Examination of Witnesses (Questions 33-39)
Sir John Houghton
18 JANUARY 2005
Q33 Chairman: Good afternoon, Sir John, and
thank you very much for coming along to talk to us this afternoon.
We are very near the beginning of our inquiry and I think most
of us are anticipating we will understand the subject a bit better
when we have made a bit more progress, so you will have to forgive
us if we are not as expert as we need to be. I am told that I
have to say please speak up and speak slowly so that we can get
a proper record, and you know something of the questions that
we were thinking about asking you. I wonder, before I ask you
the first question, whether there is anything you want to say?
Sir John Houghton: Thank you very much for inviting
me. I appreciate the opportunity to come and tell you a little
about the science of global warming and the like. I have nothing
else I particularly want to say. Your questions were very comprehensive
so I am happy to work through those.
Q34Chairman: As we all know, you have written
widely on the science of global warming and I wonder if you could
summarise what you regard as the main scientific justifications
for supposing that past and projected global warming temperature
changes are due to human-induced emissions of greenhouse gases?
Sir John Houghton: Yes. There are the things
we know. First of all, carbon dioxide has increased by over 30
per cent since the beginning of the industrial revolution. We
are sure, absolutely certain, that the reason for most of that
is the burning of fossil fuels by the world's industry. Why do
I say we are very certain? Because not only does the amount of
increased concentration in the atmosphere fit with the amount
of emissions, but also there are isotope signatures on the carbon
which tell you exactly how much has come from fossil fuels and
how much from other sources, so that is something we are absolutely
certain about. Secondly, we are quite certain about the basic
science of the greenhouse effect, the fact that we have gases
in the atmosphere, carbon dioxide and methane in particular, that
absorb radiation admitted by the surface, thermal radiation, and
stop it going out to space and act as a blanket over the earth's
surface and make the earth warmer than it would otherwise be.
If it was not for the greenhouse gases, that are there for natural
reasons, the temperature of the earth's surface would be less
by about 20 degrees Celsius, and the earth would be covered with
ice rather than as it is. So we are sure about the greenhouse
effect, we know the basic physics, it has been known for well
over a century, 150 years or so. The first calculation of the
effect of doubling carbon dioxide in the atmosphere was carried
out by Arrhenius in 1896, the result, although high was in the
range of today's estimates. No physicist who understands radiation
in the atmosphere would dispute the fact that there is a greenhouse
effect, that increases in carbon dioxide increase the temperature
of the surface. So we are certain about those things. Now, the
uncertainties arise because of feedbacks. One of the feedbacks
occus because, as you warm the earth's surface, water vapour increases.
It is the greater evaporation from the surface that increases
water vapour in the atmosphere which is also a greenhouse gas;
that will increase the effect of the first warming, and I could
say more about that later on. I see you raise Professor Lindzen
later on and I will talk about his work, and he has been very
keen on that particular issue. I think the reason why we believe
in human induced climate change is because the basic science is
there, the carbon dioxide is there, and, of course, the story
as we read it in the climate record over the last century is also
there. We are observing it in the atmosphere in much the way we
expect.
Q35Lord Lamont of Lerwick: Last time Colin Robinson
of the University of Surrey said this, because he is not a scientist,
"The amount of warming does not appear to be very closely
correlated with greenhouse gas emissions".
Sir John Houghton: Well, that is just not true.
The amount of warming is closely correlated with greenhouse gas
emissions. There are other reasons also for temperature changes
in the atmosphere; there are natural variations in solar energy,
there is volcanic activity. But I could show you a diagram of
the record of the 20th century temperature as observed, as predicted
by the best models, and you will see a very close connection between
them.
Q36Lord Lamont of Lerwick: And he went on, "The
Committee could usefully question the scientists about the extent
to which they have isolated the warming effects of these emissions,
given all the other complicated things that are going on in the
climate".
Sir John Houghton: Well, we know the factors
that cause variations in climate and the best models we have at
the moment take each of these variations due to different effects
and fold them together in a way that gives you a very good result.
You may say that is contrived because you knew the answer you
wanted, but if you look at it more deeply you find it is not contrived,
so we have very good simulations of the climate of the 20th century.
Indeed, you have a question later on which goes into climates
of the past, and I will say a little about that at that time.
Q37Chairman: Which do you think are the best
climate models?
Sir John Houghton: The one which is believed
I think to be the best in the world is at the United Kingdom meteorological
office. But there are other models also elsewhere in the world;
there are 20 or 25 now which are in different sectors in the world
which compete for being in the top rank. They all compare their
data, they run in different ways; they have different modelling
methods, so there is a great deal of activity comparing those
models one way or another. Also, on the question of the science,
and I should perhaps have brought the volumes along with me, the
Intergovernmental Panel on Climate Change, of which I was privileged
to be the Chairman or co-Chairman for over 12 years, has produced
reports, three major ones, 1990, 1995 and 2001. The 2001 report
is in four volumes, all about 1,000 pages long, all with many
thousands of references, all reviewed by the science community
very thoroughly and also by government scientists. The policy
makers' summaries of those reports have been through the toughest
possible hurdle that you could possibly give them to get through,
which is an intergovernmental meeting with 100 governments there,
including states like the US, Saudi Arabia and other oil states
who are fighting like anything to weaken it; there are other states
who want to be green and they try to strengthen it. These policy
maker summaries have been agreed word by word, sentence by sentence
in intergovernmental meetings of that kind. So they have been
through tremendously strict procedures. They are not just the
work of green activists, as is sometimes said, or NGOs who can
say what they want without being questioned. They are the work
of serious world scientists including virtually all the leading
scientists in the world in the field, and including many hundreds
of scientists from very many countries. The number of scientists
who have been outside that process who are any good is very few.
Q38Lord Marsh: As you know, this is only our
second meeting here and some of us are not familiar with this
area at all. I wonder if you could explain why you think we are
looking at a trend in warming rather than some natural cycle and,
if it is the latter, can you give any indication of what you think
might be the period of the cycle to get some indication of how
fast moving the process is?
Sir John Houghton: There are various cycles
in climate and the longest of those and the most important in
some ways on a very long-term scale are the cycles of the ice
ages. We get an ice age roughly every 100,000 years and we know
what triggers ice ages; it is the variations of the earth's orbit
round the sun, the variations of the eccentricity of the orbit,
the variations in the tilt of the earth's orbit, and the variations
of the time where the earth is closest to the sun which all vary
from 100,000 years to 40,000 years to about 23,000 years. You
can find all those periods within the climatic history of the
last million years during which there have been around 10 ice
ages.
Q39Lord Marsh: But this current concern is on
a slightly shorter timescale, and can you identify anything on
that?
Sir John Houghton: Of course. I mention the
ice ages because there are people who say we are heading for another
ice age so getting warmer is a good idea. The next ice age we
know from astronomical data is due in something like 50,000 years'
time, so it is not a concern on the timescale that even you politicians
deal with! There are variations that occur on shorter time scales.
We believe that there are variations in the sun because the best
known solar cycle is 11 years long and is very small in terms
of energy charge or any other influence. There are probably variations
on a longer timescale which are not so well understood but we
have a good handle on how large they are. There are other cycles
that occur for internal reasons probably in the atmosphere and
in the oceans, especially as the atmosphere and the oceans couple
together, and you can see such variations occurring within the
record. If you take the record of the last millennium, for instance,
in the Northern Hemisphere for which we have reasonable data,
we are able to identify the Northern Hemisphere average temperature
over that period; you will find variations which show the "medieval
warm period", and the "little ice age", but these
are small variations.
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