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Research links water vapour and climate chang -

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ELEANOR HALL: The Federal Government might be shaping its climate change policy around reducing
carbon dioxide emissions but research from the US Oceanic and Atmospheric Administration has found
an increase in stratospheric water vapour may be responsible for nearly a third of the global
warming that took place during the 1990s.

It also found that a 10 per cent drop in vapour slowed down the rate of global warming by a quarter
over the last decade.

Timothy McDonald spoke about the research to Professor Steven Sherwood who is from the Climate
Change Research Centre at the University of New South Wales.

STEVEN SHERWOOD: Water vapour is the strongest greenhouse gas in our atmosphere. And the reason you
don't hear people talking about it as a cause of climate change like they do about carbon dioxide
is that it's controlled by the climate itself.

So the only way to change water vapour is to change climate. That's true of most of the water

The water that's in the stratosphere has been very unusual. It's fluctuated over from one decade to
the next and we haven't been able to figure out quite what is causing that to happen.

TIMOTHY MCDONALD: Could it be that in addition to being a driver of climate change it could also be
a symptom?

STEVEN SHERWOOD: It could be, yeah. People have suggested that for years that maybe climate change
was causing, for example there was a substantial increase in water vapour toward the end of the
21st century, some people said it was caused by warming. Other causes were suggested and then it
kind of surprised everybody by starting to drop about 10 years ago.

TIMOTHY MCDONALD: Do we have any idea of how much it tends to fluctuate of its own accord?

STEVEN SHERWOOD: Well we only have good measurements for about the last 10 or 15 years from
satellites. And before that there's just a few places around the world where balloons were launched
that can go high enough and that have accurate enough water vapour instruments to measure it.

So we just have a rough idea that it increased. It looks like it doubled over the last half of the
20th century and then in the last decade it's gone down by something like 10 or 20 per cent.

TIMOTHY MCDONALD: Does this in any way sort of challenge the main thesis of climate change and
specifically anthropogenic climate change?

STEVEN SHERWOOD: No, not at all. But what it does is it represents another, you know, small but not
completely negligible influence on climate that we don't want to forget about. And that was the
point of this paper.

They looked at these variations and they found that by doing a calculation that the effect on
climate change from decade to decade was, should be noticeable. I mean it's not large but they
calculated it would be enough to have made the warming for the end of the 20th century a bit faster
than it would otherwise have been from other greenhouse gases and then should have made warming in
the last 10 years a bit less than it would have been.

TIMOTHY MCDONALD: Aren't they suggesting in this paper that it fuelled nearly 30 per cent of the
global warming that took place?

STEVEN SHERWOOD: Well that would be an upper limit. But yeah, it could be as much as that according
to their calculations.

TIMOTHY MCDONALD: And does it really mean anything for the wider climate change debate?

STEVEN SHERWOOD: Well I think why it's interesting is you had people looking at the temperature
record over the last few decades and saying, well the warming has stopped, which is not true. If
you just look at it, you know, that claim is based on a bogus way of looking at the data.

But it is true that the warming has been slower in the last decade. But what this paper is showing
is that yes there are reasons why the warming rate isn't the same every decade and this is
something that helps to explain that variation.

And there's no reason to think that this stratospheric water vapour is going to keep dropping. It
will probably start going up again at some point.

ELEANOR HALL: That's Professor Steven Sherwood from the University of New South Wales speaking to
Timothy McDonald.