[Music]
>> The Passionate Eye now returns to The Truth
About Global Warming.
For
the first time, legendary broadcaster,
Sir David Attenborough, speaks out about 
global warming.
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>> The key question, of course, is how can
we distinguish between variations
due to natural causes and those variations
of the climate that are induced by human activity?
And the key thing that convinced me, at any
rate, was a graph like this one that we have
marked out on the floor that had been prepared
from climate scientists like Professor Peter Cox.
Now, explain to us the significance of this
graph.
>> Okay.
What we're going to do is to take a walk through
time, and the first
thing to note as we walk through is that the
climate is naturally variable.
It's a spiky beast.
Occasionally there's a downward trend that's
associated with a volcano going off to cause
a
system dam because of the dust it throws up,
but generally it just oscillates around.
And
then we get to a period, around about 1910,
where you can start seeing an upward trend,
a
warming of the climate, a global warming,
if you like.
An issue is what caused that?
Was
that humans or was that natural?
What we did to try and work that one out is
to take a
climate model and to put in the various factors.
And what we can see with this green curve
here is a climate model that includes just
these natural factors.
So this is when volcanos go off
and the output from the sun.
And you can see that the green curve
can reproduce reasonably
well this mid-century warming.
So, up to this point you could reasonably argue
climate variation
can be explained by natural factors.
But as we move on we can see that's no longer
true as
you get to the latter part of the 20th century.
From about 1970 onwards here, you can see
the red curve, the observed temperatures,
and the green curve really are beginning to
diverge.
And the question, again, is what caused this
recent warming.
So we run the model
again, and we include human factors, particularly
we include the greenhouse effect, from --
mostly from carbon dioxide that comes from
fossil fuel burning.
And then we get this
yellow curve, and we can see as well as reproducing
the mid-century warming, we get this
recent rather rapid warming reproduced.
And that tells us two things.
One is that the
model looks realistic, it looks like the real world.
And the second thing the model tells us
that this recent warming is due to human beings.
>> So there you have it.
There seems little doubt that this recent
rise, this steep
rise in temperature, is due to human activity.
If you look at the green line of natural
variability, it's clear that without the action
of human beings there would have been far
less
temperature change since the 1970s.
