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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Is the science settled?

What the science says...

Select a level... Basic Intermediate

That human CO2 is causing global warming is known with high certainty & confirmed by observations.

Climate Myth...

The science isn't settled

"Many people think the science of climate change is settled. It isn't. And the issue is not whether there has been an overall warming during the past century. There has, although it was not uniform and none was observed during the past decade. The geologic record provides us with abundant evidence for such perpetual natural climate variability, from icecaps reaching almost to the equator to none at all, even at the poles.

The climate debate is, in reality, about a 1.6 watts per square metre or 0.5 per cent discrepancy in the poorly known planetary energy balance." (Jan Veizer)

Skeptics often claim that the science of anthropogenic global warming (AGW) is not “settled”. But to the extent that this statement is true it is trivial, and to the extent that it is important it is false. No science is ever “settled”; science deals in probabilities, not certainties. When the probability of something approaches 100%, then we can regard the science, colloquially, as “settled”.

The skeptics say that results must be double-checked and uncertainties must be narrowed before any action should be taken. This sounds reasonable enough – but by the time scientific results are offered up to policymakers, they have already been checked and double-checked and quintuple-checked.

Scientists have been predicting AGW, with increasing confidence, for decades (indeed, the idea was first proposed in 1896). By the 1970s, the scientific community were becoming concerned that human activity was changing the climate, but were divided on whether this would cause a net warming or cooling. As science learned more about the climate system, a consensus gradually emerged. Many different lines of inquiry all converged on the IPCC’s 2007 conclusion that it is more than 90% certain that anthropogenic greenhouse gases are causing most of the observed global warming.

Some aspects of the science of AGW are known with near 100% certainty. The greenhouse effect itself is as established a phenomenon as any: it was discovered in the 1820s and the basic physics was essentially understood by the 1950s. There is no reasonable doubt that the global climate is warming. And there is also a clear trail of evidence leading to the conclusion that it’s caused by our greenhouse gas emissions. Some aspects are less certain; for example, the net effect of aerosol pollution is known to be negative, but the exact value needs to be better constrained.

What about the remaining uncertainties? Shouldn’t we wait for 100% certainty before taking action? Outside of logic and mathematics, we do not live in a world of certainties. Science comes to tentative conclusions based on the balance of evidence. The more independent lines of evidence are found to support a scientific theory, the closer it is likely to be to the truth. Just because some details are still not well understood should not cast into doubt our understanding of the big picture: humans are causing global warming.

In most aspects of our lives, we think it rational to make decisions based on incomplete information. We will take out insurance when there is even a slight probability that we will need it. Why should our planet’s climate be any different?

Basic rebuttal written by James Wight

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 7 July 2015 by James Wight. View Archives

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Comments 76 to 81 out of 81:

  1. @ Bob, It's just a miscommunication on my part. I couldn't think of how to word my question that's why I ended up asking that subjective question in the beginning.

    I'm not questioning the reliability of models...models are used all the time like CFD. The limitations in CFD is due to budget and computer power. I was wondering where the uncertainty in climate was mainly coming from like the uncertainty in equilibrium climate sensitivity from double co2. was it because of theory or tech? but you guys already answered my questions that the it's due to limits on current tech. thanks.

  2. engineer

    There are a range of potential feedbacks that are hard to quantify because each one of them is an entire field of study in its own right. For example:

    Vegetation response. Will the Amazon for example remain a rainforest? Become a drier forest? Grassland? Each has different implications for carbon cycle sinks and sources, surface albedo and evapotranspiratioin patterns.

    Ice sheet retreat. What are the dynamics of any decline in Greenland ice sheet cover, WAIS, EAIS? Timing and extent of this for any particular level of GHG forcing again has significant impacts on albedo.

    Ocean Circulation. Major ice sheet melt might impact on the Thermo-Haline circulation that drives ocean currents - there is some evidence this was a part of what happened during the warming from the last Glacial Maximum. If ocean currents change, this can alter the distribution of where heat is transported to. Thus cloud patterns, climate zones, all sorts of things.

    Methane release from Permafrost. How fast will permafrost melt and where? Will this produce more aerobic or anaerobic decomposition of the defrosted organic matter, influencing whether carbon outgasses as Methane or CO2. Higher rates of methane release will have a greater short term warming effect than if it is released as CO2 even though the longer term impact will be the same as ultimately the methane is oxidised to CO2

    This stuff is too hard to do at a theoretical level and even modelling involves stacking models on models. Thats why paleo climate studies are an important reference point. That is what climate has actually done in other circumstances.

  3. @engineer

    Here's a concise descpriton of how global climate models evolved and function.

    By the mid-1990s, it was possible to investigate the causal mechanisms behind changes in Earth's climate using relatively sophisticated mathematical models of Earth's climate. These models solved the same complex equations of atmospheric physics that numerical weather prediction models did. But they also took into account components of the climate system other than the atmosphere, including the oceans, the continental ice sheets, and even life on Earth (collectively known as the "biosphere"), and they attempted to account for the physical, chemical, and biological interactions among these components. Of course, no theoretical model is ever perfect; even the best model is only an idealization of the actual world. There are always real-world processes that cannot be captured—for example, in the case of a numerical climate model, individual clouds or small-scale air currents like dust devils—that are simply too small for the model to resolve. The key question is, can the model be shown to be useful? Can it make successful predictions?

    Source: How Do We Know Humans are Responsible for Global Warming? by Michael Mann, WeatherUnderground, April 22, 2013

  4. engineer:

    I don't think it is correct to just call it a technical problem. To follow on Glenn's comment, when you are trying to model today's climate, you can get away with saying "this is what the vegetation is", or "this is where the ice sheets are", etc., and simply measure the required input parameter for the climate models. You can even do that to a certain extent for past climates, as there are proxies that will give you an indication of vegetation cover, ice cover, etc. Understanding why the vegetation, ice, etc. are the way they are is a help, but not an absolute necessity to be able to develop a good understanding and a reliable model of current climate.

    Contrast that with the future: we can't measure the vegetation cover or ice sheet distributions - we have to model them. But uncertainties in how vegetation responds to a changing climate is not a problem that necessarily requires increased understanding of climate dynamics - it is a problem of understanding vegetation dynamics. Predicting something like future aerosols not only requires estimating future levels of existing emission sources (which requires economic modelling), it also requires assumptions of what future combustion technology might produce, and what social policy choices might be made. You can start by assuming they won't change, but proper policy decisions require that you also evaluate what might happen if they do change (using realistic ranges of possibiliites).

    It's a classic multi-disciplinary issue.

  5. Replying to TLander from another thread :-

    TLander, you are deceiving yourself if you propose that there is some as-yet-unknown or undiscovered dominant cause of the rapid modern global warming.   You wish to imply that that we should halt activities designed to mitigate CO2 emissions . . . until such time that your not-even-yet-hypothesized mechanism of rapid global warming gets discovered (and also shows itself to be so strong and beyond human influence, that it is futile for humans to attempt softening [through CO2 reduction] the warming effect caused by your notional new discovery).

    TLander, there are at least two counter-arguments against your proposal.

    You are already aware of (A) : That for many decades, many tens of thousands of scientists have very closely studied climate-related science (and the underlying physics).   This is not the era of science in the mid-1800's .   The chance of them entirely missing a major previously-unknown factor . . . is exceedingly small.  Indeed, so vanishingly small, that surely no sane man would gamble the health of his planet on that chance.

    But you might not be aware of (B) : That CO2 (and the other greenhouse gasses) form a superb match for the physics of modern global warming — both in theory and in empirical evidence.  Merely a correlation, you say?  No — the theory backs the observations, and the observations back the theory.   So, if you are proposing an as-yet-unimagined novel "dark cause" of Global Warming . . . then you have a big problem, a double explanation which you need to pull out of your hat.

    Firstly you need to find a "dark cause" factor which very closely matches the historical & growing effect of CO2.  Then you also need to discover another new factor — this time a cooling factor which matches and cancels out the known warming effect of CO2.

    All a very big ask.

    TLander, the implication is that you have not thought things through.

  6. Regarding the plane falling out of the sky analogy... If you had to get on one of two planes, would you pick the one with a tem percent chance of survival or the one with a 90 percent chance of survival.

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