A new paper (commentary) on the so-called “pause in global warming” puts it all together.
First let’s establish this as a starting point. When climate science contrarians refer to a “pause” or “hiatus” in global warming, they usually mean that the process of warming of the Earth’s surface caused by the human release of greenhouse gas is not a thing. They are usually implying, or overtly claiming, that the link between CO2 and other greenhouse gas pollutants and surface warming was never there to begin with, and previous warming, warming before “the pause,” was natural variation. Many even go so far as to claim that the Earth’s surface temperature will go down to levels seen decades ago.
“The Pause” is not, in their minds, a slowdown in the rate of warming. It is a disconnect, either there all along or produced somehow recently, between the physics of the greenhouse effect and reality.
Also, the evidence adduced for this “pause” is often bogus. Sometimes badly calibrated satellite data is used to show a flat Earth surface. Er, flat Earth surface temperature. Sometimes a line is drawn from an unusually warm, even under conditions of global warming, El Niño year, to later years, lately excluding record breaking recent warm years, in order to make the line flatter. So, that part of the denier pause is a different kind of lie. See this post and this post for recent research on this issue.
Having said all that, there have been frequent slowdowns and speed-ups in the rate of the planet’s surface warming throughout the entire instrumental record. (Even though the instrumental record begins in 1850 or 1880, depending on which data set you use, greenhouse gas pollution started before that, so some greenhouse warming has been happening all along).
Prior some date, like 1970 perhaps, the up and down variations in surface temperature has been a combination of natural variation and human caused variation, with both being strong factors. The human caused variation includes particulate pollution (from burning coal, mainly) which pushes the temperature down, and greenhouse gas release and its associated effects, which push the temperature up.
For the last third or so of the 20th century, through the present, while both natural and human-caused effects matter, the role of human effects has increased to be the dominant force in the overall trend. The natural variations continue to contribute to the shape of the curve, but this contribution is attenuated by the increased abundance of human generated greenhouse gas.
For the last few years, we have seen several research projects that look at the “pause.” Many of these projects helped to explain the slowdown by showing that it wasn’t really as much of a slowdown as previously thought. For example, some research showed that the surface warming in recent decades has been under-measured because the Arctic (and probably the interior of Africa) were getting warmer faster, compared to other regions, and those areas were under-sampled by the usual data sets. Also, heat has been moving in and out of the ocean all along, and that has had an effect on the surface temperaturews.
But even after accounting for all of these effects, there is still a slowdown.
John C. Fyfe, Gerald A. Meehl, Matthew H. England, Michael E. Mann, Benjamin D. Santer, Gregory M. Flato, Ed Hawkins, Nathan P. Gillett, Shang-Ping Xie, Yu Kosaka and Neil C. Swart have just published a commentary in Nature Climate Change called “Making sense of the early-2000s warming slowdown” that looks at what caused this partial flattening out of the upward trend in global surface temperatures.
Part of this investigation compares the earlier part of the 20th century, when there was a much more significant slowdown in warming, with the more recent slowdown. Fyfe et al note that there are two major contributors to variation in surface temperature aside from greenhouse gases. One is the abundance of aerosols, such as industrial pollution (more of a factor during the earlier hiatus) and the output of volcanoes (such as the 1991 eruption of Mount Pinatubo). The other is multi-decade scale variation in the interaction between the oceans and the atmosphere. The following figure compares the two periods of reduced rate of warming.
As noted in the caption, the two periods are representations of how far off from expected (based on simple greenhouse warming) each period is. It happens that these two periods of slowdown in rate of warming are associated with the negative phase of a major ocean-atmosphere interaction, during which the ocean was eating up some of the extra heat, removing it from the atmosphere. The intervening period of increased rate of warming (from the mid 1970s to about 2000) is associated with a period when this system was in positive phase, putting heat out into the atmosphere. As I’ve noted before, the ocean, which takes up most of the global warming caused heat, is the dog, and the atmosphere is the tail. This is a graph of a dog wagging its tail.
It is not clear when the multi-decade scale ocean-atmosphere interactions will shift to a positive phase. If you look at just the raw numbers, it seems like this may have started a few years ago (around late 2013) but the index for this phenomenon varies enough (goes positive and negative and back over shorter time periods, briefly) that this is not certain. More recently, we have an El Nino causing the belching of heat form the ocean to the air, heating up the surface. This may or may not be related to the multi-decade pattern. Having said all that, we may be concerned that over the next ten years or so, starting about now, we will be in a positive phase during which the rate of warming will be accelerated. This may not be the case. Or it might be the case. No one is actually betting on this yet.
You’ve heard much about the so-called “pause” or “hiatus” in global warming.
One of the implications of a multi-year “pause” in global warming is that the science of global warming must be somehow wrong, because with CO2 rising in atmosphere, due to human activity, how can the surface not warm? However, surface temperatures have been rising, but at a somewhat slower rate than at some other times.
The truth is that there is a lot of variation in that upward trending surface temperature value, measured as an anomaly above expected temperatures. Sometimes the variation pushes the rate of warming up, sometimes it pushes the rate of warming down. This has always happened, and will always happen.
So there was something of a lowering of rate of surface warming, but at the same time, no such reduction in rate of ocean warming. Most of the heat from global warming is added to the ocean, not the surface. So, the reality is, global warming has been continuing apace.
One of the factors involved in a slowdown is probably the fact that the Pacific Ocean has been absorbing more heat, for a longer period, relatively uninterrupted by large El Ninos (which reverse that trend), for longer than usual. This year’s El Nino is returning some of that heat to the atmosphere. But even before El Nino kicked in, we were having month after month of record breaking heat (with the very rare month not being a record breaker) for a long time.
Anyway, a couple of papers have recently been published that look once more at the “pause” and I wanted to point them out. The best way to get at these papers is to read the guest commentary by tephan Lewandowsky, James Risbey, and Naomi Oreskes on RealClimate.org: Hiatus or Bye-atus?
The idea that global warming has “stopped” has long been a contrarian talking point. This framing has found entry into the scientific literature and there are now numerous articles that address a presumed recent “pause” or “hiatus” in global warming. Moreover, the “hiatus” also featured as an accepted fact in the latest IPCC report (AR5). Notwithstanding its widespread use in public and apparent acceptance in the scientific community, there are reasons to be skeptical of the existence of a “hiatus” or “pause” in global warming …. We have examined this issue in a series of three recent papers, which have converged on the conclusion that there is not now, and there never has been, a hiatus or pause in global warming.
We drove north for two days, to arrive at a place that existed almost entirely for one reason: To facilitate the capture and, often, consumption of wild fish. The folks who run the facility make a living providing shelter, food, boats, fishing tackle, easy access to a fishing license, and they can be hired as guides. The whole point is to locate, capture, butcher, cook, and eat the fish. The fish themselves have little say in the matter.
And while talking to the people there we got a lot of advice as to how to find and capture the fish, and offers were made to assist with the butchering and culinary preparation of the piscine prey. After a bit of final preparation and a few final words of advice, we were ready to go fish hinting.
“Except for those, fish,” the woman we were talking to said, pointing towards a particularly long dock extending into the vast lake, one of the largest lakes in North America. “Don’t catch those fish.”
“Why?” I asked perplexed.
“We named them,” she said. “You can go down and look at them, the fish that hang out at the end of that dock. A couple of Northern Pike. Don’t catch those fish.”
“OK,” I said. And off we went in the other direction to catch some different fish. I figured there were about 200 million fish of a pound or more in size in this particular lake. We could skip the ones with names.
For many decades, probably for over a century, there has been an observable, measurable, increase in global surface temperature caused by human greenhouse gas pollution. For the first several decades, this increase is a clear trend, but a mild one, and there is a lot of up and down fluctuation, with periods of several years of decrease as well as increase. Then the upward trend becomes stronger, and some time around 1970 it becomes virtually relentless, going up a good amount every decade. But still, there are fluctuations in the curve.
What causes these fluctuations? Several things. The total amount of CO2, the main greenhouse gas causing this heating, has been going up during this period without stopping. Because CO2 added to the atmosphere stays there for a long time, so even if the amount released into the air by burning fossil fuels varies, there is always an upward trend. This causes the general increase, and it is why the increase in the last 50 years or so has been stronger; more CO2 has been released each year more recently.
There are large scale interactions between the ocean, which is also heating up, and the atmosphere and sea surface, the latter being what is measured in graphs of “surface temperature.” These fluctuations are decades long, and influence the degree to which the surface is warm vs very warm. There are shorter term ocean-air interactions such as La Nina (periods when the ocean is taking in more heat) and El Nino (periods when the ocean is pumping out more heat).
As the Arctic has warmed, it has been less icy, and other parts of the Northern Hemisphere have been less snowy, so there has been less sunlight reflected away, another source of fluctuation. Also, since we are talking about the Arctic, there are fewer measurements there so the traditional curves showing global warming have not included increased rates of warming there to the degree they should. Some of the fluctuations in the surface temperature curve are caused by this kind of bias, a shifting bias (because of relatively more warming in under-sampled areas) in the data set.
Humans and volcanoes make dust. Humans used to make a lot more dust before environmental regulation required that factories and power plants clean up their act. There are varying amounts of widespread low level volcanic activity and the occasional enormous eruption. This dust affects the surface temperature curve, and the dust varies quite a bit over time.
If the earth was simpler … a rocky surface, no ocean, no volcanoes, no vegetation (and thus no wildfires as well), but a similar atmosphere, changes in the amount of CO2 or other greenhouse gasses in the atmosphere would be reflected in changes in surface temperature much more smoothly. If that was the case, the amount of variation in energy supplied by the sun would probably be visible in the curve (that factor is so small compared to the other factors that it is very hard to see in the actual data). The curve on the simple earth would probably jiggle up and down a bit but there would be relatively smooth.
If you look at the temperature curve, you can see periods of greater or lesser upward change in temperature. You can even name them. I decided to do this. I chose common baby names, half male and half female, giving male names to the periods with slower increase, female names to the periods with faster increase. It looks like this:
In recent years, in what is at the root a corporate funded, and rather nefarious effort to delay addressing the most important existential issue of our time, climate change caused by human greenhouse gas pollution, science deniers have come up with their own name for one of the fluctuations in the ever increasing upward march of global surface temperatures. They call it “hiatus” (aka “pause”). The purpose of naming this part of the curve is to pretend that global warming is not real. It looks like this:
I am not impressed. And neither should you be. This is like those fish at the end of the dock. Except for the fish it is an affectation of a few people having fun, whereas with the science deniers it is a bought and paid for attempt to cause another hiatus, a hiatus in taking action to save our future.
There is a new study, the Nth in a spate of studies looking at the “Hiatus,” that asks experts on trends (economists, mainly) to look at the surface temperature trend as though it was something other than surface temperatures (they were told it was global agricultural production), to see if they identify the hiatus.
The study is by Lewandowsky, Risbey, and Oreskes, and is “The “Pause” in Global Warming: Turning a Routine Fluctuation Into A Problem For Science. It is here.
There has been much recent published research about a putative “pause” or “hiatus” in global warming. We show that there are frequent fluctuations in the rate of warming around a longer-term warming trend, and that there is no evidence that identifies the recent period as unique or particularly unusual. In confirmation, we show that the notion of a “pause” in warming is considered to be misleading in a blind expert test. Nonetheless, the most recent fluctuation about the longer-term trend has been regarded by many as an explanatory challenge that climate science must resolve. This departs from long-standing practice, insofar as scientists have long recognized that the climate fluctuates, that linear increases in CO2 do not produce linear trends in global warming, and that 15-year (or shorter) periods are not diagnostic of long-term trends. We suggest that the repetition of the “warming has paused” message by contrarians was adopted by the scientific community in its problem-solving and answer-seeking role and has led to undue focus on, and mislabeling of, a recent fluctuation. We present an alternative framing that could have avoided inadvertently reinforcing a misleading claim.
The authors show that there is no unique pause in the data. They also discuss biases in the measurements themselves which suggested a slowing in warming that actually did not occur once the data were de-biased. Finally, they reported on recent work that displayed a common error when people compare climate models to measurements (climate models report surface air temperatures while observations use a mixture of air and sea surface temperatures). With this as a backdrop, the authors take a step back and ask some seemingly basic questions.
Speaking of John Abraham, he just sent me this new graphic based on the latest surface temperature measurements. This is a good moment to have a look at it:
There are two new scientific research papers looking at variation over the last century or so in global warming. One paper looks at the march of annual estimates of global surface temperature (air over the land plus sea surface, not ocean), and applies a well established statistical technique to ask the question: Was there a pause in global warming some time over the last couple of decades, as claimed by some?
The other paper looks at the so called global warming “pause” and interrogates the available data to see if the pause is supported. It concludes that it isn’t. The paper is written up in a blog post by one of the authors, here.
I’ll give you an overview of the findings below, but first, a word from the world of How Science Works.
It’s the variation, stupid
No, I’m not calling you stupid. Probably. I’m just paraphrasing Bill Clinton to underscore the importance of variation in science. The new paper examines variation in the global surface temperature record, so this is an opportunity to make this point.
Much of the time, science is about measuring, understanding, explaining, and predicting variation. This is a point non-scientists would do well to grasp. One of the reasons non-scientists, especially those engaged in policy making (from voter to member of Congress to regulatory agent to talking head) need to understand this better is because variation is one of the most useful tools in the denier tool kit. If your objective is to deny or obfuscate science, variation is there to help you.
Global warming, the increase in the Earth’s surface and ocean temperatures caused by the human caused increase in greenhouse gas, is a system with plenty of variation. The sources of variation are myriad, and the result is that the measurement of air temperature, sea surface temperature, and deeper ocean temperature appears as a set of squiggly lines.
In many systems, variation exists at more than one scale.
So, at the centennial scale, we see global surface temperatures not varying much century by century for a thousand years, then the 20th century average is higher than the previous centuries, and the 21st century average, estimated by 15% of the years of a century, is higher still. That is the effect of industrialization, where we shift from using energy from human and animal work, together with a bit of wind and water power, to using energy stored in carbon bonds in fossil fuels. This combined with population increase and increasing demands to support a consumer-driven comfort-based lifestyle have caused us to release fossil carbon into the atmosphere at an alarming rate.
At the decadal scale, we see a few recent decades that stick up above the others, and a few that are lower than others or at least don’t go up as much as others. Over the last 100 years, the decadal average temperatures have gone up on average, but with variation. The primary explanation for this variation is two fold. First, there is an increase in the absolute amount of greenhouse gas, and the rate at which we are adding greenhouse gasses to the atmosphere, so over time, greenhouse gasses have become the main determinant of temperature change (causing an increase). Earlier on, when greenhouse gas concentration was lower, other factors had a bigger impact. The second (and related) explanation is variation in aerosols, aka dust, in the atmosphere from various industrial processes, volcanoes, and such. Decadal or multidecadal variation over the last century has been mainly caused by aerosols, but with this effect diminishing in its importance as it gives way to the increasingly important role of greenhouse gas.
At a finer scale, of a year or a few years, we see variation caused mainly by the interaction of the surface (the air and the sea surface) and the upper ocean (this is sometimes examined for the top, say, 700 meters, other times, for the top 2000 meters, etc.) When we look at just ocean temperatures or just surface temperatures, we see a lot of squiggling up and down on an ever increasing upward trend. When we look at both together, we note that the squiggles cancel out to some extent. The ocean warmed considerably during recent years when the surface warmed more slowly. This is because heat is being exchanged back and forth between the surface and the deeper sea in a away that itself varies.
That is the simple version. In reality things are more complex. Even though ocean and surface temperatures vary from year to year, with the major variations caused by El Nino and La Nina events in the ENSO cycle, there are longer term variations in how this exchange of heat trends. This time scale is in the order of several decades going in one direction, several decades going in the other direction. (see this post) Then, this sort of variation may have much larger scales of change, at century or even millennial time scales, as ocean currents that facilitate this exchange, undergo major changes, which in turn alters the interaction of the surface and the sea. And, of course, both sea and ocean temperature can affect the major ocean currents, so there is a complex causal interaction going in both directions between those two sources of variation.
This is not a digression but it is annoying
Have you ever been annoyed by someone who makes a claim about the health benefits, or negative effects, of some kind of food or other ingestible substance? You know, one of those totally non-scientific “findings” from the usual internet sources. Here is a little trick you can do if you want to challenge such a claim.
In order to truly evaluate a health related claim, and have that evaluation be credible, you have to be able to do one of the following things, depending on the claim. Being able to do this is not enough to validate your expertise, but it is a starting point. It is a gate-keeper thought experiment. If you can’t do this, then you can’t really make the claim you are making with any credibility.
Name all the parts of a cell and what they do (for many health claims, especially those that have to do with diet, energy, metabolism, etc.)
Name all the different components of the immune system and explain how they work in detail (for many disease or illness related claims).
Describe, in detail, the digestive process, i.e., the process of food sitting on a plate being ingested and eventually being used by a human body, at the molecular level (for many claims about the beneficial or negative effects of certain foods, or the benefits of various dietary supplements).
You might be a climate scientist if …
All that stuff I said above about variation is the very simple version of what happens in the climate with respect to global surface temperature imbalance and global warming. If you read what I wrote and the whole time were thinking things like “yeah, but, he’s totally glossing this” or “no, it isn’t that simple, what really happens is…” then you might be a climate scientist.
If, on the other hand, this extensive tl;dr yammering on variation seemed senseless or a waste of time, or you didn’t find it interesting or don’t get the point, the you may not be prepared to evaluate claims like the one about the so-called “pause” or “hiatus” in global warming. More importantly, there is a good chance that a person making the claim that there has been such a pause is unprepared to do so, just as the person claiming that wearing a $50 fragment of a discarded circuit board around their neck will protect them from EMF can not really make that claim because they are a total dumb-ass when it comes to energy fields and cell biology.
Or, the person making the claim (most common in the area of global warming) is just trying to fool somebody. They are like the person who sells the fragment of the discarded circuit board.
A long series of data may demonstrate the outcome of a set of variables where all the variables act in similar ways over time, and any trend plus or minus variation will be clear. But if the variables change in their level of effect over time, it may be that parts of the long term data series need to be treated separately. This requirement has led to the development of a number of statistical techniques to break a series of data up into different segments, with each segment having a different statistical model applied to it.
The statistical approaches to this problem initially arose in an effort to better understand variation in the process of making key electronic components in the telecommunications industry. An early method was the “Control Chart” developed by Walter A. Shewhart at Bell Labs. The method allowed engineers to isolate moments in time when a source of variation contributing to mechanical failure changed, perhaps because some new factor came into play.
More recently, the statistical method of “Change Point Analysis” was developed to provide a better statistical framework for identifying and assessing the statistical significance of changes in sources of variation. The process involves determining whether or not a change in the sources of variation has occurred, and also, estimating if multiple change points have occurred. The process is pretty complicated, numerically, but is automated by a number of available statistical tools.
The new paper attempts to assess the reality of a “pause” or “hiatus” in global surface temperature increase using change point analysis. The change point analysis used four of the major commonly used data sets reflecting surface temperature changes. In each case, they found three change points to be sufficient to explain the medium to long term variation in the data. Most importantly, the most recent detectable change point was in the 1970s, after which there is no detectable change in the trend of increasing global temperature.
The results of the analysis are summarized in this graphic:
Figure 1. Overlaid on the raw data are the mean curves predicted by the three CP model. The grey time intervals display the total range of the 95% confidence limits for each CP. The average rates of rise per decade for the three latter periods are 0.13 ± 0.04 °C, ?0.03 ± 0.04 °C and 0.17 ± 0.03 °C for HadCRUT, 0.14 ± 0.03 °C, ?0.01 ± 0.04 °C and 0.15 ± 0.02 °C for NOAA, 0.15 ± 0.05 °C, ?0.03 ± 0.04 °C and 0.18 ± 0.03 °C for Cowtan and Way and 0.14 ± 0.04 °C, ?0.01 ± 0.04 °C and 0.16 ± 0.02 °C for GISTEMP.
Those who claim that there was a pause in global warming point to certain dates as the origin of that pause. The authors tested that idea by forcing the change point analysis to assume that this was correct. The alleged starting points for a global warming hiatus failed the statistical test. They are not real. The authors determined that the change point analysis “…provides strong evidence that there has been no detectable trend change in any of the global temperature records either in 1998 or 2001, or indeed any time since 1980. Note that performing the CP analysis on the global temperature records excluding the 2013 and 2014 observations does not alter this conclusion.”
In addition, even though the alleged starting points for a global warming hiatus were found to be bogus, they were found to be more bogus in one of the four data sets, that developed by Cowtan and Way, which in turn is generally thought to be the data set that eliminates most of the biases and other problems found in this sort of information. In other words, using the best representation available of global surface temperature increase, the so called hiatus is not only statistically insignificant, it is even less significant!
But that wasn’t enough. The authors took it even a step further.
Finally to conclusively answer the question of whether there has been a ‘pause’ or ‘hiatus’ we need to ask: If there really was zero-trend since 1998, would the short length of the series since this time be sufficient to detect a CP? To answer this, we took the GISTEMP global record and assumed a hypothetical climate in which temperatures have zero trend since 1998. The estimated trend line value for 1998 is 0.43 °C (obtained by running the CP analysis on the original data up to and including 1998). Using this, we simulated 100 de-trended realizations for the period 1998–2014 that were centered around 0.43 °C. We augmented the GISTEMP data with each hypothetical climate realization and ran the four CP model on the augmented data sets. This allowed us to observe how often a fourth CP could be detected if the underlying trend for this period was in fact zero. Results showed that 92% of the time the four CP model converged to indicate CPs in approximately 1912, 1940, 1970 and a fourth CP after 1998. Thus, we can be confident that if a significant ‘pause’ or ‘hiatus’ in global temperature did exist, our models would have picked up the trend change with a high probability of 0.92.
One is forced, sadly, to think about what deniers might say about any new climate change study. In this case, I think I know what they might say. Look again at the graph shown above. We see two periods when temperatures seem to be going down, and two periods when temperatures seem to be going up. So, half the time, they are going down and half the time they are going up, right? So, what happens if, as suggested by some climate deniers, we are due for a downward trend? Maybe there will be enough multi-decadal downward trends over the next century or so to significantly attenuate the overall trend. Hey, we might even see cooling. Right?
Well, no. For one thing, as mentioned above, the overall pattern has been an increase in the importance of greenhouse gasses as the variable controlling surface temperatures. Whatever factors caused the flattish or downward trends many decades ago may still be in place but are relatively less important from now on, even if we quickly curtail CO2 output. Also, one of those factors, aerosols, is reduced permanently (we hope). Industrial pollution, in the past, caused a lot of aerosols to be released into the atmosphere. This has been reduced by changing how we burn things, so that source attenuation of surface temperatures is reduced. Also, as noted above, there are multi-decadal changes in the relationship between the surface (air and sea surface) and the ocean, and at least one major study suggests that over coming decades this will shift into a new phase with more surface heating.
I asked author Stefan Rahmstorf to comment on the possibility of a future “hiatus.” He told me that one is possible, but “I don’t expect that a grand solar minimum alone could do it (see Feulner and Rahmstorf ERL 2010). Maybe an exceptionally large volcanic eruption could do it but it would have to be far bigger than Pinatubo, which did not cause one.” He also notes that some IPCC climate models have suggested a future slowdown, and the possibility of cooling in not non-zero. The key point, he notes, is “it just has not happened thus far, as the data analysis shows.”
Author Andrew Parnell noted, “I think anybody who claims that these current data demonstrate a hiatus is mistaken.”
Think there is a global warming hiatus? Slow down a second…
The second paper is “Lack of evidence for a slowdown in global temperature” by Grant Foster and John Abraham. Foster and Abraham start out by noting that there is a widely held belief, even among the climate science community, “…that the warming rate of global surface temperature has
exhibited a slowdown over the last decade to decade and a half.” They examine this idea “…and find no evidence to support claims of a slowdown in the trend.”
The authors note that most of the discussion of global warming involves reference to “ the relatively small thermal reservoir of the lower atmosphere” (what I refer to above as the “surface”), but since this is only a small part of the planets heat storage, this can be misleading. When the ocean is taken into account, we see no slowdown in warming. The paper by Foster and Abraham refers to the above discussed paper on change point analysis, so I’ll skip that part. The remaining thrust of the paper is to apply some basic statistical tests to the temperature curves to see if there is a statistically valid slowdown.
They derived residuals, using the GISS data set, for the last several decades, indidating the divergence of each year from an expected value given an upward trend. This looks like this:
They then took sets of adjoining residuals, and tested the hypothesis “This set of numbers is different from the other numbers.” If there was a statistically significant decrease, or increase, in temperature change for several years it would show up in this analysis. The statistical test of this hypothesis failed. As beautiful as a pause in global warming may seem, the idea has been killed by the ugly fact of ever increasing temperatures. To coin a phrase.
As a last attempt to find evidence of a trend in the residuals, we allowed for models in which not only the slope (the warming rate) changes, but the actual value itself. These are discontinuous trends, which really do not make sense physically … but because our goal is to investigate as many possible changes as is practical, we applied these models too. This is yet another version of change-point analysis, in which we test all practical values of the time at which the slope and value of the time series change. Hence it too must be adjusted for multiple trials.
Again, no statistical significance. If you look at the global temperature curve, and see a pause, what you are really seeing is noise.
Foster and Abraham conclude:
Our results show that the widespread acceptance of the idea of a recent slowdown in the increase of global average surface temperature is not supported by analytical evidence. We suggest two possible contributors to this. First, the natural curiosity of honest scientists strongly motivates them to investigate issues which appear to be meaningful even before such evidence arrives (which is a very good thing). Second, those who deny that manmade global warming is a danger have actively engaged in a public campaign to proclaim not just a slowdown in surface temperature increase, but a complete halt to global warming. Their efforts have been pervasive, so that in spite of lack of evidence to back up such claims, they have effectively sown the seeds of doubt in the public, the community of journalists, and even elected officials.
There is no such thing as a “pause” or “hiatus” in global warming.
There is, however, variation as the earth’s surface temperature steadily rises as the result of the human release of greenhouse gas pollution. Every now and then that variation results in a period of several years when the rise in global temperature is relatively slow, and a recent such period has been termed a “hiatus” or “pause.” But that signifier mainly comes from those who deny the reality of global warming, and is often used by them as an argument that global warming is somehow not real. It is real, and they are wrong.
Looking at an upward shift in global surface temperatures, seeing some variation, and claiming that this variation obviates the long term and rather startling trend can only mean one of two things. One possibility is that the person making that observation is ignorant of both the nature of the Earth’s climate system and of the nature of measurements of this kind. The other possibility is that the person making that observation is willfully obfuscating the science, in an effort to distract from the reality of human caused climate change. Neither speaks well of the individual making this observation.
Having said all that, variations in the upward trend of surface temperatures are both interesting and important. There are several possible causes, and understanding these causes is important in understanding the overall system. Part of this variation may be difficult to quantify or track natural variation in the system. Part of this variation may be issues with the measurement system. After all, the long term trend is derived from the stringing together of different data sets collected with different methods, so we would expect some measurement effects. A good part of this variation, maybe most of it, is thought to be the shifting of heat between the global surface (the sea surface and the bottom of the atmosphere) and the deeper ocean, through a variety of mechanisms. A long term trend has recently been nailed down by several studies including work by Michael Mann and his colleagues, and has to do with the ocean-air interaction as well. From a post by Mann on Real Climate:
In an article my colleagues Byron Steinman, Sonya Miller and I have in the latest issue of Science magazine, we show that internal climate variability instead partially offset global warming….
[That] natural cooling in the Pacific is a principal contributor to the recent slowdown in large-scale warming is consistent with some other recent studies, including a study … showing that stronger-than-normal winds in the tropical Pacific during the past decade have lead to increased upwelling of cold deep water in the eastern equatorial Pacific. Other work by Kevin Trenberth and John Fasullo of the National Center for Atmospheric Research (NCAR) shows that the there has been increased sub-surface heat burial in the Pacific ocean over this time frame, while yet another study by James Risbey and colleagues demonstrates that model simulations that most closely follow the observed sequence of El Niño and La Niña events over the past decade tend to reproduce the warming slowdown.
And now, there is a paper just out in Science that explores the measurement part of the variation in increasing global surface temperature. Possible artifacts of data biases in the recent global surface warming hiatus by Thomas R. Karl, Anthony Arguez, Boyin Huang, Jay H. Lawrimore, James R. McMahon, Matthew J. Menne, Thomas C. Peterson, Russell S. Vose, and Huai-Min Zhang looks at changes in various measurements used to generate the basic data to track global surface temperatures. From the abstract:
Much study has been devoted to the possible causes of an apparent decrease in the upward trend of global surface temperatures since 1998, a phenomenon that has been dubbed the global warming “hiatus.” Here we present an updated global surface temperature analysis that reveals that global trends are higher than reported by the IPCC, especially in recent decades, and that the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.
An example of a measurement issue has to do with how sea surface temperatures are obtained. In the old days, most temperature measurements taken from boats involved the “bucket technique” which involves directly sampling the surface water. Later, this shifted to using thermometers measuring water temperature at intakes on board ship. The two methods measure the same thing, but because they are slightly different measurements, produce results slightly biased in relation to each other. Adjustments to these measurements apparently assumed that all the ship measurements had gone to the intake method, while in fact, some had not. This requires a small adjustment in how the numbers are used in the surface temperature estimate. The authors assert that similar small changes in the data are required for some other measurements as well.
The new analysis produced in this paper shows a consistant difference between the data as previously adjusted and how the authors feel it should be adjusted, with the older methods generally resulting in lower temperatures than the newer adjusted data. The difference between the two is especially large during the so-called “Hiatus” period. Here is the fancy graphic from the paper that shows this:
This is important and valid work. But, with respect to the public and policy-related conversation about anthropogenic global warming, this is really dotting of i’s and crossing of t’s. The difference between a temperature curve without this new adjustment and with it is very small. This graphic at the top of the post is the top part of the paper’s Figure 2, comparing the proposed new corrections and the previous corrections for the instrumental record from 1880 to the present. You can see that the better estimate of temperatures is in fact higher for the so-called hiatus, and varies from the older method noticeably here and there, but there is nothing in this new curve that changes anything important.
The end result is that the temperature trends over the past 17 or so years has continued to increase with no halt. In fact, it has increased at approximately the same rate as it had for the prior five decades. But the authors went further by trying to cherry-pick the start and end dates. For instance, they stacked the cards against themselves by purposefully picking a very hot year to start the analysis and a cool year to terminate the study (1998 and 2012, respectively). Even this cherry-picked duration showed a warming trend. Furthermore, the warming trend was significant.
The Earth’s climate is warming. The upper oceans are warming, the sea surface temperatures are elevated, the air in the lower Troposphere, where we live, is warming. This warming is caused almost entirely by the increase in human generated greenhouse gasses and the positive (not positive in a good way) feedbacks caused by that. The effects that increase the global heat imbalance and the effects that decrease it (such as greenhouse gas increase and aerosols — dust — from volcanoes, respectively) vary over time in their effect, which causes some variation in the upward march of global surface and ocean temperatures. Meanwhile heat is going back and forth between the surface (atmosphere and sea surface) and the deeper (but still upper) ocean, which causes either of these parts of the Earth system to wiggle up and down in temperature. There are other effects causing other wiggles.
Every now and then the wiggling results in a seemingly rapid increase in temperature. Every now and then the wiggling results in a seeming slowdown in increase in temperature. When the latter happen, those who wish you to believe that climate change is not real point, jumping up and down, giddy, and say, “See, there is a pause in global warming, therefore global warming is not happening!” They are wrong for the reasons just stated.
Dana Nuccitelli (author of this book), over at the Guardian, has written a blog post that nicely summarizes current scientific thinking on the so-called pause (or so-called hiatus) which has been named by witty climate scientists as the #FauxPause. Because it is faux.
First, there is no hiatus. Climate science skeptics claim that warming stopped in 1998. It didn’t. Stefan Rahmstorf has a nice post placing 2013 in context with the most recent data, HERE. Just click the “translate” button to read it in your favorite language.
Stefan has a bunch of great graphics that you will enjoy. Following his lead I’ve decided to make a graphic or two myself.
First, the data. NASA has this data to which people often refer when discussing global warming. I took that database and fixed it up a bit. I deleted the first year because there’s some missing data and who cares about only one year anyway. Then, I converted all the values to degrees C rather than hundreds of degrees off a baseline. I also calculated a rank for each year in reference to the entire database. You can download the data as a comma delimited file here. Let me know if that link doesn’t work for you, I’ll be happy to send you the file. Please cite the original (linked to above) if you use this.
Using these data I made this handy graphic showing “surface temperatures” (air and sea surface) over time from 1881 to the present.
When people talk about the hiatus in climate change, or the pause in climate change, what this means is that the slope of the temperature curve for a particular period of time is at or near zero, or negative. What actually happens is that the slope of the curve for a given interval, say 10 years, goes up and down over time. If the temperature was varying around a mean, and not going up over time, the sum of those slopes would be zero, but if there is an average increase in temperature the sum of all the different slopes (of a given interval) one can calculate will be positive.
This is actually a slightly strange way of looking at the data, but I think it is constructive, especially given that the so-called-pause is a dead horse and we are hear to beat it. Look at the chart above. Imagine taking any given ten year period and calculating a slope for that period. Then another and another and another, until you’ve measured out a slope for every ten year period … not just every ten years, but every possible interval of ten consecutive years. This would be a “moving slope” and a graph of it would look like this:
What this shows is that for the vast majority of ten year intervals since 1889 (so the first interval is 1880-1889) the slope of the temperature curve is positive, going up, increasing. It also shows what looks like a remarkably periodic increase and decrease in this slope, with only a few dips below zero. That’s presumably due to oscillations such as ENSO or other factors. Also, most of those dips are from fairly far back in time, and this happens rarely in recent years. We are currently in a period of positive change (upward temperature swings) but currently reduced. But if you look at this graph you can see that there are OFTEN periods of time when the upward slope is very high and other periods when it is very low but still above zero almost always. I hope this helps put the “hiatus” into perspective.
I also made this graph of each year’s rank for the entire period represented by the data set.
Again, this is a slightly unusual way of depicting the data, but it may be helpful. All of the highest ranked years … top ten or so … are from very recent time. The graph has grid lines at every 10 ranks. This lets you quickly identify the period of time over which the top 10, or 20, or 30, or whatever, warmest year according to this data set occurred. There are no top ten years prior to about 1998. All of the top 30 warmest years post date the early 1970s. And so on.
OK, so let’s look at the hiatus again. The hiatus is supposed to be a period of no global warming since 1998. Here’s a closeup of the original chart (above) for that period of time:
What we see here, with the trend line included to make it easier to read, is an increase in global temperature, on average, during this so called hiatus period. But, by picking 1998 as a starting point, climate science denialists have managed to flatten the curve out quite a bit. That’s called cherry picking.
Now let’s arbitrarily double the period of interest, to include the entire so-called hiatus and the same amount of time back before the so-called hiatus. What does the graph look like then? Here, I’ve tried to keep all the scales the same so you can see the shorter “hiatus” period as part of this larger graph. You can also see that 1998 was an exceptionally warm year, which is why you’d want to pick it as the beginning of your fake hiatus period if you were a damn liar. Have a look.
Let’s look at those so called hiatus years in yet another way. Here, we have the graph of the temperature by years (with the upward sloping trend line indicating continued warming even though it is supposed to be a “pause”) and at each node I’ve written in the rank order of the year for the instrumental record. Note that tied years share a number. Basically, this period of “hiatus” is a very very warm period indeed, with temperatures trending upward during the entire period, looking only at the earth’s surface. (Elsewhere we’ve discussed how there is also heat going into the oceans. See links below.)
Since the climate science denialists have chosen a period of time of 16 years to describe a so-called “hiatus” which is not really a hiatus, I thought it would be fun to chunk out the data for the entire time period into 16 year intervals, starting with the most recent and going back to 1886. When viewed using these time intervals, we see overall warming with the most recent years seeing accelerated warming. Have a look:
These are all first drafts and if I get reasonable suggestions I may make new versions with corrections, additions, etc.