In thinking about those temperature targets, it turns out that
a lot of the complexity of the carbon cycle goes away.
And that's illustrated by these schematic cases
of releasing a slug of carbon either doing it very quickly, all
at once, or doing it slowly, but for a longer time.
The total amount of carbon is released is the same in either case.
There's a fast slug of carbon
released, and a slow slug of carbon released.
If you release the carbon quickly, it reaches the
CO2 peak at the end of
the slug time, and then it slowly drifts down
as it is neutralized by the oceans.
If you release it more slowly,
it reaches a peak that's later, and probably a lesser peak, because
as you've been releasing it's had time to dissolve in the oceans.
The temperature response goes up with rising CO2, but there's a lag time for the
change in the temperature, of about 1,000 years,
because you have to heat up the ocean.
The temperature rises to a plateau. And then it basically stays there.
As far as we're concerned, it stays there forever.
But the crucial part to this is that the plateau temperature...
that it reaches for these two scenarios is about the same.
It turns out that the higher CO2
is counter-balanced by the fact that it started dropping
by the time the earth's temperature is catching up.
And so, the time dependence of the absorption of CO2 is
kind of compensated for by the time dependence of the warming.
This means that the temperature that you're going to reach
doesn't depend on whether you release the CO2 quickly or slowly.
You're going to reach
the same temperature in either case.
You'll reach it more quickly or more
slowly depending on how quickly you release it.
It's a much simpler picture now because we can just draw a one to one
relationship between the amount of carbon ever burned
and the temperature that the Earth will reach.
It turns out that, about 1,000 gigatons of carbon would take the earth about
to this two degree C threshold.
500 gigatons of carbon would take it about to one degree C.
For comparison, we've already burned and cut down about
300 gigatons of fossil fuels and 200 gigatons of trees.
We're kind of near the one degree C mark already,
and we're about half way toward the two degree C mark.
What would it look like to actually try to cut a emissions, and
stick to a one-thousand gigatons of carbon target?
Here is a plot of emissions of carbon per year.
Business as usual is this line here; it's growing a rate of 3% per year.
That's the trajectory that we continue to be on.
And the total amount ever burned goes as the area under this curve.
Let's say that in 2010 we had decided, instead of growing at 3% per year,
we would cut emissions by about 2% per year.
The emission flux would drop in this gradual way.
And the total area under this curve
would add up at about 1,000 gigatons of carbon.
If we wait longer, we have to cut more quickly, in order for
the total area under the curve to stay under a thousand gigatons.
Here's a plot of the percent per year cuts that would be
required in order to keep to 1000 gigatons of carbon for the total slug,
as a function of the year that they start.
Starting in 2010 cuts of about 2% per year would have done it.
2020, we would have to cut by 4% per year.
2030, about 10% per year.
And pretty soon, by around 2040 or so, basically we'd have to
quit cold turkey, in order to keep under a 1000 gigatons of carbon.
It's kind of like,
you know, you're in the army, and your sergeant, has caught
you doing somethin wrong, he says, drop and give me 20 pushups!
Or, if you don't want to do em today, you can do em tomorrow, but you gotta do 40.
Or you can do them this weekend, but you've got to do 100.
What are you going to do?
It's much easier, cheaper to convert our energy infrastructure
at a slow rate of a few percent per year, because that's kind of the rate
at which power plants become obsolete and need to be replaced anyway.
Whereas, if you wait until the end, you've
got to basically build everything all from scratch,
all at once, It's a much bigger and much more expensive proposition.