What are our readers to make of the slow-down in global surface temperature rise after 1998? What’s often heard in my neck of the woods is something along the lines of what Dr. Neil Frank, former KHOU-TV meteorologist and beloved Houston personality, wrote in the Houston Chronicle last Friday:

If global warming will be as catastrophic as some suggest, I have one question: Why has there been no warming of the globe for the last 17 years? Carbon dioxide has continued to increase and current carbon dioxide levels are the highest they have been in the last 150 years – yet no warming for the past 17 years! …If models can’t produce accurate forecasts for 17 years, why should they be believed for projections out to 100 years? Perhaps the relationship between carbon dioxide and the global temperature that has been built into the models is not valid. If this is true, then the importance of carbon dioxide in determining the Earth’s temperature is not nearly as important as we have been lead to believe.

There’s been a lot of discussion on the “hiatus” or “pause” or “slow down” in global surface temperature rise in the Forum, so instead of asking the participating scientists to repeat themselves, I’ve compiled a list of all mentionings of the topic thus far, starting with the most recent and working back. I think it’s a good smorgasbord of what climate scientists have to say about the hiatus. It definitely provided me with a lot of insight and context. If any scientist in the CCNF community would like to respond to the above statement by Dr. Frank or share a new thought on the hiatus, I’ll add their comment to the top.

Note: Instances where any substantive dialogue was clipped for purposes of brevity are noted in brackets. Consult the links for the exact quotes (there are more acronyms), order, and context.

Dr. Judith Curry: At the heart of the recent scientific debate on climate change is the ‘pause’ or ‘hiatus’ in global warming – the period since 1998 during which global average surface temperatures have not increased. This observed warming hiatus contrasts with the expectation from the 2007 IPCC Fourth Assessment Report that warming would proceed at a rate of 0.2oC/per decade in the early decades of the 21st century. The warming hiatus raises serious questions as to whether the climate model projections of 21st century have much utility for decision making, given uncertainties in climate sensitivity to carbon dioxide, future volcanic eruptions and solar activity, and the multidecadal and century scale oscillations in ocean circulation patterns.

– ‘Climate Sensitivity Uncertainty‘ by Dr. Curry

Dr. Andrew Dessler: Most of the major predictions of the man-made global warming theory have all be validated. The hiatus falls in the category of ‘unpredicted but surprising occurrence’. It is something that we don’t completely understand, but the idea that “climate change has stopped” is completely bogus. If you look at the data, the climate is still shifting. If you look at most of the globe, it’s still warming. There’s just this slice of eastern Pacific that is cooling. And that cooling there is offsetting the warming. If you look at it seasonally, except for Northern Hemisphere winter, the planet is still warming. Now obviously these can’t continue. You can’t have spring, summer, and fall warming, and the winter cooling. And you can’t have the eastern Pacific cooling and the rest of the globe warming, so this is going to stop at some point, and we are going to figure out what it is. But make no mistake—weird things are afoot in the climate system.

Are there are any other significant gaps in knowledge or big known unknowns? [question in the video]

Oh yeah, there’s lots of them. We don’t understand how aerosols effect clouds. There’s the cloud feedback, which is still a reasonably big unknown; I’ve done a lot of work on that for the last 5 years. There’s the heat going in the ocean—that’s the hiatus; that’s ocean variability. There are lots of things that are unknown, but the key thing to realize is that these unknowns are not existential threats to the theory. It’s not like we are going to find something out about clouds that is going to cause us to conclude that carbon dioxide is not a threat. I mean, we are going to find out about clouds, and it’s going to refine our understanding. But I think there is very little reason to think that we are suddenly going to decide that CO2 is not a significant warmer of the climate. We’ve established that carbon dioxide is a powerful greenhouse gas.

– CCNF Video Interview with Dr. Andrew Dessler (5:11-7:45)

Dr. Andreas Schmittner [referring to an op-ed by an advisor to the Austrialian Government]: It is sad that people who deliberately distort science have so much influence in politics and public media. The only good thing is that once warming of surface temperatures resumes, which I expect to happen soon, those people will all be completely discredited.

—

– Comment in the Scientists’ Comment Thread (SCT), ‘Fears of Freezing: The 1970s are calling; They want their climate policies back’

Dr. Scott Denning: The so-called “hiatus” has been wildly oversold by the usual suspects… If you really want to learn about decadal changes (a.k.a. the “hiatus”), a good place to start is with England et al (2014, Nature, doi:10.1038/nclimate2106) who show that observed changes in winds over the tropical Pacific and the resulting change in ocean warming can explain the observed changes in decadal temperature trend.

The fundamental reason for confident predictions of substantial warming in the 20th Century is the First Law of Thermodynamics. The underlying physical mechanisms for greenhouse warming are extremely well understood and firmly grounded in both laboratory spectroscopy and atmospheric observations. The physics is fully consistent with a huge range of radiation measurements both from the surface and from orbit, and also with paleoclimate data on a variety of time scales.

Decadal changes in climate are fascinating phenomena well worthy of study, but they are NOT an important way to estimate climate sensitivity over a century or more in response to forcing of 3 to 8 Watts per square meter.

Between 1997 and 2012, CO2 increased from 364 ppm to 394 ppm, providing log(394/364)/log(2) * 3.7 = 0.42 Watts/m^2 of radiative forcing. 15 years is too short a time for the climate to adjust to the small increase in the forcing, but even at equilibrium if we allow for 3 degrees of warming per doubling of CO2, we would only expect 0.3 Celsius of warming response to 0.4 Watts/m2 of forcing. Whether the amount of warming from 1997 to 2012 is 0.1 or 0.2 degrees Celsius has almost no bearing whatsoever on whether we expect 2 or 3 or 5 degrees of warming for a forcing 10 to 20 times as large.

– SCT comments, fact checker post on ‘Outsider Finds Missing Global Warming’ article.

Dr. Dessler: I think it is quite clear that the hiatus represents a deficiency of our knowledge of internal variability. That said, I doubt the hiatus poses an existential threat to the mainstream theory of climate and it will not have much of an impact on long-term (e.g., centennial) warming. Nor do I think it indicates that climate sensitivity is substantially lower than present estimates.

– SCT comment, fact checker post on ‘Outsider Finds Missing Global Warming’ article.

Dr. Schmittner: The statement that “there has been no significant warming over the most recent fifteen-or-so years” is wrong (or misleading at best) because the ocean heat content shows uninterrupted warming. Because 90% or so of all heat goes into the ocean, this is the place to look for warming. See Global Ocean Heat Content 1955 to present (0-2000m).

Dr. Curry: The uncertainties in ocean heat content below 700 m are very substantial, see http://judithcurry.com/2014/01/21/ocean-heat-content-uncertainties/… The role of internal ocean behavior in climate change was almost completely neglected up the AR4; the AR5 takes it a bit more seriously. But recent research over the past year has resulted in this being a huge issue. Everyone seems to be jumping on the bandwagon to say that internal ocean variability is causing the pause, but this leads to questions about the role of oceans in the warming of the last quarter of the 20th century.

– SCT comments, fact checker post on an article by Australian scientist Dr. Garth Paltridge. [See Dr. Schmittner’s response and subsequent post looking into whether deep ocean warming could have been a substantial cause of the observed surface warming over the past 50 years.]

Dr. Bart Verheggen [sharing thoughts on a discussion on climate sensitivity in another forum]: James Annan writes: “The recent transient warming (combined with ocean heat uptake and our knowledge of climate forcings) points towards a “moderate” value for the equilibrium sensitivity, and this is consistent with what we know from other analyses. Overall, I would find it hard to put a best estimate outside the range of 2-3°C [per doubling of preindustrial levels of CO2eq].”

Nic Lewis [who recently co-authored a paper with Dr. Curry, which Dr. Curry recently blogged about on CCNF] prefers a subset of studies in the “instrumental” category over model-based and paleo-climate approaches and concludes that “the soundest observational evidence seems to point to a best estimate for ECS of about 1.7°C, with a ‘likely’ (17-83%) range of circa 1.2–3.0°C.”

Annan’s first response to Lewis hits the nail on the head in my honest opinion:

“Nic Lewis appears to be arguing primarily on the basis that all work on climate sensitivity is wrong, except his own, and one other team who gets similar results. In reality, all research has limitations, uncertainties and assumptions built in.”

A few recent studies (Shindell, 2014; Kummer and Dessler, 2014) suggest that the “efficacy” of cooling aerosols may be larger than expected, because they are largely confined to the landmasses of the Northern hemisphere. This could explain the difference in estimated climate sensitivity (ECS) as deduced from the instrumental period vs from climate models and paleo-climate. After all, such differences need explaining. If correct, this would mean that ECS [per doubling of the preindustrial level of carbon dioxide and other greenhouse gases] is indeed somewhere in the middle of the IPCC range (~3 degrees C). In turn, this would mean that the warming should pick up speed again in the near future, James Annan argues, since the current rate of warming is close to the lower end of the model range:

“If these results are correct, then the current moderate warming rate is a bit of an aberration, and so a substantial acceleration in the warming rate can be expected to occur in the near future, sufficient not only to match the modeled warming rate, but even to catch up the recent lost ground.”

However, Schmidt et al (2014) show that when accounting for the timing of El Niño Southern Oscillation events, for the actual evolution of climate forcings (e.g. volcanic aerosols) and for inaccuracies in global temperature datasets, observations and the model ensemble mean are very similar. That weakens Annan’s conclusion about the future warming trend, as cited just above.

–‘ClimateDialogue on Climate Sensitivity‘ by Dr. Verheggen.

Dr. Curry [responding to Dr. Denning’s post, ‘Cause and Effect‘]: Since 1998, 25% of the post-industrial CO2 has been emitted, and there has been little to no increase in global surface or atmospheric temperatures. The issue is not just sensitivity; it is the timescale of the response.

Dr. Denning.: Judy, since 1998, atmospheric CO2 has increased a mere 8%, for a total forcing of about 0.4 per square meter. Temperatures have continued to rise, but as you point out the time scale to reach an equilibrium response is uncertain. It seems unwise to base estimates of climate sensitivity on such a small forcing over such a short time interval. Much more reliable estimates are available from larger changes in forcing over longer times.

JC: Scott, yes 8% of total CO2, but 25% of the total anthropogenic CO2. The main period of temperature rise is 1976-1998, a very short time also. .

SD: Judy, the whole point of the article is that expectations of future climate change are NOT based on recent temperatures! We expect a warmer future for the same reason we expect summer to be warmer than winter, or day warmer than night. We believe in the First Law of Thermodynamics. The reason we expect a warmer climate under 2x or 3x preindustrial CO2 is that we expect the added heat to change surface temperatures. Solar fluctuations of even a few tenths of a Watt per square meter seem to cause noticeable climate perturbations. At deglaciation, global temperatures warmed about 5 degrees for a 6.5 Watt per square meter change in forcing, and sea levels rose by hundreds of feet. Natural cycles prove that climate can change, has changed, and will change again if several Watts of additional heat are added to every square meter.

I suppose there are people who “believe” that the climate system can discriminate between “natural” and “anthropogenic” photons. But those people are not called “skeptics.”

JC: The issue is ‘how much’ warming. Inferences of ‘how much’ depends on climate models that don’t agree with observations, with the inference that the fast thermodynamic feedback processes are too strongly positive. .

SD: Judy, perhaps you didn’t read the article you’re commenting on? It’s not about models. It’s about physics.

.

– SCT comments, ‘Cause and Effect’.

Dr. Schmittner: Here are my “fab four” up-to-date climate change observations. … In my opinion one of the most important is ocean heat content. The reason is simple. The ocean’s heat capacity is enormous. It takes up almost all of the additional heat the Earth accumulated in the last decades. Here is the image [see above graph on global ocean heat content] and link.

– ‘My Fab Four Up-to-Date Climate Change Observations‘ by Dr. Schmittner.

Dr. Verheggen [responding to a statement by Senator Jeff Sessions (R-AL) during an EPW Committee hearing on the President’s Climate Action Plan]: The Senator wrongly suggests that the timeseries of surface air temperatures over 16 years is indicative of the longer term (multi-decadal) forced trend (what John calls 16 data points vs the process in his comment under the fact checker post on the Heartland Institute’s NIPCC report). He also wrongly suggests that surface air temperatures is all there’s to global warming (whereas most excess energy ends up in the oceans which appear to be warming, including over this same period). Furthermore, it is something that has more recently been quantified, so it can’t really be held against him, but the surface temperature data have a cool bias by ignoring large parts of the (rapidly warming) Arctic (see also http://www.realclimate.org/index.php/archives/2014/01/global-temperature-2013/).

– SCT Comment, fact checker post on statement by Sen. Jeff Sessions (R-AL).

Dr. Dessler [explaining the “standard model,” which Dr. Dessler calls the “mainstream theory of climate influence’]: The standard model explains just about everything we’ve seen and has successfully predicted phenomena in the climate system. In fact, it can explain just about everything we have observed in the climate system, both in the present day and during the geologic record. It has also made many successful predictions, which are the gold standard of science. If you can successfully predict phenomena that are later observed, one can be supremely confident that the theory captures something essential about the real world.” So as an example… [see full article for Dr. Dessler’s list of successful predictions, which include stratospheric cooling, the Arctic warming faster than the Antarctic, and the “water vapor feedback”.]

This explains why the bulk of the scientific community is so confident in the standard model. It explains just about everything and it makes many successful predictions.

Obviously, this doesn’t mean that our knowledge is perfect. This is reflected in uncertainty estimates that are provided in the consensus reports.

Now, a caveat: I said above that the standard model explains virtually everything, which means that there is a small number of observations that are not necessarily well explained by the standard model, just as there are a few heavy smokers who do not get lung cancer.

An excellent example of this is the so-called “hiatus,” which has been mentioned several times: the slow warming of the surface temperature record over the last decade or so. This is frequently presented as an existential threat to the standard model. But as I describe below, this greatly exaggerates its implications. Before I explain why, I think it is worth recognizing that skeptics have a track record of overstating the importance of these challenges to the standard model. A few years ago, for example, strong claims were made about the surface temperature record. It was argued that siting issues – for example, a thermometer too close to a building – meant that the surface record was hopelessly biased. This was portrayed as an existential threat to the standard model. Subsequent research, however, has resolved this issue. It is now clear that it was never a threat to the standard model at all.

So why do I think that the hiatus – the slow warming of the last decade – is not much of a threat to the standard model? To begin, a lack of a decadal trend in surface temperatures does NOT mean that the warming has stopped. Observations show that heat continues to accumulate in the bulk of the ocean, indicating continued warming.

Also, in my written testimony, and in the plot that Senator Whitehouse showed, the surface temperature shows frequent periods of short cooling, even while it is undergoing a long-term warming trend.

In addition, one of the Senators said that climate models do not predict periods of no warming. That is not correct. Climate models do predict periods where there is no warming. Now, that does not mean that we understand the hiatus perfectly. I view the hiatus as an opportunity, not as an existential threat. I think short-term climate variability is an area where our understanding could improve, and the hiatus will help us do that. Papers are already coming out on a monthly basis it seems. I suspect that in the next few years our understanding of this phenomena will be greatly improved. At that point, I predict that arguments about the hiatus will disappear, just like arguments of the surface temperature record have.

– ‘What We Know About Climate‘ by Dr. Dessler.

Dr. Nielsen-Gammon: You draw a good distinction between the solid science and the things that are still yet to be pinned down. The hiatus, for example, doesn’t magically mean that carbon dioxide has lost its radiative properties and doesn’t seem to have any other such dramatic implications.

However, would you agree that the behavior of global temperatures over the past 15 years has made it less likely that we’ll hit 6 C above preindustrial by the year 2100, in the absence of substantial mitigation?

The IPCC basically punts on this one, saying until we know the cause of the hiatus we can’t say anything about its long-term consequences.

I think that’s a cop-out, for the following reason: One possible implication of the hiatus is that climate sensitivity has been overestimated. Certainly the observation record points toward a lower climate sensitivity now than it did in the year 2000, or even 2007. In the absence of countervailing evidence pointing toward higher sensitivity, shouldn’t this mean that the chances of very high sensitivity have decreased a bit? And shouldn’t that, plus our behind-schedule 21st century warming, make the high end of temperatures in 2100 less likely?

AD: Given how fast science on this is moving, I think it’s a little unrealistic to expect the IPCC to have a nuanced and current view of this issue. I’m sure the next IPCC report will have incorporated this issue more completely into our understanding.

My opinion: I don’t think it’s clear what the “hiatus” tells us about the future trajectory of the climate. If it’s a mode of variability that averages to zero in the long run, then it will alternatively increase and decrease the rate of warming and my have little impact on the temperature around the turn of the next century. Is it possible that it has caused the climate sensitivity to be overestimated? Sure, but I don’t think the peer-reviewed arguments on that are convincing yet.

– SCT comments, ‘What We Know About Climate’ .

Dr. Nielsen-Gammon: Several times during the past few months, I’ve heard generally incredulous statements such as, “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? 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. What’s going on here?”

The people who ask me such questions seem to think that the IPCC’s increase in confidence is politically motivated, that the process requires a unidirectional progression of confidence in such a key area, and that this provides reason enough to account for the IPCC going against the mounting evidence. Indeed, to them this seems like a no-brainer that proves that the whole IPCC process is corrupt.

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.

How can this be? To see how, I’ll analyze Prof. Judith Curry’s recent written testimony for the hearing on Obama’s Climate Action Plan at the Senate Committee on Environment and Public Works on January 16, 2014. Her testimony provides the most comprehensive, scientifically accurate, and seemingly logical example of the argument that the IPCC’s increase of confidence is counterfactual. The complete written testimony is available here; I’ll be extracting the key arguments… [see full article for breakdown and critique by Dr. Nielsen-Gammon].

…But here’s why none of this means that the warming over the past 60 years is less likely anthropogenic. Suppose we make a generous assumption for the role of natural variability. Suppose we assume that the stadium wave hypothesis is correct. As Curry alludes to, the stadium wave hypothesis posits that there is a regular, propagating, approximately 60-year oscillation in the climate system that, among other things, affects global temperatures. Let’s suppose that it has a pronounced effect, as big an effect as Curry thinks it might have, and is thus responsible not just for the present hiatus but also the hiatus centered on the 1950s.

…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.

Note that such a statement only works if you look at the temperature change over one complete natural cycle. Over a shorter period, particularly one in which a natural cycle would have contributed to net warming, confidence would necessarily be lower.

…. 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.

– ‘Your Logic Escapes Me‘ by Dr. Nielsen-Gammon.

Dr. Verheggen: Judith Curry is too quick in jumping to the conclusion that climate sensitivity is overestimated. That judgment is based on a very limited selection of the available evidence. Taking all evidence into account (e.g. also from paleoclimate and from climatological constraints), the picture regarding climate sensitivity remains very much similar to what it’s been for decades (http://klimaatverandering.files.wordpress.com/2013/10/ipcc_ecs_pdfs_box12-2_fig11.png ). I’m puzzled by Curry on the one hand exaggerating uncertainty to the point of claiming ignorance (e.g. regarding the pause, for which many plausible mechanisms have been proposed), and on the other hand so confidently stating that climate sensitivity must be at the lower side of the range.

Disagreement between models and observations could have many causes:

– Smaller radiative forcing

– Smaller climate sensitivity

– Slower response time

– Cool bias in observations

– Internal variability

She only focused on one preferred cause without mentioning other possibilities. And there are indications that over the past decade or so there are some additional negative forcings (e.g. from the quiet sun and from volcanic and Chinese aerosol); that there is a cool bias in the main surface temperature datatsets (by excluding the rapidly warming high Arctic); that internal variability was predominantly in the cooling direction (La Nina). Perhaps most importantly, the climate system has continued to build up heat, predominantly in the oceans. To put the so-called hiatus in the perspective of the planetary energy budget, look e.g. at the purple line at the bottom of this graph (http://www.easterbrook.ca/steve/wp-content/IPCC-AR5-WG1-Box-3.1-Fig-1.png ), and compare it with the other contributions to the increase in energy content.

Dr. Mauri Pelto: I do not agree with the characterization that there is a hiatus. If we look at the global temperature record from NASA from 2001-2013, every year would have broken the global temperature record up to 1998, for global temperatures since 1880. We went from 1998 being a once in a century event for the 20th century, to every year now being near that level. The decadal temperature record does not show a hiatus. We have had five of the last six years influenced by La Nina , which is why I am amazed at how warm 2012 and 2013 have been globally.

N-G: What’s your definition of a hiatus? Your statistics argue for the absence of a decline, not the absence of a hiatus. Unless the hiatus spans two full decades, it won’t show up on the decadal temperature record.

If you’re trying to argue that it wouldn’t exist at all without the effect of ENSO variability, then I’m with you. But: the 15-17 year trend is near zero, and no such interval has occurred since the 1970s. This is an observed feature of the climate record. We have to call it something in order to talk about it.

MP: In the geologic context a hiatus is a discontinuity in a temporal record. I do not see a discontinuity in the record of warming since 2001-2010 is the warmest decade, and 2003-2012 is an even warmer decade, it is not up to the selection of a single year, the last decade is the warmest. That does not amount to a hiatus. This is also true in other data sets that are temperature controlled. Glacier mass balance had its most negative decade 2003-2012, 2013 data not in yet.

N-G: I think we’re splitting hairs. In GISTEMP, 2002-2011 was the warmest decade, 2001-2010 was second warmest, and 2003-2012 and 2004-2013 tie for third. These past two overlapping decades are the first since the resumption of sustained warming in the mid 1970s that did not either set a new record or come in second. In RSS, 2004-2013 is the seventh-warmest ten-year period. Warmest was 1998-2007. In HadCRUT4, 2004-2013 is slightly cooler than 2003-2012, which was slightly cooler than 2002-2011, which was slightly cooler than 2001-2010. The UAH temperatures behaved the way you reported.

MP: Realclimate’s new article on global temperature examines the data sets a bit, Cowtan and Way (2013) end up without a hiatus with a scheme that better accounts for areas with poor spatial coverage. They do have 2013 warmer than 1998. .

N-G: Estimation of surface temperature changes in poorly-covered regions is enough to reverse the slight cooling in the recent decadal averages in HadCRUT4, turning it into a slowing rather than a hiatus according to your definition. That plot from NASA on the RealClimate post showing the effect of El Nino and La Nina? You saw it here first: http://blog.chron.com/climateabyss/2012/04/about-the-lack-of-warming/

–SCT Comments, ‘Your Logic Escapes Me’.