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When looking at the long term ice-core temperature record, it very clearly shows a ~100,000 year cyclical trend with warming spikes and gradual cooling. To the untrained eye, it seems that such a trend has not been significantly deviated from so far (in the present day). This would indicate to me that the current upward temperature trend is not human-caused. What refutes that idea?

Also a relevant question: what is the confidence of this temperature record and what are the probable deviations from the temperatures indicated in that graph?

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    $\begingroup$ All that that tells you is that eyeballing a low-resolution graph of 800,000 years and a 20 Kelvin swing won't pick out a 1 Kelvin change over ~150 years. Surely that's no surprise? $\endgroup$ – EnergyNumbers Sep 1 '14 at 19:00
  • $\begingroup$ @EnergyNumbers So are you implying that the temperature today is significantly higher than time 0 looks to be on that graph? $\endgroup$ – B T Sep 1 '14 at 19:50
  • $\begingroup$ No. I'm saying that that graph is such low-resolution that you cannot eyeball a 1 Kelvin change over ~150 years, because the x axis spans 800,000 years and the y-axis spans 20 Kelvin. If you look at time 0, the bar spans ~ -1.5 to ~ +5 degrees C. It would be rather odd to draw any conclusion that overturned what we know from all of climatology, on the basis of a ~ 1 Kelvin shift within in a ~ 6.5 Kelvin bar on a graph. $\endgroup$ – EnergyNumbers Sep 1 '14 at 21:03
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    $\begingroup$ We are already on the downslope of the the top of the curve. So we should be cooling. Yet, we are not. It's easier to see if you look at the finer resolution plot. en.wikipedia.org/wiki/… If you want to look at temperature change due to anthropogenic influence, you are going to need to zoom in to the last 130 years, not a graph showing 800,000 years. $\endgroup$ – farrenthorpe Sep 2 '14 at 5:41
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    $\begingroup$ The comments section is getting too long, so please David and EnergyNumbers, and anyone else, just create an answer. It really doesn't matter how complete your answer is, its better than having a giant comment thread. I would like to respond to you guys in individual threads so the conversations don't tangent uncontrollably. $\endgroup$ – B T Sep 2 '14 at 18:40
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No, ice cores do not refute human-caused warming. In fact, they support it. Your logic really doesn't follow because the rate of warming now compared to some arbitrary previous date is not related to whether the warming is driven by humans. Instead, it is important to consider the natural variation of physical factors that affect climate (collectively known as Malenkovitch cycles; http://en.wikipedia.org/wiki/Milankovitch_cycles ), and then recognize any large deviations from those cycles.

An image of the EPICA and Vostok ice cores versus age obtained from http://en.wikipedia.org/wiki/Ice_age , clearly shows influence of the Malenkovitch cycles. The most notable of which is the strong cycle that is approximately 70,000 years and is associated with geologic ice ages. In fact, the Earth is historically on average much cooler than it is now, and the retreat of glaciers has been periodic but not sustained. We were supposed to have cooled off some time ago, but clearly we are not in a global cooling period. Many would even argue that humans started affecting the climate over 7,000 years ago when land management and fire was wielded prevalently.
EPICA and VOSTOK Cores

In order to attribute climate change to humans, climate models are run with and without anthropogenic climate forcing. When they do so, climate models run without anthropogenic forcing are unable to duplicate the current warming trend. I've attached an image of this from the IPCC AR5 Technical Summary draft which shows a full climate simulation, one without anthropogenic climate forcers, and one with no aerosol forcing for comparison.

climate model results for history

Finally, I thought someone might value this link that discusses climate change atrribution: http://www.realclimate.org/index.php/archives/2014/08/ipcc-attribution-statements-redux-a-response-to-judith-curry/comment-page-4/

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  • $\begingroup$ Could you please elaborate on what makes it so clear that the EPICA and Vostok ice cores indicate that we should be in a cooling phase of the Malenkovitch cycles? It doesn't seem so obvious to me. $\endgroup$ – B T Nov 14 '14 at 23:16
  • $\begingroup$ @BT Sure no problem. I will revise the answer now. $\endgroup$ – farrenthorpe Nov 14 '14 at 23:37
  • $\begingroup$ I still don't quite see why you're asserting that we definitely should be cooling. The previous spike 150,000 years ago looks pretty similar to the most recent spike ~10,000 years ago, along with a cooling downtrend and subsequent minor spike again. I'm still not quite seeing what about that is so out of the ordinary for Milankovitch cycles. $\endgroup$ – B T Nov 15 '14 at 20:51
  • $\begingroup$ @BT the point is that the natural state of the Earth is normally with large glaciers. I suppose if the Earth suddenly started cooling then the graph wouldn't be too far off. To me, the sustained warm period we are currently in appears to be much longer duration than those in the past. Surely, though, you can see how another ice age is imminent if only natural forces were at work. $\endgroup$ – farrenthorpe Nov 17 '14 at 17:40
  • $\begingroup$ Ya, so it took about 11,000 years from the peak of the last malenkovitch spike until it really started cliffing temperature. We're about 12,000 years since the most recent peak, so I agree that I would expect the temperature to cliff in the next thousand years or two, but a less than 10% longer period of time doesn't strike me as being an obvious overrun. Its also not clear to me whether data about more recent temperatures would look different on the graph than data about older temperatures, which would be another factor in interpreting the graph in the last 10k years $\endgroup$ – B T Nov 18 '14 at 1:09
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My current understanding is that, while the trend over hundreds of thousands of years has not (yet?) been significantly deviated from, the rate at which the temperature has been rising over the last 100 years has been unprecedented* (I concluded its not in fact unprecedented, see below).

I'm not entirely sure that the rate is in fact unprecedented, but I could certainly see it being that way. Look at the graphs at https://en.wikipedia.org/wiki/Temperature_record , it looks like:

  • Between about 14 thousand years ago and 7 thousand years ago, the temperature rose by 8 degrees, which is 1.15 degrees per 1000 years on average.
  • Between 132 thousand years ago and 124 thousand years ago, the temperature rose by about 14 degrees, which is a 1.75 degree per 1000 years on average.
  • In the last 100 years, the temperature has increased on average about 1.1 degrees. So that's a good 11 degrees per thousand years, which is about 6 times what looks like one of the highest rates on the long term ice core record.

So because of the different lengths of times, and different scales, its a certainty that there was variance in the rate of temperature change between 132 thousand years ago and 124 thousand years ago. This means that there were definitely spans of 100 years in the those few thousand years in which the rate of increase was greater than the average.

So this lead me to looking at the actual data the graph on wikipedia was derived from. Here's the source: ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/epica_domec/ . Looking specifically at the spreadsheet called edc3deuttemp2007.xls , I see that indeed between between 132 thousand years ago and 124 thousand years ago, there are about 12 approximately hundred-year periods with greater than a 1.1 degree increase per hundred years. I took periods of three data points, which means they're not all the same exact length of time. In any case, this was from 76 3-data-point periods, so a full 15% of the time periods in that section of the data had greater temperature growth than we've seen in the last 100 years.

In fact, since 133 thousand years ago, there have been over 150 non-overlapping 100-year periods where the temperature has been rising faster than it has in the last 100 years. This is about 11% of the 100-year periods in the last 133 thousand years.

So in conclusion, based on this data, I have to conclude that the rate of temperature rise in the last 100 years is certainly not unprecedented, and thus my question still stands: why is this not compelling evidence that the temperature rise is largely not anthropogenic?

Here are the calculations I added to the spreadsheet to come to my conclusions: http://www.btetrud.com/files/edc3deuttemp2007%20-%20annotated%20with%20~100%20year%20temp%20diffs.xls . Note that the data is all in the second spreadsheet tab (the 'data' tab).

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  • $\begingroup$ @DavidHammen What's your opinion about this? I'd still appreciate a full answer-post from you as well. $\endgroup$ – B T Nov 14 '14 at 21:40
  • $\begingroup$ I'd appreciate a comment about why this was downvoted. $\endgroup$ – B T Nov 14 '14 at 22:50
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    $\begingroup$ I wasn't the one who downvoted... but in general you are mixing apples and oranges, givine evidence of one thing and concluding another. $\endgroup$ – farrenthorpe Nov 15 '14 at 0:06
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    $\begingroup$ B.T. It seems like you are focused on one aspect of the temperature record... and to whether the change is "unprecedented", but the question asked is whether the attribution of that rise is anthropogenic. If you want to ask about temperature change extremes in geologic history, I suggest you post another question to get at that point. You posted an answer that ends with "my question still stands". Which is not an answer at all... so I have now downvoted. $\endgroup$ – farrenthorpe Nov 15 '14 at 20:53
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    $\begingroup$ Does the long-term ice-core temperature record provide reasonable evidence that the current temperature trend is not anthropogenic? No it doesn't $\endgroup$ – farrenthorpe Nov 18 '14 at 0:57

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