Global Warming Models

[ponderingturtle]

I am currious, does anyone know if the models predicting human caused global warming have been run backwards to see if they predict the little ice age and the medieval climatic optimum?

I hear all these predictions, but never any real testing to see how the models fit with historic known climatic changes.

 

[GreedyAlgorithm]

I'm afraid that's not quite how models work, except in low-level math courses. We don't just fit a function to data points and then extrapolate in both directions. It's a very rare model that can just be "run backwards". As an example of such a model (no, this isn't an analogy for global warming): suppose I told you that I put dry wood, kindling, and a lit, lighter-fluid soaked rag in a fireplace. You would be able to predict what the fireplace would look like half an hour into the future fairly accurately, but would have no idea what the fireplace looked like two days ago. The right question is something like: "do we have enough information about the state of the earth just before the little ice age to apply our model, and if so, does it predict the little ice age?"

 

[ponderingturtle]

I'm afraid that's not quite how models work, except in low-level math courses. We don't just fit a function to data points and then extrapolate in both directions. It's a very rare model that can just be "run backwards". As an example of such a model (no, this isn't an analogy for global warming): suppose I told you that I put dry wood, kindling, and a lit, lighter-fluid soaked rag in a fireplace. You would be able to predict what the fireplace would look like half an hour into the future fairly accurately, but would have no idea what the fireplace looked like two days ago. The right question is something like: "do we have enough information about the state of the earth just before the little ice age to apply our model, and if so, does it predict the little ice age?"

Or high level Physics courses. Similarity T and all that of the PCT similarities.

So in other words they are fundamentaly incapable of being tested agenst historic changes in the known climate? We know that at times the climate was warmer than now and colder than now in well recorded history, why shouldn't the models be able to account for those data points?

 

[Dave1001]

The right question is something like: "do we have enough information about the state of the earth just before the little ice age to apply our model, and if so, does it predict the little ice age?"

okay, that's convincing. So, do we have an answer to that question?

 

[casebro]

"Model" is just another word for "Theory". Let's gather facts for another century and see if those facts fit the theory.

I wonder what the "AGW Theory" time line is? Somebody had to say to themselves "Sure is hot today. Hotter than last summer. I wonder if man is causing it?" And they have spent 20 years trying to find facts that fit their theory, and forcing computer models to fit. It don't take a PHD to look at a graph from NOAA and see how much annual variation there is, so much so that the 1/2 degree rise is meaningless wihin all that variation. As if ice cores, sediment layers, and tree rings could have 1/2 degree of acurracy...

 

[a_unique_person]

Evolution is just a theory.

 

[casebro]

Evolution is just a theory.

So is gravity.

But they both have held up to ALL AVAILABLE DATA. For hundreds of years. Let's wait another 50 and see if the AGW theory holds up.

 

[shemp]

Damn, I came here expecting to see pictures of Heidi Klum. She can warm my globe anytime!

 

[T'ai Chi]

So is gravity.

But they both have held up to ALL AVAILABLE DATA. For hundreds of years. Let's wait another 50 and see if the AGW theory holds up.

How can something that says 'things change' not hold up?

 

[Diamond]

A: No.

Nor can they predict the future, nor predict the present from fifty years back. The only way that climate models "work" is that they have someone who can select from the model runs which one is "correct". Then they publish.

 

[kevin]

I am currious, does anyone know if the models predicting human caused global warming have been run backwards to see if they predict the little ice age and the medieval climatic optimum?

I hear all these predictions, but never any real testing to see how the models fit with historic known climatic changes.

They don't run the models backward, but they do set a date in the past and run the model forward from that point to see how the model compares to observations. Unfortunately the further back you go the less is actually known about the climate so they can't be sure the model matches exactly what happened further back.

Here is a general description of how they validate models, it includes sevreal graphs of observations vs. models from 1850's on:
http://www.realclimate.org/index.php/archives/2005/01/is-climate-modelling-science/

Here's a paper where the "results from a series of Goddard Institute for Space Studies (GISS) General Circulation Models (GCMs) are used to assess climate variability in the pre-anthropogenic Holocene, the interval following the end of the last glacial beginning roughly 11.5 kyr BP."

http://pubs.giss.nasa.gov/abstracts/2004/SchmidtShindellMMR.html

 

[CapelDodger]

We know that at times the climate was warmer than now and colder than now in well recorded history ...

Colder certainly, but not warmer, not in recorded history.

 

[ponderingturtle]

They don't run the models backward, but they do set a date in the past and run the model forward from that point to see how the model compares to observations. Unfortunately the further back you go the less is actually known about the climate so they can't be sure the model matches exactly what happened further back.

Here is a general description of how they validate models, it includes sevreal graphs of observations vs. models from 1850's on:

Interesting, but it does not do much to show that the theories and models have been robustly tested.

 

[ponderingturtle]

Colder certainly, but not warmer, not in recorded history.

I had heard that durring the medieval climatic optimum it was warmer than now. Now that was before modern thermometers, so the actual temp might not be recorded, but what was the temp like before the little ice age? Now it might have been local effect just in europe.

 

[CapelDodger]

I had heard that durring the medieval climatic optimum it was warmer than now. Now that was before modern thermometers, so the actual temp might not be recorded, but what was the temp like before the little ice age? Now it might have been local effect just in europe.

The Medieval Warm Period was, like the Little Ice Age, essentially a North Atlantic phaenomenon, not a global one. It didn't melt the permafrost of the period, but permafrost is melting now, quite rapidly. Permafrost that's been accumulating since the end of the Younger Dryas (if not before) is now melting. Tropical glaciers and icecaps have retreated beyond any point reached since the start of this inter-glacial. Those observations are as good a measure as any thermometer. And there's no sign of the world cooling down, in fact quite the opposite.

 

[ponderingturtle]

The Medieval Warm Period was, like the Little Ice Age, essentially a North Atlantic phaenomenon, not a global one. It didn't melt the permafrost of the period, but permafrost is melting now, quite rapidly. Permafrost that's been accumulating since the end of the Younger Dryas (if not before) is now melting. Tropical glaciers and icecaps have retreated beyond any point reached since the start of this inter-glacial. Those observations are as good a measure as any thermometer. And there's no sign of the world cooling down, in fact quite the opposite.

I thought that they represented legitimate if relatively minor alterations in climate in general, if not nessacarily either warmer in a location or colder in that location. I have heard such a change as being the cause of a civilization collapseing in central america as the rain patterns shifted.

And look at it this way, we have finaly found the north west passage.

 

[CapelDodger]

I thought that they represented legitimate if relatively minor alterations in climate in general, if not nessacarily either warmer in a location or colder in that location. I have heard such a change as being the cause of a civilization collapseing in central america as the rain patterns shifted.

Global warming models are just that, global. They do predict certain regional effects, which the passage of time is confirming.

The Mayans may well have contributed (through deforestation) to the regional climate variation that sparked off their decline. The LIA was probably caused by a variation in the North Atlantic Drift, since the effect was most pronounced in the North Atlantic littoral.

And look at it this way, we have finaly found the north west passage.

Hooray for us. On the other hand, bean yields in Europe are way down. Including mine, and I eat beans more often than I travel to Japan. A lot more often. On yet another hand, Russia has warm-water ports without having to conquer Iran. Swings and roundabouts, I guess.

 

[kevin]

Interesting, but it does not do much to show that the theories and models have been robustly tested.

Robustness comes from peer reviewed examinations such as I linked to. There are additional papers available.

The Real Climate blog is not intended be a completely through research tool. That link was to provide an overview of how they validate. The second link was to provide an example where they validated models against longer time spans than the real climate information talked about.

The models require initial conditions on a great number of variables to begin modeling, more than just temperature. We don't have those numbers so it's hard to initialize the model. It's possible to come up with a set of numbers that make the model work but with nothing to compare against you can't say for sure that the initial conditions were valid.

 

[Cuddles]

I am currious, does anyone know if the models predicting human caused global warming have been run backwards to see if they predict the little ice age and the medieval climatic optimum?

I hear all these predictions, but never any real testing to see how the models fit with historic known climatic changes.

All scientific models and theories are based on observation, in this case of the past climate. A model is then produced based on this to try to predict the future conditions. If a global warming model failed to predict the little ice age, it would be disagreeing with the very data it was based on and would be completely worthless. The only way to test a scientific model is to first ensure it agrees with the known data, in this case the past, and then test it's predictions for the future.

 

[a_unique_person]

In the forty years since the 70s, 40% of Arctic ice has melted. In under 70 years it will all be gone. And what about all this added fresh water in the North Atlantic? Well, things can change fast. And it's happened before. About 8,000 years ago, two vast freshwater lakes in central Canada burst and drained into the Hudson Straight, then into the North Atlantic. There was so much fresh water that water circulation in the North Atlantic stopped. Warm currents which normally travel north from the equator didn't arrive, and temperatures across Europe plummeted. This is the scenario in the film The Day After Tomorrow. Water melting from Arctic ice is thought to pose a similar threat.


At the other end of the globe we're taken on aerial swoops of Antarctic ice cliffs, 700 feet high, and they're falling away, crashing into the ocean. More photos show ice shelfs; there in the 70s, gone today. Powerful graphics show the devastation we can expect, as sea levels rise. Hundreds of millions of people, Gore says, will lose their land, especially in densely populated, low-lying Asian regions. We see how rising sea levels will affect the state of California. This goes some way to explaining the interest of the insurance industry in a warming planet.

According to Science Magazine, December 2004, of all peer-reviewed scientific studies on climate change, 928 papers supported global warming and 0% denied it. In a similar sampling of stories from the mass media, 53% suggested global warming is unproven. In other words, the message people are getting doesn't match the facts. Apart from clearly outlining the problem, An Inconvenient Truth doesn't suggest reasons for Al Gore's perceived failure. That's for other films to explore.

http://www.abc.net.au/rn/scienceshow/stories/2006/1685580.htm#

If the models don't convince you, then the events in progress might.

Melting glaciers and permafrost, for example.

The models are just trying to say what will probably happen, but the warming is clearly evident without them. Unless some magic negative feedback mechanism appears, warming is what it will be.