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[defacto Climate Change thread] Orbital forcing of tree-ring data
electricitylikesme
Registered User regular
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So while browsing Slashdot this (don't read this) caught my eye, notably because there is an actual Nature Climate paper involved (note: this is not the same as Nature the journal in terms of general regard, but the Nature group is held in pretty high-esteem on that name alone).
So you'll be seeing it everywhere soon. My academic position does give me full access to the paper, but in the interests of keeping it for the time being I can't reproduce the full text here.
Link to Full-Text. Digital ID: doi:10.1038/nclimate1589 (use with http://dx.doi.org/)
Now, I am not a climate scientist, but there's a lot of things people should know in dealing with this:
Orbital Forcing
Orbital forcing is how changes in the shape of the Earth's orbit, and changes in the tilt of the Earth on it's axis, affect the amount and distribution of sunlight received over the surface. Both change with time. This paper deals with detection, and notable absence in other data to date, of orbital forcing signals based on the observation that the long-term trend should be a decline in solar insolation, since the Earth's orbit is in fact slowly moving away from the sun (and tilt axis changes would vary where the tropical regions actually are).
A worthwhile quote from the full-text:
Again, I am not a climate scientist - and I suspect the whole area tries to be very careful with words these days. The paper itself makes no claims whatsoever about AGW. It is principally a discussion on the reliability and stability of tree-ring data, supported by simulation, based on a new tree-ring record which has been constructed, covering a long term time-span of 2,000 years (which, reading the paper, is quite impressive since no one actually does it that long normally).
The key finding here is that they've got a tree-ring proxy data set which shows the long term cooling trend one would expect from the known changes in orbital forcing of the climate - and by inference, that the medieval warm period was warmer then previously thought and on a long-term cooling trend since then.
The significance of this to historical data they speculate on, but note there's not enough evidence to comment: if climate variability in the past was stronger, then the orbital-forcing changes probably don't matter - they would mostly be swamped with noise in the temperature records. If the variability is more constrained, then it would indicate an underestimate of temperature previously.
Figures (I may or may not be able to link these, we'll see):
What does this mean?
I really don't know yet - it needs to be matched against other data, but the most important thing I can say is that this does not invalidate the post-instrumental temperature record. That is, since scientists started using thermometers to measure the temperature since 1880 (overlayed on the inset in the above graphic).
It possibly might help us figure out the divergence between tree-ring data and the instrumental temperature record though - I don't know enough about either, or what work on that has been done, to say much more then "hey, maybe an underestimate explains a contradiction!".
It also possibly means we can adjust our interpretation of how much warming the global climate might sustain without causing dramatic side-effects - but the medieval warm period may only have been a few tenths of a degree warmer - which is a far cry from 2 - 6 degrees C of warming we may see from current emissions levels.
This is complicated work, but the media is going to be reporting it in the most dramatic fashion possible (it is, pretty cool work, judging from my superficial understanding) and I suspect it won't be long till you start seeing "it was 4 times warmer then they said it was!" (from the abstract, it's noted that the orbital forcing value is 4 times higher then the forcing provided by AGW CO2 - this does not translate to 4 times the temperature, which would be stupid high levels).
Please do the world a favor, and think and discuss carefully - but I suspect the climate research community is going to be going over this and debating it in the literature quite a lot in the near-ish future.
tl dr; if you can't be bothered reading, you should avoid having an opinion on things like this.
So you'll be seeing it everywhere soon. My academic position does give me full access to the paper, but in the interests of keeping it for the time being I can't reproduce the full text here.
Solar insolation changes, resulting from long-term oscillations of orbital configurations, are an important driver of Holocene climate. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m−2 net anthropogenic forcing since 1750 (ref. 4), but the trend varies considerably over time, space and with season. Using numerous high-latitude proxy records, slow orbital changes have recently been shown to gradually force boreal summer temperature cooling over the common era. Here, we present new evidence based on maximum latewood density data from northern Scandinavia, indicating that this cooling trend was stronger (−0.31 °C per 1,000 years, ±0.03 °C) than previously reported, and demonstrate that this signature is missing in published tree-ring proxy records. The long-term trend now revealed in maximum latewood density data is in line with coupled general circulation models, indicating albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes. These findings, together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions relying on tree-ring data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times.
Link to Full-Text. Digital ID: doi:10.1038/nclimate1589 (use with http://dx.doi.org/)
Now, I am not a climate scientist, but there's a lot of things people should know in dealing with this:
Orbital Forcing
Orbital forcing is how changes in the shape of the Earth's orbit, and changes in the tilt of the Earth on it's axis, affect the amount and distribution of sunlight received over the surface. Both change with time. This paper deals with detection, and notable absence in other data to date, of orbital forcing signals based on the observation that the long-term trend should be a decline in solar insolation, since the Earth's orbit is in fact slowly moving away from the sun (and tilt axis changes would vary where the tropical regions actually are).
A worthwhile quote from the full-text:
These findings together with the trends revealed in long-term CGCM runs suggest that large-scale summer temperatures were some tenths of a degree Celsius warmer during Roman times than previously thought.
Again, I am not a climate scientist - and I suspect the whole area tries to be very careful with words these days. The paper itself makes no claims whatsoever about AGW. It is principally a discussion on the reliability and stability of tree-ring data, supported by simulation, based on a new tree-ring record which has been constructed, covering a long term time-span of 2,000 years (which, reading the paper, is quite impressive since no one actually does it that long normally).
The key finding here is that they've got a tree-ring proxy data set which shows the long term cooling trend one would expect from the known changes in orbital forcing of the climate - and by inference, that the medieval warm period was warmer then previously thought and on a long-term cooling trend since then.
The significance of this to historical data they speculate on, but note there's not enough evidence to comment: if climate variability in the past was stronger, then the orbital-forcing changes probably don't matter - they would mostly be swamped with noise in the temperature records. If the variability is more constrained, then it would indicate an underestimate of temperature previously.
Figures (I may or may not be able to link these, we'll see):
a, The reconstruction extends back to 138 BC highlighting extreme cool and warm summers (blue curve), cool and warm periods on decadal to centennial scales (black curve, 100-year spline filter) and a long-term cooling trend (dashed red curve; linear regression fit to the reconstruction over the 138 BC–AD 1900 period). Estimation of uncertainty of the reconstruction (grey area) integrates the validation standard error (±2 × root mean square error) and bootstrap confidence estimates. b, Regression of the MXD chronology (blue curve) against JJA temperatures (red curve) over the 1876–2006 common period. Correlations between MXD and instrumental data are 0.77 (full period), 0.78 (1876–1941 period), and 0.75 (1942–2006 period).
What does this mean?
I really don't know yet - it needs to be matched against other data, but the most important thing I can say is that this does not invalidate the post-instrumental temperature record. That is, since scientists started using thermometers to measure the temperature since 1880 (overlayed on the inset in the above graphic).
It possibly might help us figure out the divergence between tree-ring data and the instrumental temperature record though - I don't know enough about either, or what work on that has been done, to say much more then "hey, maybe an underestimate explains a contradiction!".
It also possibly means we can adjust our interpretation of how much warming the global climate might sustain without causing dramatic side-effects - but the medieval warm period may only have been a few tenths of a degree warmer - which is a far cry from 2 - 6 degrees C of warming we may see from current emissions levels.
This is complicated work, but the media is going to be reporting it in the most dramatic fashion possible (it is, pretty cool work, judging from my superficial understanding) and I suspect it won't be long till you start seeing "it was 4 times warmer then they said it was!" (from the abstract, it's noted that the orbital forcing value is 4 times higher then the forcing provided by AGW CO2 - this does not translate to 4 times the temperature, which would be stupid high levels).
Please do the world a favor, and think and discuss carefully - but I suspect the climate research community is going to be going over this and debating it in the literature quite a lot in the near-ish future.
tl dr; if you can't be bothered reading, you should avoid having an opinion on things like this.
electricitylikesme on
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Liberals are stifling debate
Why don't YOU go read the Bible, before claiming that the world is older than 6,000 years?
This reminds me a bit of a few years ago when Craig Loehe had to revise a paper on historical temperature trends using tree ring data, and the AGW denial community jumped all over it.
the "no true scotch man" fallacy.
I also believe the standard global warming/climate change argument holds still, that regardless of the global, natural patterns, humans are contributing in some part to that change, and in an upward fashion. I'd feel comfortable stating that while historical records of global temperature are useful and interesting, the major element regarding climate change has to do with industrialization. We know that pollution contributes to warmer local climates and that it alters weather patterns, and that wasn't present pre-industralization. That, and we only have 1 actual model to measure.
What if I read and just didn't understand?
I've decided that I believe the world is actually cooling because tree-ring gnomes are counteracting global warming.