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David Kirtley

I am not a scientist, but I've had a life-long interest in the sciences and in learning how the world works, from astronomy and geology to biology and evolution. When "climategate!" exploded on the internet in 2009, I was amazed at the mental hoops some people seemed to use to avoid understanding climate science. With a keen interest in making this complex subject matter easier for other "non-scientists" to understand, I joined the Skeptical Science team in 2013, helping out "behind the scenes" answering emails from readers, offering suggestions and editing blog posts. I live and work in St Louis, Missouri, USA.


Recent blog posts

Canada's ClimateData Web Portal: Normal Science, Not Fake

Posted on 17 October 2019 by David Kirtley

In August, the Canadian government launched a new website called which "provides engineers, public health professionals, urban planners, mayors, and anyone else doing long-term planning with user-friendly climate change information, data, resources and tools" (press release). The website uses past weather data from 1950-2011 along with computer modelling to project the possible future impacts of climate change in Canada. Françoys Labonté, the Executive Director of Computer Research Insititue of Montréal (CRIM), one of the developers of the new web portal, said the website's purpose is "to provide Canadian leaders and managers with easy access to useful, actual and future climate data and information to assist them in anticipating and managing climate change-related issues over the medium and long term."

ClimateData Website

Figure 1. The home page of Canada's new website: Click for larger image.

It didn't take long for climate science denialists to notice the lack of weather data before 1950 and assume that Canadian government scientists must be up to something nefarious. Blacklock's Reporter was the first to claim, "Canada omitted 100 years’ worth of weather data from a federal website intended to illustrate climate change." Their story is behind a rather expensive paywall (even for us AGW alarmists who are "rolling in grant money"). But the story was picked up by Toronto Sun journalist, Lorrie Goldstein, claiming that Environment and Climate Change Canada (ECCC) "omitted a century’s worth of observed weather data in developing its computer models on the impacts of climate change." James Delingpole blared from Breitbart that the ECCC "has erased a century’s worth of observed temperature data, claiming its modelled computer projections are more accurate." Delingpole goes further in his conclusion:

Others less committed to green activism might find it somewhat sinister that the international agencies charged with maintaining the world’s temperature records are destroying them because the factual evidence doesn’t support the global warming scare narrative.

These wild claims about omitted, scrapped, erased, or destroyed historical weather data are completely false, and they are a total misrepresentation of the types of computer modelling used for ClimateData.



But their Emails!

Posted on 30 November 2018 by David Kirtley

But their Emails

Here we go again. It's always emails with these people.

First there was "Climategate!" — the misquoting, selective quoting, and uninformed quoting of stolen emails from the University of East Anglia's Climatic Research Unit (CRU) in Great Britain. Emails between CRU scientists and other climate scientists around the world promised to peel back the curtain and reveal the global warming scam. Alarmist scientists had used "tricks" to "hide the decline"! They "can't account for the lack of warming" so they have to fake the temperature data! The whole thing is a hoax!

Not so much.



Coming full circle: from study to comedy sketch to study

Posted on 1 August 2018 by BaerbelW

Over five years ago, our team published "Quantifying the consensus on anthropogenic global warming in the scientific literature" (Cook et al. 2013) which caused quite a stir - and that, even though it wasn't the first peer-reviewed paper to find a 97% consensus on human-caused global warming. In 2014 we learned, that our study had been voted ERL's best article of 2013 and as of right now it has been downloaded a whopping 829,000+ times.

Last Week Tonight: Climate Change Debate

One of the best and arguably funniest treatments our study received, was a sketch put together in 2014 for John Oliver's HBO show "Last Week Tonight". In the segment, John Oliver illustrated to great comic effect what a statistically representative climate change debate would look like. You can view it below (warning: the video includes some profane language).

From sketch to study

As of this writing, the video has been viewed 7.8+ million times. With this many views, it's not too surprising that some researchers got curious and wanted to find out if a comedy sketch like this could have an impact on how people think about human-caused climate change. And this is exactly what Paul R. Brewer from the Center for Political Communication at the University of Delaware and Jessica McKnight of the School of Communication at Ohio State University set out to do at the end of 2014. Their study "A Statistically Representative Climate Change Debate: Satirical Television News, Scientific Consensus, and Public Perceptions of Global Warming" was published in the Atlantic Journal of Communication on June 30, 2017.

Here is the study's abstract:



Book Review: A Global Warming Primer, by Jeffrey Bennett

Posted on 11 July 2018 by David Kirtley

Our knowledge of how and why the climate is changing comes from many different scientific fields, ranging from the physics of how greenhouse gases interact with infrared radiation, to the chemistry and biology of how carbon dioxide cycles between lifeforms, rocks and ocean waters, to the geology of volcanoes and Earth's past ice ages. Even astronomy comes into play because of slow changes in the Earth's orbit and tilt which can cause the climate to change. All of this (and much, much more) may be intimidating to some folks, and the topic can be even more off-putting when you add in the political "debate" surrounding global warming.

Global Warming Primer coverWhat to do? One approach is to give folks just enough information about the science to give them a clear understanding of climate change. Jeffrey Bennett's latest book, A Global Warming Primer, does just that. Dr. Bennett has written science books for every age group, from children's books to college textbooks. His new primer is written for anyone who wants to learn about the basic facts of global warming. The goal is to give readers a "big picture" overview of the science without getting bogged down by endless details.

Bennett achieves his goal using a helpful Q&A format, which supplements his main text, throughout the book. An expanded "Detailed Table of Contents" lists all of these questions, making it easy for readers to simply look for answers to their own similar questions. This style makes the book feel "more like a personal discussion", just as Bennett intended. This format, in some ways, mirrors Skeptical Science's taxonomy of climate myths and rebuttals.

Another useful format choice is his use of two different font sizes to denote two different levels of complexity in his descriptions of the science. The larger ("normal") font is "for general text...and the big picture ideas that should be of interest to all readers". Bennett uses smaller font for more detailed discussion of the science. If readers want to just focus on the "big picture" overview they can skip over the small font sections as they read along.

Text size example
Image of sample page (p. 26) showing different fonts, and Q&A formats



The 1970s Global Cooling Zombie Myth and the Tricks Some People Use to Keep it Alive, Part II

Posted on 8 May 2018 by David Kirtley

In Part I of this look back at 1970s climate science I reviewed the findings of Peterson, Connolley, and Fleck's seminal 2008 survey of 1970s peer-reviewed literature (hereafter referred to as PCF08) which found no 1970s "consensus" about a future global cooling/ice age. I also looked at a "skeptic" critique of this paper from the blogsite No Tricks Zone, penned by Kenneth Richard (hereafter referred to as NTZ), and showed some of the errors and fallacies he used to distort 1970s science. In this post I'll take a closer look at some of the papers used by NTZ to claim that there was a global cooling "consensus" in the 1970s.

The primary critique by NTZ is given in a blog post which highlights 35 "sample global cooling/low CO2 climate influence papers" from his full list of 285 papers. Like the full list of 285 papers, these samples are not given in any apparent order, not by date or by author. I rearranged these by date of publication in order to see the progression of the science as the various threads of climate research (see Part I) developed throughout the period. Below is a screenshot of the first page of a spreadsheet (click on image for pdf) of the 35 sample papers.

NTZ 35 Papers page 1 pdf

The first thing to notice is that not all of these samples are peer-reviewed scientific papers. Two of these are RAND Corporation documents (Fletcher, 1969 and Libby, 1970), one is a book review (Post, 1979), and one is from a popular science magazine (Douglas, 1975). This article was mentioned in PCF08 in their "Popular Literature of the Era" sidebar but not included in their survey. Two other sample papers are perhaps "borderline" between grey and peer-reviewed literature: two Master's thesis papers (Cimorelli & House, 1974 and Magill, 1980). And there are a few others I'm not sure about.

Contrast this with PCF08's survey which excluded anything from the "grey literature" and focused exclusively on the peer-reviewed literature.1 The best place to find out what the scientists of the time were saying about the future climate trajectory is in the peer-reviewed literature. Things can get muddy once you include documents from the popular press. Yes, these other sources may offer insight into what the "sense of the times" were, but they can also misrepresent or distort what scientists thought at the time. NTZ expands the goal posts to include some of this grey literature and thus gathers more "papers" for his "global cooling consensus".



The 1970s Global Cooling Zombie Myth and the Tricks Some People Use to Keep it Alive, Part I

Posted on 3 May 2018 by David Kirtley

Ten years ago, Thomas Peterson, William Connolley and John Fleck published a paper in the Bulletin of the American Meteorological Society which looked back at the climate science of the 1970s: "The Myth of the Global Cooling Scientific Consensus" (hereafter called PCF08). The goal of the paper was to look at the peer-reviewed literature of the time to see what scientists were saying about the future projections of climate. In the decades since the 1970s, some "skeptics" of global warming/climate change have made claims that "all the scientists" in the 1970s were predicting "global cooling" or an "imminent ice age". But, the PCF08 survey of papers from 1965 to 1979 showed that while there were some concerns about future "cooling", especially at the beginning of the time period, there were many more concerns about future warming caused by human emissions of carbon dioxide.

1970s papers

Figure 1. The number of papers classified as predicting, implying, or providing supporting evidence for future global cooling, warming, and neutral categories. During the period from 1965 through 1979, the PCF08 literature survey found 7 cooling, 20 neutral, and 44 warming papers. (Peterson 2008)

Many of today's "skeptics" of AGW remain unconvinced. A few years ago, Kenneth Richard, at the blog site NoTricksZone, penned a critique of PCF08, claiming to find hundreds of papers in support of a 1970's "cooling consensus": "Massive Cover-up Exposed: 285 Papers From 1960s-’80s Reveal Robust Global Cooling Scientific ‘Consensus’" (hereafter called NTZ). PCF08 only found seven peer-reviewed papers supporting a future cooling consensus. Did NTZ really stumble upon a "massive cover-up": a treasure trove of 285 more peer-reviewed papers foretelling a future cooling trend leading to the next ice age?

The short answer, of course, is "no". But, I thought it would be good to take a close look at PCF08 and NTZ to see "what's going on". What were scientists in the 1970s saying about the possible future trajectory of the climate? What facts were well-known and what questions were scientists still trying to answer? How could PCF08 and NTZ come to different conclusions about 1970s climate science from looking at the period's many peer-reviewed papers?



How to Change Your Mind About Climate Change

Posted on 7 February 2018 by David Kirtley

What causes someone to change their mind about climate change? It is no secret that climate change is a highly polarized subject with some people accepting what climate scientists tell us, and others who think those scientists are only in it for the money, or that the globe isn't really warming, or that, if it is warming, it's nothing to worry about. But occasionally, some people change their minds and move from "doubtful" or "dismissive" about climate change to "concerned" and "alarmed". What moved these people? Was it a particular argument or a specific graph? Was it because of a sudden "aha!" moment when the light bulb flashed and all of climate science made sense, or was it something else?

Over the past several years there have been a number of reports from some of these "converts" explaining how they changed their minds. This post looks at some of these accounts with the goal of discovering what led these people to change their minds. This is by no means an exhaustive list. If you know of other examples not seen here please add to the list in the comments section.

Two Scientists - "Total turnaround"

Scientists are naturally, and appropriately, skeptical. They ask questions about how the world works; and when they find answers they ask further questions about those answers, always striving to get to better explanations of the natural world. So it is no surprise that scientists would turn their skeptical eyes to anthropogenic climate change (AGW).

Kasra Hassani was a scientist working in microbiology and immunology who had a  skeptical view of climate science. At first he thought there were more immediate problems facing humankind than climate change. For a time, he toyed with conspiracy theories about AGW (thanks to Michael Crichton's State of Fear), but more and more evidence for climate change forced him to face reality:

I created a list of every question and doubt I had about the physics, chemistry, biology, economics and politics of climate change, and I started reading. I took online courses. I listened to podcasts. Every myth in my head popped and floated away.

Hassani also said,

No singular bit of evidence unequivocally proved to me that humans were responsible for climate change, which makes sense if you're a science nut like me. Science works on multiple proofs. One experiment or piece of evidence supports a theory, it doesn’t prove anything.

Over time, as different researchers gather more evidence, a theory becomes refined and a more acceptable explanation for natural phenomena. It also took time because I was never astonished by a piece of evidence or a big news story; when you are in denial, evidence is unlikely to change your mind. On the contrary, it might persuade you to cover your ears and pretend you're not listening.

Hassani realized that denial was the easier answer. The real effort was in recognizing his own biases, and seeing that his own strong-held beliefs and stubbornness went beyond a healthy skepticism:



From the eMail Bag: Carbon Isotopes, Part 2: The Delta Notation

Posted on 28 December 2017 by David Kirtley

We occasionally receive excellent questions and/or comments by email or via our contact form and have then usually corresponded with the emailer directly. But, some of the questions and answers deserve a broader audience, so we decided to highlight some of them in a new series of blog posts.

In Part 1, we learned about carbon isotopes: how 14C forms in the atmosphere, how different isotopes move through the Carbon Cycle, and how isotopic measurements reveal clues about our changing climate. In this post we will look at how measurements of changing isotopic ratios are described.

Here, again, is the IPCC graph (Figure 1) illustrating the rise in atmospheric CO2 (panel a, black saw-toothed line) and the decreasing 13C:12C ratio in the same CO2 (panel b, red line).


Figure 1. Recent CO2 concentrations and emissions. (a) CO2 concentrations over the period 1970 to 2005 from Mauna Loa, Hawaii (black) and Baring Head, New Zealand (blue). In the lower right of the panel, atmospheric oxygen (O2) measurements from flask samples are shown from Alert, Canada (pink) and Cape Grim, Australia (cyan). (b) Annual global CO2 emissions from fossil fuel burning and cement manufacture in GtC yr–1 (black) through 2005. Annual averages of the 13C/12C ratio measured in atmospheric CO2 at Mauna Loa from 1981 to 2002 (red) are also shown. The isotope data are expressed as δ13C(CO2) ‰ (per mil) deviation from a calibration standard. Note that this scale is inverted to improve clarity. (IPCC, AR4)

One of our readers was puzzled by this graph (which appears in our rebuttal "How do human CO2 emissions compare to natural CO2 emissions?") and emailed us these questions about it:



From the eMail Bag: Carbon Isotopes, Part 1: The Basics

Posted on 20 December 2017 by David Kirtley

We occasionally receive excellent questions and/or comments by email or via our contact form and have then usually corresponded with the emailer directly. But, some of the questions and answers deserve a broader audience, so we decided to highlight some of them in a new series of blog posts.

For the average citizen, who perhaps has a rudimentary grasp of general science, one problem with the topic of climate science is that it is such a huge subject which draws on a wealth of knowledge from many different science disciplines. It is easy to get lost in the weeds of this detailed knowledge which informs our conclusion that the current changing climate is due to human emissions of greenhouse gases. The broad strokes of climate science may be grasped with a limited amount of scientific knowledge: carbon dioxide and other greenhouse gases absorb and release heat (infrared radiation)...burning fossil fuels releases CO2 into the atmosphere and this increasing amount can be measured...more CO2 in the atmosphere will mean more heat energy in the climate system causing climate to change. But that basic knowledge is grounded on many more complicated lines of evidence: the weeds, which can often be very confusing to the science novice.

Recently, our inbox has seen a fair number of emails from folks asking about carbon isotopes. Some emailers asked basic questions while others asked about some finer details of the more complicated science. This post will offer a primer on some of the basics about what isotopes are, where they come from, and what isotopes can tell us about Earth's climate. In a second post we will get into the weeds a bit and explain how carbon isotopes are measured.

Atoms and Isotopes

All atoms are made up of protons (positive charge), neutrons (no charge) and electrons (negative charge). The number of protons in an atom's nucleus defines what element that atom is. For the element carbon, all atoms have 6 protons and 6 electrons (Figure 1). Most carbon atoms also have 6 neutrons, hence 6 protons + 6 neutrons = carbon-12 or 12C. But a small number of carbon atoms have an extra neutron, carbon-13, or 13C and an even smaller number of carbon atoms have two extra neutrons: carbon-14, or 14C.

Carbon atoms

Figure 1. Structure of the three naturally occurring carbon isotopes.



From the eMail Bag: A Deep Dive Into Polar Ice Cores

Posted on 11 April 2017 by David Kirtley

We occasionally receive excellent questions and/or comments by email or via our contact form and have then usually corresponded with the emailer directly. But, some of the questions and answers deserve a broader audience, so we decided to highlight some of them in a new series of blog posts.

The Keeling Curve (Figure 1) is one of the most iconic graphs illustrating modern climate change. Since 1958, the Keeling Curve has shown the increase in atmospheric CO2 from about 315 ppm to the present value of 405 ppm. CO2 measurements at Mauna Loa are taken hourly so this record has a very high time-resolution and is remarkably complete, even though there are occasional short gaps in the record.

Figure 1. The Keeling Curve. Source:

Another iconic graph shows CO2 concentrations going back 800,000 years through eight of the Earth's ice age cycles (Figure 2). Our sudden anthropogenic CO2 spike is almost literally "off the charts" compared to the rest of the ice ages record. At no time during the last 800,000 years has there been any comparable increase in CO2 concentrations.



How Green is My EV?

Posted on 9 March 2017 by David Kirtley

One of the largest sources of CO2 pollution from the average American consumer is the family car. The EPA states that 26% of greenhouse gas (GHG) emissions come from all forms of transportation (2014 figures). The largest source of GHG emissions, at 30%, is the electric power sector. However, just recently the Energy Information Administration (EIA) announced that transportation emissions have now surpassed those from electric power generation. Whatever the exact numbers, it's clear that if we want to reduce our GHG emissions we need to work to reduce them from these two sectors.

As individuals we can make choices which help decrease our electricity usage: LEDs over incandescent light bulbs, smart thermostats, etc. If we can afford it, and if we have the right house orientation, we can take an even bigger bite out of our CO2 emissions by installing solar PV panels to produce some or all of our electricity. But to get the biggest bang for our buck, perhaps the single best thing we can do to decrease our emissions is to switch from a normal car (internal combustion engine, or ICE, vehicle) to an all-electric vehicle (EV).

About two years ago my wife and I needed a second car and we decided to buy a 2013 Nissan Leaf. But because we live in Missouri, where most of the electric power is generated by coal, I was concerned that I would just be switching from ICE CO2 emissions to coal-electric emissions. Would we really be making any difference?

To answer that question (and others about how to compare MPG, fuel costs, etc. between a typical ICE and an EV) I kept track of data and made some calculations over a year-long period from September 2015 until August 2016. We bought the Leaf in June 2015 but it took me a few months to come up with a system and get a handle on what information I needed and where to find it.

Gasoline vs. Kilowatt-hours

We have a good grasp of what a gallon of gas means in terms of how much it costs and probably how many miles it will take us in our ICE cars. The "fuel" for an EV is kilowatt-hours (kWh) of electricity. Comprehending something as amorphous as a kWh stored in my Leaf's battery took a bit of a mental leap for me. The Leaf has a gauge on the instrument panel which displays the battery's amount of charge in 2 kWh increments, which isn't a very fine gauge. But the Leaf also records and uploads information on every trip made: kWh consumed, miles driven, etc. I can access this finer-detailed information from a Nissan website (figure 1). Now that I knew how many miles and how many kWh I used each month, I could look at my monthly electricity bills to see how much the EV "fuel" cost.

Figure 1. Portion of a screenshot of the Nissan Leaf website showing data for August 1, 2016. I travelled 25.1 miles and used 6.2 kWh. The "CO2 Savings" is not very accurate because it doesn't take into consideration the CO2 emissions from the electricity produced to power the car.



From the eMail Bag: Abrupt Climate Change in Greenland's Past?

Posted on 25 January 2017 by David Kirtley

We occasionally receive excellent questions and/or comments by email or via our contact form and have then usually corresponded with the emailer directly. But, some of the questions and answers deserve a broader audience, so we decided to highlight some of them in a new series of blog posts.

In 2008 the U.S. Department of Energy's six National Laboratories began a new research program investigating "abrupt climate change" called IMPACTS: Investigation of the Magnitudes and Probabilities of Abrupt Climate Transitions. A blogpost on the Lawrence Berkeley Lab's website describing this program had this to say about the Younger Dryas, one of Earth's best-known periods of abrupt climate change:

[C]limate change has occurred with frightening rapidity in the past and will almost certainly do so again. Perhaps the most famous example is the reverse hiccup in a warming trend that began 15,000 years ago and eventually ended the last ice age. Roughly 2,000 years after it started, the warming trend suddenly reversed, and temperatures fell back to near-glacial conditions; Earth stayed cold for over a thousand years, a period called the Younger Dryas (named for an alpine wildflower). Then warming resumed so abruptly that global temperatures shot up 10 °C in just 10 years.

A Skeptical Science reader noticed this passage and wrote to us asking if this was accurate, especially that last sentence about an alarming 10 degree jump in just 10 years. I googled around a bit and came upon a NOAA Paleoclimatology webpage (also from 2008) which had a very similar passage:

About 14,500 years ago, the Earth's climate began to shift from a cold glacial world to a warmer interglacial state. Partway through this transition, temperatures in the Northern Hemisphere suddenly returned to near-glacial conditions. This near-glacial period is called the Younger Dryas, named after a flower (Dryas octopetala) that grows in cold conditions and became common in Europe during this time. The end of the Younger Dryas, about 11,500 years ago, was particularly abrupt. In Greenland, temperatures rose 10° C (18° F) in a decade.

The NOAA webpage had a graph of Greenland temperatures, snow accumulation and other climatological measurements. This graph also listed a number of references, including Alley, R.B. 2000. The Younger Dryas cold interval as viewed from central Greenland. Quaternary Science Reviews 19: 213-226. In that paper Alley described the abrupt increase in Greenland temperature at the end of the Younger Dryas as a "5-10°C step warming...with much of the change in a few decades or less".

Greenland Ice Core Stations

Figure 1. Locations of Greenland Ice Core Stations. From: 800,000 Year Ice-Core Records of Isotopes 2H, 18O, and other isotopic species, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy.



From the eMail Bag: CO2 in the air and oceans

Posted on 6 December 2016 by David Kirtley

We occasionally receive excellent questions and/or comments by email or via our contact form and have then usually corresponded with the emailer directly. But, some of the questions and answers deserve a broader audience, so we decided to highlight some of them in a new series of blog posts. This is the first one of those posts and more will follow every once in a while in the future.

We recently received an email from Jeffrey Middlebrook who asked about the dynamics of CO2 transport from the atmosphere to the oceans: atmospheric water vapor increases with CO2-driven atmospheric warming, there will be more CO2 capture by the increased water vapor (yielding more carbonic acid) which will transfer more CO2 to the oceans, thereby decreasing the effects of atmospheric CO2 as a greenhouse gas, and with greater precipitation due to more atmospheric water vapor more atmospheric heat will be transferred to the oceans and terrestrial landscapes. It seems plausible that the increases in atmospheric water vapor due to increases in atmospheric CO2 might just produce a strong negative feedback.

About 46% of human emissions of CO2 stay in the atmosphere, while ~26% makes its way to the oceans, and ~28% is used by plants. Our emissions of CO2 may be good for plants (at least for now) but the additional CO2 in the oceans is leading to climate change's "evil twin": ocean acidification. (For much more about ocean acidification see our series: OA is not OK.)

Figure 1. CO2 added to the atmosphere is absorbed by the oceans due to Henry's law. More CO2 added since pre-industrial times leads to more CO2 absorbed by the oceans.



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