When you talk to people, do you sometimes point out the difference
between weather and climate? Could you start with that as a basic
He is an influential climatologist and senior scientist at the National Center
for Atmospheric Research. Wigley was part of a team whose analyses of the
earth's average surface temperature, and how it has risen, has become the data
most cited by climate experts. He also was a lead author of the
landmark 1995 Intergovernmental Panel on Climate Change (IPCC) report
which said that human activity is a likely cause of the warming of the
earth's atmosphere. In this interview he summarizes the
evidence pointing to the human factor in global warming, addresses
skeptics' criticisms about this evidence, and explains how societies are
vulnerable to even relatively small changes in climate.
It's an interesting distinction, really, because climate models are
basically weather models that concentrate on different time scales. The
standard difference between weather and climate is that climate is, in simple
terms, the average weather. So climate is what we can expect to happen for a
particular season or a particular year, but it's not going to tell us what's
going to happen day by day.
It's interesting that sometimes people confuse. They think that a climate
forecast is going to tell us what's going to happen in the year 1999 on January
3, or something like that. But, of course, we can't. We can only tell people
what the average expectation is going to be. And even there, there's a lot of
So a weather forecast would talk about things that might happen over the
next day or two. But climate would be something like a more average pattern
over a longer period of time, like a season?
Or longer than a season, really. Just to give you an example, one of the
concerns about future climate change, particularly in Europe, is changes in
storminess. That doesn't mean we're going to be [able] to say, you know, there
will definitely be more storms, they will be more intense, and they'll happen
this time of the year, or they'll last for this certain period of time. But
what we can say is that there may be, on average, more storms.
So if we looked at the average weather situation in the winter over a
period of ten years, there would be more storms. That doesn't mean there are
going to be more storms in any individual year. It means that the long-term
prediction, the average prediction over a decade or more, is that there would
be more storms. I'm not saying actually that there will be more storms, but
that happens to be an area where there's a considerable amount of uncertainty.
But that's just an example of the difference between weather and
You've been talking about within a decade, from decade to decade. And we
get bigger patterns going over longer periods of time as well? Century to
century? Millennium to millennium?
Yes. Climate does affect all time scales, all time scales beyond the time
scale of one year to another. An interesting example of the short end of the
time-scale hierarchy is El Niño. And that's a quasi-oscillation of
weather conditions that happens on a three- to eight-year time scale. And that
means that, roughly, every three to eight years, it might be wetter in one
region of the world than normal, and then at the other end of the cycle, it
might be drier than normal.
The sorts of time scale that we're interested in for human influences on
climate are longer than that. It may be that human influences will affect the
way El Niño operates, but the primary concern is what's going to happen
on time scales of 10, 20, 50, 100, or maybe many hundreds of years.
The issue at stake here is, before we decide whether or not humans are
driving the climate into an abnormal state, we need, as scientists, to know
what is a normal state in order to figure out whether we can detect a signal of
a human influence which is, I guess you'd call it, abnormal, since it hasn't
really happened before.
Yes. We do need to know the background against which the human influences
might be superimposed. But in terms of identifying a human influence, the most
important thing is not so much what the background is, but how much the
climate normally varies from year to year and decade to decade.
If that variability were very large, then we would need a very big
human-induced change in order to identify the effect of human activities. If
that variability was zero, then any small change would be easily identifiable.
So, really, it's the background variability or noise that is the concern. And
this is, in technical jargon, referred to as a signal-to-noise problem. We need
to know what the signal is, what the changes wrought by human activities might
be--and we use complicated climate models to do that--but we also need to know
what the noise is against which this signal is appearing.
Let's talk about some of the sources of data we have to determine this
issue. When people talk about the instrumental record, what do they mean by
The instrumental record spans, roughly, the last few centuries. And the
instruments that we're referring to here are instruments that measure
temperature, instruments that measure precipitation or pressure at the earth's
surface, sometimes instruments that measure temperature above the earth's
surface. The first thermometers were invented many, many centuries ago.
Apocryphally at least, Galileo was one of the first people to take temperature
measurements and to develop a practical way of measuring temperature. Rainfall
measurements go back even further than Galileo's thermometers. The earliest
rainfall records, I think, go back to some very crude measurements made in
Korea around about the fourteenth century or so. Still, quantitative
So the critical thing about instrumental climatology is that these are
numbers. These are not just descriptions of what the climate or the weather was
like, but they're actual firm numbers measured by scientific
But t's one thing to say that one person might have measured something
at the end of the sixteenth and early seventeenth century, but you need to
know: Was it done accurately? Was it done on a sufficient number of sites of
the globe? Measured at the same time of day? There's all kinds of problems
about using that data.
There are certainly many problems associated with using old instrumental
records. An interesting example is the Galilean thermometer, because those
thermometers have been preserved, or some of the thermometers he used have been
preserved. And, in that case, it's possible to use them to measure temperatures
now, and calibrate them or compare them with modern thermometers and see what
the differences are, and see how accurate they are. And if there are
differences, then those can be applied as corrections to the early records. We
don't always have the luxury of being able to do that.
How accurate are they? They're not very accurate, are they?
Those thermometers are certainly accurate to better than one degree
Celsius. The changes that we're talking about due to human activities, however,
are substantially less than one degree Celsius. Or at least the changes that
have occurred so far--may be half a degree Celsius. The future changes may be
much larger. But you can always then ask the question: Well, if thermometers in
the past were only accurate to one degree, how can we possibly identify a half
a degree change in the temperature of the globe that people claim has occurred
over the last 100 years? And the reason why we can do that is although
individual thermometers in the past may not be terribly accurate, if there are
enough of them, and if their inaccuracies are random, by averaging them all
together, we can reduce that error substantially and get a fairly precise
measure of temperatures over a large area, maybe of a substantial fraction of
If you could just address some of the criticism of the surface record
that have been made...take the issue of sample bias. What do people mean by
"sample bias"? Is this because it's easier to measure on land than sea?
Certainly there are a lot of problems with using temperature data from
different sources, different countries, over land or over ocean, and so on. In
fact, temperatures over the ocean, or the temperatures that they used in trying
to get a global picture of temperature changes, are not air temperatures but
usually sea surface temperatures, temperatures measured by ships, by dipping
some sort of bucket into the ocean and then bringing the bucket up on deck and
then letting it sit for a while, and measuring the temperature of that water.
So there are all sorts of problems with different instrumentation, different
methods of measuring temperature, different ways of exposing thermometers. The
meteorological instrumentation exposure used in the nineteenth century was very
different from what is used today.
There's another problem associated with temperature measurements, and that
is urbanization. As cities have grown, so their own activity causes a so-called
urban heat island, so temperatures in an urban area would be expected to
increase with time, simply because of this urban heat island effect.
Is that because of the concrete?
Well, it's partly because human activities--vehicles, heating, industrial
activity and so on--actually releases heat. And so there is an actual heat
source that forms up a city or an urban environment, and that can spill over
into the surrounding countryside, you know, maybe for tens of kilometers. Of
course, another issue is that the concrete of roads and buildings and parking
lots is able to absorb more heat from the sun's rays, and then give that heat
off to the atmosphere in the urban environment and cause additional heating.
So, there are a lot of reasons urban areas are warmer than outlying country
When you add these up, there are a lot of problems. Are you satisfied
that it's been possible to take the errors or control for the [errors] in these
measurements, to get this number?
A fantastic amount of work has gone into trying to correct for all of these
so-called biases, these instrumentation, exposure, and so on, effects. Part of
the battle here is knowing, you know, what the problems are. And I think the
problems are pretty easy to identify. Then the next step is to try and correct
for those sources of uncertainty or sources of bias. And one can do
Just to give you an example: In measuring temperatures at the sea's surface
or the upper layers of the ocean, many years ago people threw a particular type
of bucket into the ocean and measured the temperature in that. It was small,
not really like a household bucket, but a special instrument that was used to
house the thermometer for measuring the temperature of the sea surface. And,
over the years, the construction methods for those so-called buckets changed.
And as they changed, bias crept into the measurement of sea surface
temperatures, associated with the different shape and size and construction of
the bucket. Well, in that particular case, we can actually look at the old
buckets and measure that bias, [and] compare [it] with modern methods for
measuring temperature in the ocean environment. We can also make little
mathematical models of the buckets and determine how rapidly they cool if
they're set on the deck and exposed to the wind. And we can apply both those
empirical and theoretical correction methods, and we can compare them to see
that they're consistent. We can apply those to the early instruments to make
them compatible with modern instruments.
When you do all this and you end up with this curve that's sometimes
called the Jones-Wigley curve, or whatever, what does it tell us? This is an
average global temperature since about the late nineteenth century?
That record goes back to around about 1860 or so. And it is a record that
combines temperatures over the oceans and temperatures over the land. What it
shows most strikingly is that the temperatures during this decade are about
six-tenths of a degree warmer than they were at the end of last century, or
during the period 1860 to 1900.
So that's a bit more than half a degree Centigrade rise over 100
It doesn't seem much to people. So I might say that in the place where I
live, Massachusetts, the temperature would vary ninety degrees maybe over the
year, or vary a lot within a day. So half a degree Centigrade over a century
doesn't sound like a lot, because my perception is of local weather. Explain to
me why that should be cause for concern, or why that should be seen as a
The primary reason that's a cause for concern is that it seems to be
outside the range of natural variability. So it's pretty clear that human
activities are partly to blame for that rise--relatively small rise. Okay. So,
how do you put that into a realistic context? Well, one way to do that is to
look at records over past thousands or tens of thousands or millions of years,
and see how much the climate of the globe or the global mean temperature
changed on that very long time scale.
If we go back 20,000 years, a fair fraction of the world in the Arctic
regions was covered by huge ice masses. That was the last glacial period. The
temperature during that last glacial period was about four or five degrees
Celsius less than today. And yet the environment was just radically different.
Not that we're expecting such massive cooling to occur in the future. Quite the
contrary. We expect warming of that order of magnitude to occur over the next
few hundred years. If the difference between the Ice Age and the present was so
large in terms of the physical environment, the vegetation, the amount of ice,
the areas where people could live, the amount of rainfall, and so on, if there
were such large differences between 20,000 years ago and now, and we anticipate
similar differences--but in a different direction, the opposite
direction--might occur over the next few hundred years, then I think that
is cause for concern.
So it's not so much just the temperature changes, but it's the changes in
all the other aspects of the environment: amounts of precipitation, the ability
for vegetation to maintain its status quo, the amount of water that's available
for agriculture and for water resources, and so on.
If we upped it from the half a degree to five or six degrees positive.
Right? That's the logic?
Well, I thinks if we have a few degrees' warming, then that's half the
amount of temperature change that occurred over the last 20,000 years, but it's
going to occur in a few hundred years or even 100 years. So there's also an
acceleration of the amount of warming, compared with what has occurred in the
geological and historical past.
Now, a couple of things that sometimes skeptics say, I'd just like to
get on record here. One of the things they say is that of this half a degree, a
good fraction of it occurred earlier in the century, before 1945, and then
followed by a sort of cooling period. And so it's difficult to make the
argument that greenhouse gases are really responsible for all of this, or
particularly the first part.
Well, it may be difficult for those skeptics to be able to make that
association, but it's certainly not difficult for scientists who work in the
Anybody who works on trying to understand just, say, the temperature
changes over the last 100 years is not going to attribute those temperature
changes to a single causal factor. In fact, there are really three categories
of factor that one has to consider.
One is the influence of human activities---the buildup of carbon
dioxide in the atmosphere, and other greenhouse gases--and also the effect of
sulfate aerosols. Aerosols are very small droplets, primarily made of sulfuric
acid or ammonium sulfate, that are suspended in the air, and they arise from
fossil fuel combustion, which produces sulfur dioxide. Sulfur dioxide is
oxidized to these sulfates, and that produces these very small droplet
particles in the atmosphere. And those particles have a cooling effect. They
have a regionally specific cooling effect because they don't stay around in the
atmosphere for more than a few days to a week. But they're continually
replenished in areas where there's industrial activity.
So, essentially, from the human-influence point of view, we have two
opposing effects. One is the fairly globally uniform effect of greenhouse
gases--particularly carbon dioxide--and then counterbalancing that,
but with an entirely different spatial pattern, the effect of sulfate aerosols.
And the irony is that both the carbon dioxide and the source for the sulfate
aerosols is fossil fuel burning. Okay. So that's just one side of the
The next important issue is, what about natural external factors that might
cause temperature to change on a global basis? There are really two primary
factors. One is the sun, and if the output of the sun were to change--and we do
believe that it does change. In fact, we know from satellite measurements that
the output of the sun changes on a decadal time scale. But we also suspect that
the sun's output changes on times scales of 30 years to 100 years or more.
Okay. So we have to account for that solar influence.
And then the other external forcing factor, or external influence, is the
effect of volcanic eruptions. And a prime example there was Mt. Pinatubo, which
erupted not so long ago and caused, for a year or two, a substantial amount of
Now, in addition to those two factors that we refer to as external
forcing--driving mechanisms that are outside the climate system--the
climate system itself can vary from year to year and decade to decade, as the
amount of heat in the atmosphere is transferred to ice masses or to the ocean,
and there's continual interchange between ocean atmosphere and the land surface
and ice masses and so on.
These are kind of internal cycles knocking around?
Well, they're not cycles in the sense of having any strict periodicity,
although El Niño is an example. I mean, that does have a reasonable
periodicity. But there are other interactions that are going on, purely
internally, within the climate system that could cause, for example, the globe
to warm by one- or two-tenths of a degree, or cool by one- or two-tenths of a
degree, over a 100-year period. So, what you have to do in unraveling the
reasons for global warming is, you have to account for all of these factors.
And at different times, different factors have more or less importance. So if I
go back now and look at that record of global warming, it's true that over the
period from about 1910 to 1940, there was very substantial warming, so much so
that it cannot have been due only to human activities.
But there are two other possibilities. It could be due to changes in the
output of the sun. And, in fact, we believe that this is the primary reason for
That it was reduced, you're saying?
The output of the sun during that period increased in the early twentieth
century, and that added to a small amount of warming due to human activities.
If we go then to the period from 1940 to 1970, when there wasn't much increase
in global mean temperature, and then you say, "Well, well, at that time the
emissions of carbon dioxide were going up very rapidly, so why didn't the world
warm?" Well, part of the reason is that the output of the sun declined slightly
during that period, so we believe, and also that the emissions of sulfur
dioxide increased dramatically. So the amount, or number, of these little
droplets increased, and they had a cooling effect that partially offset the
warming effect that would have occurred just due to greenhouse gases.
And then since 1970, there's been a really dramatic warming, you know, more
rapid than has ever occurred. And we believe that this is partly because the
emissions of sulfur dioxide have been controlled in certain parts of the world,
so that cooling effect has diminished slightly. There's tremendous growth in
emissions of greenhouse gases, particularly carbon dioxide, over the last
twenty years or so. And, interestingly, the output of the sun has increased a
little bit over that period, too. So all those things together cause a very
So, basically, if you look at that whole record of global mean temperature
changes over 100 years or so, then account for, or try to account for that,
using all of these external factors, you can account for it tremendously well.
In fact, the agreement between model expectations and the observed record of
warming is spectacularly good. It's so good that it's clearly a bit of a fluke
that it came out so well.
But one criticism could be that you could always, with so many
factors--especially ones you don't have a good handle on, like the sun and the
sulfate aerosols--you could really just cook the books. You can make it come
Cooking the books would be a concern if that was what scientists were apt
to do. But I don't think scientists generally operate that way. So, in
explaining this past temperature record, the way it has been done is first to
take independent estimates of all of the contributing factors, and put them
together, and then run a climate model with an independent estimate of how
sensitive that climate model is to external forcing, carbon dioxide, or
whatever. So that all of the input information is essentially given to the
climate modeler, and then you run the climate model, and then it
comes out with this amazingly good agreement.
Now, after doing that, it's important to go back and say: Now, what are the
uncertainties associated with these calculations? You can say, for example,
that we don't really know what the cooling effect of sulfate aerosols is. We
have a best-guess number, but we have a range of uncertainty. So, you consider
and concatenate all of the upper and lower bounds of these uncertainties, and
that gives you a range of possible temperature changes due to these factors.
Well, the observed temperature change is right in the middle. But of course,
you know, it's possible that all of the uncertainties acted in the same
direction, and that the model calculation is overestimating the amount of the
human influence, or the amount of the solar influence, or whatever.
But, of course, it's equally likely that it's in the other direction as
well. So we, you know, we try to play the game as straight as possible, and
then also to look at the range of uncertainties.
But some of the uncertainties, like the indirect effect of aerosols, are
really quite large. And it's difficult to imagine how you're really going to
narrow them, isn't it? Some of these questions are very difficult ones to
Well, I'd like to quantify the uncertainty. I mentioned that the amount of
observed global mean warming is roughly six-tenths of a degree Celsius over the
last 100 years. If we put all of our best estimates of the parameters into a
climate-model calculation, then the amount of warming is pretty close to
six-tenths of a degree. The range of uncertainty in this model calculation is
about plus or minus two-tenths of a degree. So that means...what the model
says is that the warming due to all of the factors--and that includes the solar
term, by the way--is, say, between four-tenths of a degree Celsius and
eight-tenths of a degree Celsius. Now, part of that uncertainty is associated
with the cooling effect of sulfate aerosols. And it's clearly important to try
and reduce that particular uncertainty, because that will give us a better
handle on how well the model and the observations agree.
But reducing many of these uncertainties is a really formidable task. Okay.
So I'll go back to sulfate aerosols. The way sulfate aerosols have a cooling
effect is very complicated. In fact, there are really three different
components of that cooling.
The first is that in the clear sky atmosphere, in the absence of clouds,
these little droplets reflect incoming solar radiation. And that obviously has
a cooling effect. If less solar radiation gets to the surface, then the surface
is going to be cooler. The second factor--which is, we believe, the most
important factor--is what these aerosols do to clouds. They're sufficiently
small that they can act as nuclei on which cloud droplets can condense. And
that means that if you put more of these sulfate aerosols into the atmosphere,
then clouds on average should have more droplets. If they have the same total
amount of water, but it's distributed over a larger number of droplets, then
the average diameter of a cloud droplet will be less.
Well, it turns out that when clouds have very small droplets, they're much
more reflective than clouds that have big droplets. And you can actually see
that when you go outside, because clouds...on a stormy day there are white
clouds and gray clouds, or even black clouds, and it's usually the darker
clouds that are about to rain. And the reason they're about to rain is they
have bigger droplets. The white clouds are the clouds that have smaller
droplets, and they reflect more sunlight. So we know that that's empirically
The difficulty is in quantifying how much the reflectivity of clouds will
be increased as cloud droplets become smaller. And in order to do that better,
we need to have measurements of droplets in clouds--that's kind of a difficult
thing to do--measurements of the sizes of these little aerosols, estimates of
the physics of how the cloud radiation properties change, how the
re--reflectivity changes and [so on]. So it's a tremendously difficult
scientific problem to understand the processes that are going on, quantify
them, and reduce the uncertainties involved.
Now, just to make things even messier: Not only do clouds become more
reflective when there are more aerosols around, but also their residence times
change. Quite clearly, if a cloud has a lot more small droplets, then it's not
going to rain as readily. So if it doesn't rain as readily, it's going to stick
around for longer. So what we say is that with a lot of aerosols in the
atmosphere, the lifetime of a typical cloud will be longer. It'll stick around
for longer. That means it can actually reflect incoming solar radiation for a
longer period of time.
So, somehow we've got to put all of these factors together. It's a
horrendous theoretical modeling computer calculation.
You were saying earlier that the six-tenths of a degree Centigrade is
significant because it shows that something's happening at a faster rate than
would be normal. But there are people who talk specifically about certain
regions in the past, evidence of periods in, say, northern Europe that were
called the Little Ice Age, or earlier, the Medieval optimum, when it was very
cold or very warm, seemingly by more than this...stories which say that
Greenland was colonized in the twelfth thru thirteenth century, then became
unlivable by the fifteenth. So, if those things could happen before there was
really any CO2, isn't the natural variation bigger than what you
have said we should be alarmed about?
These past changes in climate that we believe occurred, and which seem to
have an influence on human populations, are certainly important. It's possible
to look at those and say: Well, there was a lot of variability in the past. And
how do we know that the present changes are not just another manifestation of
similar types of natural variability? I'll come back to that in just a second.
But you know, what's more important is that those past records show really how
vulnerable societies are to relatively small changes in climate.
Now, let's get back to this issue of natural variability and those past
records. What we have for the twentieth century is an estimate of how the whole
global mean temperature, has changed. The past records are all from specific
regions. For example, from Greenland or Iceland or parts of Scandinavia, and so
on. We don't have a true global picture of how temperatures changed in the
past. We have fragmentary records. We have records that are indirect and quite
difficult to quantify in many cases. So it's a rather uncertain record, and I
think it would be a very brave scientist who claimed that they knew what the
past record of global mean temperatures was more than 100 years ago, or what
the past variability in that record was.
You know, we have interesting evidence that suggests that the climate
system is quite variable, and we take account of that evidence in trying to
evaluate and assess the changes that have occurred in the twentieth century.
And our judgment is that measured against that yardstick of rather imperfectly
known past changes, the present changes are substantially greater than have
occurred in human experience.
The global changes?
The global changes.
Here's another argument I've heard: If regional fluctuations can occur
in the absence of a mean global change as significant as this--say, for
instance, in Greenland or Mill Creek, in Iowa--if those things could occur
naturally, does increasing the global temperature increase or decrease the
regional variance? And make this more alarming or less alarming?
There's certainly evidence of societies being affected by changes in
climate in the past,. And not just changes in weather from one year to another,
but changes in climate that appear to have been prolonged. And there are some
classic examples. I think when you actually delve deeply into these examples,
you'll find that there were many factors other than just climate change that
caused those societies to either just disappear, move, or change rather
Now, a very good example is the settlements in Greenland. And they
apparently died out quite dramatically, and people have suggested that that was
associated with a change in climate. Now, when you actually look at the climate
records, it's quite difficult to quantify those records with enough precision
to associate cause and effect. And if you look at the archaeological remains,
you can see that there are many other factors that could have contributed to
the demise of those little outposts of civilization. Now, it might be that
climate was the trigger. It might be that, in the past, those societies were
vulnerable to relatively small changes in climate, and you know, that something
came along and that was the end. But the societies themselves were vulnerable.
I think that those past societies were much more vulnerable than most modern
societies, certainly, you know, societies in Europe and North America and the
developed countries. We would be able to stand those natural changes, I think,
So the concern is not so much the natural changes. I mean, that's the
backdrop against which human-induced changes are going to occur. But the
human-induced changes that are expected over the next 100 years are much, much
greater than any changes that societies experienced in the past. Much
So, given that we know these greenhouse gases have some radiative
properties, how do we know that actually (1) it's increasing, (2) that the
source for this increase is anthropogenic, and (3) by how much has it
increased? Just give me those facts, the data.
We know, without a shadow of doubt, that the concentrations of the
important greenhouse gases have increased dramatically over the last few
hundred years. Those gases are carbon dioxide, methane, nitrous oxide, and
other gases like the halocarbons or the so-called CFC's. The CFC's are
interesting because, for most of those, they didn't exist before human beings
came around. They didn't exist until the 1930's. And now they exist in
considerable abundance, so we can be absolutely sure that they are due to human
But what about carbon dioxide? That's the classic greenhouse gas. First, we
know that the concentration of carbon dioxide has increased over the last forty
years, because we have been measuring it with very high precision since 1957 or
so, at initially just a couple of sites, but now there's a global network, and
so we know that these changes are ubiquitous and very strong over the last
forty years. When we first began these measurements, the concentration of
carbon dioxide was 315 parts per million in the atmosphere, and now it's 365
parts per million. That's a really big increase.
Now, to go further back, what we have to do is go to Antarctica or
Greenland--but the best records come from Antarctica--and drill a hole into the
ice, and then within that ice core, we find trapped a fossil record, bubbles of
air that were trapped when the ice was laid down, going back hundreds of
thousands of years. And we can extract the bubbles in the ice, we can measure
the atmospheric composition, we can measure the amount of carbon dioxide, and
we can get a very accurate record of carbon dioxide changes going back
hundreds-- literally hundreds of thousands of years, or more particularly,
since preindustrial times.
Now, if we just look at the last 10,000 years, the level of carbon dioxide
up to about 1700, or 1750 or so was reasonably constant...varied by a few
percent, maybe 5 percent or so. Since the middle of the eithteenth century,
there's been about a 30 percent increase in carbon dioxide. It seems like more
than coincidence that this marks the period of industrialization and population
But couldn't that just have bubbled out of the oceans because it got
warmer, because of the sun's heating?
Yes. So how do we explain this very rapid and large rise in atmospheric
carbon dioxide concentration? There are a lot of possible explanations, and one
is that it was just purely a natural phenomenon.
Well, we can determine that it's not natural by looking at different
isotopes of carbon or carbon dioxide in the atmosphere. And the most important
one of these is radiocarbon, or carbon-14. That's the constituent of carbon
that is used for radiocarbon dating. There's a very, very small amount of
radiocarbon in the atmosphere. It's produced by cosmic rays hitting the upper
atmosphere. And normally the concentration of radiocarbon in the atmosphere is
roughly constant. But we can measure how that has changed over the last 100
years. And we find that it's changed very dramatically, along with the increase
in carbon dioxide. And the change is toward less radiocarbon. Now, how can that
Well, if the increase in carbon dioxide were due to burning fossil fuels,
you have to remember that coal and oil and gas have been in the ground for a
long, long time, and they have no radiocarbon left. That's all decayed away,
due to radioactive decay processes. So if we burn fossil fuels, then what we're
doing is, we're injecting carbon dioxide into the atmosphere that has very
little or zero radiocarbon. And that would dilute the amount of radiocarbon in
the atmosphere by a predictable amount. And lo and behold, the amount of
observed dilution agrees with the prediction. In other words, because of this
dilution of radiocarbon in the atmosphere, we can be very sure that a large
component of the increase is due to burning fossil fuels.
So it's gone up by about a third, and the source is industrial
fossil-fuel burning, primarily. This is new. So then we have to say: By itself,
can the CO2 really do that much? It's just one element in your
climate system, right? You've talked about forcings and feedbacks. How do you
distinguish between the two? How would we call a forcing? Forcings lie outside
What I can do is give you a simple explanation of how the climate system
works. And the analogy I like to use is of a motor vehicle. And the effect of
carbon dioxide or the effect of the sun's output changing is a bit like putting
your foot on the accelerator of a car. So that, in a sense, is an external
forcing of the motor vehicle. By putting our foot on the accelerator, we're
activating this engine, which drives the wheels and makes the car move. And in
the same way, by turning up the output of the sun or by adding carbon dioxide,
it's like putting our foot on the accelerator of the climate system. Now, one
of the big uncertainties in the climate system is just how powerful that
accelerator action is. Essentially, we don't know whether we've got a
Volkswagen or a Porsche.
How sensitive it is, you mean?
We don't know how sensitive the climate system is to external forcing. I
mean, we know that it's sensitive. We know that at least we've got a
Volkswagen. But we might actually have a Porsche. And if we had a Porsche, then
we'd have to worry a lot, because then a small amount of external forcing or a
small amount of pressure on the accelerator would cause a very large climate
change. So you know, my guess is, we're somewhere in between.
Given that you've got these forcings, what can either diminish or
amplify the effect of you putting your foot on the accelerator? These are
called feedbacks, right? So, that's your analogy of the car. So for instance,
with a Porsche, presumably, that's where you see lots of positive
Yes, that's right. So the difference between the Porsche and the Volkswagen
is related to these processes called positive feedbacks. And I'll give you one
example of a positive feedback, and that is that if we were to add carbon
dioxide to the earth's atmosphere and cause warming, then the oceans would
warm, and the amount of water evaporating from the oceans would increase. And
it happens that water vapor is also a power greenhouse gas. So that by putting
carbon dioxide into the atmosphere, we increase the amount of water vapor, and
so we increase the total amount of greenhouse gases in the atmosphere and
amplify the effect of carbon dioxide alone. And that amplification is called a
positive feedback. And there are a number of different feedback processes in
the climate system. And the difficulty is knowing just how strong those
feedback processes are. Some of them are relatively easy to quantify. The water
vapor feedback is one.
But other feedback processes are much more difficult to quantify. An
example is the effect that warming might have on clouds. You can make up a very
simple argument. You could say: Well, more warming would mean more water vapor
in the atmosphere, therefore more clouds, and clouds reflect incoming solar
radiation, and that should have a cooling effect. And that would be a negative
feedback. But, of course, clouds affect the climate system in very, very
complicated ways. And you just have to look at the difference between day and
night. In the daytime, clouds might make the day cooler. But in the nighttime,
the presence of clouds makes the night warmer. So it's not immediately clear
whether clouds cause warming or cooling, or whether they're a positive or a
negative feedback effect.
Critics such as Richard Lindzen say that you have to have water vapor
feedback in order to get up to the big numbers of concern. They say, if you
couldn't depend on that, then CO2 by itself wouldn't really do all
that much. Is that true?
Lindzen has this argument that says that all of the other feedback
process--all the positive feedback processes--are dependent on the water vapor
feedback having a certain magnitude. So it's really important to quantify the
water vapor feedback. And Lindzen has a theory that says that this is less than
other people have suggested. It's a rather complicated theory, but essentially
it boils down to this: More warming causes more evaporation, and therefore the
clouds become more active, and stronger, convective clouds, or cumulus clouds
or storm clouds, have greater, stronger updrafts, and inject water from the
surface up into the upper atmosphere.
Now, what Lindzen says is that there's a counteracting effect that...okay,
so the clouds are bringing water into the upper atmosphere, water vapor into
the upper atmosphere, and that is acting as a greenhouse effect and amplifying
the warming due to carbon dioxide. But, in addition, the updrafts... eventually
they come out of the top of the clouds. The dry air then is denser than the
surrounding environment, and so that air--that dry air--sinks down into the
atmosphere, eventually back down to the surface. In fact, this is a very
important process. These downdrafts of dry air outside of clouds can be
extremely serious for aviation, for example. So there's no doubt that they
exist. Now, what Lindzen says is that this causes a net drying effect that is
greater than the effect of the moisture injected into the upper atmosphere by
the clouds. And the drying effect, he claims, will reduce the water vapor
feedback, and then that will have a knock-on effect and cause all the other
feedback effects to be smaller.
So it's a nice hypothesis, and it's something that we can test. We can test
it observationally in very simple ways by actually looking at water vapor
changes in the atmosphere, or we can do things rather more sophisticated and
actually quantify the effect of water vapor changes on an intra-annual time
scale. In other words, we can look at the variability of water vapor in the
atmosphere throughout the year and see whether, on that short time scale, the
Lindzen hypothesis works.
Well, it turns out that, in both cases, the effect that Lindzen claims to
be important is, we believe, not important. You know, we've tried to test his
hypothesis in different ways, and we find that it actually doesn't work. It's
physically realistic, but it is outweighed by other processes.
So the climate is more sensitive than he claims.
The climate's a lot more sensitive than he claims.
There's another argument, made by the coal industry, that another
feedback is just the greening of the world. Basically, you put the
CO2, plants grow, trees grow; it will take it up. What's the problem
with this? That would be a negative feedback. They argue in their extreme
version that it's a good thing.
I've been talking about feedbacks within the climate system. But it's
possible to think of feedbacks that are within the whole global environmental
system. And one of those is the fact that plants grow better with higher
amounts of carbon dioxide and, that as plants grow better, they absorb more
carbon dioxide from the atmosphere. So that one might expect that if you burn
fossil fuels, the amount of carbon dioxide stimulates the growth of vegetation
and that slows down the rate of increase of carbon dioxide in the atmosphere,
and that will therefore slow down the rate of climate change.
Now, that's a process that's been know for, you know, 100 years or so. And
it's a process that can be reasonably well quantified. And it is incorporated
into the models that we use to predict how much carbon dioxide will increase in
the future. In other words, I mean, we have fully accounted for that so-called
"CO2 fertilization process" in projecting CO2 increases
in the future.
But...I was just going to say that that's actually not their argument.
Their argument is that CO2 is good for you, and you know, it's the
impact on the vegetation--
Given that you can have positive and negative forcings and positive and
negative feedbacks, is there any way, logically, that you can escape from this
process? Given that we accept greenhouse gases are positive forcing, could ever
the indirect effect of sulfate aerosols...could you offset that perpetually?
Can you escape from the dilemma?
That's a pretty broad question.
Because you could say, for instance, even if the climate was less
sensitive than you said, in time, if you pumped enough in, the figures would
come out. You couldn't escape from it unless you had negative forcings which
could keep up. Can negative forcings ever keep up? The natural ones, obviously,
fluctuate both ways, but I'm talking about the anthropogenic forcings.
It's difficult to answer that question. But I can say something else that's
kind of off the track a little bit but...
The leading issue here is not really how much has the climate changed in
the past. Of course, we have to understand what those changes have been caused
by. But the real issue is what the changes are going to be in the future. And
not only what our best estimate of those changes might be, but what the range
of uncertainty is. And it's possible to run a climate model and make the
positive feedback effects very small--minimize them, essentially--and then get
a very low estimate for future warming. And if you do that, over the next 100
years, with some reasonable estimate of what the emissions of carbon dioxide
and other greenhouse gases might be, and what the emissions of sulfur dioxide
might be, the warming over the next 100 years is still pretty substantial. The
lowest possible amount of warming would be 1 to 1.5 degrees Celsius.
In other words, at the real bottom end of the range of possibilities, we
anticipate a warming that would be at least double the rate of warming that has
occurred over the last 100 years. At the other end, though, the warming could
be as much as five degrees Celsius over the next 100 years. And that would be
horrific, I would say.
Are you saying that's the best you can do? You can't, say, invoking the
indirect effect of aerosols, knock it back even lower?
Let's just consider the effect of aerosols in the future. In the past,
what's happened is, because of burning fossil fuels, the loading of these
aerosols in the atmosphere has increased tremendously. The emissions of sulfur
dioxide have increased tremendously over the last 100 years, globally. Now, in
North America and Europe, those emissions have declined over the last 10 to 20
years. The real growth in the emissions of sulfur dioxide today is in countries
like China and India. Now, what's going to happen to those countries in the
Now, as we become richer as a society, we tend to become much more
environmentally conscious. We have different priorities, and we tend to be more
concerned about issues like acid rain, urban air quality, and so on. And
basically it's because we can afford to do something about sulfur dioxide
emissions and sulfate aerosols that we have done something about it.
Those economies of countries like China and so on are growing very rapidly. The
societies are becoming richer, and we expect that they soon will be able to
afford to do something about the emissions of sulfur dioxide. So, we don't
expect their emissions to continue to go up. What we anticipate is that over
the next twenty to thirty years or so, they will have the concern about the
environment, coupled with the wealth to be able to do something about it, and
they'll do what we've done. They will reduce the emissions of sulfur
So what's going to happen in the long term, then, is that the emissions of
sulfur dioxide and the loading of sulfate aerosols in the atmosphere will go
down. It might go up for another twenty years or so, but after that, it's going
to go down. Now, these aerosols have a cooling effect. If there are less
aerosols, the net effect has to be warming. So, what's going to happen in the
future is that sulfur dioxide will have the opposite effect to what [it has]
had in the past. In the past, sulfur dioxide emissions have offset greenhouse
gas warming. In the future, as they reduce, they will add to greenhouse gas
They'll reveal it.
They will just make the warming stronger.
Given that, then, if we reach a position where you've convinced people
there's cause for concern, and then the discussion goes on to asking: What do
we do about it. You've mentioned residence time. This CO2 we've put
in the atmosphere stays in the atmosphere for quite a long time, doesn't it?
So, it's not enough to simply cap emissions at current levels if we have a
world that uses 90 percent of its energy from fossil energy. Really, to change
these buildups in atmospheric gases, you have to cut massively these emissions,
In deciding what to do about future global warming, the motor car analogy's
actually quite useful, because we're in this car, it's zooming along at a
fairly high speed, and we're worried that there might be a cliff down at the
end of the road there that we're going to go over. So what do we do about it?
Well, we don't just leave the accelerator pedal at the same position. In other
words, for the climate system, we don't just keep emissions at their present
level. We actually have to take our foot off the accelerator. And doing that in
a car isn't going to stop the car immediately. The car has a tremendous amount
of inertia and is just going to keep going.
In the same way, even if we reduce the emissions of carbon dioxide and
other greenhouse gases--we take our foot off the accelerator--the climate
system's going to keep on going. It's going to keep on warming for a number of
years. I don't mean number of years, by the way. I mean decades or centuries.
So we have to do something pretty dramatic in order to stop global warming.
We don't necessarily believe that all global warming is bad, by the way. We
don't necessarily believe that the increase in carbon dioxide is uniformly bad,
because carbon dioxide accelerates plant growth and can be good, potentially,
for agriculture. But what we're afraid of is that if the planet warms too much,
we're going into unknown territory. We can't predict the climate well enough to
know what to expect. So we certainly don't want to go too far down the road,
down the pathway of global warming. We have time to think about what to do. But
eventually, we have to do something dramatic.
We really have to move completely away from using fossil fuels for energy.
Either that or we have to somehow sequester the carbon dioxide that's being
emitted into the atmosphere, which would be a very costly thing to do. I think
that, you know, the only solution really is to, in an economically sensible
way, reduce our dependence on fossil fuels as sources of energy. You know. I
think that might be quite difficult to do, but--
When you look at something like a Kyoto proposal, which is politically
unacceptable to many people, how much difference would it make to the problem
if it were enacted tomorrow?
If everybody were to come on board with the Kyoto protocol, then that would
slow down the rate of warming of global mean temperature, slow down the rate of
climate change by a very, very small amount. So we have to clearly, in the
future, do something a lot more dramatic than just the Kyoto Protocol. It's a
very small step along a pathway that is long and difficult.
And it's a step we're almost certainly not going to make, the way things
are going. When you look at the scale of the problem from a carbon point of
What do we have to do? We have to stop as much getting into the
atmosphere. Otherwise it will double, treble. What will happen if we just carry
on exhausting fossil supplies?
Okay. If we were just to follow the Kyoto Protocol...if everybody came on
board--and .I think it's unlikely that the United States will--if all countries
did, that would not stop the increase in the growth of carbon dioxide
In other words, of course the Kyoto Protocol would do something: It would
slow down the rate of growth. But don't forget that the Kyoto Protocol only
applies to the developed countries, and right now, the other countries of the
world are growing quite rapidly, and their emissions are growing, and that will
continue over the next century unless something much broader than the Kyoto
Protocol is enacted. If emissions of carbon dioxide continue to grow, then the
level of carbon dioxide in the atmosphere will continue to grow.
Following the Kyoto Protocol would reduce the level of carbon dioxide in
the atmosphere in the year 2100 by a relatively small percentage. Just to give
you some numbers, what we think as a best guess, in the absence of any policies
to reduce the missions of carbon dioxide...as a best guess, we think that the
concentration of carbon dioxide in the year 2100 would be about 700 parts per
mission, which is roughly double what it is today. If we enact and follow the
Kyoto Protocol, then we might reduce the buildup by a few tens of parts per
million carbon dioxide. In other words, instead of doubling the amount of
carbon dioxide, we might only increase it by 90 percent. So it's clearly a very
But we still have molecules of CO2 in the atmosphere that
were produced in James Watts' steam engine, don't we? The stuff hangs around
for a long time. So the question is, to turn the curves of concentration, what
do you have to do?
So the Kyoto Protocol will cause a small slowdown in the rate of buildup of
concentration in the atmosphere.
It won't stabilize it though, you're saying?
It will not stabilize it. Now, the guideline for the Kyoto Protocol...the
background document is the United Nations Framework Convention on Climate
Change. And that has as its ultimate objective the stabilization of the
concentrations of greenhouse gases in the atmosphere. There are a lot of
conditions associated with that stabilization. You know, for example,
stabilization should be done in a way that is not economically disruptive. That
may not be spelled out exactly that way in the Framework Convention, but
nevertheless it's quite clear that we've got to make sure that the economy of
the world isn't damaged by our desire to save the environment. There's got to
be a balance between these things.
Stabilization of carbon dioxide concentration in the atmosphere...how do we
do that? What level do we want to stabilize carbon dioxide in the atmosphere
at? You know, clearly, the lower that stabilization level, the more we've got
to do. If we were to stabilize the level of carbon dioxide in the atmosphere
at, say, 50 percent above what it is today, then we could allow emissions of
carbon dioxide to grow over the next ten or twenty years--or maybe the next ten
years, I should say.
But after that, the emissions of carbon dioxide worldwide would have
to decline rapidly. And over the next fifty years, you know, from 2010 to 2050
or 2060, the emissions of carbon dioxide would have to drop down to, say, about
a third of what they are now. That's an overwhelmingly difficult task. How on
earth do we achieve that? What sort of technology is required in order to go
away from the burning of fossil fuels? How do we develop that technology? I
mean, these are massive problems that we face.
Just what scenario is more likely? I'm just trying to get the sense of
how difficult this is. What other way is there to drive a car, apart from
gasoline? Hydrogen economy? That's not going to come in ten years.
How much time do we have to do these things? Is it one year? Ten years?
Fifty years? I think that the planning has to begin now, and it already has
begun now. I mean, the Kyoto Protocol is a small step. But the Kyoto Protocol
doesn't say anything about how we're going to actually get to this ultimate
objective of stabilizing the level of carbon dioxide in the atmosphere. Somehow
we've got to build in some incentive for technological development in order to
move away from fossil fuels. That's my view, and that's the view of some of my
And you know, that raises the issues of how on earth do we do that? How do
you stimulate innovation? Technological growth? Do you do it by so-called
command and control, by taxing fossil fuel usage? Or do you do it by providing
incentives for technological innovation? I really don't know the answers.
I think that we probably have another ten or twenty years to figure out how
to get on the pathway that will eventually lead to stabilization. It's going to
require a global effort. It's not just going to be the developed world. We've
got to do this in a way that embraces all of the countries of the world,
because climate change is not just going to affect us; it's going to affect
everybody. In fact, ironically, it's probably going to affect North America and
Europe less than these developing countries. They're the people who are going
But, on the other hand, you know, we're the people to blame, because we're
the people who've put most of the carbon dioxide into the atmosphere. So, you
know, somehow we've got to get together and realize that, you know, we're to
blame, and the developing countries may not be so much to blame for where we
are now, but the people are going to suffer, those in the developing countries.
So that we and them, we together have a responsibility to do something about
Last question. If nothing was done, and global temperatures went up by
as much as five or six degrees, are we talking about something pretty extreme
there, in your view?
If the world were to warm by five degrees Celsius, and there were a really
big increase in the level of carbon dioxide in the atmosphere...I mean, suppose
it got up to three times the preindustrial level, which is not impossible. I
mean, if we do nothing about it, that's almost sure to happen. What will that
do to the environment? Well, a five-degree warming will just change the whole
climate system radically. Precipitation patterns will be entirely different.
The amounts of precipitation will be entirely different. You know, storm tracks
would be unrecognizable from what they are today. You know, it might take a few
hundred years to get there, which would give us time to plan for the changes,
but the changes would be really enormous.
The worst prospect, however, is sea level rise. If we were to warm the
world by five degrees, then I strongly believe that large parts of the
Antarctic ice sheet would flow into the ocean and melt, and cause sea level to
rise by potentially many meters. And even if that didn't happen, a five-degree
warming would cause all of the smaller glaciers in the world to melt, would
cause substantial melting from Greenland. It would cause the ocean to warm up
and expand by a large amount. Even without parts of Antarctica falling into
the ocean, sea level would still rise by meters. And that means that a lot of
coastal communities would suffer tremendously. Island communities in the
Pacific and the Indian Ocean would be dramatically affected. People may not be
able to live in those low-lying areas.
> the debate
> carbon diet
> stories in ice
> beyond fossil fuels
> water world
> program excerpt
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