Taking up yesterday's question, here's my answer. The graph above shows the data from Nemani et al, along with black data points that I took from their graph and their Excel trend line. The OLS estimate of the trend (assuming all the data points were independent and normally distributed) is 0.188 +/- 0.044 GT/yr of carbon (compared to the -0.05 +/- 0.1 trend for the 2000 to 2009 data).
It turns out it's reasonable to ignore the autocorrelation. For example, if you take the residuals from the trend line above, and then plot them against their neighbor at one year lag, here's the scattergram:
With a correlation coefficient of less than 10%, we are pretty safe in ignoring this. Then, the way you'd compare these two trends is to compute the difference in the slopes (0.188 - -0.05 = 0.238), and compare that to the combined error (sqrt(0.1^2+0.044^2) = 0.11. That would be 2.2 sigmas and so (just) statistically significant (p = 0.03).
So using the IPCC likelihood terminology, I think it's fair to say that "It is very likely that the previously demonstrated trend of increasing NPP has now slowed. It is possible that NPP is now actually decreasing, but the evidence for that is equivocal."
15 comments:
Stuart, I think you made an effort to digitize the points of the graphs of both articles. Would it be possible to have the file with the data? I think several of your readers (including myself) would like to try to work on the data themselves. If you can, thanks a lot in advance!
Ugo:
I have
1982.5 -2.05
1983 -3.15
1984 -2
1985 -0.45
1986 -0.8
1987 -0.8
1988 -0.9
1989 0.5
1990 1.6
1991 0.15
1992 0.2
1993 1
1994 0.8
1995 1.2
1996 0.8
1997 2.25
1998 -0.2
1999 -0.2
for Nemani et al, and
2000 1.2
2001 0.3
2002 -0.8
2003 -0.05
2004 1
2005 -1.85
2006 0.05
2007 -0.3
2008 0.2
2009 0.35
for Zhao and Running. These are anomalies in GT/yr, and are not off the same base
Thanks Stuart. As soon as I have finished tuning the cloning machine, I'll see if I can play with these data.
Ouch.... I just noticed: these are CO2 anomalies, right? Would you have also the data for the NPPs.....? (sorry)
No, those are the NPP anomalies (expressed in gigatonnes/year of carbon fixed by plants).
Oops.... sorry, that was my clone. Not yet perfectly tuned up... :-)
Stuart: Damn, you beat me to it! Regards, don
peak NPP anomaly 1997? :-)
A lot of scatter in the data, but there is a clear maximum around 1998
Alex: "Peak NPP"? I guess that is what we are debating isn't it? Good turn of phrase. Time will tell.
A quick stab at the whole set of data - very rough, it was made by one of my clones!
On my Italian blog at
http://ugobardi.blogspot.com/2010/08/il-picco-della-produttivita-terrestre.html
What's the difference between NPP and "natural sinks"? See Fig 5.1. These take up the slack when CO2 fluctuates, to give that more-or-less continuous buildup we see in the Keeling curve; or so I'm led to understand. It all seems a bit fishy, are these fluctuations in sinks being inferred? Wouldn't variations on that scale manifest themselves in an apparent fashion, huge spikes in crop burning or something? Yet you see the same charts in published papers, likely I just haven't found an appropriately entry level explanation for this.
KLR:
What we are discussing here is the carbon taken up by terrestrial plants. The other main sink is the ocean.
Longer-term data based on the portion of emitted carbon that actually ends up in the atmosphere indicates that the amount of carbon taken up by plants, while varying significantly over the years, shows a long-term increase. See a graph that I borrowed from p. 119of James Hansen's latest book, Storms of My Grandchildren, at http://michaelaucott.blogspot.com/search/label/Climate
Of course, this rate of carbon capture could be changing. But we don't know the size of the error bars around the individual data points in the studies you refer to, right? If these are big enough, even your highly-caveated conclusion implies too much certainty.
The NPP is supposedly changing due to increased temperatures causing draughts and thus less plant growth, right? Has anyone got any data on actual rainfall around the world to see if these draught conditions really are increasing? Do we have any way to measure plant growth in past eras in which global temperatures went up or down (regardless of whether these temp changes were related to changes in CO2 levels in the atmosphere)? I'm not dispuing that NPP may be changing, but I am reminded of the debate among ancient Greek philosophers concerning how many teeth a horse has; they never went out to find an actual horse and count the teeth...
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