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Milieu Hier kunnen alle discussies woden gevoerd over milieu, kernenergie, klimaatswijziging, ....

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Oud 9 mei 2007, 11:20   #1
Pieke
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Standaard De Problemen van Globale Climate Modelling

Computermodellen worden gebruikt om trends in het klimaatgebeuren te verklaren en te voorspellen. Meer nog, ze worden gebruikt om ons de noodzaak van dringende, structurele, globale maatregelen te verduidelijken.
Alles staat of valt dus met de accuraatheid van de klimaatmodellen, niet alleen om het recente verleden te verklaren, maar ook om voorspellingen te doen naar de toekomst toe. En daar wringt het schoentje. De complexiteit van het klimaat, en de complexe interacties van feedbacks en feedforwards, bemoeilijken de omzetting naar computermodellen.
Roger Pielke heeft dat beschreven in volgend artikel, verschenen in Natuur, wetenschap en techniek in januari van dit jaar.

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Oud 17 mei 2007, 17:16   #2
Groentje-18
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Even though the climate is chaotic to some extent, it can be predicted long in advance.
Climate is average weather, and it can vary unpredictably only within the limits set by major influences like the Sun and levels of greenhouse gases in the atmosphere. We might not be able to say whether it will rain at noon in a week's time, but we can be confident that the summers will be hotter than winters for as long as the Earth's axis remains tilted.
The validity of models can be tested against climate history. If they can predict the past (which the best models are pretty good at) they are probably on the right track for predicting the future – and indeed have successfully done so.
Clouded judgement

Climate modellers may occasionally be seduced by the beauty of their constructions and put too much faith in them. Where the critics of the models are both wrong and illogical, however, is in assuming that the models must be biased towards alarmism – that is, greater climate change. It is just as likely that these models err on the side of caution.
Most modellers accept that despite constant improvements over more than half a century, there are problems. They acknowledge, for instance, that one of the largest uncertainties in their models is how clouds will respond to climate change. Their predictions, which they prefer to call scenarios, usually come with generous error bars. In an effort to be more rigorous, the most recent report of the IPCC has quantified degrees of doubt, defining terms like “likely” and “very likely” in terms of percentage probability.
Given the complexity of our climate system, most scientists agree that models are the best way of making sense of that complexity. For all their failings, models are the best guide to the future that we have.
Finally, the claim is sometimes made that if computer models were any good, people would be using them to predict the stock market. Well, they are!
A lot of trading in the financial markets is already carried out by computers. Many base their decisions on fairly simple algorithms designed to exploit tiny profit margins, but others rely on more sophisticated long-term models.
Major financial institutions are investing huge amounts in automated trading systems, the proportion of trading carried out by computers is growing rapidly and some individuals have made a fortune from them. The smart money is being bet on computer models.

http://environment.newscientist.com/...change/dn11649
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Oud 17 mei 2007, 21:20   #3
Stratcat
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Citaat:
Oorspronkelijk geplaatst door Groentje-18 Bekijk bericht
Even though the climate is chaotic to some extent, it can be predicted long in advance.
Climate is average weather, and it can vary unpredictably only within the limits set by major influences like the Sun and levels of greenhouse gases in the atmosphere. We might not be able to say whether it will rain at noon in a week's time, but we can be confident that the summers will be hotter than winters for as long as the Earth's axis remains tilted.
The validity of models can be tested against climate history. If they can predict the past (which the best models are pretty good at) they are probably on the right track for predicting the future – and indeed have successfully done so.
Clouded judgement

Climate modellers may occasionally be seduced by the beauty of their constructions and put too much faith in them. Where the critics of the models are both wrong and illogical, however, is in assuming that the models must be biased towards alarmism – that is, greater climate change. It is just as likely that these models err on the side of caution.
Most modellers accept that despite constant improvements over more than half a century, there are problems. They acknowledge, for instance, that one of the largest uncertainties in their models is how clouds will respond to climate change. Their predictions, which they prefer to call scenarios, usually come with generous error bars. In an effort to be more rigorous, the most recent report of the IPCC has quantified degrees of doubt, defining terms like “likely” and “very likely” in terms of percentage probability.
Given the complexity of our climate system, most scientists agree that models are the best way of making sense of that complexity. For all their failings, models are the best guide to the future that we have.
Finally, the claim is sometimes made that if computer models were any good, people would be using them to predict the stock market. Well, they are!
A lot of trading in the financial markets is already carried out by computers. Many base their decisions on fairly simple algorithms designed to exploit tiny profit margins, but others rely on more sophisticated long-term models.
Major financial institutions are investing huge amounts in automated trading systems, the proportion of trading carried out by computers is growing rapidly and some individuals have made a fortune from them. The smart money is being bet on computer models.

http://environment.newscientist.com/...change/dn11649
Nu ben ik overtuigd.
Hoe kon ik ooit twijfelen?
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Oud 30 juni 2007, 13:56   #4
Pieke
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De eerste audit van AR4 is verschenen. Hoofdstuk 8 wordt tegen de lamp gehouden door J Scott Armstrong, Marketing professor en specialist in "scientific forecasting mehods". Hij schreef een aantal boeken over voorspellingen 'Long-range forecasting" en 'Principles of Forecasting: A Handbook for Researchers and Practitioners'. Hij is dus een autoriteit op het gebied van 'evidence-based forecasting'.

Hoofdstuk 8 handelt over de klimaatmodellen. Hoofdstuk 10 over de voorspellingen.

Uit het artikel:

Citaat:
These forecasts are contained in Chapter 10 of the Report and the models that are used to forecast climate are assessed in Chapter 8, “Climate Models and Their Evaluation” (Randall et al. 2007). Chapter 8 provided the most useful information on the forecasting process used by the IPCC to derive forecasts of mean global temperatures, so we audited that chapter.
Citaat:
The Intergovernmental Panel on Climate Change’s Working Group One Report predicts dramatic and harmful increases in average world temperatures over the next 92 years. We asked, are these forecasts a good basis for developing public policy? Our answer is “no”.
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The forecasts in the Report were not the outcome of scientific procedures. In effect, they present the opinions of scientists transformed by mathematics and obscured by complex writing.
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We found no references to the primary sources of information on forecasting despite the fact these are easily available in books, articles, and websites.
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We found that the forecasting procedures that were used violated 72 principles. Many of the violations were, by themselves, critical. We have been unable to identify any scientific forecasts to support global warming.
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Those who are not familiar with the research on expert forecasting believe that surveys of experts provide useful forecasts.
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Comparative empirical studies have routinely concluded that judgmental forecasting by experts is the least accurate of the methods available to make forecasts.
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International surveys of climate scientists from 27 countries, obtained by Brat and von Storch in 1996 and 2003, were summarized by Bast and Taylor (2007). Of over 1,060 respondents, 35% agreed with statement, “Climate models can accurately predict future climates,” and 47% percent disagreed.
Citaat:
Carter, et al. (2006) examined the Stern Review (Stern 2007). They concluded that the Report authors made predictions without any reference to scientific forecasting.
Citaat:
Pilkey and Pilkey-Jarvis (2007) concluded that the long-term climate forecasts that they examined were based only on the opinions of the scientists. The opinions were expressed in complex mathematical terms. There was no validation of the methodologies.
Citaat:
We examined the references in Chapter 8 (on evaluation) of the 2007 Intergovernmental Panel on Climate Change’s Working Group One report. There were 788 references. Of these, none had any apparent relationship to forecasting methodology.(...)It is hard to understand how scientific forecasting could be conducted without any reference to the literature on how to make such forecasts.
Citaat:
Carter (2007) examined evidence on the predictive validity of the general circulation models (GCMs) used by the IPCC scientists. He found that while the models included some basic principles of physics, scientists had to make “educated guesses” about the values of many parameters because knowledge about the physical processes of the earth’s climate is incomplete.
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Further, individual GCMs produce widely different forecasts from the same initial conditions and minor changes in parameters can result in forecasts of global cooling
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We are not suggesting that climate change cannot be forecast, only that this has yet to be demonstrated.
Citaat:
The Stern Review concluded that, “The scientific evidence is now overwhelming; climate change presents very serious global risks, and it demands an urgent global response” (Stern 2007, p. xv). We have not been able to find any scientific evidence to support such a claim. We can only hope that before committing resources, decision makers will insist on scientific forecasts rather than accept the opinions of some scientists.
Citaat:
Those who advocate various positions on the climate owe it to the people who would be affected by the policies they recommend to base their advocacy on scientific forecasts that address all four of the key areas that are necessary for a rational analysis of the problem.

Laatst gewijzigd door Pieke : 30 juni 2007 om 13:58.
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