Tuesday, August 24, 2010

Climate Alarmism at Science Magazine?

I feel obliged to begin this piece with a disclaimer.  I am not a climate sceptic.

That said, I believe that the case for climate change, and action on climate change, should be made with good science and honest arguments.  And in that spirit, there's a paper in Science (along with Nature, one of the worlds most prestigious science journals) last week that's bothering me quite a lot.  The abstract says this:
Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.
In other words, the claim is that, whereas in the past, increasing amounts of carbon dioxide was causing land plants to fix more carbon (and by implication grow more and better), now land plants are overall fixing less carbon because the planet was getting hotter and drier in the 2000s.

That's obviously scary, since all 6.5 billion (and on the way to 9 billion) of us humans have to eat from those plants, and if they are drying up and doing less well because of climate change, that's clearly a trend in the wrong direction.  And sure enough, various newspapers, magazines, and blogs picked up this paper. Here's the UK Guardian:
Rising temperatures in the past decade have reduced the ability of the world's plants to soak up carbon from the atmosphere, scientists said today.

Large-scale droughts have wiped out plants that would have otherwise absorbed an amount of carbon equivalent to Britain's annual man-made greenhouse gas emissions.

Scientists measure the amount of atmospheric carbon dioxide absorbed by plants and turned into biomass as a quantity known as the net primary production. NPP increased from 1982 to 1999 as temperatures rose and there was more solar radiation.

But the period from 2000 to 2009 reverses that trend – surprising some scientists. Maosheng Zhao and Steven Running of the University of Montana estimate that there has been a global reduction in NPP of 0.55 gigatonnes (Gt). In comparison, the UK's contribution to annual worldwide carbon dioxide emissions was 0.56Gt in 2007, while global aviation industry made up around 0.88Gt (3%) of the world total of 29.3Gt that year, according to UN data.
and here's Climate Progress, with the characteristically blaring headline Science shocker: Drought drives decade-long decline in plant growth, which concludes:
And so we have another potential of an amplifying feedback in the carbon cycle.

This new study is the latest in a series of truly alarming scientific studies published just this year that suggest we may be closer to dangerous carbon-cycle tipping points and irreversible thresholds than anyone realized...
Ok, so I go to read the paper itself with a view to blogging about the latest data point in the overall scary trend of humans poking the angry beast with sticks.  Now, before we go on, let's take apart the paper abstract above - what are the main claims this paper is making?  The first sentence says:
Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass.
Ok, so that's just defining what we're going to be talking about here - the total amount of carbon which plants extract out of the air and fix into their own tissues (as opposed to immediately using in their own respiration: including that would make it gross primary production instead of net primary production).

Next we get to the first major claim of the paper:
Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon.
Ok, so NPP used to be trending up, now it's trending down. We even have a number for how much it's trending down: 0.55 petagrams between 2000 and 2009 (petagrams = Gigatonnes, billions of metric tonnes of carbon).  So that's the big claim of the paper.  Whereas the world's plants used to be going from strength to strength, at least as measured by total mass, now they are starting to get in trouble.

Note that there's no error bar quoted on that "0.55", a point we will come back to.

Now the rest of the abstract:
Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere.
So the reason for this trend reversal is droughts, especially in the southern hemisphere. And finally:
A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.
This is scientist speak for BAD AND SCARY.  A) if plants stop absorbing so much carbon because of climate change, our own emissions will be buffered less, giving rise to a nasty vicious circle of escalating CO2 in the atmosphere, and B) we humans need/want those plants for own ends, and it's not going to be so great if there are fewer of them.  Hence the Climate Progress headline of "Science Shocker", etc.

Ok, so I get into the body of the paper (which unfortunately you can't read unless you have a subscription or buy the paper from Science), and in due course I come to the argument for why there is now a reduction in NPP (the 0.55 petagrams, remember):
Satellite data can generally provide realistic information on vegetation dynamics, including land cover change (5, 6), disturbances, and recovery (7), which may help to reduce uncertainties in carbon-budget estimates. In this study, we investigate terrestrial NPP and climate variability over the past decade (2000 to 2009) by analyzing satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite and global climate data.

We used the global MODIS NPP algorithm (8) [see supporting online material (SOM) text S1] to examine spatially explicit NPP changes from 2000 through 2009. We used collection 5 (C5) 8-day composite 1-km fraction of photosynthetically active radiation (FPAR) and leaf area index (LAI) data from the MODIS sensor (9) as remotely sensed vegetation property dynamic inputs to the algorithm. Data gaps in the 8-day temporal MODIS FPAR/LAI caused by cloudiness were filled with information from accompanying quality-assessment fields (SOM text S2) (10). For daily meteorological data required to drive the algorithm, we used a reanalysis data set from National Center for Environmental Prediction (NCEP) (SOM text S3) (11). A Palmer Drought Severity Index (PDSI) (12) at 0.5° resolution was used as a surrogate of soil moisture (13) to measure environmental water stress by combining information from both evaporation and precipitation (SOM text S4). A lower PDSI generally implies a drier climate.

Global NPP slightly decreased for the past decade by –0.55 Pg C (Fig. 1). Interannual variations of the global NPP were negatively correlated with the global atmospheric CO2 growth rates (correlation coefficient r = –0.89, p < 0.0006) (Fig. 1) (14), suggesting that global terrestrial NPP is a major driver of the interannual CO2 growth rate. Carbon isotopic measurements have indicated that the exchange of CO2 with terrestrial ecosystems is the dominant cause of the CO2 interannual growth rate (15). Though NPP is a part of carbon exchange between the land and atmosphere, the strong correlation may imply that the process of heterotrophic respiration depends ultimately on the substrate supply from NPP (16), as shown by the strong correlation between gross primary production (GPP) and ecosystem respiration derived from eddy-flux towers (17). Although also a major contributor, global fire emissions do not correlate as strongly as NPP with interannual CO2 growth rate (SOM text S5) (18).
My emphasis.  The bolded sentence is the main argument in the paper for why the 0.55 reduction is true, and it pretty much entirely consists of a reference to Figure 1.  Ok, so Figure 1 looks this:


And the caption in the paper in the paper reads: Interannual variations from 2000 through 2009 in anomalies of annual total global terrestrial NPP (green circles) and inverted global atmospheric CO2 annual growth rate [red squares and (14)]. Global average annual total NPP is 53.5 Pg C/yr.

Ok, so we're looking at the green curve.  And the claim is it's trending down by 0.55 in the units on the left hand side.  Now, there's a major problem here to my way of thinking.  The authors didn't give us a trend line, but one thing that's immediately clear is that the bouncing around from year to year in that green curve is quite large compared to any overall trend in it.  So that raises the question: are we really sure that it's genuinely trending down?  Or is it just bouncing around and happened by chance to be a bit lower at the end than the beginning?  To give you some feel for this, I took the numbers off their graph (which might be slightly inaccurate but that will be small compared to the noise in the data), and made my own graph, along with a trend line:



The trend line is in black.  My numbers come out to a slope of -0.05 GT/year, which is -0.045 -0.45 over the nine years.  That's the reduction in NPP.  However, to illustrate the concern about the robustness of this trend, suppose it had happened that the satellite was launched a year later, so the 2000 data point was missing:


Oops!  Now it's going up!  So in other words, this "trend" of "reduction" in NPP is extremely sensitive to exactly which data points we look at.  It's not a robust trend, but rather is sensitive to noise.  Suddenly our 0.05/year reduction has become a 0.03/year increase.

Now, there is a whole long history in science of how to assess this kind of thing - to test whether a particular number or trend is statistically significant, as opposed to the situation where it could well just be a fluke.  It's a normal part of doing science that one analyzes the statistical significance of trends, and analyzes the likely range of uncertainty around a particular estimate.  However, in this paper, there is no analysis in the paper of whether the reduction in NPP is statistically significant, and, as I noted, no error bar is provided for the "0.55 petagram" reduction.  

I find it very strange for a paper published in a top journal to provide no evidence that the main claim in the abstract is actually statistically significant.

So I'm driven to doing my own analysis.

The simplest possible method is based on the theory of ordinary least squares.  I entered the data above (including the 2000 point) into an online regression calculator here, and it tells us:


Look at the "Slope" part.  The slope is -0.05 (in agreement with what Excel came up with in the graph above), but the standard error in the slope is 0.1 - in other words the negative slope is less than the expected uncertainty in the slope, and the result is very much not statistically significant.  Roughly what this means is that, given this level of year-to-year noise, even if there was no real trend up or down, we would expect to get a trend reading between -0.1 and +0.1 about 2/3 of the time.  So the -0.05 trend is entirely consistent with just being noise, and is not good evidence of a reduction in NPP.

I should stress here that we are not doing advanced theoretical ecology here.  This is the kind of stuff scientists learn in their first undergrad stats class, or in my case, in first year undergraduate physics labs.  Uncertainty analysis, OLS regressions for straight lines, etc, is absolutely basic stuff if one is a scientist.

So, I read the rest of the paper to see if I'm missing something, and when it seems like I'm not (the rest of the paper is mainly concerned with the drought/NPP connection and is quite interesting), I email the authors of the paper.  I start off with Maosheng Zhao, the listed correspondence author, who is a research scientist with the Numerical Terradynamic Simulation Group at University of Montana, Missoula.  My initial email was as follows:
Hi:

I blog about energy/resource issues at

http://earlywarn.blogspot.com/

I read your Science paper on the decline in global NPP in the 2000-2009 timeframe.

I notice that you don't seem to have any analysis of whether the decline (the green line in your Figure 1) is statistically significant given the substantial year-to-year fluctuations. A quick OLS suggests that the slope of that data is -0.05 +/- 0.1 GT/yr, which certainly doesn't suggest statistical significance for the decline. Taking account of auto-correlations would seem like it could only make it less significant.

Am I missing something? Did you assess the statistical significance of the trend somehow?

Thanks,

Stuart Staniford.
So, given that this is not my field, I was kind of expecting that they'd come back and point out that I'd missed the importance of the paragraph on page XYZ in the supplemental material, which explained how they analyzed the trend a different way and it really is significant.  Instead, Maosheng gets back to me essentially saying that yes, they agree with me that the data isn't statistically significant, but this is all the data that's available from the satellite.  He and I go back and forth a couple of times, and then he brings in his boss Steve Running, the second author on the paper, who is a Professor at UMT, and the Director of the Numerical Terradynamics Simulation Group.  He gave me permission to quote his emails verbatim, so here is the first one:
Hello Stuart;

We have a continuing difficulty in Earth science when doing time series
research of how to separate real system interannual variability from
measurement error. And the problem is even worse at global scales where
there is only one dataset from only one instrument. The large real world
variability generates a very high statistical threshold of significance for
defining changes in trend trajectory. Because terrestrial carbon sink issues
are of such high policy significance, we felt that this new global trend was
an important message that could not wait longer to give a first look. And
obviously Science magazine agreed. It will probably take another decade of
data to answer this question to full statistical satisfaction, which will
then be with a new satellite, as they don't last that long. So then we will
have to wrestle with changing sensor characterstics too. There are also a
number of other papers from other groups with different methodologies
showing similar trends in related variables, adding to our confidence that
these trends, for now, are real.

Sincerely,

Steve Running
To which I responded:
Hi Steve:

I appreciate you guys taking the time to respond.

I completely understand the "this is all the data we have and it's an important problem" line of reasoning, and that that might well justify publication of a paper, but I still find it very surprising that the paper lacks an explicit discussion of the statistical limitations of the evidence supporting the main claim of the abstract (and certainly, as a longtime subscriber, my surprise extends to the Science editorial and refereeing process). By my calculation, just on the data in the paper, you've got something like a 5-10% chance of being wrong about the "there is a trend break" part, and more like a one third chance of being wrong about the "NPP is now trending down" part. That's weaker evidence than most fields of science will accept as clear evidence of an important claim. The abstract is not phrased as "our evidence is statistically weak but consistent with other papers", but more like "we discovered that this trend has reversed" - that's why it's in Science, right, not some other minor journal that I would never have paid attention to?

I intend to blog about this over the weekend. May I quote your emails publicly, or would you prefer that I just paraphrase the gist?

Thanks,

Stuart.
And he responded:
Stuart;

Some research findings are so important that society really cannot afford to
wait another 10+yr for 95% or 99% statistical confidence. We (and I suppose
Science) felt this result was one of them. And recognize that we are not
advocating this result, merely reporting what we measured and why we think
it is happening. I actually hope in 10-20yr that some young scientist proves
we are wrong, and that NPP trends have turned back up. Humanity will be much
better off if that occurs.

And yes, if you feel my comments are interesting enough, you can quote them
directly, I speak publicly on these topics all the time. For Maosheng you
need to get his permission.

Steve Running
Ok.  So here we have a statistically non-robust result, that the authors are well aware is not statistically robust, being published because it's of "high policy significance".  However, and critically, the authors included no discussion whatsoever of the statistical limitations of the evidence.  The "-0.55" in the abstract is not "-0.55 +/- 1.1" or something like that to give the reader a heads up that there is a lot of uncertainty here.  There is no calculation of the "p-value" of that trend (how likely it was to occur by chance), even though the rest of the paper is littered with p-values of subsidiary results.  They know perfectly well how to calculate this, they know it's not statistically significant, but they chose to put their readers in a position where we have to take the data off the graph and do our own statistical analysis to realize what's really going on.

And the refereeing and editorial process at Science allowed the paper to be published like that.

I think that sucks.

Just to summarize, here's some things I'm not saying:
  • I'm not saying that climate change isn't real, or humans aren't causing it.
  • I'm not saying that trends in NPP aren't incredibly important, they are.
  • I'm not saying NPP is still trending up - the evidence in the paper is rather equivocal and it's hard to say for sure.
  • I'm not saying a paper on this satellite data shouldn't have been published, or that they should have waited ten years to do so.
  • I'm not even saying it shouldn't have been published in Science.
Instead, what I'm saying is:
  • The paper should have been much more explicit that this is a very preliminary result with weak statistical evidence - weaker than is normally accepted in scientific publications.  There should have been an explicit error bar on the trend.
Maybe that would have resulted in less press and blogosphere attention.  So be it.  In the end, in the great debate with all manner of moneyed interests and massive human inertia, the only things climate scientists have going for them are the truth and the moral high ground.  Those things shouldn't be thrown away lightly.

36 comments:

Sam Norton said...

Well done Stuart - you're being holy again ;-)

(BTW you might find this post of mine - tangentially related - of some interest: http://elizaphanian.blogspot.com/2010/05/time-for-reformation-of-science.html )

kjmclark said...

Wow. Yeah, you're right, someone blew that. What are we going to say in two years when the data is just as unclearly trending back up, and the deniers claim this paper was bunk and NPP is rising after all? It's hard enough getting the public to listen without journals jumping the gun.

And now we're in the predicament of either hoping future data confirms this result so that the deniers have no ammunition - with the side effect that NPP is trending down and we're in more trouble; or hoping that NPP isn't trending down yet - and the deniers get to postpone action by pointing out the scientists were wrong.

Didn't they think about that?

galacticsurfer said...

Applause all round for your correctness.

It is a shame but only the deniers get to be lying dirty bastards paid off by coal and oil industry and the scientists have to be Jesus Christ squeaky clean vest types.

Like Nero burning Christians at the stake and taunting them about turning the other cheek, some would obviously lose their temper at some point. The whole situation is pretty nasty as Truth Must Out and Time is Short.

What to do?

Burk Braun said...

Thanks very much for your excellent analysis. You should write a letter directly to science if you want this spread more widely.

I could imagine that they may have some rationale for some of the year-to-year variation that might escape just being dumped into a least-squares error analysis. But if they don't way what it is, then you are correct in your argument.

One more thing to say is that if primary productivity fails to positively respond to CO2 in a strong way, and that is what this data says, whether marginally positive or negative, then that also says some very dire things about our climate future.

J.W. Hamner said...

I'm no climate skeptic either, nor is this my field... but the authors should admit this is negative finding and present it as such. I notice that they carefully used the word "suggest" to describe their results, so it's not like they didn't know that the difference was non-significant. I guess they might argue that a trained scientist would know the difference between a "suggestion" and a true finding... but it still strikes me as fairly dishonest presentation that clearly confused some journalists.

Fixed Carbon said...

Stuart and Burk:
Note the "CO2 (inverted)" label on the green and red figure that you reproduced: when the NPP anomaly decreased, the CO2 anomaly increased.

The paper doesn't say (re Stuart's early paragraph) "whereas in the past, increasing amounts of carbon dioxide was causing land plants to fix more carbon..." The limitations on NPP discovered by these authors are inferred to be owing to decreased water availability to global vegetation. Presumably, drought was caused by increased temperature, thus evaporation and evapotranspiration increased without correspondingly increased precipitation. This is a major implication of the work because increased temperature puts more moisture into the air, ceteris paribus. This is a huge concern for agriculture, does geographical variation in precipitation change with global warming?

A decade ago, growth chamber and green house studies with crops suggested that the increasing anthropogenic CO2 would stimulate NPP and we would all be saved from ourselves! Alas, it turned out not to be true, as demonstrated by the FACE (Free Air Carbon Enrichment) experiments. NPP in the natural environment is increased by more soil water, more sunlight (day length, annual total radiation, etc), more soil nutrients such as N, P, etc, and less competition among plants (crowded root systems and tangled vegetation slow plant growth). But CO2 increases over the relevant ranges do not cause natural vegetation to fix more carbon. CO2 increases do change the character of vegetation by selecting for species adapted to higher CO2, but this apparently has not impacted global NPP yet. So, more NPP causes the CO2 anomaly to decrease, but more CO2 does not cause NPP to increase measurably.

Stuart Staniford said...

Fixed Carbon:

My interpretation of the red curve in Fig 1 is that when the NPP is lower than usual in a particular year, the CO(2) in the atmosphere jumps up more than usual (and vice versa). The high correlation between the two is basically saying that fluctuations in the NPP are the major control on the short term fluctuations in the CO(2) growth (after factoring out the trend growth in the CO(2)). I don't see a reason to question that conclusion.

Stuart Staniford said...

I should have qualified "terrestrial NPP", not just NPP. So this goes to why this trend (or lack of it) is an important question.

Alexander Ac said...

Stuart,

good post. They should have included the uncertainty into that result. And of course, if you take away one point out of 9, the trend will change. And not to say anything about the uncertainty in the global NPP estimation! However, they provide supplementary material for some explanation.

I am afraid that in 10 years we will not need any proof of statistical significance, unfortunately. Just check drought in Russia or Canada this year, or Australia last year, etc...

no paper is perfect and I agree that if it is Science or Nature, it should be more balanced.

Would be interesting to see how the trend would change after including 2010 :-)

Alex

Fixed Carbon said...

Stuart: Your response seems ok to me. What I questioned was your statement, "whereas in the past, increasing amounts of carbon dioxide was causing land plants to fix more carbon..." Burk used similar language.
Regards, Don

Stuart Staniford said...

Fixed Carbon:

Oh, I see what you are saying. Zhao and Running attibute the prior increase trend to "increasing temperature and solar radiation" and I took at least the temperature part to be due primarily to CO(2) trends and solar radiation trends to be of secondary importance. I wasn't intending to take a position one way or the other on the importance of direct CO(2) fertilization, but I see how my words could be read that way.

Wise Bass said...

Excellent post, Stuart. I probably would not have learned about this without you posting on it.

Kevin Drum made a pretty good point in addition to this over at his blog, which was basically "I can understand why they think it's important, but in the wake of the whole "climategate" debacle, we can't afford to be overly vague on stuff like this."

Lars-Eric Bjerke said...

Stuart,

Do have have a comment to the NASA article?
http://www.nasa.gov/topics/earth/features/plant-decline.html

Eric Hacker said...

Well done. Thank you for taking the time to research and write it up so thoroughly.

Mike said...

0.55 is an empirically derived number, not a statistic. If you weighed me 20 yrs ago (165) and today (180), you could conclude I gained 15 pounds. What is their to put an error bar around unless 20 years ago you put me on a scale with chickens and today you put me on a calibrated digital scale? Does it mean I'll weigh 195 in 20 more years--no, and I've made no such claim. Could I weigh 181 tomorrow? Sure, but that does not invalidate today's measure.

Whatever Climate Progress has to say is irrelevant to analyzing the claims in the paper--you conflate the two.

The authors don't give a trendline because they are not intending to say anything about the future. They are not preparing a forecast. Note in their response the address that they cannot do "trend trajectory."

Their is nothing in the paper to suggest they make more of their data than what they have. You have TRIED to read more into the paper than there is--attack Climate Progress if you have to but not the authors of this paper.

Stuart Staniford said...

Mike - your "0.55 is an empirically derived number" interpretation will not fly since the difference between their 2000 and 2009 points is 1.2 - 0.35 = 0.85, not 0.55. Even if that was the correct interpretation, it's still a meaningless number unless you distinguish what is likely due to the underlying trend and what is due to noise.

(If the abstract of the paper had been "Fluctuations in NPP driven by droughts" I wouldn't have had a problem with it.)

I mention the Guardian and Climate Progress coverage purely to illustrate the kind of attention that the paper had gotten. These outlets interpreted the paper in a way that is entirely natural, given the abstract. The problem is that the claims of the abstract are not adequately justified by the data presented in the paper.

brett said...

" My numbers come out to a slope of -0.05 GT/year, which is -0.045 over the nine years."

I believe you mean -0.45 over 9 years

Mike said...

Stuart--OK, point taken on the math. Not sure where -.55 comes from since it does look more like -.85 (typo?). But the point is the same. The abstract never claims that there is a trend. It says a "reduction," which from 2000 to 2009 it clearly is (you can also compare the 1/2 decade averages). This contrast this with an expected increase. They also are clear that they do not know what will happen in the future. "Noise" is only relevant if you are trying to make a prediction. Read my example again.

Alexander Ac said...

Hi Stuart, OT, have you seen these excellent graphs regarding food/arable land/population? See here: http://www.landcommodities.com/index.cfm?fuseaction=main.dspInvestmentFundamentals

maybe worth commenting...
Alex

Stuart Staniford said...

Brett - thanks, fixed.

Stuart Staniford said...

Mike:

I agree there is a reduction, but the point is that it's not a statistically significant one, and the norms of science dictate that they should disclose and discuss that fact.

www.gregor.us said...

Stuart thank you so much for a great post. Apart from my own email discussion about the Science/Nature magazine findings, your post is like a free and short seminar on statistical analysis, which I found incredibly useful. I'm going to save this post as a PDF, and use it as a teaching tool. Wonderful stuff.

Best,

G

Ugo Bardi said...

Stuart, good post and you are making several correct points. But I think the significance of this paper is best understood if one looks at reference 2 that they cite. That previous study reported an increase in NPP over the 2 decades from 1980 to 2000. They didn't report the standard deviation on their measurements, either but, if you compare the two studies, there is a rather evident change in trend. During the past decade NPP didn't go up any longer, but it started to flatten out and perhaps go down. That's most likely because global warming is inducing droughts and other disturbances.

So, I do think that the paper deserved to be published; which is what you are saying, too. Unfortunately, the authors have been somewhat careless in pushing this paper throughout, doing things in a hurry is never a good idea. I think I understand them: these findings ARE important. But at times scientists don't realize what kind of wolves there are, out there, ready to attack science a the smallest chance.

But these data are not only important, they are truly worrisome. They indicate that we may be seeing "peak productivity" of the world's ecosystem. It needs more data for a confirmation, of course, but you don't need to be 100% sure to be worried.

Ugo Bardi said...

You may also wish to give a look to my comment on this paper on my blog (alas, it is in Italian!)

http://ugobardi.blogspot.com/2010/08/in-calo-la-produttivita-dellecosistema.html

Stuart Staniford said...

Ugo:

I didn't put it in the post, since I was trying to keep it accessible, but I did do that analysis roughly in email with the authors, and I came up that the trend change from Zemani et al was just short of statistical significance (they have not disputed that conclusion so far). However, it's close - probably around p = 0.1 or so depending on what kind of yardage you want to allow for autocorrelation (which is likely to be present to some degree here since the NPP is heavily climate influenced and so multi-decadal oscillations like ENSO are likely to factor into it). Still, the statement "The NPP trend has changed for the worse" is on somewhat more solid ground than "The NPP is now reducing" But they said the latter, not the former...

I have good intentions of blogging further on this point soon.

Alexander Ac said...

You mean Nemani et al.,

Alex

Sleepyfloyd said...

I don't see a link to this at climateprogress. If you posted it I would predict a 49.9% chance of moderator rejection, 49.9% chance of vicious flaming, and .2% chance of respectful disagreement and concurrance in the view of the study authors.

Understanding of statistics is so important because global anything is a statistcal construct - temperature/precipitation/foliage...

The obligation to exagerate abstracts, summaries, and headings has been claimed by many researchers and IPCC authors/editors. As in this case, the entire paper is not fraudulent and has integrity, but the abstract is garbage.

Walker said...

Good on you. Very much appreciate seeing someone give such a clear explication of why the data deserve a second look and some thinking about the implications.

I think Science was correct to publish the paper, but that there is something wrong with the peer review process that did not flag the need for a sensitivity analysis.

All that said -- the other thing missing (and missing from your dissection) is "What would NPP have been had the prior trend simply continued (i.e., the prior increases continued at the prior rate, despite increased CO2) and then also what should NPP have been if the increased CO2 was driving NPP to increase at an increasing rate.

When I think of it that way, I can see why this is such a disturbing result. If the carbon cycle is working the way we should hope it is (increased atmospheric levels leading to increases in uptake at various sinks) then the fact that there is not a strong, clear increase in NPP 2000-2009 is scary enough.

Like you, my disclaimers: I'm not saying NPP is dropping. For me, the fact that the data suggest that it MIGHT highly likely be dropping, in fact (if you accept 90% = highly likely) is scary enough. If the paper prompts an intense search for better data and measurements of this critical question, then good on them too.

Stuart Staniford said...

Alex - yes, Nemani, thx for correction.

physecol said...

I agree with Burk Braun. You should write a letter to Science pointing out the lack of statistical significance in the trend.

nigguraths said...

Hi,
I've been a little late getting to your excellent post.

I would like to point out that Zhao and Running's derivation of 'q10' - the rate of evapotranspiration increase per every 10 degree celsius rise in temperature, and their use of fixed value q10, both, are >2 or =2.

Mahecha et al 2010, published a month earlier in Science, showing q10 values are ~1.4 across all types of ecosystems. This parameter affects significantly, estimates of 'carbon loss' or metabolic demand/stress as calculated in Zhao and Running.

Regards

Belette said...

I'm late too (I'm coming via Romm, in case you wondered) but liked your post.

http://scienceblogs.com/stoat/2010/11/time_to_opine.php

Peter T said...

Stuart

Your take on the paper is perfectly reasonable. But I think it assumes that the context - which the authors take for granted - is unimportant. The context is the general scientific acceptance that global climate change is a real and urgent problem. In considering possible responses, we cannot assume that vegetation will behave as normal, nor can we wait ten years or so to find out if the vegetation response is slightly positive instead of, as suggested here, slightly negative or worse. The latter likelihood needs to be factored in as early as possible.

As for confusing journalists - that has been shown repeatedly to be very, very easy. It's not something the scientists can reasonably be asked to take into account.

Broken Yogi said...

Good post. I'm a climate skeptic/denier/paid oil company shill/hater-of-life-who-wants-our-planet-to-burn-in-hell, so take what I say with a grain of salt, but the slight decline or sideways trend these findings indicate shouldn't be surpising at all, given that the same sideways trend is evident in the temperature record over the last ten years also.

One would generally assume that rising temperatures would lead to more plant growth and thus more carbon fixing going on. Whereas, steady or declining temperatures would lead to steady or declining carbon fixing, which seems to be what's going on.

The problem with the trend lines here is the same as the problem with the last ten years the of temperature record. It's a fairly short data set, and the starting points yield different slopes. If one starts in very warm 1998, the slope goes downward up to the present. If one starts in the cooler 2000, the slope trends upward. If 2002, the slope goes down again. The general indication is that over the last decade the temperature trends have broken, resulting in what is mostly a sideways trend. What's not clear is if this is a true break in rising temps merely a temporary interruption.

THe point is that it's this fundamental problem with interpreting the temperature record that underlies this same problem with the carbon fixing record. We just don't know how significant it is yet. But on the other hand, this is also relatively good news. It would be a different story if the temperature trends had continued up during this period, and yet the carbon fixing had gone sideways or down. That would indicate a fundamental break with the general climate principle that increased warmth will also increase plant growth and thus carbon fixing. So that's another issue that needs to be addressed in this paper, but isn't. They are trying to suggest that there's something scary about their findings, even if they are accurate, when that is not the case due to the same sideways trend in the temperature record

Hank Roberts said...

Hm. I was hoping one of the scientists would comment on

"We just don't know how significant it is" (I thought that was the point, we do know: 'not very yet'; something about dealing with trends assuming no change, or assuming change in one direction, and when variability is changing -- not easy but being worked on, I think.

And

"general climate principle that increased warmth will also increase plant growth"
which seems to be missing an "all other things held the same" caveat.

Jin-Soo Kim said...

I really agree with you.

GPP = ε_max [parameter]* Temperature_Scalar [obs] * Water_Stress_Scalar [obs] * APAR [obs-model]
Here, using independently caculated PAR, APAR = FPAR x PAR ( cos (SZA)).

However, in my guess, it has a problem that weaker greening signal after 2000 in high latitude due to "Sensor degradation". (This signal reported from GIMMS data).

FLUXNET GPP data has a significant positive trend while MODIS NPP has a negative.