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There is lots of evidence that I have seen showing correlation between human activities and climate change but what evidence is there to support causation?

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This question is ingenuous enough that I have to ask what research you actually did before asking. –  David Gerard Feb 25 '11 at 0:39
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@David I have done no more research than what is on the TV / newspaper websites / what is taught at school. –  david4dev Feb 25 '11 at 0:50
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Do not be afraid, others will vote, as this is one of the hottest questions nowadays. Be prepared to lengthy, sometimes disorganized and emotional (probably on both sides) discussion. –  Suma Feb 25 '11 at 13:02
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@Suma What do you mean by “correlation was not proved sufficiently”? The current trend is obvious: Temperature is rising, man-made greenhouse gases (CO2) are rapidly rising. There are hard numbers for both. Historical records could show that this correlation is not causation. But the correlation is there in plain eyesight. –  Konrad Rudolph Mar 7 '11 at 22:11
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Correlation is itself evidentiary support for causation, but certainly not proof of causation, but it's only indirect evidence. However, there is massive, overwhelming direct evidence of AGW, which is why 975 of climate scientists and 100% of reputable science organizations accept AGW. –  Jim Balter Mar 16 '11 at 13:28
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5 Answers

up vote 72 down vote accepted

Humans affect the weather in mainly the following ways:

Direct emissions of various gasses

Typically CO2 is considered, but also other greenhouse gasses. The greenhouse effect of carbon dioxide was first measured in 1859.

Greenhouse effectsource

In the 19th century, scientists realized that gases in the atmosphere cause a "greenhouse effect" which affects the planet's temperature. These scientists were interested chiefly in the possibility that a lower level of carbon dioxide gas might explain the ice ages of the distant past. At the turn of the century, Svante Arrhenius calculated that emissions from human industry might someday bring a global warming. Other scientists dismissed his idea as faulty. In 1938, G.S. Callendar argued that the level of carbon dioxide was climbing and raising global temperature, but most scientists found his arguments implausible. It was almost by chance that a few researchers in the 1950s discovered that global warming truly was possible. In the early 1960s, C.D. Keeling measured the level of carbon dioxide in the atmosphere: it was rising fast. Researchers began to take an interest, struggling to understand how the level of carbon dioxide had changed in the past, and how the level was influenced by chemical and biological forces. They found that the gas plays a crucial role in climate change, so that the rising level could gravely affect our future.

The Carbon Dioxide Greenhouse Effect

List of greenhous gasessource

Farming

Another man-made source is the direct emission of greenhouse gasses through farming (funny, but true!): manure (and cows) produce methane which is a pretty effective greenhouse gas.

US cattle methane emissionssource

Increasing atmospheric concentrations of methane have led scientists to examine its sources of origin. Ruminant livestock can produce 250 to 500 L of methane per day. This level of production results in estimates of the contribution by cattle to global warming that may occur in the next 50 to 100 yr to be a little less than 2%.

Methane emissions from cattle

Deforestation

Plants "fix" carbon (a phenomena called "Carbon sequestration"), the less plants, the less fixing (and the more carbon released by fires).

Forest carbon cyclesource

Carbon sequestration

Carbon sequestration: Forest and soil, by Jukka Muukkonen, Statistics Finland

The Oceans

Changes to the biological equilibrium of the oceans affect the climate because marine biology is known to have a large carbon-fixating effect

Carbon sequestration by the oceansource

One of the most promising places to sequester carbon is in the oceans, which currently take up a third of the carbon emitted by human activity, roughly two billion metric tons each year.

Carbon Sequestration in the Ocean

Conclusion

All four of these effect can be shown in a laboratory and no model is required to do so, but we have very very good models to explain the lab experiments.

Differently from the lab, the whole climate system is much less understood. And, yes, the model are not as reliable as we would like.

However — due to our knowledge of chemistry — it is undeniable that we are affecting climate. Note that nobody has asserted that human intervention is the only cause of climate change, but it can be said, with a straight face, that humans are changing climate. A very simple example, the rise in temperature melts ice at the pole - which is not only responsible for reflecting some light out of the atmosphere, but also contains methane, which is then released.

The debate can only be on "how much" and "how well can we reverse the trend (even beyond our contribution)".

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+1 -- nice breakdown showing some parts we can be sure of. –  Russell Steen Mar 7 '11 at 22:39
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"Plants "fix" carbon, the less plants, the less fixing (and the more carbon released by fires)" My understanding is that as wood rots it also releases CO2, so fire is not necessary. –  jozzas May 17 '11 at 4:31
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@jozzas: that is indeed the case. this is the real dangers of threshold events: we may calculate that by certain amount of CO2 emission, we'll only raise the mean temperature by such and such, but there are milestones along the way, such as the temperature at which the tundra will melt, which will cause massive emissions on its own, by rotting the peat that has been constantly frozen. –  David Hedlund May 26 '11 at 8:05
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We have been doing thing some of those thing for centuries. Nothing in that is evidence that the human contribution is more that negligible. –  Guillaume Coté Apr 10 '12 at 14:54
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You should call it ranching, or animal husbandry, not farming. If you are going to count cows as a direct effect, you need to count the plants that we grow as a direct negative effect as well. –  user1873 May 5 '12 at 23:50
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Yes, humans cause climate change (each doubling of C02 causes about 1C increase). It's really a meaningless question. Any input to any chaos system will cause some effect on that system. Do we know what effect we are having? Can we measure/predict it? Do we have any idea how to alter/change/control that change? And really, what the hubbub is about is not "will the client change", but "will it change in a really bad way"

According to Peter Stott models failed to predict current temperatures (though he echoes the recurring claim that they'll be correct in the future), which means that no existing model has predicted, correctly, any significant amount of future climate change, and new research is steadily revealing flaws in existing catastrophic prediction models, so the answer to those questions should be no.

A model which has yet to make an accurate prediction cannot be said to be an accurate model. Therefore, we don't know what effect we're having, we can't predict it, and as a result of those two, we do not know how to alter or control that affect.

Causation on a chaos system is nigh impossible to prove with our current abilities, so we rely on modeling. Unfortunately, instead of insisting that a model make a prediction and have it come true before accepting it, we accept models as true if they accurately predict past events (not kidding), which is trivially easy.

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@Odd -- A chaotic system is one for which small changes to input values cause large and hard to predict changes in output values, or said another way, systems that are highly sensitive to small changes. An example of known small changes that affect climate would be sunspots, CO2, ozone, atmospheric dust... –  Russell Steen Feb 25 '11 at 5:57
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I understand what a chaotic system is (and I concede that tomorrow's maximum temperature is based on one) but we are debating average temperatures over years, which may be predictable even from an underlying chaotic system. I can't predict heads or tails from one coin flip, but I can from 10,000. –  Oddthinking Feb 25 '11 at 6:00
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I agree that it may be predictable. Anything is possible. But so far no one has succeeded at it. I'm waiting for the first citation to a model which has successfully predicted a future climate change. -- I provided examples of known factors of high sensitivity to small changes. –  Russell Steen Feb 25 '11 at 6:03
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Now I don’t like the current last sentence.Because it heavily implies that predictions can only be valid if applied to future events (and even ridicules this). Which is false, an shows a gross misunderstanding of science. Predicting past events is absolutely fine. Many scientific theories started out this way. Once again, look at evolution. Only quite recently have we successfully predicted future observations using it, and it was accepted as true (and justly so) long before that. –  Konrad Rudolph Mar 8 '11 at 7:40
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According to that we should have also seen a 1.1C increase by 2010, and a .58 increase as of 1990. His prediction was off by .2C in 1990 (almost 50% of his projected increase) and .5C in 2010, warming only about half what he predicted. I'm sorry, but making 35 years of predictions and being right one year isn't very good prediction. –  Russell Steen Mar 11 '11 at 20:20
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I can’t answer the question directly.

However, there has been at least one large-scale review on the scientific consensus. And it can safely be said that the scientific consensus is overwhelmingly that the current trend in global warming is caused by mankind. It would be weird if this consensus came to be without good evidence.

The review did a literature mining for peer-reviewed literature published between 1993 and 2003 with the words “global climate change” in their abstracts. They found 928 abstracts. Of those, 75% explicitly or implicitly endorsed AGW. 0% rejected it. 25% did not take a position.

As Russell has noted in the comment, these also include mitigation proposals which shouldn’t be counted towards the consensus (since they merely refer to other papers) but were. Furthermore, the review only used one key phrase for their search, excluding parts of the available literature.

So the review contains one systematic error (inclusion of mitigation proposals) and one unsystematic error. Nevertheless, because of the large number of papers it is still safe to assume that these will not change the reported consensus significantly.

Note that this does not mean that there are no dissenting opinions in the scientific community – there are – merely that the overwhelming majority of experts accepts AGW and that they probably have good reasons to do so.

(Still, this “answer’ is more of an FYI than an actual answer since, I want to stress again, it does not provide any of the evidence asked for.)

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Ideally, this should be a comment. (But awesome comment.) –  Borror0 Mar 7 '11 at 22:12
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The large scale review you link is very skewed. For instance, they include mitigation proposals as supporting papers. So if one paper demonstrates a cause, and five papers use that paper as a reference for which to show mitigation, it would be counted six times. Also the study only included 1993 to 2003. They only showed papers using the words "climate change", instead of doing a thorough analysis. It looks, all in all, like someone just googled a journals database, with a specific goal in mind, as noted by the fact that anything that didn't specifically counter AGW was counted as support –  Russell Steen Mar 7 '11 at 22:26
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@jwenting “there is no ‘consensus’” – well, I have cited sources that show the opposite. What are your sources? As it stands, your statement is ridiculous. || “even if there were ‘concensus’ isn't scientific evidence” – exactly, that’s what I also said in the answer. The other consensuses you cite rely on ignorance rather than data and don’t pertain to the discussion. –  Konrad Rudolph Mar 8 '11 at 8:54
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@jwenting There is a consensus; the claim to the contrary is a lie. And consensus is the consequence of there being scientific evidence -- in this case, overwhelming scientific evidence. Against this consensus, you have ... nothing. Your analogies are irrelevant -- this consensus is among climate scientists, whereas the consensus against Galileo was among people who had never looked through a telescope. –  Jim Balter Mar 16 '11 at 13:43
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@vartec “consensus”, according to the dictionary and common usage, means “general agreement”. “unanimity” means “ absolute agreement”. They may be used synonymously, or they may denote different degrees of agreement. “Overwhelming consensus” is not newspeak – and I resent the accusation. I used it to make it clear that there is more than “just” general agreement, i.e. that the agreement is almost unanimous. There is nothing “newspeak” about this, it’s a normal juxtaposition of words to intensify a meaning. –  Konrad Rudolph Jun 26 '12 at 9:19
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The IPCC report gives the following probabilities:

The total radiative forcing of the Earth’s climate due to increases in the concentrations of the LLGHGs CO2, CH4 and N2O, and very likely the rate of increase in the total forcing due to these gases over the period since 1750

What do they mean when they say very likely? They mean 0.95 < p < 0.99. When someone says that the evidence for climate change is comparable to the evidence for evolution they are either advocating that the IPCC is wrong by an order of magnitude or they are gravely insulting academic biology.

255 members of the US National Academy of Sciences including 11 Nobel Price winners issued a letter that claims:

For instance, there is compelling scientific evidence that our planet is about 4.5bn years old (the theory of the origin of Earth), that our universe was born from a single event about 14bn years ago (the Big Bang theory), and that today's organisms evolved from ones living in the past (the theory of evolution). Even as these are overwhelmingly accepted by the scientific community, fame still awaits anyone who could show these theories to be wrong. Climate change now falls into this category.

In an attempt to defend orthodox wisdom mainstream scientists seem to be willing to pretend that the evidence is for climate change is a lot better than it actually is.

Other people who see themselves in defense of climate change think that the IPCC is a bit overconfident.

There are a lot of reasons why that might be the case:

  • Humans typically suffer from confirmation bias. Even a friendly reading of the climate gate emails that Wikileaks published suggest that they don't engage in mental strategies to reduce their vulnerability to confirmation bias.
  • The computer code that they use to generate the models has low standards. It has probably a lot of bugs that throw extra inaccuracy into the models that aren't accounted for.
  • Some data isn't openly available to allow for independent verification.
  • We have seen in the financial crisis that complex computer models often include a lot of assumptions that make them overconfident.
  • Climate scientists test their models on past data and generally don't make predictions about the future to test their models. As the models have a lot of parameters that makes the models to appear better than they are.

That doesn't mean that we should assume p=0 but it might be reasonable to use a p value that's a bit lower than the IPCC value. If we go from 0.95 < p < 0.99 to 0.80 < p < 0.90 the p value for the climate change claims isn't statistically significant anymore. Even if we just go to 0.90 < p < 0.95 it's not significant anymore.

Why does that matter? Isn't p=0.80 enough for starting to reduce CO2 emissions? That might be true. If we however start geoengieering the confidence in our models matter a great deal. Starting geoengieering on the assumptions that our models are magnitudes better than they really are is dangerous.

Part of being a good skeptic should be to avoid being more confident in your beliefs than the data warrants. We should move past binary classification. Instead of showing tribal loyalty we should call out our friends when they overstate the evidence.

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Some datasets are not available publicly, but there is still a lot of data that is available for independent verification. –  Fabian Mar 8 '11 at 13:44
    
@Fabian: Okay, I changed the statement to saying from "the data" to "some data". –  Christian Mar 8 '11 at 14:36
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I don’t understand your allegation in connection with evolution. (1) who is claiming that there is comparable evidence for both? (2) why would this mean the IPCC were wrong by an order of magnitude? Magnitude of what? The p-value? The confidence? –  Konrad Rudolph Mar 8 '11 at 19:02
    
(1) The quoted paragraph. I provided a link. Saying that two theory are in the same class of evidence suggests that both have an equal chance of being true. (2) According to the IPCC this climate change claim has a likelihood of being true that's under 0.99. I think we can agree that the likelihood for evolution being true should be over 0.99999999. –  Christian Mar 8 '11 at 19:53
    
The comparison of economic models to models in any physical science is laughable. Also, if you have ever read any scientific code, you will know that it is almost uniformly of poor coding standard (computer science may be an exception here). Note that coding standards refers only to stylistic conventions, which certainly improve maintainability, but are totally independent from whether the code works as intended or not. The link you provide is mostly specious remarks, with little or no actual criticism of code functionality or quality. –  naught101 Nov 5 '12 at 6:05
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Trends in solar radiation don't match up with trends in temperature. One of the arguments from skeptics of climate change is that rising global temperatures are a natural phenomenon caused by the Sun. However, most measures of total solar irradiance (also known as solar radiation, the electromagnetic energy incident on Earth's surface) show that, on the whole, it is falling. (This, of course, necessitates taking a step back to see larger TSI trends, beyond the valleys and peaks caused by the solar cycle.)

ACRIM's Irradiance Data from 1978-1984

In short, it looks like the Sun is actually cooling. Not dramatically, but it's certainly not becoming hotter, and certainly not enough to account for rising global temperatures. In fact, when we juxtapose climate temperature with solar irradiance, as shown below, we find that they have little to do with one another. This is a basic, common sense approach, but if you require mathematical proof, then Skeptical Science has put together a digestible calculation and analysis. Anyhow, just a graph:

From atmospheric scientist Bart Verheggen's blog, *Our Changing Climate*

So you may not agree that global warming is anthropogenic. But as scientists look at solar irradiance as just one piece of evidence that correlates with various others that fellow commenters have left, it's becoming increasingly clear that it's not caused by the Sun. What does that leave?

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Welcome to Skeptics. This answer demolishes one alternative hypothesis to anthropogenic climate change, but that isn't enough to demonstrate it is anthropogenic. –  Oddthinking Nov 7 '13 at 23:29
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As Hume argued, it is impossible to gain certain knowledge of causation through empirical means (as we can only directly observe correlation, not causation). This means science essentially boils down to a search for the best explanation, and the way that is done is by first showing the alternative explanations are flawed, and then argue that amongst the hypotheses that remain one has better support from theory. This is not the kind of question that can be given a definitive answer based purely on observations. –  Dikran Marsupial Nov 8 '13 at 10:06
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At this stage (there already being a good answer higher up) it seems more useful to demolish the alternatives, rather than to have another go at answering the question completely, which would just end up with a great deal of repetition. Just my opinion, of course. –  Dikran Marsupial Nov 8 '13 at 10:18
    
Why look at only the last 35 years or so? Your link to "Historical Total Solar Irradiance" lasp.colorado.edu/lisird/tsi/historical_tsi.html from year 1600 until now to me seems to explain a lot more of the temperature changes. –  dontomaso Dec 29 '13 at 14:27
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