Friday, December 26, 2008

Thoughts on Long Term Climate



The graph shows global temperatures over the past quarter of a million years; the horizontal scale is in thousands of years, the vertical in degrees C. The source is Jouel et. al. 1996. Two things strike me about it:

1. We are in an unusually warm period, and have been for about the past ten thousand years. It has only gotten this warm once before over the period shown.

2. Judging at least by the previous peak about a hundred and twenty-five thousand years ago,and the smaller peaks between, this warm period has already lasted for an unusually long time.

From point 1 one might conclude that current worries about global warming are misplaced—on the long term evidence, we ought to be worrying about cooling. One might also include that arguments about anthropogenic warning are misplaced, since the current warm period started long before modern technological society with its large consumption of fossil fuel.

Point 2 suggests a somewhat different conclusion. The beginning of the long warm period we are still in roughly coincides with the invention of agriculture and the consequent large expansion in human population as we went from hunter/gathering to farming. One can imagine a variety of mechanisms by which that change might have affected climate, some described in a recent blog post at FuturePundit, one of my favorite blogs.

18 Comments:

At 1:46 PM, December 26, 2008, Blogger haastk said...

So what is the conclusion now? The sheer existence of human civilization destroys the climate?

 
At 6:01 PM, December 26, 2008, Anonymous BlackSheep said...

The beginning of the long warm period we are still in roughly coincides with the invention of agriculture and the consequent large expansion in human population as we went from hunter/gathering to farming.

Maybe the correlation is in the other direction... Warmer weather increased productivity of agriculture making the sedentary life style more attractive than the nomad. Further social and technological productivity improvements compensated the following climate cooling.

 
At 9:51 PM, December 26, 2008, Blogger Karl said...

Somewhere in my library I have a book titled Climates of Hunger. It makes he point that the climate we consider "normal" is in fact abnormally warm. His thesis was that when (not if) the climate returned to its historical normal temperature, we were in for tough going on the agricultural front.

My personal theory, based on absolutely nothing, is that we will eventually learn that high levels of carbon dioxide in the atmosphere actually cool the planet.

 
At 9:02 AM, December 27, 2008, Blogger montestruc said...

This has been discussed a lot before, and probably has some truth. The end of the medieval warm period for example came soon after the discovery of the new world and the consequent infection of natives with smallpox which caused a massive die-off of humans in the Americas (~90%), and so reduced human agricultural activities in the New World. That was not the only thing going on, but it is reasonable to think that it contributed.

Reference:


http://en.wikipedia.org/wiki/File:2000_Year_Temperature_Comparison.png

 
At 7:29 PM, December 27, 2008, Anonymous Steve B said...

The medieval warm period, depending on how you define it, started ending around 1300, considerably before any significant European/American contact. Although it's certainly possible that European diseases killing off native Americans gave it another push in the direction it was already going.

 
At 7:39 PM, December 27, 2008, Blogger chriscal12 said...

This may be a silly question, but why are the years in irregular intervals?

 
At 7:57 PM, December 27, 2008, Blogger Beastin said...

If the planet were currently cooling, I'm sure we would be worried about global cooling. A relatively narrow band of temperatures is suitable for sustaining life as we know it; the earth must neither get too hot nor too cold. Since the planet is currently warming, it makes sense to worry about overheating, particularly given that we are warming it in a novel way.

That being said, the concentration of carbon dioxide in the atmosphere during the Cretaceous Period was over four times the present concentration. This would seem to indicate that pumping more carbon dioxide into the air will not lead to a runaway greenhouse phenomenon. I think the main question is whether we can deal with the effects of the warming we are likely to produce.

 
At 11:54 AM, December 28, 2008, Blogger Steve_Roberts said...

Warm is good. Colder means less agricultural land at higher latitudes and higher altitudes. C02 is good, it is essential to plant life, and therefore to animal (including human) life. Civilisation is good, it enables us to be more efficient in our use of resources. Rather than using warming as a reason to cripple our economies, we should be working out what to do if/when cooling takes hold

 
At 12:27 PM, December 30, 2008, Anonymous Paul Birch said...

Point 1 is not quite correct; it has been this warm twice before over the graphed period (the other is right at the beginning).

Point 2 also seems to be inaccurate; apparently, the scale of the graph has been expanded twofold (or a bit more) in the recent warm period. Take that into account and its duration is not very different from the previous one.

Some order of magnitude calculations - the subject of a recent blog post here - would show the absurdity of some of the theories being thrown around. Differences in the number or lifestyles of human beings ten thousand years ago, or in the medieval period, cannot possibly have caused the warming; the causal effects, such as they are, are all in the other direction.

 
At 10:43 AM, January 01, 2009, Anonymous Anonymous said...

The more recent part of the global temperature data correlates well with the amount of carbon dioxide in the atmosphere in most of the reported measurements I have seen. (This part of the problem seems pretty well understood.)

But there is a huge amount of carbon in various forms in a lot of other places on Earth (e.g. biological material, oil, coal, other rock, the sea, etc.) The global carbon balance problem is not that well understood and much harder than just what amount of carbon dioxide is in the atmosphere: the carbon balance must inform whatever decisions come next.

The longer term changes are more difficult to evaluate. But they also contribute even if these behaviors are not understood.

Human actions clearly change the amount of carbon dioxide in the atmosphere and therefore influence the recent global carbon balance. The longer term problem cannot be caused by human activity. I am not even certain if the longer term global climate behavior is dominated by the carbon balance problem, as opposed to solar variations or something else (alone or in combination). The human activity questions going forward involve both the knowns and the unknowns.

 
At 4:04 AM, January 02, 2009, Anonymous Paul Birch said...

Over thousand year timescales CO2 correlates reasonably well with temperature with a lag, that is, rises and falls in CO2 follow temperature changes rather than causing them. This is primarily due to the solubility of CO2 in water as a function of temperature (warmer equals less soluble) and the equilibrium between carbonates and silicates CaCO3+SiO2<->CaSiO3+CO2 in aqueous solution and on the sea floor. However, over the past century the correlation is very poor; the biggest rise in CO2 production and atmospheric concentration occurred in the period when temperatures were falling from their mid-century peak.

 
At 3:59 PM, January 02, 2009, Blogger Damien Sullivan said...

The Sun has been getting warmer over time; that high CO2 levels were stable in the past need not mean equal CO2 levels will be stable now.

Global heating potentially opens up new land to agriculture, yes. It also dries up land and reduces glaciers, whose year-round melting is a key part of the renewable fresh water supply in much of the world. New land isn't much good if you can't water it, or if you can but you're losing old land to drought. As it is, a lot of development in the US Great Plains and in China has been fueled by fossil water and is liable to collapse at some point if not replaced.

And the "new lands" might not have the loess soils of the existing breadbaskets; just because it's warm enough to plant doesn't mean the plants will grow as well.

Years ago I looked at climate charts, and was surprised to see that Anchorage got only as much precipitation as Los Angeles. But the evaporation rate in Anchorage is a lot lower, so the available water supply is higher.

Fact is, our agriculture may well be adapted to a sweet spot -- not too frozen, not too hot -- with large changes in any direction being bad or at least pretty risky. Odd that 'conservatives' are so blithe about unknown climate changes.

 
At 5:29 PM, January 02, 2009, Blogger Andrew said...

The main conclusion i'd reach from this post is that very smart people still need to do a lot of study before they can say intelligent things about long-term climate. I wonder how many people writing about climate change could make a basic black-body thermal model of the earth/sun.

 
At 10:11 AM, January 04, 2009, Blogger Damien Sullivan said...

Robin Hanson once complained that amateurs felt qualified to make pronouncements about economics, but that's clearly not the only field suffering from a lack of respect for expertise.

 
At 1:54 PM, January 13, 2009, Anonymous Mark said...

Even if the science on global warming is still unclear (and I'm not saying it is or it isn't), I've never heard anyone dispute the link between air pollution (especially from fossil fuels) and negative health consequences, such as increased rates of lung cancer and asthma.

This alone would seem to be a good reason to curb pollution, even if it slows economic growth.

 
At 3:31 PM, January 16, 2009, Anonymous BlackSheep said...

Mark, it sounds like the strategy and the targets for that effort you suggest may not overlap that much with the current effort to cool the Earth. Besides, that one can be more effectively approached locally than with a world government.

 
At 4:05 PM, January 16, 2009, Anonymous Anonymous said...

A LOT of VERY cold weather right now. Al Gore deserves a frozen mukluk in the ass.

 
At 11:39 AM, February 24, 2009, Blogger B. Kalafut said...

Nobody who is anybody is arguing that all warming is anthropogenic or that the long-term effect you see has anthropogenic cause. There is some good argument out there for the unusually long length of the current long-timescale warm period being anthropogenic, although that is still controversial.

The current concern is over an unusual short-timescale warming and the damage it is predicted to cause.

This is all in the IPCC report, and I respectfully recommend that you familiarize yourself with it or perhaps track down your nearest climatologist and figure out through conversation what's going on and why people who study this are bothered by it. No sense fumbling about in the dark, and less sense broadcasting opinion on this before familiarizing one's self with expert understanding.

 

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