2010-08-26 01:20:04Basic rebuttal for 9: "We are heading into an ice age"
Anne-Marie Blackburn
Anne-Marie Blackburn

Can't quite put my finger on it but not that happy with this one. Any input appreciated.


 We’re heading into an ice age


According to ice cores from Antarctica, the past 400,000 years have been dominated by glacials, also known as ice ages, that last about 100,000. These glacials have been punctuated by interglacials, short warm periods which typically last 11,500 years. Figure 1 below shows how temperatures in Antarctica changed over this period. Because our current interglacial (the Holocene) has already lasted approximately 12,000 years, it has led some to claim that a new ice age is imminent. Is this a valid claim?



 Figure 1: Temperature change at Vostok, Antarctica (Petit 2000). The timing of warmer interglacials is highlighted in green; our current interglacial, the Holocene, is the one on the far right of the graph.



To answer this question, it is necessary to understand what has caused the shifts between ice ages and interglacials during this period. The cycle appears to be a response to changes in the Earth’s orbit and tilt, which affect the amount of summer sunlight reaching the northern hemisphere. When this amount declines, the rate of summer melt declines and the ice sheets begin to grow. In turn, this increases the amount of sunlight reflected back into space, increasing (or amplifying) the cooling trend. Eventually a new ice age emerges and lasts for about 100,000 years.


So what are today’s conditions like? Changes in both the orbit and tilt of the Earth do indeed indicate that the Earth should be cooling. However, two reasons explain why an ice age is unlikely:


1.    These two factors, orbit and tilt, are weak and are not acting within the same timescale – they are out of phase by about 10,000 years. This means that their combined effect would probably be too weak to trigger an ice age. You have to go back 430,000 years to find an interglacial with similar conditions, and this interglacial lasted about 30,000 years.

2.    The warming effect from CO2 and other greenhouse gases is greater than the cooling effect expected from natural factors. Without human interference, the Earth’s orbit and tilt, a slight decline in solar output since the 1950s and volcanic activity would have led to global cooling. Yet global temperatures are definitely on the rise.


It can therefore be concluded that with CO2 concentrations set to continue to rise, a return to ice age conditions seems very unlikely. Instead, temperatures are increasing and this increase may come at a considerable cost with few or no benefits.



2010-08-26 02:52:45


I would be more forceful in the last sentence - with CO2 emissions accelerating, a natural return to ice age conditions is all but ruled out....

 You might want to include the following image from the last 2000 years of arctic temps taken from proxies that shows a natural long term cooling trend up until the onset of the 20th century so that people can visualize how CO2 is a greater forcing agent that orbital forcing -


Two Millenia of Arctic Temperatures 


Maybe also include a reference to Milankovitch so people can look up the science on their own easier.


Hope that helps some!




2010-08-26 03:07:30
Anne-Marie Blackburn
Anne-Marie Blackburn

Thanks AL. I always err on the side of caution - I find it saves squibbles from taking over the discussion.

Good graph - do you have a link to it so I look into it a bit more? Ta.

As for Milankovitch, I included the cycles in a previous post but we all agreed in the end that it wasn't necessary in the basic rebuttals. Maybe once we get the glossary going we'll be able to include more complex concepts?

2010-08-26 03:19:18Devil's advocate



Is 2,000 years long enough to draw ANY conclusion about what the orbital forcing is trying to do? (So that you could say whether or not the AGW is overcoming it?)


2010-08-26 03:20:26


The source for the image is originally from the Kaufman paper i believe.


but i grabbed the image from the BBC:




2010-08-26 03:27:59



  My feeling is that at the very least it is consistent with what orbital forcing should do (but i could be wrong).  There is also an image from Global Warming art that covers the whole Holocene and from what i can tell also shows a long term cooling trend starting about 8K years ago that abruptly ends around the modern era:

Holocene Temperature Variations


also the abstract of the Kaufman paper states:

 "A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000."


2010-08-26 04:52:29Comment
Robert Way

The kaufmann et al. 2009 data can be found here:


I suggest using Temperature Change instead of Anomaly and so on. It would be a very good graphic to have in there however.

I tried the data
here is the jpg

2010-08-26 18:57:48
Ari Jokimäki

Thumbs up #002.
2010-08-27 06:16:34


last about 100,000. - last about 100,000 years.

which typically last 11,500 years.   - I'd include the fact that they can be much longer here, in preparation for the main point.  something like - which typically last 11,500 years, but can last more than 30,000 years.  

Would you consider explaining what "out of phase" means?

 Looks good to me.


2010-08-27 09:59:28timescales should be appropriate to causes


I don't see much point in using a graph that shows 2000 years of time, when discussing a phenomenon (orbital variations) that takes place on scales of 22,000 years or so. It looks like cherry-picking, which is exactly what we want to discourage in the deniers. Even if that short 2000-year segment is entirely characteristic of the 22,000-year period, there's no way to tell unless you have the graph for something like 22,000 years.

Just as we don't display a set of temperature measurements from 2:00 to 2:10 pm to illustrate temperature variation due to the zenith of the Sun during the day: It might give the right direction of change, but it's a completely inappropriate time scale. You would need several hours, at least.


2010-08-27 14:49:03



 You might be correct that it could appear like cherry-picking.  However, the other graph i showed of the Holocene indicates that it would only be indicative of a trend since the beginning of the most recent deglaciation ~12K years ago.  I was thinking of it more as a visualization of the way that C02 warming has clearly overtaken the cooling effect of orbital forcing, hence the delaying of the onset of another glacial period. 




2010-08-27 18:25:34Some concerns
Graham Wayne

Good morning Anne-Marie.

I share your initial concerns. I didn't find it simple enough - and suspected that some very simple graphs might aid you regarding periodicity of climate cycles - but it's the take home points that have me worried. To illustrate this, I was going to put on my denier hat, but I've run out of tin-foil. Here instead is a devil's advocacy, based on a response to the last line "It can therefore be concluded that with CO2 concentrations set to continue to rise, a return to ice age conditions seems unlikely":

"So what you are saying is that we are overdue for the next cooling phase or actually in it, and it is only global warming that is staving this off. Ergo: AGW is a good thing, right?"

2010-08-31 20:36:48
Anne-Marie Blackburn
Anne-Marie Blackburn

Thanks all for the input. I've added a graph for clarity to illustrate the glacial-interglacial cycles during the past 400,000 years or so. A graph of Holocene temperatures would be great - I like the graph from the Kaufmann paper, it's just a shame it ends when it does. It would be more powerful with the latest temperatures included. I'm off to see if I can find an up-to-date one.

I'm still working on the out-of-phase bit to try and make it clearer. I've changed 'synchronously' to 'within the same timescale' but I'm not sure it's an improvement. I'm sure I'll get there eventually.

Graham, I've also added a sentence at the end with a link to one of your previous articles. My previous ending was too abrupt, and of course I'd temporarily forgotten how good contrarians are at twisting everything.

2010-09-01 01:10:44


This is a reasonable explanation, not overstated. A neutral person prepared to integrate the difference between this interstade and others will get the point. As to a "skeptic" we know there's no point in wishful thinking.

Not -every- rebuttal is going to come out like a 300 pound wrestler pinning a 100 pound opponent to the mat.  

2010-09-01 01:54:47
Robert Way

Jouzel et al. 2007 800KYr δD Data and Temperature Reconstruction, text or Microsoft Excel format.


There you can find the glacial/interglacial information for 800,000 years (and data to plot up in excel)

2010-09-01 03:11:38OK with our without this suggestion...
Jim Meador


I suggest changing...

"These two factors, orbit and tilt, are weak and are not acting within the same timescale – they are out of phase by about 10,000 years. This means that their combined effect would probably be too weak to trigger an ice age."


"The cooling from orbit and tilt variations is weak overall, and these factors will not peak at the same time in this cycle. Their combined effect would probably be too weak to trigger an ice age." 

2010-09-01 07:48:04
Nicholas Berini

This version is excellent.
2010-09-01 09:58:58Published
John Cook

Tweaked the HTML code, turned the 1,2 points into a HTML ordered list