What weather would falsify the current consensus on climate change?
In January, Roger Pielke Jr. asked
What behavior of the climate system could hypothetically be observed over the next 1, 5, 10 years that would be inconsistent with the current consensus on climate change?
Meanwhile, Gavin Schmidt explained that we really can’t look at short-term weather to assess models. While Gavin’s answer is perfectly correct , it falls short of answering Roger Jr.’s specific question: Roger would like to know if we can state, in advance, any sort of weather that is inconsistent with the ‘current consensus’ on climate change.
In fact, no matter how variable weather, it is possible to answer Roger’s question– provided it is made a bit more precise. In my opinion, the plot below answers the question:
“What trend in GISS Land/Ocean temperatures over the next 5, 8 or 10 years, would be inconsistent with the most recent IPCC projections of climate change?”
This question can be answered, because it nails down a metric– GISS Land/Ocean, and specifies the “current consensus” with projections that are published, and, so, knowable. The answer is a bit complicated, since the IPCC provided a range of projections, described the probabilities in somewhat vague terms and has different projections for both short and long terms trends.
In my opinion, the short answer to the question is: If, the weather is such that an ordinary least squares fit to GISS Land/Ocean data for the next decade shows any negative trend, this would be inconsistent with the IPCC’s short term projection for temperature which appears to be 2.0°C per century.
A trend of 2.0°C per century, evidently, represents the mid-point of model projections based on SRES scenarios. So, in some sense, falsifying a 2.0°C per century predicted trend would amount to falsifying the current GCM’s prediction for the central tendency, while accounting for the range of uncertainty introduced by weather.
Details about IPCC spread.
In reality, the IPCC provides a range of predictions/projections for warming trends, the magnitude of the projected temperature trend varies both according to time frame and emissions scenario,. The most commonly cite values are increases expected to occur during the 21st century: The full range, including the most extreme scenarios is 1.1°C/century to 6.4°C/century. The most probable range is said to be 1.8°C/century to 4.0°C/century, with, evidently 2.0C/century representing the midpoint of SRES model projections.
After performing a statistical analysis, I plotted the trend in GISS Land/Ocean temperatures trends calculated using ordinary least square (OLS) using data collected over the next 5,8 and 10 years that I think would be inconsistent with the IPCC range of projections in climate change. That plot is shown below.
In today’s blog post, I’ll explain how to read the graph. Some time soon, I’ll explain how I concocted the graph. I may also post a few other graphs and explanations of statistical tests next week.
How to read the graph.
First, assume we believe “consensus” position is that global mean surface temperatures (GMST) will rise at a rate of 2.6°C per century — the midpoint of the “most probable range” suggested by the IPCC. Further, let’s assume we believe temperatures are already rising at 2.6°C per century. Then, of course, one would expect that the probable rate of rise for GMST over the next decade is about 2.6°C a century, right?
But, as Gavin correctly pointed out, due to weather noise (and volcano eruptions and other unpredictable phenomena), even if the underlying trend due to AGW is, 2.6°C/century the trend in temperature averaged over decade might be higher or lower than 2.6°C/century. But, how much higher or lower?
This question can be answered using some statistical reasoning- and I’ll explain that later. For now, let’s assume my method is sound and read the answer of my graph.
Draw a vertical line from “2.6°C/century” — the supposed “consensus” value, stop at the yellow line corresponding to a 10 year trend, and then read “0.6°C” to find the lowest possible trend over 10 years that is ‘not consistent with’ 2.6°C/century being true.
So, according to my graph, a mean temperature rise of 0.6°C obtained by Ordinary Least Squares (OLS) using data collected during the upcoming 10 years is “not inconsistent” with a prediction of 2.6°C/century.
In contrast, if the measured trend over the upcoming 10 years, is less than 0.06°C over 10 years, then this will be inconsistent with an assumed trend of 2.6°C/century .
(For those wondering, a measured trend of 0.6°C a century measured over 10 years also ‘not inconsistent with’ the real trend being 0°C/century. Weather being what it is, all sorts of trends are ‘not inconsistent with’ a variety of hypotheses. )
Other values read from the chart are also provided below.
| Rise per Century | Minimum rise per century over period. | ||
| 5 year | 8 year | 10 year | |
| 1.1°C | -6.3°C | -1.9°C | -0.9°C |
| 1.8°C | -5.6°C | -1.2°C | -0.2°C |
| 2.0°C | -5.4°C | -1.0°C | 0.0°C |
| 2.6°C | -4.8°C | -0.4°C | 0.6°C |
| 4.0°C | -3.4°C | 1.0°C | 2.0°C |
| 6.4°C | -1.0°C | 3.4°C | 4.4°C |
For any given predicted or projected rate of temperature increase show in the left hand column, you can read the minimum rate of temperature increase, calculated using ordinary least squares, measured over 5, 8 or 10 years periods is ‘not inconsistent’. Measured rates below the appropriate value to the right are inconsistent with the projected value.
Oh, by the way. My analysis includes variability due to volcanic eruptions. So, and I’ve made choices that make it difficult to falsify. I’ll explain how I did this later. Few people want to read math unless they are interested in understanding a result, but I strongly suspect others will want to understand what I did so they can figure out if there are any analysis choices they would dispute.
So, is there any weather that could falsify IPCC projections?
Yep, in principle, some sorts of weather could falsify IPCC projections.
Unfortunately, it’s not very easy. Weather, even when averaged over 10 years is so highly variable, that the range of weather that is ‘not inconsistent’ with even 10 years worth of projections is quite large.
In particular, if the trend over the next 10 years falls below “0°C/century” of warming, this will be inconsistent with 2°C/century of warming, which appears to be the midpoint of model predictions for the next two decades. (See Wikipedia; look for discussion of “SRES” projections.)
It is a bit ironic however, that the break even point for ‘falsifying’ the IPCC claim happens to correspond precisely to no-warming. This was, as it happens, a complete coincidence, but there you go!
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44 Responses to “What weather would falsify the current consensus on climate change?”
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John V February 27th, 2008 at 3:46 pm
lucia,
Very interesting post. Another potentially useful way to look at this would be to find the range of underlying temperature trends that is permissible given the observated OLS trend. Basically read your graph backwards. Start with the observed trend and find the range of permissible trends.
Using your example above, if the observed trend is 0.6C/century for 10 years then the maximum underlying trend is 2.6C/century.
Assuming your algorithm is symmetric, the “minimum rise” could be easily augmented with a “maximum rise”. Assuming symmetry and using your example again, an observed trend of 0.6C/century for 10 years is compatible with an underlying trend between -1.4C/century and 2.6C/century.
Used in this way, the results could falsify claims in both directions.
lucia February 27th, 2008 at 3:49 pm
John V–
This is a one tailed test, so the error bars aren’t symmetric. My reasoning is: If the trend is higher than claimed by IPCC, no one calls them “falsified”.
John V February 27th, 2008 at 4:51 pm
lucia,
I think it’s interesting to attempt to falsify in both directions. Claims that warming has stopped could be falsified with the other side of the test. That adds a little spice to the analysis.
lucia February 27th, 2008 at 5:06 pm
John V–
Sure. And if, in 3 years, there is enough data to provide one falsification, you can be sure arguments for the two -tailed test will kick in!
Right now, there is not enough data to achieve any sort of falsification. I’ll probably post the test your asking for— but even before that, I plan to discuss β error– a topic that will surely make all readers eyes glaze over!
Steven mosher February 27th, 2008 at 5:53 pm
Lucia, To be clear. radiative physics would not be disconfirmed. ( dont go C02 loony on me)
lucia February 27th, 2008 at 6:00 pm
Steven–
The radiative physics would not be disconfirmed. Also, rates of temperature increases less than 2.0C/century would not be disconfirmed.
As I see it the arguments aren’t over whether or not AGW is possible at all the questions are: What’s the rate? Are the IPCC estimates on the high side? The low side? Or about right?
Paul Linsay February 27th, 2008 at 6:15 pm
Lucia,
Nice discussion. The problem of course is that the simple GMST buries a lot of information about an extremely complex system. A far better test would be to have the modelers publish global temperature maps for the next five to ten years in advance with error bars. RSS and UAH would then independently compare the predictions with the satellite measurements of temperature.
David, 831. The only test that matters is (1), it’s a genuine prediction. The other two can be fudged by a model with enough tunable parameters.
lucia February 27th, 2008 at 6:23 pm
Paul– Sure. But there is no mechanism to force scientists to publish anything in particular. Currently, the main metric used by the IPCC, modelers and pretty much everyone prognosticating is GMST. So, really, that’s what has to be used.
For the most part, if AGW is such that the trend is greater than 2C/century (or less), this fact would be more-or-less reflected in all possible metrics. These things will be at least somewhat positively correlated. You might confirm or falsify a signal faster with one or the other, but they’ll all tend to move the same way.
Paul Linsay February 27th, 2008 at 6:31 pm
Lucia,
That’s not obvious to me. The entire increase could be due to a few hot spots like Waldo that Steve McIntyre keeps searching for in the temperature records. Some of the models produce unphysical results, freezing temperatures along the equator is one I’m aware of.
lucia February 27th, 2008 at 6:37 pm
Paul Lindsey-
Fair enough. But when validating projections, I focus on measured temperatures for the real earth, not model predictions. Also, statistically, we get much faster falsification or validation with global measures like GMST.
In principle, if we look at a local temperature– like my back yard near chicago, we will eventually see validation or falsification, but it takes much longer because the variability of any local measure is larger.
There is a limit where, if the whole world incrases 6C, you aren’t likely to see many spots getting colder.
Ian Castles February 27th, 2008 at 8:05 pm
Lucia, In your comment 827, you say that ‘A validation of 2000-2010 against data would involve using 70% data that were already incorporated into the “prediction”’.
With respect, I don’t think that this is so. The IPCC “prediction” in question is based on simulations which make use of the projections of future emissions specified in the Panel’s Special Report on Emissions Scenarios (SRES). This Report was approved and published by the IPCC in 2000. As the Panel decided at its Plenary Meeting at Vienna in November 2003 that ‘the SRES scenarios provide a credible and sound set of projections, appropriate for use in the AR4’, NO post-2000 data have been incorporated into the ‘prediction’ that you are seeking to test. It is therefore valid to use post-2000 data to test the ‘prediction’ for 2000-2010 in AR4.
As there has been a good deal of misunderstanding of the matter at issue, I’ll key in here the text of a letter sent to the Chair of the IPCC, Dr. Pachauri, by Dr. John Mitchell on 30 October 2002. Dr. Mitchell was writing in his capacity as Chair of a Group which included representatives from all of the major modelling groups, and his letter was tabled at the 28th Session of the IPCC Bureau in Geneva in December 2002.
“The World Meteorological Organisation JSC/CLIVAR Working Group on Coupled Models (WGCM), which includes representatives from almost all the major climate modelling centres contributing to the Third Assessment Report, recently considered what work needed to be done to ensure that the best modelling advice will be available for the next IPCC Assessment, due to report in 2007. For each emission scenario, it is necessary to run an ensemble of simulations to define the uncertainty due to natural variability, and to do this with as many models as possible to define the range of uncertainty in modelling the earth system. These uncertainties mean that there is little scientific justification in running new scenarios since the resulting climate change outcome is unlikely to be indistinguishable (sic) from existing scenarios with similar radiative forcing. Hence the WGCM unanimously urge IPCC to retain the current SRES scenarios without change, to make sure a sufficient number of model runs are available for the next assessment;
- to define the uncertainty range associated with current scenarios; and,
- to ensure that these simulations are available in time in a wide range of impact studies in the 4AR.
“We appreciate that small changes in the emission scenarios may require large economic and social changes, and that the effect of the social and economic changes could be assessed in time for the next report. However, unless the accompanying changes in radiative forcing are likely to produce detectable changes in climate, we believe that [it] is better not to try and run new model experiments, but to stick to the scenarios used in the TAR. This will allow a better definition of the range of uncertainty in projected changes due to model uncertainty and natural variability, which are likely to dwarf any difference due to tweaking the existing emissions scenarios. This we believe will provide the best scientific basis for the next IPCC Assessment. Please feel free to contact me if you need clarification or further information.”
Thus the evidence suggests that the IPCC retained the SRES scenarios without change in order to allow “a better definition of the range of uncertainty in projected changes due to model uncertainty and natural variability” - i.e., in order to facilitate the type of analysis you are undertaking. The fact that the IPCC authors “knew” of changes in estimated observed temperatures after 2000 is irrelevant, because these data were not incorporated in the projections. I think that your comments 826 and 827 reflect some misunderstanding of the way in which the temperature ‘predictions’ in AR4 were produced.
.
Paul Linsay February 27th, 2008 at 8:08 pm
Lucia,
A last comment and then we’ll just agree to disagree. There are an infinite number of wrong temperature maps that will produce a given trend but there’s only one map that matches the physical climate. I think that maps would quickly sort out which, if any, of the models are correct. My bet is none of them.
I looked up Lisle, IL. Do you work at Argonne or FermiLab? I spent time at both in the long ago.
kim February 28th, 2008 at 3:13 am
I agree with Ian. Foolish as it may seem, I don’t think they included 21st century data.
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lucia February 28th, 2008 at 4:41 am
Paul–
I agree there are lots of maps. I guess if you had one that was some how “the official map” and we could do the work to falsify or validate against the GISS full earth temperatures. But…. it’s a lot more work for a blogger just doing this as a hobby.
Still, it appears that bloggers doing things as a hobby can at least talk about HOW it could be done.
I do, indeed, work for ANL. But part time, which gives me lots of time for hobby blogging.
lucia February 28th, 2008 at 4:42 am
@Paul–
Ok. Then, if these predictions were made in 2000, then 2001 is a fair start date to validate. That’s convenient, because we don’t have to wait 10 years to start!
kim February 28th, 2008 at 6:37 am
And so, what kind of temperatures would it take in the next three to four years to keep their prediction from being invalidated? I know I’m on thin ice, here, and my knowledge base is leaden rather than bouyant.
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lucia February 28th, 2008 at 6:58 am
Kim,
This means the answer I already got is unchanged.
In contrast, if they’d said solar variations did still matter, and the sun got ’stuck’ in a maunder, they could attribute a flat spot to the sun.
What Does NASA Mean by Solar Variations Don’t Matter? | The Blackboard February 28th, 2008 at 7:59 am
[...] Comments: What weather would … [...]
Steve Milesworthy February 28th, 2008 at 9:29 am
Fascinating stuff. It could mean that the debate might be going on for quite a while longer, but I guess better decadal prediction will tie it down before then.
The range of 2100 projections is based on scenarios with different changes in emissions through the century. All scenarios have similar projected emissions and therefore the midrange projected rise up to 2030 is about 2.6C/century. Therefore it would be valid to take the 0.6C/century target over the next 10 years rather than the zero warming target.
lucia February 28th, 2008 at 9:55 am
Steve,
Realistically, some aspect of the debate will go on forever. Even after we all agree on the science, there is still the question of what policy would best solve any problems.
The 2.0 is in the guide for policy makers. So, that’s why I picked it. Otherwise, I would also have picked 2.6 C/century.
Philip_B February 28th, 2008 at 2:07 pm
lucia, I’m not sure Gavin is right about weather being noisy.
Of course, at any given location weather is noisy, but we are talking about the aggregate weather across the entire planet. How noisy is that?
Given that weather is just heat and moisture moving across the Earth’s surface. I don’t see how there can be much noise in it (over the entire planet).
Most of the ‘noise’ will result from measurement issues, i.e. we can’t accurately measure the temperature across the entire planet.
And I’d add, that if the noise isn’t due to measurement issues then the Earth’s atmospheric heat content is fluctuating up and down (ignoring seasonality), which would seem to invalidate the IPCC’s Forcings model, or perhaps show there are significant forcings not recognized by the IPCC.
lucia February 28th, 2008 at 2:19 pm
How noisy?… it depends what you call “noise”. Weather is variable, or “fluctuates” or what have you. Gavin likes the word weather noise to describe variability. I’m not sure I’d use that precise word, since the weather is the weather. Still, when posting on a blog and trying to describe the difference between climate and weather noise is as good a word as any.
How big is the noise in GMST? Counting measurement and weather “noise” together, the residuals to straight line fit to annual average GMST appear to be ±0.1C. That’s my estimate of noise!
Weather– as measured by temperature, is a bit noisy, even averaged over the surface of the planet and over a year. After all, there is heat in the ocean etc. Given the equations that govern weather, it should be noisy– the only question is how noisy, and on what time and spatial scales.
Fluctuations in surface temperatures don’t falsify the IPCC. We expect temperature to fluctuation around a “mean” value even when constant forcing is applied. We see the same things in engineered systems involving transport of mass, momentum and energy. It just happens.
Andrew February 28th, 2008 at 3:05 pm
David, two things. Firstly, we are talking about GISS here (technically unfair, at least HadCrut would make since as it’s used by the IPCC, as far as I’m aware) which has plenty of positive trendiness in the last ten years (although a quick look at the monthly anomalies from Jan 1998-Jan 2008 in the RSS lower troposphere data reveals a very slight negative trend(and yes, I did actually do the linear regression for this, but I already knew since I had created a similar chart the month before with basically zero trend). The second thing is that the point is to have about twenty years, I think, of basically trendless data (no? I actually think not, becuase then the trend in the next ten years would have to negative, strongly, to overcome the positive trend in the last ten years of GISS).
Interesting to know what would be inconsistent with the most dire predictions, but I should add that a positive ten year trend wouldn’t necessarily prove the catastrophic point. Pat Michaels tends to argue that warming will probably be exactly on the long term linear trend line, which wouldn’t be much at all.
Andrew February 28th, 2008 at 3:08 pm
Opps, didn’t realize the discussion here went further than I read. Ignore the above comment.
MarkR February 28th, 2008 at 4:49 pm
Hi Lucia
The maths here is way out of my league, but my question is, if the IPCC current predictions for mid rate of temp growth were applied to the last5, 8 and 10 years for which data have been available. Would the IPCC forecast have been falsified?
lucia February 28th, 2008 at 4:53 pm
Mark R:
No, no and no.
With respect to the 5 years– there is just too short an amount of time.
Philip_B February 28th, 2008 at 5:22 pm
Having now read Gavin’s post, he basically asserts there is weather noise at the global scale and then goes on to discuss sampling issues, which for me is just a subset of measurement issues.
Anyway, let’s assume Gavin is right and aggregate global weather is noisy/chaotic and we do have sampling problems, then it would appear to be a simple problem (simple theoretically, not necessarily logistically). Take truly random samples of temperature over the earth’s surface.
My knowledge of statistics is fairly basic, but I don’t believe it would take a large number of random temperature measurements to get a measure of global temperature with a high statistical confidence. This would at least answer the question - How noisy is global aggregate weather?
Or am I missing something here?
BTW, I agree with you. The only significant source of (aggregate) weather noise I.e. not external forcings) I can think of is variations in ocean/atmosphere heat exchange (and perhaps water exchange).
lucia February 28th, 2008 at 7:13 pm
Phil_B.
Theoretically, the issue of getting random samples is easy. But, for climate science, as a practical matter, it’s a pain in the neck.
If this were a lab experiment, you just wait a long enough time between data samples to ensure each is uncorrelated with the previous sample. The problem is, with the earth’s climate, you need to wait about 3 years to space samples in a way that ensure they are independent.
In a lab experiment studying pipe flow, you might work .1 second.
So, you can see the problem.
David Stockwell (34 comments.) March 7th, 2008 at 10:05 pm
Hi Lucia, comments arising from this post can be found here:
http://landshape.org/enm/surfa.....he-trends/
Cheers
IPCC Projections Overpredict Recent Warming. | The Blackboard March 10th, 2008 at 7:53 am
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