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Something I hear often from climate scientists, and indeed even politicians, is that we "don't understand climate change very well". And, I'm sure this is true since the climate is a complex adaptive system with thousands of dependent variables, one that we don't yet have the computational sophistication to accurately model. But that doesn't mean we don't know, to a high degree of accuracy, what is going to happen if we make certain changes to the atmospheric composition or take certain actions. We have paleoclimate data ranging extensively far back many hundreds of thousands of years, encompassing global temperature, climate cycling, atmospheric composition, and more.

Sure, the global temperature 100,000 years ago wasn't measured using a thermometer, but we've used various independent proxy methods that all agree with each other to a reasonable degree, right? And so do we not then have a reasonably accurate idea of Earth's climate sensitivity by simply looking at precedent? And paleoclimate will be close to exactly right since it takes all the variables into account exactly in the amount they're needed to be accounted for. Why are we still "uncertain" about the realities of climate change? Why are we not moving on and taking decisive action based on this--what seems to me--reliable, accurate data?

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    $\begingroup$ The problem, at least in the context of AGW, is that nothing similar has happened in the past. At least not since the Permian-Triassic Extinction event en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event The causes of that aren't entirely known/agreed on, due to the very limited data. $\endgroup$ – jamesqf Sep 18 at 18:57
  • $\begingroup$ "Why are we not moving on and taking decisive action" is more of a political question... $\endgroup$ – jeffronicus Sep 18 at 22:05
  • $\begingroup$ What makes you think models aren't used to reconstruct paleoclimate? $\endgroup$ – John Sep 19 at 1:07
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We need both paleodata and models, in fact the relationship between paleodata and the climate at a certain time is typically evaluated through models. Paleodata does not actually measure climate observables directly (tempreture/pressure/salinity/other) but instead is (e.g.) a geochemical quantity related to one of those.

For example, the two most frequently used records are the Antarctica EPICA ice core record and the Lisiecki Raymo benthic stack. The isotopes in these records are affected by temperature at a point on the planet and global ice volume, but how that relates to global temperature and the size of individual ice sheets requires inference from climate models.

Further, climate (or earth systems) models can allow us to probe what could happen in addition to what did happen. Current CO2 levels are reaching a point not seen in the ice core record (our most direct records) and so we need increasingly complex models to get a handle on the range of possibilities.

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It is true that we don't understand climate change very well for the reasons you give in your question, namely that there are so many variables involved and some of them are unknowns. Indirect methods of assessing climate composition and temperature are reasonably accurate for the last few tens of thousands of years, but the further you go back, the more scope there is for error to creep in.

Computers are helpful, but not the be-all and end-all. Garbage in, garbage out, as the old saying goes. There comes a point in the prehistorical climate record where palaeontology is the best guide to what ancient climates were like. Paleoclimatic data can't be exactly right when you get to this point because it is too far back, and although palaeontology can tell you what sort of climate we had millions of years ago, it can't explain what part CO2 abundance played.

Changing the composition of the atmosphere is more difficult than some people imagine. Oxygen and nitrogen are present in countless trillions of tons, and the oxygen is constantly being replenished by photosynthesis. Most photosynthesis takes place in the oceans. This same photosynthesis is also removing CO2 from the atmosphere. Because CO2 is only present in trace amounts, it is the only atmospheric constituent we can do anything about, with the possible exception of methane.

The fact that plants grow more vigorously in an atmosphere where CO2 content is enhanced adds another complication. There is no serious doubt that atmospheric CO2 has increased since the industrial revolution got well underway, that climate change is real, and that we are currently in a warming phase, but whether CO2 is the sole cause of it is another thing. Action is being taken to reduce CO2 in the atmosphere, but not enough to make a measurable difference. The reason much sterner measures will not be taken is that the major industrial countries which are the main emitters are worried about the damage to their industries and economies if they do everything that environmentalists want.

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