability to reflect the sun's energy back into space something that science bods call albedo
I'm not going to delve into the actual numbers and units of measurement because we already did that
in quite a lot of detail in our video on the grand solar minimum last year but I'll leave a link to
that program in the description box below so you can go and have a look at it if you want to or
what you can click up there to jump back to it now if you prefer anyway if we focus on this
section of the chart we can see that there are several aerosols that have a cooling effect in
our atmosphere especially sulfates which are derived predominantly from sulphur dioxide so
how does sulfur dioxide convert to a sulfate and why does it have such a significant cooling effect
well we can turn to nature for a bit of a steer on that one on the 18th of May 1980 mount St. Helens
in the northwest of the United States erupted with an energy equivalent to 1,500 Hiroshima atom bombs
sending superheated gas 15 miles up into the atmosphere at speeds of up to 400 miles an hour
on the 15th of June 1991 Mount Pinatubo exploded in a cataclysmic eruption that ejected more than
5 cubic kilometers of material 22 miles high both those eruptions spewed millions of tons of sulfur
dioxide high up into the atmosphere up there it reacted with water vapor to make sulfuric acid
which in turn condensed in the stratosphere to form sulfate aerosols which acted like billions of
tiny mirrors reflecting sunlight back into space the results in both cases was global dimming our
planets skies literally dimmed a little and the average global temperature went down by about
half a degrees Celsius for several months and that temporary dimming effect is exactly what
our anthropogenic industrial emissions of these particles also causes but unlike greenhouse gases
which can stay up in the atmosphere for decades or even centuries industrial aerosols drop back out
of the sky after only a few days or weeks and it's this difference in timescale that causes
some observers to speculate that if we rapidly shut things down like coal power stations as
part of decarbonization then we'd get a massive warming spike as all the aerosols disappeared the
implication being that this warming spike would be so severe that it would quickly take our planets
very sensitive closed system past crucial tipping points causing irreversible catastrophic runaway
feedback loops so what do our scientists tell us about this theory this paper published on the 18th
of September 2019 in the scientific publication Nature was co-authored by Dr. Chris Smith research
fellow at Leeds University and Professor Drew Shindell of the Earth Sciences Department at
Duke University in Durham northeast England in this interview for the website Carbon Brief Dr.
Smith sets out the context of the report he says we believe that man-made aerosols have offset a
significant amount of greenhouse gas warming up until now perhaps in the region of North
0.5 to 1 degrees Celsius and it seems to be this historical data that drives the logic behind the
global dimming paradox theory the Smith maintains that there's a flaw in this logic and that's the
assumption of an instantaneous removal of all the emissions which he says it's simply not realistic
in our complex interdependent world according to co-author Professor Shingle the model simulations
show that even under the most rapid conceivable transition away from fossil fuels it takes a good
deal of time to actually move the entire planets energy systems to clean energy these slightly
complicated spaghetti-like graphs show how their models stack up on the left we've got a bunch of
lines all representing slightly different emission reduction strategies all of which are aimed at
keeping global warming to around the 1.5 degrees Celsius mark as we go towards 2100 the even
busier chart on the right shows the corresponding temperature variance for each strategy it's a bit
of an eyeful at first sight but if we zoom in to the top of the right-hand chart during the years
between 2020 and 2030 you can see that none of the model runs predict an initial temperature increase
that reaches even as high as 0.3 degrees Celsius as we expand back out again you can see that some
of the lines actually break through the horizontal zero line by the 2040s and that represents an
overall average atmospheric cooling effect by then all sounds fairly positive doesn't it but
there is a significant caveat to these findings which is highlighted quite clearly in this paper
published in the American Geophysical Union in 2018 the research finds that there are likely
to be massive variations in regional temperature and extreme weather patterns as a result of huge
disparities in the existing aerosol concentrations from one part of the world to another these charts
show how the northern hemisphere is far more sensitive to aerosol removal unsurprisingly the
heavily industrialized regions like the United States it's Europe and China all see a marked
increase in local temperatures as a result of reductions in atmospheric aerosols with local
temperature spikes reaching their peak way up in the Arctic Circle there's also a noticeable
increase in precipitation either as rain or snow over North America and northern Europe and to a
very significant degree across southern Asia and China the paper points out significant variance
between different climate modeling methods and technologies and the authors offer this
typically technical conclusion the differences in sensitivity shown here highlight the need
for studies of future climate change to carefully consider not just their net forcing and resulting
global mean surface temperature but also the detailed balance between greenhouse gases and
aerosols in emission pathways the differences in spatial pattern between climate forcing and
response due to greenhouse gases and aerosols mean that for low global warming scenarios for example
consistent with the 1.5 degree target the realized climate impacts will depend significantly on the
path we take toward global temperature goals all climate modelers and researchers readily
concede that the enormously chaotic interactions between our atmosphere and the oceans coupled
with variations in weather patterns as systems move from land to sea over varying timescales
make it extremely difficult even for our most advanced supercomputers to provide a hundred
percent guarantees about future consequences of aerosol removal but taking an aggregate of all
available data and modeling techniques gives the scientists a clear enough overall picture
for the vast majority of them to offer a pretty unambiguous prognosis as Dr. Shindell explains
in his interview with Carbon Brief we know there are enormous risks associated with continuing to
burn fossil fuels what this work shows is that it's mistaken to think that the transition to
clean energy also has large environmental risks instead it provides huge public health benefits
while also mitigating climate change I know this one's a pretty emotive and contentious subject so
no doubt many of you will be keen to voice your opinions in the comments section below and as
always I'll be keen to read through as many of them as I can I do come across some discussions
that get pretty heated from time to time though and while there's nothing wrong with a frank
exchange of views I would gently suggest that during these somewhat more troubling times it
might be worth as all aiming for constructive dialogue in preference to some of the more
robust communication methods that occasionally get demonstrated down there in the spirit of
freedom of expression though I will of course leave that decision entirely up to you that's
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as always thanks very much for watching I hope you're all staying safe and well through these
troubling times have a good week if you can and remember to just have a think see you next week