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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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David MacKay's letter

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 that

'sea level could rise by 0.3-1.3 m this century',

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 century',

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 m'. 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 can.

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 reading!

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 derivative!

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 feedback 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 fluctuations.

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 of.

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, 775-778.]

(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. p709.]

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


David MacKay FRS

Chief Scientific Advisor, DECC