0:00

The near future climate model is intended to try to

Â capture some of the dynamics of what's going on the nowish,

Â this decade, this 100 years with rising CO2.

Â The radiative forcing that that imposes on the climate.

Â And then the time evolving planetary response to that

Â change in the energy balance.

Â [COUGH] So, the CO2 concentration in the atmosphere, we're sort of

Â decomposing into a natural constant amount that was there before we were.

Â And then an exponentially growing part that's due to the industrial activity.

Â So given the CO2 concentration, we can calculate the radiative

Â forcing from that, which is number in watts per square meter.

Â Which indicates how much the change in CO2 from the initial

Â value has changed the energy balance.

Â So, eventually the planet warms up and makes the energy balance go back to zero.

Â So, this radiative forcing is defined after you put the CO2 in the air,

Â but before the temperature has had a chance to change at all.

Â So the radiative forcing is directly,

Â linearly proportional to the equilibrium temperature change.

Â Now it could be that in reality the temperature change could be some more

Â complicated function of the radiative forcing.

Â But to at first approximation, one watt per square meter

Â radiative forcing gets you three-quarters of a degree of temperature change,

Â and that proportionality is the climate sensitivity.

Â There's uncertainty to that, of course.

Â So the equilibrium temperature then is the temperature

Â that the planet is relaxing to given enough time.

Â But there's a long, non-neglible time scale for how long it takes for

Â the planet to reach the equilibrium temperature.

Â So, if the equilibrium temperature were to just suddenly change like this,

Â the transient temperature kind of relax torted on some longer timescale.

Â So this transient temperature is the one that's actually is what's controlling our

Â weather.

Â My python version of the world without us code looks like

Â it's got some variables initialized at the beginning and

Â a bunch of lists that are initialized and get filled up in a series of for loops.

Â And if we run it.

Â This is the CO2 concentrations as a function of time.

Â So, here is the business as usual, exponential ramp up.

Â And then here is the world without us,

Â where the CO2 is sort of slowly declining as it dissolves in the ocean.

Â 3:21

So this is the radiative forcing for masking.

Â It is proportional to the rate of increase of CO2,

Â just kind of call it industrial activity.

Â And then this suddenly drops to 0 at the world without us,

Â because the smoke all gets cleaned out of the air and that happens quickly.

Â So, in the business as usual scenario, the masking

Â is assumed to just stay at this value.

Â And so the temperature, really the total radiative forcing

Â starts to go up faster due to the rising CO2 concentration.

Â Here is the radiative forcing for the world without us simulation,

Â so it's following the business as usual up to the present day.

Â And then when the CO2 sort of stops rising but declines slowly that's here.

Â And then you also get this extra boost because the masking

Â effect goes away quickly.

Â So what we'll see

Â