Global emission of carbon dioxide:
the contribution of natural sources
Murry L. Salby
Professor of Climate
Macquarie University, Sydney, Australia
Lecture given at the Sydney Institute, Australia, 2 August 2011
 Professor Salby, now Chair of Climate at the Faculty of Science at Macquarie University,
has held Visiting Professorships in Paris, Stockholm, Jerusalem and CNRS, and has
spent two sabbaticals at the Bureau of Meteorology in Australia. He is a former reviewer
for the UN Intergovernmental Panel on Climate Change (IPCC).
 The diagrams that accompanied Professor Salby’s lecture are omitted from this
transcript because they are under a pre-publication embargo. They will be included in
the transcript as soon as they are available.
Thanks to the Sydney Institute and Tom Parker for inviting me. You are aware of the topic, at
least in generality. I should tell you that for most of my career of 30 years in atmospheric physics
on this topic my position was agnostic, like many competent scientists in the field. The findings I
will present were recently unveiled at an international conference. They have compelled me, and
others in the scientific community, to rethink that position.
In view of the public debate on carbon dioxide, the Australian community should have the
benefit of these findings now, while they are relevant.
Much of the debate surrounds human emission, by Australia, in relation to other countries. But
what matters physically is not emission: it is carbon dioxide in the atmosphere. That is what
influences the Earth’s temperature. The debate has been obsessed with emission, because the
presumption is that observed changes of CO2 are due exclusively to human emission. In reality,
atmospheric CO2 follows not just from Australian emission, not even from worldwide human
emission: it follows collectively from all sources and sinks, human and natural. And that’s what
we’re going to discuss.
I was drawn to this topic not naturally but out of necessity. After relocating to Australia, I found
myself waiting for resources. Eventually I decided, “Well, I’d better do something.” So I
undertook the production of a new book. Now, as these things go, a book never proceeds as
quickly as one would like, because when it comes to putting pen to paper one finds that the
understanding of key issues is inadequate. And on this subject one of the most conspicuous is the
evolution of CO2.
This panel encapsulates our view of how CO2 has changed. Plotted in green is proxy CO2 – in ice
from a core over Antarctica. It is thought to represent CO2 that was previously in the atmosphere
when that compressed snow fell. Now, CO2 is measured in units of parts per million by volume: 1
ppmv means that out of every million air molecules one is a CO2 molecule.
From the 19th century, CO2 increases steadily to the 1960s, when actual atmospheric
observations began. Current values are about 380 ppmv. In red is the so-called delta-13: the
relative concentration of the two isotopes, carbon-13 to carbon-12. On Earth, 99% of all carbon
atoms have a molecular weight of 12, but 1% have an additional neutron that gives them a
molecular weight of 13. It turns out that, because photosynthesis is more efficient with the lighter
form of carbon, vegetation prefers carbon-12 over carbon-13. We will revisit that in a moment.
The relative concentration of carbon-13 – that’s the red curve – evidences a decline, which
mirrors the increase of CO2. Together, these proxy records comprise the smoking gun of human
emission. For reasons just seen, vegetation is slightly leaner in carbon-13 than the atmosphere.
So is its ancestor, fossil fuel: coal and oil each derive from ancient vegetation.
Emission from combustion of fossil fuel, therefore, adds CO2 to the atmosphere, enriching
atmospheric CO2 – that is the green curve. The CO2 added, however, is leaner in carbon-13 than
the atmosphere, thereby diluting its concentration in the atmosphere – that is the red curve. The
glove fits.
In truth, however, it is but one finger. Combustion of fossil fuel is one of many sources of CO2.
Other sources involve a wide range of natural processes. Many are poorly documented. The
historical interpretation is that opposite changes of CO2 and carbon-13 – the green and the red
curves – are the signature of human emission.
For this interpretation to be valid, other sources of CO2 – natural sources – must have the same
concentration of carbon-13 as the atmosphere, which would then be left unchanged: that is, CO2
emitted by natural sources must not dilute carbon-13 in the atmosphere. The observed increase
of CO2 and decrease of carbon-13 can follow only from the human source.
In reality, our knowledge of natural sources is limited. The little we do know is that natural
sources are dynamic: they depend intrinsically upon environmental conditions – cloud,
moisture, temperature, even on the prevailing ecosystem.
Why should we care? This is the estimated contribution from all sources. The human source is of
order 5 gigatonnes per year. By comparison, the ocean emits of order 90 Gte/yr; land emits of
order another 60 Gte/yr. Total emission from natural sources is thus of order 150 Gte/yr. It is
approximately balanced by natural sinks, which absorb about as much. The key word is
“approximately”. Because natural sources and sinks are two orders of magnitude stronger, even a
minor imbalance can overshadow the human source. Moreover, if those sources involve carbon13 leaner than in the atmosphere, as many do, all bets are off.
In truth, only one component of the CO2 budget is known with any certainty: human emission,
implicitly through records of extraction: how much coal and oil were dug up. The rest, about
96%, is vapourware. It remains largely obscure. We have only isolated field measurements: that
is because many of the responsible processes operate beneath the Earth’s surface, beyond the
grasp of global observation by satellites. They leave the natural sources and sinks – the big
arrows – not sharp but fuzzy. If they change, we haven’t the observations to know – at least not
directly.
Fortunately, we don’t care. What’s relevant is net emission – that which is collected from all
sources and sinks, human and natural. That is what ultimately controls atmospheric CO2.
Net emission is just the sum of the CO2 added from all sources minus the sum of the CO2
removed by all sinks. In turn, net emission is proportional to the global mean (or globallyaveraged) CO2; that is, the rate at which CO2 increases, averaged over the Earth. So the net
emission rate essentially controls how quickly CO2 increases, averaged around the Earth.
Now, CO2 is produced and destroyed at the Earth’s surface. But once it enters the atmosphere, it
is inert: CO2 is, therefore, efficiently mixed by the atmospheric circulation – homogenized, like
cream stirred into your coffee.
This is the global distribution of CO2, only recently available from satellites. Even these modern
observations are limited to continents, but that’s where the human source is. Large values – in
yellow and red – are a signature of source regions. Notice that they are not found in the
industrialized centres – the Ohio River valley of the US; northern Europe; not even China.
Rather, they appear over the Amazon Basin; tropical Africa; and south-east Asia. Those regions
have little human population, let alone industrialization.
What I really want you to notice is that even in those source regions CO2 deviates from its global
mean by less than 5%. That’s a good thing, because it means that local values are a good
approximation of the global average, which in turn provides an average of net global emission.
In fact, we have a long record of CO2 from only one site: Mauna Loa, Hawaii. The local record for
Mauna Loa, therefore, approximates the global mean, which, through its growth-rate, chronicles
the history of net global emission - collectively from all sources, human and natural.
And here it is – This is the instrumental record of CO2 from Mauna Loa. It is characterized by a
conspicuous trend. That is what you saw earlier in the ice proxy. Superimposed on the trend is a
small, innocuous annual variation, which has been largely ignored.
You may recall the film Men in Black, in which there was a small canine character named Frank,
whose size was overshadowed by his persona. He said, and I quote, “Humans think that if
something is important it must be big. The galaxy is tiny – like a marble.” Well, maybe Frank had
something.
This is the mean annual variation. CO2 increases during some months, when sources prevail
over sinks. More CO2 is then added than removed. It decreases during other months, when sinks
prevail over sources. More CO2 is then removed than added. But the variation is not cyclic. More
CO2 is emitted during some months of the year than is absorbed during others. This leaves a net
change. CO2 in one January is 1.5 ppmv higher than CO2 in the preceding January. It represents
the annual increment to the trend. Each year, CO2 picks up another 1.5 ppmv.
Now, this is just the average picture. Here is the collection of all annual variations. Emission and
absorption vary widely between years. So, then, does net annual emission, highlighted in blue. In
some years, it increases by 100%. In others, it decreases to almost zero.
These features of the instrumental record establish the following fundamental characteristics of
atmospheric CO2:
1. The trend is inexorably intertwined with the annual variation;
2. Net annual emission – that is, the annual increment to the trend – is a small residual of
seasonal emission and absorption; and
3. Changes in net annual emission are large – as large as the mean annual increment to the
trend.
Therefore, it would be a good idea to understand them. The difficulty is that CO2 emission is
coupled to other properties of climate. A change in one climate property induces changes in
others. Ideally, we would evaluate how that emission of CO2 changes through a controlled
experiment, wherein we perturb the climate system in a known way and observe its response.
Unfortunately, we do not have that capacity. Only one party can perform that experiment –
Mother Nature.
And she has. For over three decades, the Earth-atmosphere system has been observed globally by
satellites. The satellite record is now long enough to have collected a population of climate
perturbations, wherein the Earth-atmosphere system was disturbed from equilibrium. Those
events reveal how perturbations in one climate property influence others, including net emission
of CO2.
To explore the large changes in net emission, we filtered the CO2 record to changes that operate
on timescales of two years and longer. The respective growth rate – that is, the rate at which CO2
increases – chronicles the history of net global emission. It is plotted in green. Net annual
emission is an average of about 1.5 ppmv/yr. We are on the right planet – that’s the average
annual increase you just saw. But it varies between years dramatically, by 100% - from almost 0
ppmv/yr in some years to almost 3 ppmv/yr in others.
Do I need to tell you that human emission does not do anything like this? Plotted in red, human
emission increases slowly. It mirrors the increase of human population, with deviations of only a
couple of percentage points.
By contrast, observed deviations of net emission exceed 100%. They are uncorrelated with
human emission: that is, net global emission of CO2 changes independently of the human
contribution.
The record of net emission does, however, resemble surface properties of the general circulation.
During 1991/92, net emission decreased – by 70%. It gradually rebounded over the following
three years. That episode coincides with the eruption of Mount Pinatubo, which decreased global
temperature in the same fashion. During 1997/98, net emission increased – by 200%. That
episode coincides with the El Niño event, which increased global temperature.
The correspondence during these events indicates that net emission is shaped by internal
properties of the climate system which characterize the general circulation. The observed
correspondence is, in fact, consistent with field measurements of CO2 emission. They reveal a
strong sensitivity to temperature, supported over continents by soil moisture. An increase of
either leads to increased emission of CO2 - from natural sources.
To get a handle on the behaviour generally, we analysed the record of net emission – that is the
green curve – in terms of global temperature from the satellite microwave sounding units (MSU)
and soil moisture from climate analyses. Those surface properties regulate emission of CO2 from
natural sources.
In blue is the respective component of net emission – the part that operates coherently with the
general circulation (that is, dependently on it), but incoherently with human emission. Although
it accounts for only two key properties, the natural component closely tracks observed emission.
It achieves a correlation to the observed record of 0.93, accounting for nearly the entire history
of net global emission. Most of the dependence comes from temperature alone, which achieves a
correlation of 0.80.
The same analysis has been performed on 𝛿13C, the relative concentration of carbon-13. Observed
emission of carbon-13 is displayed in red. It has a mean that is negative, consistent with the
decline of carbon-13 that you saw earlier. But, like CO2, it varies dramatically, by more than
100%.
In blue is the part that operates coherently with the general circulation, but incoherently with
human emission. Like CO2, it varies coherently with observed emission of carbon-13 – just
opposite. When temperature increases, emission of carbon-13 decreases, and vice versa.
Likewise when CO2 emission increases (that is the green curve) carbon-13 emission decreases
(that is the red curve).
Does this ring a bell? Remember the smoking gun? Opposite changes of CO2 and carbon-13 are
the same ones in the ice proxy. The latter is regarded as a signature of human emission, but in
the satellite record the changes are clearly not from human emission – not unless you believe
that human emission causes volcanic eruptions and El Niños.
The blue curves are, to first approximation, just temperature. They describe the component of
that emission that operates coherently with the general circulation but incoherently with human
emission. Those curves embody the sensitivity of CO2 emission – how much net emission of CO2
is increased by an increase of temperature or soil moisture. Because they are incoherent with
human emissions, the blue curves and related sensitivity can only represent emission from
natural sources. If we know that component of that emission, we can add, from one year, next
year and so forth. The total then represents the contribution to observed CO2 from natural
sources.
In blue is the circulation-dependent component of CO2 – the part introduced by natural sources.
It is determined entirely by temperature and soil moisture. In green, the observed monthly
record of CO2 from Mauna Loa. [The figure shows they track one another.] At the bottom is the
circulation-dependent component of 𝛿13C, part-introduced by natural sources. It, too, is
determined entirely by temperature and soil moisture. In red is the observed monthly record.
[The figure shows they track one another.]
The two components from natural sources evolve in opposite directions, just like the ice proxies
of CO2 and carbon-13. Now, in principle, the sensitivity of emission observed during the satellite
era enables the analysis to be extended backward to earlier years – in fact, over the entire 20th
century. But before the satellite era, global observations are lacking. Nonetheless, most of the
sensitivity of net emission comes from temperature alone. Global temperature before the
satellite era is estimated from surface thermometers. They provide only partial coverage of the
Earth, over land, near centres of population. But that is all we have.
In blue is the circulation-dependent component of CO2 – the part from natural sources, again
derived from the satellite record, but now determined only from temperature: no soil moisture.
As before, the natural component tracks the observed record of CO2, which is superimposed in
green. In black is the same information, but now based on the surface estimate of global
temperature: thermometers. The sampling uncertainty from that estimate is shaded. The black
curve follows from just two things: the sensitivity of emission observed during the satellite era
and the record of global temperature. Representing natural emission, it too tracks observed CO2
during the satellite era. In fact, it tracks it throughout the instrumental era, back to 1960, when
atmospheric observations began. Observed CO2, that’s the green curve, and natural emission,
that’s the black curve, then both approach 300 ppmv.
Before 1960, the natural component continues to decrease, approaching 275 ppmv in 1880. The
uncertainty, which increases backward, is then wide, ranging between 220 and 330 ppmv.
During this early period, observations of atmospheric CO2 are non-existent: we have only proxy
CO2.
In green is the ice proxy you saw at the start. It too decreases backward, approaching 290 ppmv
in 1880. The modest discrepancy that develops before 1940 – about 10 ppmv – lies well within
the sampling uncertainty of surface measurements. But in light of the agreement during the
instrumental era in the last half century, it may actually reflect error in the ice proxy. It may also
reflect limitations of the surface network. At the bottom is coverage of the Earth by surface
thermometers. It maximizes near 1960 at 35% – that is how much of the Earth is land. Before
1960, coverage plummets. By 1880, coverage of the Earth by the surface network is less than 8%.
From the observed behaviour, it is clear that net global emission of CO2 depends intrinsically on
temperature. Human emission does not.
Plotted here is a proxy reconstruction of global temperature – just one of many such
reconstructions. Like proxy CO2, proxies of temperature disagree in many respects, but they
share two major features. Between the years 900 and 1300, global temperature was anomalously
warm, known as the mediaeval warm period. This excursion of climate is chronicled in anecdotal
evidence, the most familiar being the Norse colonization of Greenland. Between 1400 and 1800,
global temperature was anomalously cold, known as the Little Ice Age. It, too, is chronicled in
anecdotal evidence, including chronic freezing of the Thames and New York Harbour. Since
then, global temperature has been increasing. A pivotal issue is just how much of the modest
warming in the 20th century is accounted for by the rebound from the Little Ice Age. That issue
remains under debate. The observed behaviour of CO2 shows that, regardless of how much, this
warming must be attended by an increase of CO2 emission. That’s what you saw in the
contribution from natural sources.
What the observed behaviour reveals is a large – but hitherto invisible – elephant in the room.
Remember the budget of CO2? You just thought it was CO2. It is really something with which
you are more familiar – your monthly household budget. See the little red arrow? That credit is
the varying interest on your bank account. Now, if your bank account is like mine, it is not a big
player. See the big red arrow? That credit is your variable earnings. Unlike me, you work on
commission: that is why you earn more. And, like your bank interest, it varies from one month to
the next. Anyone want to guess what is the large debit – the big green arrow? Oh, come on, what
is the biggest drain on your monthly budget? That’s right – your variable mortgage. What I have
presented today is your net income. The presumption has been that it equals your bank interest.
That presumption follows by balancing your cheque-book - with only 5% of the entries. Bank
interest. Guess what? Your net income does not behave like your bank interest. It behaves like
the difference between your variable earnings and your variable mortgage.
The correspondence to observed changes of CO2 on timescales of a couple of years over the
satellite era, and, to the degree seen, even over the 20th century, makes it difficult not to conclude
that sources involved in changes of CO2 on short timescales are also involved in its change on
long timescales. The popularized view has been that CO2 is driving the bus, and that climate is
along for the ride. The observed behaviour reveals just the reverse. Climate is at the wheel and,
to a significant degree, CO2 is at the back of the bus.
Climate projections rely on an ability to project CO2. It is the one thing believed to be known –
because of the presumption that “we control it”. Namely, future observed CO2 is determined
entirely by human emission. That is what is specified in climate models, which then predict how
climate will respond – so-called climate scenarios. The observed behaviour reveals that, much as
we might like it, the real world does not work that way. Net emission includes a substantial
contribution from natural sources. If you do not control CO2, you cannot predict it. And if you
cannot predict CO2, you can hardly predict how climate will respond.
The climate framework just described is the cornerstone of the IPCC – the UN
Intergovernmental Panel on Climate Change. I used to be a reviewer. Much of the public debate
stems from the IPCC’s last report. The behaviour you have seen was not known at the time of
that report. Were it known, the IPCC could not have drawn the sweeping conclusions that it did.
Here is why.
As you have seen, emission from natural sources is integral to observed changes of CO2.
Its contribution has not been recognized: nor is it represented in climate models.
Because it involves emission that is not solely human, future atmospheric CO2 is only marginally
predictable and, in significant part, not controllable. That means that changes of
human emission will not be tracked by changes of atmospheric CO2. They never have been.
I will close on a philosophical note. When I hear politicians speak on this fashionable topic, and
cite the science, as do those of a certain bent, it stimulates my involuntary gag reflex. There is no
such thing. Science is dynamic. It’s predicated on discourse – questioning. That is how we get to
the truth. Were it not for discourse, we should still be in the Dark Ages. Excluding discourse from
the equation is not science: it is advocacy. And, if it is from an advocate that you hope to
understand nature and how it will evolve, pick up your phone and call Canberra. There are
thousands of them.
Beyond what you have seen today, and irrespective of how much of it you have grasped, there is
one thing – one overarching theme that this audience should carry forward – with unanimity.
Anyone who thinks that the science of this complex system is settled is in Fantasia.
Thank you.