Antarctica: ice gain or loss?

November 25, 2015 by

Guest post by Dr. Jan Wuite, Enveo, Innsbruck

A new study released by NASA scientist Jay Zwally and colleagues in the Journal of Glaciology, receiving wide coverage in the media last month, reports an 82 Gigatons per year increase of land ice in Antarctica during the period 2003-2008. The study received much skepticism from other leading scientists in the field, as there are many indications that point at the contrary: ice loss, possibly irreversible. How does this new study fit in that picture, what are the consequences for expected sea level rise and are these numbers correct? Glaciologist and polar scientist Jan Wuite, working at Enveo in Innsbruck and involved in various international studies related to Antarctica explains.

One of the adverse consequences of climate change is global sea level rise. At more than 3 mm per year, the current sea level rises twice as fast as during the 20th century. The expectations are a rise of at least 70 cm by the turn of this century. The principal causes are clear: global decline of land ice (mountain glaciers & ice sheets) and thermal expansion of ocean water (water expands as it becomes warmer). To clarify, land ice is resting on land and can reach a thicknesses of up to several kilometers, in contrast to seasonally restricted sea ice (mainly just frozen ocean water), that is typically only a few meters thick and has no direct influence on sea level. Studies have indicated an increasing contribution of the two largest ice sheets, the Greenland and Antarctic ice sheets, to sea level rise.

The largest unknown for future sea level rise is caused by uncertainty in the predicted response of the Antarctic ice sheet to global warming. As warmer air can hold more moisture, it is possible that increasing snow accumulation compensates part of the sea level rise. On the other hand it is also possible that ice drains faster to the oceans accelerating it.

There is a lot of ice in Antarctica; in some places the ice thickness reaches well over 4 km. There is enough ice to, when melted completely, raise global sea levels with roughly 58 m. But even if only a small part of that melts it could have a significant impact on coastal communities, or ocean circulation. For this reason scientists are very interested in mass changes of the ice sheets: the mass balance.

Figure 1. An illustration of key processes determining ice sheet mass balance. Source: Zwally et al figure 1.

Read the rest of this entry »

From complex clarity to nuanced misunderstanding: Response to Hollin and Pearce

October 24, 2015 by

Earlier this year, Greg Hollin and Warren Pearce published a short letter in Nature Climate Change entitled Tension between scientific certainty and meaning complicates communication of IPCC reports.

In contrast to their claims, we demonstrate in our comment on this article that the IPCC correctly placed the hottest decade in the context of long-term trends. The IPCC did not dismiss the recent slowdown in surface warming, the so-called “hiatus” or “pause”, as scientifically irrelevant.

Hollin and Pearce’s central premise is nicely encapsulated in the abstract of their paper:

Here we demonstrate that speakers at the press conference for the publication of the IPCC’s Fifth Assessment Report (Working Group 1) attempted to make the documented level of certainty of anthropogenic global warming (AGW) more meaningful to the public. Speakers attempted to communicate this through reference to short-term temperature increases. However, when journalists enquired about the similarly short ‘pause’ in global temperature increase, the speakers dismissed the relevance of such timescales, thus becoming incoherent as to ‘what counts’ as scientific evidence for AGW.

This observation leads them to theorize about the tension between scientific certainty and meaning. But did they actually demonstrate what they claimed they did? We argue in a comment to this article that they did not.

They base their statement that IPCC speakers attempted to communicate ‘meaning’ by reference to short-term temperature increases on this statement made by Pachauri:

the decade 2001 onwards having been the hottest, the warmest that we have seen

This statement was explicitly placed in the context of long-term, climatically relevant trends, as indicated by Pachauri’s preceding words and the graph below (IPCC AR5 SPM fig 1) that was prominently shown at the press conference:

each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850


That is an entirely different ballgame than the short-term variability that underlies the slowdown of the surface warming trend (often referred to as “the pause”). So no, they did not demonstrate in the least that IPCC speakers “relied on temporally local events to increase public meaning”. Hollin and Pearce’s premise is based on misunderstanding the timescales that were discussed.

What about the claim that IPCC speakers dismissed the relevance of the so-called “pause”?

As we write in our comment, five of the 18 journalists asked a question about recent temperature trends; none were ignored. Also David Rose’s question, which is featured prominently in Hollin and Pearce’s argument, was not dismissed. Stocker responded to Rose’s question, followed by Jarraud explaining why he regarded it as “ill-­‐posed”, reframing it as a well-posed question, and responding to that. See the (freely available) Supplemental Information for more details.

So no, Hollin and Pearce did not demonstrate that the relevance of the slowdown was dismissed. Hotwhopper hammers home this and related points with lots of extra detail, showing that the IPCC message was clearly received by most journalists and that only one journalist who asked a question at the press conference “condemned” the IPCC for supposed dismissal. This journalist, as you might suspect, was David Rose, for whom IPCC bashing is a modus operandi.

The entire premise for their argument thus seems to rest on shaky ground. Their conclusions about e.g. the IPCC’s credibility being somehow eroded by their (in Hollin and Pearce’s eyes) mixed messages are thus not supported.

I’m actually surprised that in their reply to our comment, Hollin and Pearce don’t acknowledge their mistaken interpretation regarding timescales, but rather keep digging their heels in regarding that point. E.g. they reiterate that Pachauri’s quote above was “illustrative of references to the warmest decade made by all three speakers”, apparently without realizing, or not considering it as relevant, that this and other such references were made in the context of the long-term trend. Very peculiar.

They wrote a blogpost in which they try to reflect on the meaning of this back and forth in the scientific literature. It’s an interesting read, and they basically argue that their initial letter was based on inductive research: starting with the data, seeing patterns or interesting things, and the theories and broader claims are integrated later. They claim that this is more common in qualitative social science than it is in natural science and that this difference may be at the root of our disagreement. I’m actually quite comfortable with such inductive style research. I have often started my research by looking at “what the data told me” as my PhD advisor used to say, especially for a large body of field observations. That was also the approach we took in analyzing the data from our climate science survey.

And Then There’s Physics, also co-author of the response to Hollin and Pearce, goes into more detail on this point, and rightly wonders

why it makes any difference whether one’s approach is inductive or deductive. What you present should be a reasonable representation of reality, whether you approached it inductively (“the data looks interesting, why is that?”) or deductively (“I have a theory/hypothesis, let me collect some data to test it”). For example, either the IPCC fell into a trap by using one indicator to stress the certainty of AGW while dismissing another essentially equivalent indicator, or they didn’t; either the IPCC dismissed the so-called “pause”, or they didn’t. It can’t really be both.

I would hardly think that the difference in interpretation between Hollin and Pearce on the one side and Jacobs et al (myself included, but also e.g. cognitive psychologist Stephan Lewandowsky) on the other side boils down to a typical difference in approach between social science and natural science. Rather, it boils down to the former misinterpreting statements relating to timescales and basing the whole remainder of their argument on a false premise.

Update (27 Oct 2015)

Over at Pearce’s blog I replied to Greg Hollin:

I re-read your reply incl the SI and I’m still struggling to see understand your point of view regarding time scales.


In the SI for example you quote Jarraud as saying
“more temperature records were broken than in any other decade”

with the emphasis (italics) on the word “any”. Doesn’t that point to these temperature record being presented in the context of the longer-term trend (“than in *any* other decade”)? To me the answer to that would be a clear yes, but apparently that’s not the same to you. Could you clarify your position in that respect?

It may very well be that speakers mentioned (spatially and temporally) local events as *examples* of what climate change might mean to a person’s live, so yes, to make it more societally meaningful. I’m not challenging that. What I’m challenging is your premise that in doing so the IPCC speakers provided an incoherent picture of timescales, on the one hand presenting a decade’s worth of data as scientifically meaningful and on the other hand as not meaningful. The former, the warmest decade, was consistently put in the context of the long term trend, even in the quote that you mentioned yourself. So there was no such incoherence.


Wanted: blog readers to be interviewed as part of PhD research into climate blogging

October 10, 2015 by

Guest post by Giorgos Zoukas. As part of his research on climate blogging he would like to interview blog readers. Please contact him if you’d like to participate. He has interviewed me as well as some other climate scientist bloggers. BV


Invitation to participate in a PhD research project on climate blogging

My name is Giorgos Zoukas and I am a second-year PhD student in Science, Technology and Innovation Studies (STIS, at the University of Edinburgh ( This guest post is an invitation to the readers and commenters of this blog to participate in my project.

This is a self-funded PhD research project that focuses on a small selection of scientist-produced climate blogs, exploring the way these blogs connect into, and form part of, broader climate science communication. The research method involves analysis of the blogs’ content, as well as semi-structured in-depth interviewing of both bloggers and readers/commenters.

Anyone who comments on this blog, on a regular basis or occasionally, or anyone who just reads this blog without posting any comments, is invited to participate as an interviewee. The interview will focus on the person’s experience as a climate blog reader/commenter.*

The participation of readers/commenters is very important to this study, one of the main purposes of which is to increase our understanding of climate blogs as online spaces of climate science communication.

If you are interested in getting involved, or if you have any questions, please contact me at: G.Zoukas -at- (Replace the -at- with the @ sign)

(Those who have already participated through my invitation on another climate blog do not need to contact me again.)

*The research complies with the University of Edinburgh’s School of Social and Political Sciences Ethics Policy and Procedures, and an informed consent form will have to be signed by both the potential participants (interviewees) and me.

Richard Tol misrepresents consensus studies in order to falsely paint John Cook’s 97% as an outlier

September 24, 2015 by

John Cook warned me: if you attempt to quantify the level of scientific consensus on climate change, you will be fiercely criticized. Most of the counterarguments don’t stand up to scrutiny however. And so it happened.

The latest in this saga is a comment that Richard Tol submitted to ERL, as a response to John Cook’s study in which they found 97% agreement in the scientific literature that global warming is human caused. Tol tries to paint Cook’s 97% as an outlier, but in doing so misrepresents many other studies, including the survey that I undertook with colleagues in 2012. In his comment and his blogpost he shows the following graph:

Richard Tol misrepresenting existing consensus estimates

Richard Tol comes to very different conclusions regarding the level of scientific consensus than the authors of the respective articles themselves (Oreskes, 2004; Anderegg et al., 2010; Doran and Kendall Zimmerman, 2009; Stenhouse et al., 2013; Verheggen et al., 2014). On the one hand, he is using what he calls “complete sample” results, which in many cases are close to meaningless as an estimate of the actual level of agreement in the relevant scientific community (that counts most strongly for Oreskes and Anderegg et al). On the other hand he is using “subsample” results, which in some cases are even more meaningless (the most egregious example of which is the subsample of outspoken contrarians in Verheggen et al).

The type of reanalysis Tol has done, if applied to e.g. evolution, would look somewhat like this:

  • Of all evolutionary biology papers in the sample 75% explicitly or implicitly accept the consensus view on evolution. 25% did not take positon on whether evolution is accepted or not. None rejected evolution. Tol would conclude from this that the consensus on evolution is 75%. This number could easily be brought down to 0.5% if you sample all biology papers and count those that take an affirmative position in evolution as a fraction of the whole. This is analogous to how Tol misrepresented Oreskes (2004).
  • Let’s ask biologists what they think of evolution, but to get an idea of dissenting views let’s also ask some prominent creationists, e.g. from the Discovery Institute. Never mind that half of them aren’t actually biologists. Surprise, surprise, the level of agreement with evolution in this latter group is very low (the real surprise is that it’s not zero). Now let’s pretend that this is somehow representative of the scientific consensus on evolution, alongside subsamples of actual evolutionary biologists. That would be analogous to how Tol misrepresented the “unconvinced” subsample of Verheggen et al (2014).

Collin Maessen provide an detailed take-down of Richard Tol on his blog, quoting extensively from the scientists whose work was misrepresented by Tol (myself included). The only surveys which are not misrepresented are those by Bray and von Storch (2007; 2010). This is how I am quoted at Collin’s blog RealSkeptic:

Tol selectively quotes results from our survey. We provided results for different subsamples, based on different questions, and based on different types of calculating the level of agreement, in the Supporting Information with our article in ES&T. Because we cast a very wide net with our survey, we argued in our paper that subgroups based on a proxy for expertise (the number of climate related peer reviewed publications) provide the best estimate of the level of scientific consensus. Tol on the other hand presents all subsamples as representative of the scientific consensus, including those respondents who were tagged as “unconvinced”. This group consists to a large extent of signatories of public statements disapproving of mainstream climate science, many of whom are not publishing scientists. For example, some Heartland Institute staffers were also included. It is actually surprising that the level of consensus in this group is larger than 0%. To claim, as Richard Tol does, that the outcome for this subsample is somehow representative of the scientific consensus is entirely nonsensical.

Another issue is that Richard Tol bases the numbers he uses on just one of the two survey questions about the causes of recent climate change, i.e. a form of cherry picking. Moreover, we quantified the consensus as a fraction of those who actually answered the question by providing an estimate of the human greenhouse gas contribution. Tol on the other hand quantifies the consensus as a fraction of all those who were asked the question, including those who didn’t provide such an estimate. We provided a detailed argument for our interpretation in both the ES&T paper and in a recent blogpost.

Tol’s line of reasoning here is similar to his misrepresentation of Oreskes’ results, by taking the number of acceptance papers not just as a fraction of papers that take position, but rather as a fraction of all papers, including those that take no position on current anthropogenic climate change. Obviously, the latter should be excluded from the ratio, unless one is interested in producing an artificially low, but meaningless number.

Some quotes from the other scientists:


Obviously he is taking the 75% number below and misusing it. The point, which the original article made clear, is that we found no scientific dissent in the published literature.


This is by no means a correct or valid interpretation of our results.

Neil Stenhouse:

Tol’s description omits information in a way that seems designed to suggest—inaccurately—that the consensus among relevant experts is low.


To pull out a few of the less expert groups and give them the same weight as our most expert group is a completely irresponsible use of our data.

You can read their complete quotes at RealSkeptic.

See also this storify of my twitter discussion with Richard Tol.

Rick Santorum misrepresents our climate survey results on Bill Maher show

September 2, 2015 by

In our survey of more than 1800 scientists we found that the large majority agree that recent climate change is predominantly human induced. The article where we discuss our results is publicly available and a brief rundown of the main conclusions is provided in this blogpost.

We were quite surprised to hear a US Presidential candidate, Rick Santorum, make the opposite claim based on our survey. On the Bill Maher show he said:

The most recent survey of climate scientists said about 57 percent don’t agree with the idea that 95 percent of the change in the climate is caused by CO2. (…) There was a survey done of 1,800 scientists, and 57 percent said they don’t buy off on the idea that CO2 is the knob that’s turning the climate. There’s hundreds of reasons the climate’s changed.

What did we actually find in our survey?

In our survey of 1868 scientists studying various aspects of climate change, we asked two questions about the causes of recent global warming. Of all scientists who provided an estimate ~85% think that the influence of human greenhouse gases is dominant, i.e. responsible for more than half of the observed warming. ~15% think greenhouse gases are responsible for less than half of the observed warming. If you zoom in to those respondents with arguably more expertise, the percentage agreeing with human dominated warming becomes 90% or larger.

The existence of a strong scientific consensus about climate change is also clear from previous surveys of scientists and of the scientific literature and from statements of scientific societies. A scientific consensus is a logical consequence of the evidence for a certain position becoming stronger over time.

Rick Santorum’s claim is misleading and wrong because:

1) it is based on a wrong interpretation of just one of the two survey questions about the causes of recent climate change.

2) it is based on the argument that respondents who didn’t provide a specific estimate for the contribution of greenhouse gases (22% of the total number) think that this contribution is small. That is a wrong inference.

3) it is based on the argument that respondents who think it is “very likely” or “likely” or “more likely than not” that greenhouse gases are the dominant cause of recent warming disagree with this dominant influence. That is a wrong inference.

Politifact also did a fact-check of Santorum’s claim and found it to be false. It gives a very good overview of what’s wrong with Santorum’s claim. Several people are quoted in their article, myself included as well as the blogger Fabius Maximus who came up with the 43% consensus figure used by Santorum to claim a 57% dissensus:

Read the rest of this entry »

A warm 2015 and model –data comparisons

August 7, 2015 by

Guest post by Jos Hagelaars. Dutch version is here.

Discussions on the Internet regarding climate change are sometimes about scientific details, sometimes about the climate sensitivity regarding the equilibrium situation hundreds of years from now, but the most prevalent discussion topic is probably: the global average temperature. Will it get warmer or colder, is there a temporary slowdown or acceleration in the rise in temperature, are the models correct or not, will the eventual warming of our earth in the future be large or small? New numbers are released on a monthly basis and every month megabytes of text are generated about them. My forecast is that 2015 again will lead to a discussion-spike.

The graph above shows the evolution of the global surface temperature anomaly for three datasets, where the average of the period 1981-2010 is defined as 0. For the year 2015 only data are presented up to and including June. So far 2015 exceeds all other years and the evolving El Niño makes it likely that 2015 will set a new world record.
Read the rest of this entry »

PBL survey shows strong scientific consensus that global warming is largely driven by greenhouse gases

August 4, 2015 by


(5 Sep 2015): US Presidential candidate Rick Santorum used an erroneous interpretation of our survey results on the Bill Maher show. My  detailed response to Santorum’s claim is in a newer blogpost. Politifact and Factcheck also chimed in and found Santorum’s claims to be false. The blogpost below goes into detail about how different interpretations could lead to different conclusions and how some interpretations are better supported than others.

As Michael Tobis rightly points out, the level of scientific consensus that you find “depends crucially on who you include as a scientist, what question you are asking, and how you go about asking it”. And on how you interpret the data. We argued that our survey results show a strong scientific consensus that global warming is predominantly caused by anthropogenic greenhouse gases. Others beg to differ. Recent differences of opinion are rooted in different interpretations of the data. Our interpretation is based on how we went about asking certain questions and what the responses indicate.

To quantify the level of agreement with a certain position, it makes most sense to look at the number of people as a fraction of those who answered the question. We asked respondents two questions about attribution of global warming (Q1 asking for a quantitative estimate and Q3 asking for a qualitative estimate; the complete set of survey questions is available here). However, as we wrote in the ES&T paper:

Undetermined responses (unknown, I do not know, other) were much more prevalent for Q1 (22%) than for Q3 (4%); presumably because the quantitative question (Q1) was considered more difficult to answer. This explanation was confirmed by the open comments under Q1 given by those with an undetermined answer: 100 out of 129 comments (78%) mentioned that this was a difficult question.

There are two ways of expressing the level of consensus, based on these data: as a fraction of the total number of respondents (including undetermined responses), or as a fraction of the number of respondents who gave a quantitative or qualitative judgment (excluding undetermined answers). The former estimate cannot exceed 78% based on Q1, since 22% of respondents gave an undetermined answer. A ratio expressed this way gives the appearance of a lower level of agreement. However, this is a consequence of the question being difficult to answer, due to the level of precision in the answer options, rather than it being a sign of less agreement.

Moreover, the results in terms of level of agreement based on Q1 and Q3 are mutually consistent with each other if the undetermined responses are omitted in calculating the ratio; they differ markedly when undetermined responses are included. In the supporting information we provided a table (reproduced below) with results for the level of agreement calculated either as a fraction of the total (i.e., including the undetermined answers) or as a fraction of those who expressed an opinion (i.e., excluding the undetermined answers), specified for different subgroups.

Verheggen et al - EST 2014 - Table S3

For the reasons outlined above we consider the results excluding the undetermined responses the most meaningful estimate of the actual level of agreement among our respondents. Indeed, in our abstract we wrote:

90% of respondents with more than 10 climate-related peer-reviewed publications (about half of all respondents), explicitly agreed with anthropogenic greenhouse gases (GHGs) being the dominant driver of recent global warming.

This is the average of the two subgroups with the highest number of self-reported publications for both Q1 and Q3. In our paper we discussed both ways of quantifying the level of consensus, including the 66% number as advocated by Tom Fuller (despite his claims that we didn’t).

Fabius Maximus goes further down still, claiming that the level of agreement with IPCC AR5 based on our survey results is only 43-47%. This result is based on the number of respondents who answered Q1b, asking for the confidence level associated with warming being predominantly greenhouse gas-driven, as a fraction of the total number of respondents who filled out Q1a (whether with a quantitative or an undetermined answer). As Tom Curtis notes, Fab Max erroneously compared our statement to the “extremely likely” statement in AR5, whereas in terms of greenhouse gases AR5 in Chapter 10 considered it “very likely” that they are responsible for more than half the warming. Moreover, our survey was undertaken in 2012, long before AR5 was available, so if respondents had IPCC in mind as a reference, it would have been AR4. If anything, the survey respondents were by and large more confident than IPCC that warming had been predominantly greenhouse gas driven, with over half assigning a higher likelihood than IPCC did in both AR4 and AR5.

PBL background report - Q1b

Let me expand on the point of including or excluding the undetermined answers with a thought experiment. Imagine that we had asked whether respondents agreed with the AR4 statement on attribution, yes or no. I am confident that the resulting fraction of yes-responses would (far) exceed 66%. We chose instead to ask a more detailed question, and add other answer options for those who felt unwilling or unable to provide a quantitative answer. On the other hand, imagine if we had respondents choose whether the greenhouse gas contribution was -200, -199, …-2, -1, 0, 1, 2, … 99, 100, 101, …200% of the observed warming. The question would have been very difficult to answer to that level of precision. Perhaps only a handful would have ventured a guess and the vast majority would have picked one of the undetermined answer options (“I don’t know”, “unknown”, “other”). Should we in that case have concluded that the level of consensus is only a meagre few percentage points? I think not, since the result would be a direct consequence of the answer options being perceived as too difficult to meaningfully choose from.

Calculating the level of agreement in the way we suggest, i.e. excluding undetermined responses, provides a more robust measure as it’s relatively independent of the perceived difficulty of having to choose between specific answer options. And, as is omitted by the various critics, it is consistent with the responses to the qualitative attribution question, which also provides a clear indication of a strong consensus. If you were to insist on including undetermined responses in calculating the level of agreement, then it’s best to only use results from Q3. Tom Fuller’s 66% becomes 83% in that case (the level of consensus for all respondents), showing the lack of robustness in this approach when applied to Q1.

Verheggen et al - Figure 1 - GHG contribution to global warming

Some other issues that came up in recent discussions:

See also the basic summary of our survey findings and the accompanying FAQ.


Responses to the Climate Science Survey

April 12, 2015 by

Appeared in similar form on the PBL website

In the Spring of 2012, the Netherlands Environmental Assessment Agency PBL held a survey among 1868 scientists studying various aspects of climate change, including physical climate, climate impacts, and mitigation. The main results of the survey were published in an article in Environmental Science and Technology (ES&T) in August 2014: “Scientists’ views about attribution of global warming”. It showed that there is widespread agreement regarding a dominant influence of anthropogenic greenhouse gases on recent global warming. This agreement is stronger among respondents with more peer-reviewed publications.

A background report with the results for all 31 questions has now been made available. The total number of responses for each answer option is provided and a subdivision into seven groups for five questions. The background report contains previously unpublished data. Some highlights are provided below.

Climate sensitivity

Respondents were asked for their opinion regarding the best estimate and likely range for equilibrium climate sensitivity (ECS). This is an important quantity for projections of global warming, as it gives the expected warming that would follow from a doubling in atmospheric CO2 concentration after the climate system has equilibrated to the new conditions. Thus, expected warming in the future depends on the combination of total emissions and climate sensitivity.

The figure below gives the average estimates of ECS from 7 groups of respondents, including authors of the Working Group I report of the fourth IPCC Assesment Report (AR4), respondents who signed public declarations critical of mainstream climate science as embodied by IPCC (‘unconvinced’), and four different subgroups distinguished according to their self-declared number of climate related peer-reviewed publications (0–3; 4–10; 11–30; more than 30). Results from most groups were very close to the IPCC range (1.5-4.5 °C) mentioned in the fifth assessment report (AR5) – except those tagged as ‘unconvinced’ which strongly deviated from the other groups, and to a lesser extent the group of respondents with three or less publications. For all subgroups the ‘best estimate’ was slightly lower than the ‘best estimate’ reported in AR4 (i.e. 3 °C). AR5 provided no best estimate.

Scientists views on climate sensitivity - PBL

Role of climate science in society

Respondents were also asked their opinion about seven statements regarding the role of climate science in society and how the science should be communicated. There was a strong consensus that scientists themselves should communicate with both policymakers and the general public about climate change and that communication with the general public should focus on solid knowledge. To a lesser extent there was agreement that risks and uncertainties should be emphasised during such communication. Responses varied more strongly about whether or not existing uncertainties in climate science strengthen the case for mitigation (i.e. to avoid potential low probability, high impact events). There was (strong) disagreement with the statement that climate science would be too uncertain to be useful for policymaking on climate change.

Scientists views on role of science in society - PBL

The role of the sun in global warming

In the public debate about climate change the role of the sun is often put forward as an alternative explanation for global warming. Question 17 asked what fraction of recent global warming could be attributed to the sun. Those tagged as ‘unconvinced’ had the lowest fraction of respondents that indicated that they don’t know (together with AR4 authors) and the highest fraction that said that the role of the sun is unknown. As expected they also had by far the highest fraction (27%) that believed that the sun caused more than half of recent global warming.

As with the attribution questions (see the ES&T article), there appears to be a trend in responses going from the group with fewest publications to those with most. The more publications about climate change respondents report to have written, the larger fraction of them agree with the IPCC position that the sun hardly played a role in recent global warming, since the solar output decreased slightly over that period.

Scientists views on the role of the sun in global warming - PBL

More information:

PS: I’ll have a poster presentation about the survey at the EGU conference this week, in session EOS6 “Communication and Education in Geoscience” on Thursday evening.

Climate researcher Bart Strengers wins wager with climate sceptic Hans Labohm

January 23, 2015 by

Guestpost by Bart Strengers. Originally appeared as a news item on the PBL website.

Late 2009, in the run-up to the international climate conference in Copenhagen, PBL climate researcher Bart Strengers had an online discussion with climate sceptic Hans Labohm on the website of the Dutch news station NOS (in Dutch). This discussion, which was later also published as a PBL report, ended in a wager. Strengers wagered that the mean global temperature over the 2010–2014 period would be higher than the mean over 2000 to 2009. Hans Labohm believed there would be no warming and perhaps even a cooling; for example due to reduced solar activity.

At the request of Labohm, it was decided to use the UAH satellite temperature data set on the lower troposphere (TLT) (roughly the lowest 5 km of the atmosphere). These data sets are compiled by the University of Alabama in Huntsville. Satellites are used to measure radiation in the atmosphere, after which the temperature of the various layers of the atmosphere is derived using a complex algorithm.

According to the UAH today, temperatures appear to have been an average 0.1 °C warmer over the past five years than over the 10 years before that. Thus, Strengers has won the wager. The stakes: a good bottle of wine.

PBL temp comp Eng - 0040_001g_adhoc
The UAH temperature series since 1979 (no satellites were available for the period before then). The green lines represent the mean over periods of 10 years. The purple line on the far right is the mean over the 2010–2014 period.
UAH satellite data series shows the greatest warming

Precisely these UAH data, incidentally, show by far the most warming. The 4 other main global temperature series also show warming over the last 5 years, but one that is markedly lower (between 0.03 and 0.05 °C).

What causes the differences between the data series?

The table below shows the global warming, in °C, over the past 5 years, compared to the 10 years before that, for the five main global temperature series: the satellite series of the University of Alabama in Huntsville (UAH) and of the Remote Sensing Systems (RSS), and the surface temperature series of NASA, Climate Research Unit (CRU) and the National Climatic Data Centre (NCDC). CRU’s series are based on surface temperature measurements up to and including November 2014, as data on December were not yet available.

The large difference (by more than a factor of 3!) between the UAH and RSS satellite series is remarkable (also see the graph below). According to the UAH team, in which two well-known climate sceptics are involved, the difference is mainly caused by the fact that RSS partly bases its series on an old satellite (NOAA-15) with an increasingly lower orbit around the earth. This causes an error in measurements that is insufficiently corrected by RSS. All in all, it is a technical and complex issue, which possibly causes the differences, but it mainly shows how complicated the procedure is for determining global temperatures on the basis of satellite measurements. The three surface measurement series provide a much more consistent image of between 0.04 °C and 0.05 °C warming.

Satellite temperature measurements difficult to compare with surface measurements

In addition, it is important to note that satellite and surface measurements are difficult to compare. This is due to the fact that satellite series are based on the temperature of the entire lower troposphere (the lowest 5 km of the atmosphere). The temperature of this atmospheric layer is, for example, much more sensitive to El Niños than surface temperatures are. This is illustrated in the graph below by the relatively high peak for the two satellite series at the time of the super El Niño in 1997–1998 and the less strong El Niño of 2010. The reverse is the case for La Niñas, such as the strong one of 2008; here, satellite series typically show a lower temperature.

PBL - temp comp - 0040_002g_adhoc
Temperatures according to 2 satellite series (UAH and RSS). The purple line indicates the mean of the three surface temperature series. The satellite series show peaks in 1998 and 2010, as a result of El Niño, which are greater than those in the surface temperature series. The low satellite value for 2008 coincides with the opposite of an El Niño: La Niña. Note how the last 4 years in the RSS series are far below those in the other series. According to the surface temperature measurements, 2014 was the warmest year on record!

The graph shows that the last years in the RSS series clearly deviate from the other temperature series, with lower values of over 0.1 °C. This suggests that RSS rather than UAH is too low (as also claimed by the UAH team). The outcome of this discussion may lead to adjustments to one or both satellite series, as has been done in the past, particularly to the UAH series, on numerous occasions.

The surface temperature series further indicate that 2014 was the warmest year on record, even without an El Niño!

Contribution by cooling and warming influences.

Strengers indicated at the time that ‘in light of the scientific uncertainties, I may lose, but this is not likely to happen’. He gave four reasons why a possible reduction in warming, or even a cooling could occur. Bold indicates that the related reason more or less became a reality over the past 5 years.

  • a continued (relatively) low solar activity;
  • a relatively high heat absorption by the (deep) oceans;
  • a period of cooling due to incidental variations in the climate;
  • lower climate sensitivity than expected.

In addition, Strengers gave three reasons why he nevertheless expected to win:

  • a further increase in greenhouse gas concentrations in the atmosphere;
  • the ‘best-estimate’ by the IPCC is that of a warming of 0.2 °C per decade;
  • the chances of overestimating climate sensitivity are smaller than those of underestimation.

The sum of all factors, thus, has led to continued warming. Below each of these factors is explained in more detail.

Continued (relatively) low solar activity

Over the past 5 years, the reduced solar activity has continued and, thus, likely also has slightly reduced global warming over that period. In the discussion at the time, Strengers wrote: ‘astrophysics […] cannot rule out the possibility of a long period of relatively low activity. This could lead to a reduction in warming of up to 0.4 °C (although 0.2 °C is more likely) over the coming 20 to 30 years.’ The past 5 years, therefore, are in keeping with the idea that such a period of relatively low activity is a fact, but the degree to which this reduction will actually continue over the coming years, or for how long it will go on, is still very uncertain.

Relatively high heat absorption by the (deep) oceans

Over 90% of the heat that is added to the climate system, particularly caused by the increase in greenhouse gases, ends up in the oceans. Only a few per cent is stored in the atmosphere. The remainder is absorbed by the land surface and ice sheets (which are therefore steadily melting). Variations in heat absorption can have a large impact on surface temperatures. According to a recent study by England et al., published in December 2013 in Nature, there has been increased heat absorption by the oceans since 2001, which since then has reduced warming by 0.1 to 0.2 °C. The added heat seem to be concentrated largely around the equator in the western part of the Pacific Ocean, at a depth of around 125 to 200 metres, which means it remains ‘hidden’ from the atmosphere. England and his team do not expect this heat storage effect to continue in this way and they project that, at a certain moment, temperatures at the surface level will begin to increase more rapidly. This could happen, for example, due to an El Niño with large amounts of heat being released suddenly, possibly causing temperatures to jump, as happened in 1997–1998 during the so-called super El Niño. Over the past months, a new El Niño seems to be developing. If this continues into 2015, this year may end up being even warmer than the record year of 2014.

A period of cooling due to incidental variations in the climate

The climate knows random variations. Strengers wrote that these may lead to longer periods of no warming or even cooling, even under a steady increase in greenhouse gas concentrations in the atmosphere. During the discussions, Strengers pointed to a study which shows on the basis of climate models that periods of up to 16 years of random cooling or non-warming may occur, even in an overall warming climate. Recent research shows that a combination of random factors likely has led to a reduction in temperature increases over the past 15 years (see the section below, ‘IPCC’s ‘best-estimate’ is that of a warming of 0.2 °C per decade’, for more details). However, this reduction in warming was not high enough for the past 5 years to be cooler than the decade before that.

Lower climate sensitivity than expected

The IPCC – the scientific body that inventories all knowledge on climate change every 5 to 7 years –stated in 2007 in its fourth assessment report (AR4) that climate sensitivity was likely (i.e. with a likelihood of 66%) between 2.0 and 4.5 °C, with a ‘best estimate’ of 3 °C. The fifth assessment report (2013) stated a range of 1.5 to 4.5 °C without giving a ‘best estimate’. The reason for the downward adjustment of the lower limit to 1.5 °C (at which it had been estimated since 1990) originated from a number of studies that pointed to the possibility of a low climate sensitivity. The ‘best estimate’ was not provided “because of a lack of agreement on values across assessed lines of evidence and studies” (i.e. based on all studies up to and including July 2012). All this, however, does not mean that climate sensitivity was ‘less than expected’. In fact, the only thing that can be concluded is that the value of climate sensitivity has become more uncertain.

Further increase in greenhouse gas concentrations in the atmosphere

Greenhouse gas concentrations in the atmosphere have steadily increased over the past 5 years. By late 2014, CO2 concentrations were at 399 ppm (399 molecules of CO2 per million molecules of air). Five years ago this level was 388 ppm. The increase is a direct result from an ever faster increase in CO2 emissions, particularly in countries such as India and China.

IPCC’s ‘best-estimate’ is that of a warming of around 0.2 °C per decade

At the time of IPCC’s fourth assessment report, in 2007, a global warming of 0.2 °C was assumed for the current decade (2010–2019), particularly on the basis of climate model results. As discussed above, the degree of warming according to the UAH series, which is based on satellite measurements, was 0.1 °C over the last 5 years, compared to the mean of the 10 years before that. If this trend continues over the coming 5 years, our current decade will register a warming of around 0.15 °C – slightly less than the ‘best estimate’, but well within the projected range by the IPCC. However, all surface temperature series show a lower degree of warming, between 0.04 and 0.05 °C, over the past 5 years (see the section on ‘What causes the differences between the data series?’). Extrapolation over the 2010–2019 decade shows a total maximum warming of 0.08 °C [typo fixed]. This is in line with the discussion on the ´hiatus´ or the finding that the rate of warming over the past 15 years has been lower than in the 20 years before that, and also lower than the average outcome of many climate models. Note though that there is no significant change in trend from 1998. If climate model calculations take into account the ´random factors´ that cannot be predicted, such as the occurrence of El Niños, solar activity, and volcano eruptions, then models and observations seem much more in agreement.

The chances of overestimating climate sensitivity are smaller than those of underestimation

The IPPC’s fifth assessment report (2013) states that climate sensitivity is likely (66% probability) to be between 1.5 and 4.5 °C. It subsequently states that it is extremely unlikely (less than 5% probability) to be smaller than 1, and very unlikely (less than 10% probability) to be higher than 6.  In other words, very low values are less likely than very high values, which substantiates the above statement.

[Note: hyperlinks added by Bart Verheggen]

Open thread – Winter 2014/2015

December 3, 2014 by

For any climate related discussion – please be civil and minimize repeating yourself. It’s okay to agree to disagree.


Get every new post delivered to your Inbox.

Join 150 other followers