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Here is the letter that David MacKay sent me following my
article in The Times and to which I replied.

(I have gone to weblinks for his charts and in one case
come up with a slightly different version — the sea ice graph I
could not find the exact one he included so I have found another
from the same source which has more years on it than his version,
but it’s the same data and the same source.)
Update: all
graphs now correct!


Once more I thank David for the exchange, and for his
willingness to address the issues I raise. He’s welcome to write
here again in response to my response.



Dear Matt,

I’m writing in response to your recent article in the
Times, which was passed to me by our Secretary of State.
 Reading your article, I enjoyed and fully supported your
insistence that science should be transparent, and that decision
making should be based on a calm assessment of credible science.
Providing honest advice based on robust science is a key
responsibility in my role at DECC.  I have lots of time for
scepticism, because I’m a scientist, and scepticism is the bread
and butter of science.

I’m not a climate scientist. But I have spent quite a lot
of time in seminar rooms with real climate scientists over the last
few years, and I have to say that my impression of that scientific
community is that it is in a healthy state, quite unlike the
caricatures in some of the media and blogs (which allege they are
secretive, dishonest data-manipulators, etc). The community seems
healthy in the sense that the scientists are open; they are
critical of their own community’s work; they highlight weaknesses
in colleagues’ presentations and models and papers; and they
criticise everyone – the IPCC, Al Gore, and Nigel Lawson alike –
for inaccuracies or simplifications in any direction. In
particular I have always noticed that the climate science community
knows how big the uncertainties in climate science still
are. Sadly, some science communicators and policy people seem
to find it difficult to communicate this aspect of the
science. The media machine seems to prefer to turn the normal
scientific process of slow and iterative development of
understanding into a series of ‘headline new findings’, which focus
on disagreement. For example, if one group of scientists report

‘sea level could rise by 0.3-1.3 m this

this can lead to a headline of ‘sea level to rise
1.3 m shock!
‘. (Note the omission of the uncertainty.) And,
if the following week another group of scientists describe results
suggesting that

‘sea level could rise by 0.28-1.28 m this

it’s easy to imagine a headline of ‘sea level rise
has been exaggerated, new report says it will be as little as 0.28
‘. I mention this issue of uncertainty-reporting because I
think the failure to ensure the public discussions of climate
change have included the uncertainty, and the failure to express
decision-making in terms of risk-management, have contributed to
the current climate-policy congestion.

But rather than discuss scientists and the media, what I’d
really like to discuss is the science. It would be great if we
could find agreement on the science, and I am optimistic that we

Let me highlight the paragraph in your article that I’m
particularly responding to.

“I’m not a denier: I think carbon dioxide is a greenhouse
gas. I’m not even a sceptic (yet): I think the climate has warmed
and will warm further. But I am now a “lukewarmer” who has yet to
see any evidence saying that the present warming is, or is likely
to be, unprecedented, fast or tending to accelerate. So I have
concluded that global warming will most probably be a fairly minor
problem – at least compared with others such as poverty and habitat
loss – for nature as well as people.”

OK. First, “unprecedented” and
fast” – these terms are ill-defined, so are tricky to
discuss.  If “unprecedented” refers to recent human
timescales, I’d suggest that the data do show that something
unprecedented is going on. Look at the data on Arctic sea ice,
for example. Here’s the latest graph from Boulder:

For most of the summer of 2010, the arctic sea ice extent
has been about four standard deviations below the 1979-2000 mean.
The summer ice extents in the years 2007, 2008, and 2009 were all
more than two standard deviations below the mean. I find these
sorts of data compelling.

If however “unprecedented” means “the current
warming is unprecedented on 100-million-year timescales” then I
would absolutely agree – as I’ll elaborate later,
there have been natural warming events that have
been roughly as fast, and that led to significant and lasting
climate change and mass-extinctions. So, yes, there is a
precedent, but the precedent doesn’t make for comfortable

What about “fast“? I don’t know how to
define “fast”, so let’s talk about the numbers. The measured
warming rate is about 0.8 degrees C per century, over the last
century, and 1.1 degrees C per century, over the last half century
(plus or minus error bars). Is 1 degree per century
“fast”? Maybe the right thing to do is to focus on
consequences. If we assume, just for the sake of argument,
that the warming continues at this rate of 1 degree per century for
another century, then the people of 2100 will be saddled with a
two-degree world. Whether or not we call that warming rate
“fast”, a two-degree world has global consequences about which I
would not want to be blasé.

Is the warming “tending to accelerate“?
Here, I definitely agree with you that there is no clear evidence
that warming is “accelerating”, but equally there is also no clear
evidence that it isn’t. The natural fluctuations are too big to
tell; it’s already hard enough to get a good measurement of the
first derivative, let alone the second

Let’s move to your conclusion, that climate change will
likely be a “fairly minor problem”. I’d like to challenge this in
two ways.

First, I’d like to consider what the value of the climate
sensitivity parameter might be. (That is, the long-term warming
response to a doubling of carbon dioxide.)  The consensus of
the climate science professionals is first, that this parameter is
still very uncertain, but second, that it probably lies somewhere
between 1.5 degrees and 4.5 degrees C, with a most probable value
in the neighbourhood of 3 degrees C. Now, is there any value
in this range (1.5-4.5 C) that you would judge to be a “fairly
minor problem”? I assume not, but do correct me if I’m
misintepreting. So I take it that what you are judging from
the evidence is that the consensus is wrong; and that you are
confident, based on the evidence, that the climate sensitivity
is less than 1.5 degrees – perhaps something like 1 degree or
so? I don’t want to put words in your mouth, but would like to
clarify what I understand you to be saying. This climate
sensitivity of 1 degree C does have a rational basis, namely, it
would be the sensitivity if there were zero net
from a carbon-dioxide forcing to other warming
effects – it’s the warming of the system if CO2 were
doubled, and nothing else happened. So if you believe that
“it’s a fairly minor problem” because you are convinced that the
sensitivity is about 1 degree, I guess you’re saying that you judge
the sum of all the feedbacks to be roughly zero; whereas in
contrast the consensus of the climate scientists is that the sum of
all the feedbacks is significantly positive, albeit with large
error bars.  [A cartoon of why the feedback is expected to be
significantly positive: one dominant response to warming is that
warmer air holds more water vapour; and water vapour is a strong
greenhouse gas (stronger than CO2); so there is
feedback from warming (caused by CO2 or any other
forcing) to more warming (by water vapour). Some evidence for this
physical model comes from the fact that computer models of
atmosphere and ocean reproduce this effect and thus fit the last
hundred years of global data pretty well.]

The view that the sum of the feedbacks is not
significantly positive but is near zero or even
negative isheld by a small number of climate
scientists – for example, Roy Spencer, an atmospheric scientist who
specialises in satellite observations. His view that the feedbacks
are not large and positive is based on a belief
that clouds contribute a big net negative
feedback. It’s widely agreed that clouds are one of the
least-well-understood parts of the system, so this is
conceivable. Now, the smaller (or more negative) the feedback
is assumed to be, the less of the 20th century warming
can be attributed to human effects, and the more of it must be
attributed to natural fluctuations. The consensus view is that
it’s hard to see how natural fluctuations could account for most or
all the warming, but I would agree that it is conceivable that the
computer models have not yet adequately captured natural

Where am I going? Well, I’m trying to quantify your view,
as I understand it, which is that the climate sensitivity might be
only 1 degree C or so, and I want to agree that, yes, this
is conceivable, and it’s a possibility that is
indeed consistent with published climate science – climate
scientists all agree that there remain significant uncertainties
about clouds and about natural fluctuations, and the error bars on
the climate sensitivity are large. So I agree with you about
this possibility; what I don’t understand is how you can feel
so sure that the climate sensitivity is only
about 1 degree C. All the climate professionals I’ve spoken to
think it’s more probable that the climate sensitivity is
significantly bigger than 1 degree C, and they do have a detailed
physical account of why they believe this. My non-expert view
would be that yes, a sensitivity of 1 degree C is conceivable, but
so is a sensitivity of 2 or 3 or even 4 degrees.

The second tack I’d like to take is to point you to
evidence that supports the view that climate change might be more
than “a minor problem” – evidence that supports the proposition
that the sensitivity is greater than 2 degrees. It’s possible
you haven’t heard about this evidence, since the mass-media
reporting of climate science rarely goes into any scientific
detail, so it is a story that only a few lay-people are aware

The evidence is from geology, from events that
happened millions of years ago. For our convenience, nature
has already carried out experiments to test what happens when a
trillion tonnes of carbon are released in to the atmosphere over a
brief period, and the results of those experiments can be read out
of the rocks.

This photo is of a 55-M year old sedimentary rock core
from the North Atlantic, on which the publication of Norris and
Röhl (1999) was based. [Nature 401,

(The original image is available at

in case this reproduction doesn’t come out well.)

With the naked eye you can see near-periodic bands of
lighter and darker colour. These bands match Milankovic periods of
20,000 years or so (the period of some of the Earth’s orbital
wobbles) and allow the events recorded in the rocks to be dated
with great precision. Based on measurements of isotope
concentrations in these and other rocks, the reconstruction of
the PlioPaleocene-Eocene thermal maximum (which
falls at one end of this rock core) runs as follows: first, from a
natural source – either mega-volcanoes or within-ocean-floor
deposits or both – a large release of carbon took place, over about
1000-10,000 years: perhaps about one trillion tonnes of carbon.
(For comparison, human emissions have passed 0.5 trillion tonnes,
and are heading for one trillion tonnes within a few
decades.) The source of this carbon was probably in the
vicinity of the North Atlantic. In response, a large global climate
change happened, with temperatures rising by more than 4 degrees C,
and there was a mass extinction, with lots of species wiped out,
and widespread anoxic and acidic conditions in the
oceans. This climate change blip lasted about 200,000 years
before natural processes restored carbon levels and temperatures to
roughly what they had been before.

For an accessible account of this discovery by the
President of The Geological Society (a formerly sceptical
oil-explorer), I recommend “Challenged by Carbon” by Bryan Lovell
(CUP 2010).  This evidence caused Bryan Lovell to change his
mind on climate change.

In case a single experiment does not convince, nature has
repeated the test, and there is another event, the Toarcian, which
took place 183 M years ago in the Jurassic. The data can be
gathered from places like Robin Hood’s Bay in Yorkshire. I learned
about this just this week. Again, there was a natural massive
carbon release, probably triggered by a mega-volcano (the
associated basalts go by the name of Karoo Ferrar, and had a volume
of 2.5 million km3), after which there was an immense
anoxic event in the oceans, severe global warming, and a mass
extinction (the Pliensbachian-Toarcian extinction). In
addition, based on measurements of strontium in fossils, it’s
deduced that during the period of global warming, weathering rates
increased to four times normal, showing that rainfall patterns were
radically changed. The temporary global temperature rise was
believed to be about 6-10 degrees C.

Finally, I was interested to read your comments on
the IPCC’s structure and processes. While I won’t address them here
(noting that DECC has made a full response to the InterAcademy
Council’s report ahead of IPCC’s imminent Plenary), I think it is
worth noting that there are some headline issues where the IPCC
cannot be accused of exaggeration. Sea level rise is a good
example, where we can compare an IPCC projection with subsequent
data. The IPCC’s Fourth Assessment Report estimates of global
average sea level rise by 2100 were conservative because they
omitted the potential contributions from polar ice sheets.
Subsequent data on sea level (previous page, right) show that
actual sea-level rise falls at the upper range of the IPCC’s
projections (shown by the dashed grey lines). [Source: Stefan
Rahmstorf, et al. SCIENCE 316 4 May 2007.

Can I thank you again for your engagement with these
issues. I’d be delighted to discuss them further if you


David MacKay FRS

Chief Scientific Advisor, DECC

By Matt Ridley | Tagged:  the-times