There's the peak, and
there's the long tail.
And there's a couple of factors which tend to de-emphasize
the peak, and emphasize the tail.
One of them is the band saturation effect, which is the the fact
that when you add CO2 to the air the temperature response
Is not linear but it bends over like that.
Actually the temperature goes up as the logarithm of the CO2, rather then
as a direct multiplier times the CO2, which is what this shape looks like.
That means that the really high CO2 of the peak doesn't translate
into as really high a temperature as you would think, if it was just proportional.
This function acts to fatten up this tail
relative to what it looks like just in the CO2 concentration.
Looking at CO2, you say, oh, there's a big peak and
yeah, okay, kind of a long tail as an afterthought.
But when you look at the temperature, it doesn't have such
a pronounced peak.
It looks like the temperature just changes and pretty much stays there.
The other thing that tends to de-emphasize the peak
is the slow ocean heat uptake time.
To warm up the planet, you have to warm up the deep ocean.
Today, as we'll learn in future lecture, the oceans are keeping
us cool because a lot the excess heat from the greenhouse
gases is slowly invading the ocean.
And the time scale for heat invading the ocean is very similar to the timescale
for CO2 invading the ocean, because they're both
controlled by the ocean circulation, about a thousand years.
As a result, the temperature evolution from
the peak, over that thousand years there, is going to get chopped
off by the slow ocean heat uptakes.
That further de-emphasizes
the peak relative to the long tail.
There are parts of the climate system that respond
most strongly to the long tail because they take a long time to respond.
And the poster child for that is really the ice sheets
and their impact on sea level. Here is a plot of reconstructed changes
in sea level due to changes in the size of the ice sheets from the geologic past.
Present day, we are here and sea level is by definition zero.
This data point here is from the last glacial time, when it was maybe five
or six degrees colder than today, and sea
level was actually 120 meters lower than today.
Looking further back in time, about 3 million years ago, time
called the Pliocene when there wasn't much
of any ice in the Northern hemisphere,
and the temperature was was somewhat warmer then today.
Its not totally clear exactly how much
higher sea level was then or how much warmer it was; it's hard to put together
information from the past like this.
But here's the best guess.
And then further back in time, this is from a time when
when the Earth was in what's called a hothouse climate.
There was no ice at all on Earth.
The problem with understanding or figuring out
the sea level during sometime this way, in the distant
past like that, is if you find some fossil beach someplace,
you don't really know if
that's at the same elevation today as it was then.
Because the land surface is moving up and down on geologic time scales.
This 70 meters of sea level rise at
that warmer temperature is based on how much ice there is today.
If we wanted to make a hot house climate out of the earth today
with the continental geography how it is, it would raise sea level by 70 meters.
You know there's a whole lot of complexity in this line.
But there is also just a fairly clear relationship.
Especially when you compare it with the
forecast for the year 2100, which calls for about
2 to 4 or 5 degrees centigrade of warming.
And a sea level change of maybe a meter or two.
This is totally off this line from the geologic reconstructions.
And the reason why is because it takes a long time, longer than
just from now to the year 2100, hopefully, to melt the major ice sheets.
And so the forecast for the year 2100
doesn't really capture the time scale of the problem of the ice sheets.
And that's why this forecast is a little bit deceptive.
I mean, it's useful, because we're not, I mean, those of you
who are watching this video probably are not going to live beyond the year 2100.
and so this is an anthropogenic, a human time scale,
but it's not the time scale for the ice sheets.
But if the CO2 is going to continue to effect climate for, hundreds of thousands of
years into the future, there's plenty of time for the ice sheets to respond fully.
This is an example of a part of the climate system
that is mostly sensitive to the long tail of the CO2.
Other examples include a carbon cycle feedbacks,
and warming the deep ocean and
causing methane hydrate to decompose and release its carbon.
This would happen on a very long time scale of thousands of years, because
it takes a long time for heat to get into the deep ocean.
Or melting permafrost soils on land and decomposing the carbon
that's frozen there will also take centuries, presumably, to happen.
These are all impacts
that are more strongly tied to the long tail
of the CO2 than to the actual peak itself.