How to understand climate modelling and why you should care
[Applause]
lewis carroll created the absurd world
of alice in wonderland
he also wrote another story
that was about an equally odd little
place that created a perfect map
the map was so perfect that the scale of
it was one to one
and so it shut out all the sunlight
because it covered the entire town
eventually the townspeople decided that
they didn’t like this map very much and
that they would use the town itself as a
map instead
maps are a type of model
we use them to try and understand more
complicated things
and the towns map is an example of a
model that isn’t very good
the awkwardness of unfolding it aside
this map is pretty useless because it’s
just as complicated as the area it
models and it’s no easier to understand
lewis carroll was a mathematician
and he understood that the fine detail
is not so helpful when we talk about
modeling
a good model
is a simplification of reality and it
omits detail so that we can focus on
what’s relevant
and start to explore understand and
predict the system that we’re looking at
when it comes to climate
models are a really important tool
because we only have one earth
and one climate so we can’t do many real
experiments with it
but with a model we can start to answer
questions
like
what would happen if all the clouds
disappeared
or if we doubled our concentration of
carbon dioxide in the atmosphere
actually that second one is not the best
example because we’re well on our way to
doing that experiment in real life
today i want to demystify the process of
mathematical modeling
and show you how we can wrap our heads
around key climate change terms
like resilience stability and tipping
points
to do this we’re going to build a model
from the ground up
our model is based on conservation of
energy
on some hypothetical planet
and there are two key ingredients for
our model that we’re going to look at
absorbed energy from the incoming solar
radiation
and the emitted energy that’s radiated
back to space
how much of the incoming energy is
actually absorbed depends on what it
hits
so let’s consider the extreme cases
if our hypothetical planet was very cold
we’d expect it to be covered in lots of
ice and snow which would reflect well
keeping the absorbed energy low
i’ll call this the snowball state
if on the other hand our temperature of
our planet was quite high then there’d
be lots of land and deep dark ocean
which would absorb well
i’ll call this the flame ball
now with this thought experiment we’ve
already illustrated one of the key
ingredients of climate change
feedback
the reflection that happens when there’s
lots of ice and snow
reinforces and amplifies the snowball
state
keeping it cold
that is the temperature determines the
energy absorption which in turn
determines the temperature
how the absorbed energy changes between
these two extremes depends on the
underlying physics
and looks like so
the key here
is that it’s not the incoming solar
radiation that’s changing but how much
of it is actually absorbed at different
temperatures
the emitted energy can also be described
well by the underlying physics
a warmer planet
would emit radiation better than a
cooler planet and so the emitted energy
curve looks like this
they’re both just energies so i’m going
to put them both on the same plot
so that we can properly compare the
incoming and the outgoing energy
now when our curves intersect the
absorbed energy and the emitted energy
are equal
and the state is in an equilibrium
this happens at three different
temperatures in our system and each of
these temperatures would produce a very
different looking climate
the first is nice temperate climate and
it would support human life
it has ice caps but it’s not a complete
snowball
so i’ll call this earth-like
the second is a much warmer version
think
lava flowing bushfires running rampant
and i’ll call this the hot house
and the third
is something in between
now these states
are three different versions of the same
hypothetical planet
but only one of them will exist at any
given time
so let’s assume that right now it’s the
earth-like planet state that currently
exists
we’d like to understand the stability of
this climate
and so to start looking at that we
explore what would happen when the
temperature changes slightly and how it
would respond
decreasing the temperature a little bit
the absorbed energy would be greater
than the emitted energy because the red
line is above the blue line
this means more energy would be coming
into the system and so the temperature
would increase
if on the other hand we increased the
temperature slightly
now the emitted energy is greater than
the absorbed energy
which would drive the temperature
lower
for this reason the earth-like state is
currently in a stable state
because
when we give it a nudge it will be
pulled back
to where it was before
like a ball in a valley
when it’s pushed it comes back
and for this reason it’s also called an
attractor
if it were the hot house version of the
planet that currently existed
well
that’s also a stable attracting state
that’s in a valley
both of these states have resilience
because it would take quite a
substantial change in temperature
for their climate to to really shift
in between two valleys
there must be a hill
and a ball on top of the hill would be
unstable because any small change to it
would cause it to roll down the hill
in our case
this planet’s climate
if the temperature changed ever so
slightly it would be forced into one of
the alternate planets
we can summarize all this information on
a single line
this line shows the different versions
of the planet and their stabilities
let’s now assume that there is an alien
species that lives on our earth-like
planet
and this species is able to somehow
change the emitted energy from their
planet
by say changing the greenhouse gas
concentration
if they increase the greenhouse gas
concentration
well then more of the radiated energy
would be trapped in the atmosphere
and the greenhouse gas effect would get
worse
with less energy being emitted
our emitted energy curve the blue curve
will shift down
the intersections it makes with the red
curve
will change and the earth-like state
will move to the right
increasing its temperature
if the aliens continue to increase
greenhouse gas concentration
well
less energy will be emitted
so the blue curve will move down
the intersections will change
the earth-like planet will move to the
right
increasing its temperature even more
and if they continue to do this
right up until the earth-like state
collides and overlaps with the unstable
state
well now
their climate
is half stable
like a valley on one side and a hill on
the other
if the temperature is decreased
that’s fine it will come back to where
it was before
but
if the temperature is increased
this planet’s climate will run away to
the hot house
and this this is the key because this is
a tipping point
any further increase in greenhouse gas
concentration
will cause the earth-like state to be
destroyed
and only the hot house will remain
because at this level of emissions that
top summary curve that we see
there’s only one version of the planet
and it’s much much warmer
okay so the aliens
recognize their mistake they quickly
draw down carbon from the atmosphere and
reduce their greenhouse gas emissions
more energy can now be emitted so the
blue curve will move back up
the temperature of their planet
will cool
a little
but the earth-like state just because it
exists now doesn’t mean that they get to
occupy it
they’re stuck in the hot house
because remember the hot house is stable
and resilient
so if they want to move out of it it’s
going to take a lot of effort
they’re going to have to reduce their
greenhouse gas concentration
down
beyond even where it was when they first
started putting greenhouse gases into
the atmosphere
until they reach another tipping point
and only then
will they be able to cross back into the
ice cap detractor
this is called hysteresis it means that
crossing these tipping points is
irreversible
a tipping point is the straw that broke
the camel’s back
it’s a large-scale drastic change in a
system that can’t be unwound just by
removing one straw
in our earth’s climate system tipping
points such as the amazon rainforest
dieback permafrost loss and greenland
ice sheet disintegration can all be
thought of in a similar manner to our
model
there’s a good state to be in and a bad
state to be in
and
the transition from one state to another
may take place over decades or even
centuries
but once that collapse has started it
may be virtually impossible to stop
the climate is complicated
and a lot more complicated than just
absorption and emission of energy
but by reducing it to just these two
factors
we’ve been able to derive genuine
insight
into one of the climate’s main driving
factors
it’s the simplicity of this model that
let us do that
and we know that simple models like this
are good
because they’ve worked really well in
the past
in fact a model quite similar to this
was actually used to understand the
carbon dioxide’s role in our own
planet’s atmosphere and temperature
climate models have come a long way from
the simple
energy balance models that we’ve seen
today
but state of the art modeling still
can’t produce a one-to-one map of the
climate
and that’s okay
because that true future the one-to-one
map
is not so helpful to us
what’s far more valuable are the
alternate futures that modeling can show
us by considering various socioeconomic
responses and possible outcomes of our
actions
this isn’t just theoretical right now
climate scientists are modeling an
entire array of possible pathways
to help us find the best possible route
to limiting global warming to one and a
half degrees above pre-industrial levels
the good news
is that the most recent ipcc report and
the models that they refer to
show us
that avoiding catastrophic tipping
points is still possible there’s still
time
but critically
we can only do it if we stay on the
pathways that these models
our maps are giving us
they won’t be perfect of course
but unlike the town in the lewis carroll
story
these maps are really good
thank you
[Music]
[Applause]
you