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