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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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How do we know more CO2 is causing warming?

What the science says...

Select a level... Basic Intermediate Advanced

An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of empirical evidence.

Climate Myth...

Increasing CO2 has little to no effect

"While major green house gas H2O substantially warms the Earth, minor green house gases such as CO2 have little effect.... The 6-fold increase in hydrocarbon use since 1940 has had no noticeable effect on atmospheric temperature ... " (Environmental Effects of Increased Atmospheric Carbon Dioxide)

Predicting the Future

Good scientific theories are said to have ‘predictive power’. In other words, armed only with a theory, we should be able to make predictions about a subject. If the theory’s any good, the predictions will come true.

Here’s an example: when the Table of Elements was proposed, many elements were yet to be discovered. Using the theory behind the Periodic Table, the Russian chemist Dmitri Mendeleev was able to predict the properties of germanium, gallium and scandium, despite the fact they hadn’t been discovered.

The effect of adding man-made CO2 is predicted in the theory of greenhouse gases. This theory was first proposed by Swedish scientist Svante Arrhenius in 1896, based on earlier work by Fourier and Tyndall. Many scientist have refined the theory in the last century. Nearly all have reached the same conclusion: if we increase the amount of greenhouse gases in the atmosphere, the Earth will warm up.

What they don’t agree on is by how much. This issue is called ‘climate sensitivity’, the amount the temperatures will increase if CO2 is doubled from pre-industrial levels. Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

What Goes Down…

The greenhouse effect works like this: Energy arrives from the sun in the form of visible light and ultraviolet radiation.  The Earth then emits some of this energy as infrared radiation. Greenhouse gases in the atmosphere 'capture' some of this heat, then re-emit it in all directions - including back to the Earth's surface.

Through this process, CO2 and other greenhouse gases keep the Earth’s surface 33°Celsius (59.4°F) warmer than it would be without them. We have added 42% more CO2, and temperatures have gone up. There should be some evidence that links CO2 to the temperature rise.

So far, the average global temperature has gone up by about 0.8 degrees C (1.4°F):

"According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS)…the average global temperature on Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per decade."

The temperatures are going up, just like the theory predicted. But where’s the connection with CO2, or other greenhouse gases like methane, ozone or nitrous oxide?

The connection can be found in the spectrum of greenhouse radiation. Using high-resolution FTIR spectroscopy, we can measure the exact wavelengths of long-wave (infrared) radiation reaching the ground.


Figure 1: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapour is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

Sure enough, we can see that CO2 is adding considerable warming, along with ozone (O3) and methane (CH4). This is called surface radiative forcing, and the measurements are part of the empirical evidence that CO2 is causing the warming.

...Must Go Up

How long has CO2 been contributing to increased warming? According to NASA, “Two-thirds of the warming has occurred since 1975”. Is there a reliable way to identify CO2’s influence on temperatures over that period?

There is: we can measure the wavelengths of long-wave radiation leaving the Earth (upward radiation). Satellites have recorded the Earth's outbound radiation. We can examine the spectrum of upward long-wave radiation in 1970 and 1997 to see if there are changes.


Figure 2: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries 2001).

This time, we see that during the period when temperatures increased the most, emissions of upward radiation have decreased through radiative trapping at exactly the same wavenumbers as they increased for downward radiation. The same greenhouse gases are identified: CO2, methane, ozone etc.

The Empirical Evidence

As temperatures started to rise, scientists became more and more interested in the cause. Many theories were proposed. All save one have fallen by the wayside, discarded for lack of evidence. One theory alone has stood the test of time, strengthened by experiments. 

We know CO2 absorbs and re-emits longwave radiation (Tyndall). The theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur (Arrhenius).

Scientists have measured the influence of CO2 on both incoming solar energy and outgoing long-wave radiation. Less longwave radiation is escaping to space at the specific wavelengths of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth at the same wavelengths.

These data provide empirical evidence for the predicted effect of CO2.

Basic rebuttal written by GPWayne

Update July 2015:

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


Last updated on 1 August 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Further reading

A good summation of the physics of radiative forcing can be found in V. Ramanathan's Trace-Gas Greenhouse Effect and Global Warming.


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Comments 401 to 410 out of 410:

  1. I am interested in the climate debate and have noticed a pattern where the pro camp resort to insults and continue to claim its science where the deniers tend to look at the actual real time data regarding warming,sea level etc.You see it should have started happening by now.Curious? feel free to insult me


    [DB] Ideology, fake-ad-hominem and off-topic snipped.

  2. Yeah right, Duncan.  

    Please specify the actual real time data  which the scientists ignore /misinterpret.

    Yes, deniers do sometimes look at the data ~ they see it, but they do not observe it. [courtesy: Sherlock Holmes] 

  3. I looked at all the historic records for all the major cities in Canada and there has been no change.It should of started by now.I am keeping it simple and would have to go back to the source but all the predictions for as early as 2000 have not come true.The show I saw was filmed in a basement in Manhattan which according to the modelling should be under water but it is not.The data for the extreme change is all being fudged and you cant do that.I will be surprised if I am allowed to ask these questions and expect to get Moderated


    [DB]  Conspiracy-thinking, ideology and off-topic snipped.

  4. I reread the question.NASA sea level data does not corelate to the actual tide data around the globe and its different by a lot.There is no relationship with alledged climate change and bushfires in Australia.I live here and it has happened as long as I can remember.Its all about reducing fuel load.We dont get fires in the middle of the country cos there is nothing to burn


    [DB]  Off-topic snipped.

  5. Duncan, I ask again :-

    Please specify the actual real time data  which the scientists ignore /misinterpret.

  6. I will pick sea levels,The actual tide reading at locations around the world are real and recordable but the NASA data is different and the modelling even wilder.Its hard to take it serious when predictions are made but do not come true in real time.I have access to claims that areas where I live would be underwater by 2020.Well can we agree its 2020 and the areas are still there and not flooded.I was wrong about the sea levels in Fremantle they have gone up 200mm but its over 160 years and the sea is lower at French Guyana.The magnetic poles are moving and undersea volcanoes occur plus localized silting.There is a claim here locally that the land is sinking because we are using a lot of groundwater for consumption.You can easy find all the data online or would you like me to post pictures with wriggly lines on it.I am keen to find the truth and need some actual proof.On another forum a poster showed before and after pictures of a glacier the first is from 1940 and its all iced up the second is recent and it shows all the ice gone except on the top of the mountain.The only problem I picked up on was the second photo is clearly much closer than the first and the water level is way down on the first photo where they were standing at the edge of the lake.The second photo they would have been 30 feet under and it is definitely the same place.That individual  never posted again once I pointed this out.Makes it hard to take it for real.What do you have.Claims based on what should happen.Can anyone in the universe show me where its flooded and not Norfolk because that always floods on the spring tide.I have seen pictures of 1940 airbases on pacific Islands that are only 1 metre above sea level and they are still there


    [DB] You are off-topic.  There are thousands of posts here on virtually every subject pertaining to climate change science, each with it's own discussion thread.  Use the Search tool to find the most appropriate one.  If you persist in being off-topic, as you have been, your posting privileges will be rescinded.  There will be no further Warnings on this matter.

    Off-topic snipped.

  7. Sorry Duncan, but you are still not giving any real evidence that the scientists are ignoring data.

    Science is published in peer-reviewed papers in respected scientific journals.   Not by Al Gore or Christopher Monckton or Tony Heller in shonky crap like Breitbart or NoTricksZone or WattsUpWithThat.

    Reputable scientific journals, Duncan, where it gets examined and criticized by experts.   The data can also be extensively discussed on reputable websites (such as this one).   

    If you are having problems in understanding the real factual state of things, Duncan, then it is likely because the real scientists know something that you don't know about climate.   The scientists are not ignoring data.  And so far, you have not demonstrated any data that they are (allegedly) ignoring.

    Duncan, you are well off-topic for this thread, which concerns CO2 and Warming.   If you can find some genuine examples of what you believe is ignored information /data, then please bring it to everyone's attention in the proper thread for that topic.

  8. My question is about greenhouse radiation spectrum. Is the radiation spectrum different at surface of blue water ocean and surface of land?

    Any spectrum curve I've seen that has location is over land. If there is a difference then the amount of energy available to be absorbed by CO2 would be significant in the discussion.

  9. Whari @408 : Sorry, but I am confused by the wording of your question.   Can you re-state the point you are discussing?

    Am I right in assuming you are talking about the upward Infra-Red radiation spectrum detected by satellites?  Or something else?  The satellites can detect reflected radiation (visible and near-visible light) or they can detect IR radiation emitted from land / sea / clouds /  atmospheric gas (H2O; CO2; CH4; etcetera).

    From a GreenHouse point of view, the satellites are detecting upward IR radiation from the upper troposphere (so-called TOA - Top Of Atmosphere - which is at an altitude of approx 3 - 10 km, depending on which latitude and which of the GreenHouse Gasses you are considering). The upper atmosphere is a swirling mix of air (averaging of horizontal winds and vertical convections) and so is not directly  connected to the ocean or land surface below.

    Heat is lost upwards from ocean & land, by means of air convection & evaporation/re-condensation & Infra-Red radiation (from molecule to molecule in the air).

    My apologies if I am misunderstanding you.

  10. Whariwharangi - for all intents and purposes, the spectrum of the IR emitted is determined by surface temperature (Planck's law). Sea is generally cooler than land, but any modelling of emissions absolutely takes that into account.

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