So the way you test models is by hindcasting and seeing how your model, given initial conditions, predicts what you know already happened. The answer to your question is that we know what the climate response was over a century. And the models accurately fit the data already obtained. What all the models have in common is that, despite accurately modelling the data for a century, they suddenly fail to model accurately the data obtained since the 1970s without including CO2 and other gases added to the atmosphere by humans.
The models do exactly as you suggest - they model the climate of the past when human CO2 was a minor factor and fail to model recent times until they add in the effect of human CO2. If your argument is that you also need to get experimental evidence of what would have happened had we not burnt fossil fuels, I'm afraid that's impossible.
It's sort of like saying that you really need to know what would have happened had the car not hit a pedestrian in order to establish that the car is responsible after it hit a pedestrian ...
Kudos to Elizabeth ---
If you can force yourself to read her answer with a clear objective mind, you might become the first Denier here not hopelessly looking in the wrong directions – and by wrong direction, I mean not looking where a real skeptic would look for evidence that questioned AGW.
There is much I would like to add to Elizabeth’s response, but I could not let kano’s lies go unanswered.
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Kano ---
>>The IPCC's mandate was to evaluate the effect of CO2 on the climate, not to study natural climate causes<<
CO2 became a topic of interest only when - after 150 of years of scientific research on the natural causes of climate change and changes in climate variables that had identified predictable changes in temperature based on natural causes (changes in the sun, the earth’s orbit, tectonic activity, etc.) – observed temperature data stopped following all of the natural causes that it had always been known to follow.
Scientists conducted thousands of experiments using every possible combination of natural causes trying to find a “natural” explanation for the empirical temperature data. In every scientific test, the known natural causes of changes in temperature failed. Only then did scientists begin looking at anthropogenic atmospheric CO2 as a possible explanation.
Your ignorance of scientific methodology can be legitimately explained; however, your ignorance of the history of climate research that occurred during you adult life has no honest explanation. You simply have never spent one minute trying to learn anything about science or climate. And – that is what makes you both a liar and a Denier.
The IPCC's mandate was to evaluate the effect of CO2 on the climate, not to study natural climate causes (forcing is a stupid word) of course in building a model they would have to add them in (those they understood and could calculate) but the main emphasis was always the effect of CO2 and they cared little about the rest.
It was not until the hiatus, that they began to investigate ocean cycles and other phenomomen so that they could explain it away.
You are right about that being the big problem with alarmists. They simply don't know what the climate would be without our CO2 emissions. They like to pretend that they do and real scientist wouldn't. Their political motivations explains why they are so arrogant. Their models clearly haven't worked. They redraw the lines in the sand and constantly pretend that they didn't make the predictions that all of us remember them making. They want to have their wolf and eat it too.
They do not know all of the natural forcings. They make a best guess about what they know, and then attempt to build a model that matches the observed reality. They adjust this to get the best match they can. there are so many variables, that they can do a reasonable job of matching, and still produce an output that they want. Search RealClimate for Keystone XL, and you will see a post where the author works with a model that has been tuned to yield a 3C response.
Now the effects of clouds is unknown, and this unknown is enough to smash the results of the models. A 5C response can drop to .5C because of the effects of clouds.
It's being done under a "microscope" of information. The "BIG PICTURE" is a simple "trial" being shown through climate modeling. The "flexible forcing" is barely understood by climate science. How they can actually derive an answer to "how the climate works" is and always been based on a "preponderance of evidence" and "arrogant assumptions".
Neil deGrasse Tyson explains the discovery of Climate Change and the Greenhouse Effect.
From the past, with proxies.
In the present time, by direct measurement.
The basic AGW theory claims that most of the recent warming is due to increases in atmospheric CO2. To make that claim, you would need to know what the climate system response would have been without any fossil fuel burning (or land use change for that matter). I have always found it difficult to figure out how we know that or where that has been studied and explained.
What I have found are a few studies which briefly mention this:
Figure 1(b): http://thingsbreak.files.wordpress.com/2012/01/improved-constraints-on-21st-century-warming-derived-using-160-years-of-temperature-observations.pdf
Figure 2(b): http://www.andywightman.com/docs/metoffice_climatepaper.pdf
These studies are about the attribution of warming and show the results of climate models which are run with and without CO2 forcing. And frankly, the discussion in the text doesn't really describe what I am asking.
My question is how did they determine the natural forcing? How would the above "natural" model scenarios be validated with real world data?
Don't the above figures imply that ALL of the recent warming is due entirely to CO2 emissions?
