by Judith Curry

How can the IPCC increase their confidence in anthropogenic global warming at the same time their model projections are diverging farther and farther from reality? – John Nielsen-Gammon

This question is asked in a blog post by John Nielsen-Gammon entitled Your logic escapes me. Interestingly, the logic that escapes him is the argument used in my recent Senate testimony. Excerpts:

Yet, we’re in the middle (or perhaps the end, or perhaps the beginning) of a hiatus in the rise of global temperatures. The evidence seems to be mounting that natural variability is more important than the IPCC reports had previously contemplated, yet the IPCC’s confidence in anthropogenic global warming grows stronger.

Now, I don’t know the IPCC’s reasoning process, to the extent that a complex organization can even be said to have a reasoning process. However, those who think that the IPCC is acting unscientifically in this particular matter are failing to recognize something particularly important: despite the growing divergence, the evidence supports increased confidence in the IPCC’s statement.

Nielsen-Gammon says he has no disagreement with these individual points from my testimony:

Lack of warming since 1998 and the growing discrepancies between observations and climate model projections

Evidence of decreased climate sensitivity to increases in atmospheric CO2 concentrations

Evidence that sea level rise during 1920-1950 is of the same magnitude as in 1993-2012

Increasing Antarctic sea ice extent”

Nielsen-Gammon further agrees with the following statements from my testimony:

“Multiple lines of evidence presented in the IPCC AR5 WG1 report suggest that the case for anthropogenic warming is weaker than the previous assessment AR4 in 2007. Anthropogenic global warming is a proposed theory whose basic mechanism is well understood, but whose magnitude is highly uncertain. The growing evidence that climate models are too sensitive to CO2 has implications for the attribution of late 20th century warming and projections of 21st century climate.”

“If the recent warming hiatus is caused by natural variability, then this raises the question as to what extent the warming between 1975 and 2000 can also be explained by natural climate variability.”

Nielsen-Gammon then goes into a relatively lengthy argument regarding the possible importance of natural internal variability, which I summarize briefly here.

If natural cycles are regular and repeatable, the net temperature change over one complete natural cycle will be approximately zero. The warming during part of the cycle is cancelled by cooling during the other part of the cycle. What’s left is the long-term rise caused by man.

Why does the IPCC conclude that the long-term rise is caused by man? The primary logic is simple, really. Of all the things driving long-term changes in the climate system, the biggest by far over the past 60 years is greenhouse gases. Second on the list is particle pollution, or aerosols, which partly counteract the greenhouse gases. Over the past 60 years, natural forcings (sun, volcanoes) have also had a cooling effect. So arguments over the relative importance of different kinds of forcing don’t really matter for explaining the past 60 years of temperature rise: the only large one on the positive side of the ledger is greenhouse gases.

Of course, it’s not enough to say that greenhouse gases point temperature in the right direction. The magnitudes have to match, also. Here, too, the hiatus increases confidence that there’s not some unknown but significant positive forcing agent other than greenhouse gases that’s driving temperature. The smaller the rate of warming, the smaller the possibility that a separate, additional cause of warming is being missed, and that, therefore, greenhouse gases account for most or all of the total amount of warming.

Curry is correct that the hiatus and increasing model-observation divergence is evidence that the models are too sensitive to forcing agents, including carbon dioxide. She is also correct that, the stronger and longer-lasting the divergence, the greater the evidence that much of the warming pre-2000 was natural. Likewise, the stronger the evidence that models might be overestimating future warming. Exploring those issues requires evaluation of all the available evidence, not just the last 60 years of temperatures, and will have to wait for another blog entry.

But here’s the thing. If, over 60 years, natural variability averages out to zero, it doesn’t matter how strong natural variability is compared to man-made climate change, what’s left over is the man-made part. Thus the IPCC can and should consider it to be extremely likely that human influence dominates the net rise in temperature over the past 60 years.

If you’re a believer in strong natural variability, and you’re looking to criticize the IPCC, you might complain that they don’t reduce their estimates of climate sensitivity enough, or that they don’t adequately discuss the increased evidence of the importance of natural variability in affecting temperatures from decade to decade. But none of this, and none of the evidence presented by Curry or by anyone else I’ve seen, reduces the likelihood that the temperature change over the past 60 years or so was mostly anthropogenic.

JC comment: A quick comment on the above argument relative to the stadium wave. First, the stadium wave is one element of natural internal variability, focused specifically on multi-decadal time scales (it says nothing about natural internal variability on longer timescales). Even on decadal to multi-decadal timescales, there is additional natural variability not captured by the stadium wave, notably the NAO/AO (which may not be a purely internal mode).

Reasoning about climate uncertainty

Nielsen-Gammon and I agree on the first order evidence that I presented in my testimony. How is that we then disagree regarding the IPCC’s attribution statement for warming since 1950? This statement by N-G sums it up:

Now, I don’t know the IPCC’s reasoning process, to the extent that a complex organization can even be said to have a reasoning process.

The IAC review of the IPCC argued for transparency in the reasoning that went into the confidence assessment. I agree with N-G that the IPCC’s reasoning is not transparent.

So back to the original question, how can different individuals or organizations look at the same primary evidence and draw different conclusions? Insight in this is given in my 2011 paper published in a special issue on Climatic Change entitled Reasoning about Climate Uncertainty, which can be summed up in one sentence:

How to reason about uncertainties in the complex climate system and its computer simulations is not simple or obvious.

JC’s reasoning about attribution

The logic behind my reasoning is this.

The way the IPCC’s attribution argument is laid out, there are two possible contributing causes to climate change since 1950: anthropogenic forcing, and natural variability. The sum of these two contributing causes is 100%; the chief issue of interest is the relative percentage contributions of anthropogenic forcing and natural variability.

The IPCC AR5 makes an extremely confident statement that ‘most’ of the warming is attributed anthropogenic forcing, and I understand ‘most’ to cover a range of 51-95%. The IPCC implicitly recognizes that the attribution issue is uncertain by not giving a distribution of values or a ‘best estimate.’ Rather, they provide a bounded region that covers ~44% of the possible territory.

In the absence of a compelling theoretical reason for the lower bound of 51%, I infer that the IPCC does not regard a high likelihood for values of anthropogenic forcing less than 60%; otherwise it would be difficult to argue that the lower bound would not be breached. At the time of the AR4, in talking with IPCC lead authors I had the sense that most of the scientists thought that the anthropogenic contribution was >90% (this is my subjective assessment; I would be interested in other documentation on this).

So for the sake of starting somewhere in my argument, lets start with this breakdown for the IPCC AR5 statement on attribution since 1950:

anthropogenic: 75%

natural: 25%

The elements of greatest certainty in this argument are the components of anthropogenic forcing. The elements of greatest uncertainty are the sensitivity to CO2 (and the fast thermodynamic feedbacks), natural internal variability, and then there are also the unknowns such as solar indirect effects.

The uncertainty in sensitivity to CO2 forcing is acknowledged by the IPCC by the lowering of the bottom bound in their ‘likely’ range; this points to lowering the proportion of anthropogenic component of the attribution. The ‘pause’ is raising awareness of the importance of natural internal variability. Other data reflects the importance of natural internal variability, which N-G didn’t pick up on as being relevant: the importance of natural variability in 20th century sea level rise as indicated by the mid-century bump; and the importance of natural internal variability in determining both arctic and antarctic sea ice extent. With regards to arctic sea ice decline, I spotted this statement in AR5 Chapter 10:

Comparing trends from the CCSM4 ensemble to observed trends suggests that internal variability could account for approximately half of the observed 1979–2005 September Arctic sea ice extent loss.

So . . . this evidence all acts to lower the contribution of anthropogenic forcing, and increase the contribution of natural variability, and I would argue that this lowering should not be regarded as trivial. Yes the 44% provides lots of wiggle room, but it is not unreasonable to infer, based on the evidence provided by the IPCC that the anthropogenic component has dropped below 70% and even below 60%. In this light, putting an extremely confident lower bound of 51% seems insupportable. Particularly in light of the unknowns such as solar indirect effects.

N-G’s reasoning about how to think about the relative contributions of natural vs anthropogenic anthropogenically is something like the way I have approached this, but not quite. I look at it the following way. Consider two periods: 1975-1998 (warming), and 1998-2013 (hiatus). Play with the percentages of natural variability (assuming warming in the first period and cooling in the second period) and anthropogenic forcing, accounting for the relative lengths of the two periods, and see what percentage breakdown works for both periods. And what the implications are of the hiatus extending another 10 years. You will not get numbers that exceed 75% for anthropogenic.

If I were given a 44% range to work with, I would put the range at 28-72% anthropogenic. My range overlaps with the IPCC in domain 51-70%, but I also allow for numbers below 50%. The main uncertainties seem all in the direction of increasing the contribution from natural variability.

Bottom line: with the growing recognition of the importance of natural variability, it is increasingly difficult to defend the bottom bound of 51% for anthropogenic forcing, hence I find the increase in confidence to ‘extremely likely’ to be unjustified.

UPDATE: N-G has responded to this essay at the end of his post, it definitely clarifies the differences in our reasoning. Without detailing his new responses, here are some quick responses to some of his statements.

The question of relevance to policy makers is how much of the recent warming is natural vs anthropogenic. Arguing that components of anthropogenic forcing are both positive and negative, and natural variability at various times can be positive and negative, is not useful in assessing the relative contribution of humans vs nature to the warming since 1950 (and it didn’t start warming until after 1975).

There is a quibble about the meaning of ‘most’. Wisely, the AR5 uses ‘more than half’ rather than most. N-G argues that this includes up to 100%, and that the AR5 states its best estimate is 100% for the natural contribution. If this were true, why did the AR5 not say ‘virtually all’ rather than more than 50%?

N-G states: If you’re going to impute a range for them, at least have their most likely value somewhere near the midpoint of the range. For the sake of argument, a reasonable range would be 51-135%.

This kind of argument, where anthropogenic contribution is argued to exceed 100%, seems senseless to me, even if you are just looking at the period from 1975-2000 where there is a clear warming trend. It is something that arises from the over sensitivity of models to CO2 forcing combined with oversensitivity to aerosol forcing.