Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


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.

Home Arguments Software Resources Comments The Consensus Project Translations About Support

Bluesky Facebook LinkedIn Mastodon MeWe

Twitter YouTube RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

New? Register here
Forgot your password?

Latest Posts


Human activity is driving retreat of Arctic sea ice

What the science says...

Select a level... Basic Intermediate

Thick arctic sea ice is undergoing a rapid retreat.

Climate Myth...

Arctic icemelt is a natural cycle

"In 2007, the Northern Hemisphere reached a record low in ice coverage and the Northwest Passage was opened. At that point, we were told melting was occurring faster than expected. What you were not told was that the data that triggered this record is only available back to the late 1970s. We know the Northwest Passage had been open before." (Matt Rogers)

At a glance

The Northwest Passage is the sea route around the waters off northern Canada and Alaska. Its discovery and eventual navigation involves a fascinating tale of endeavour, adventure and tragedy, too, for some expeditions ended in disaster.

Of the many mishaps, by far the worst was that which befell Sir John Franklin and the 128-strong crews of his two ships: they were last heard of in 1845. It took many expeditions and almost ten years before their fate was finally pieced together. One thing became clear by then: the Northwest Passage does not take prisoners. Yet at the same time, those searches for Franklin and his crew generated lots of new chart cover of the waters between the islands making up the Canadian Archipelago.

Complete navigation of the Northwest Passage was finally accomplished by Norwegian explorer Roald Amundsen between 1903 and 1906. Amundsen's boat was relatively small at 47 tons and 70 feet long but usefully it had a very shallow draft. That meant it was able to pass through areas where a bigger boat would have fouled the bottom, thus offering a wider choice of courses to take. Amundsen's route was criticised in some circles because of that factor - what was the point of making the crossing if bigger freight ships could not? But Amundsen was motivated not by money but by science.

With his experienced crew of six, they spent two winters off the eastern side of King William Island, about halfway through the archipelago, collecting data on Earth's magnetic pole and local meteorology, traded with the Inuit and developed hunting and fishing skills. Leaving there in August 1905, they reached Nome, Alaska twelve months later. The ice had pinned them in for a third winter. There was not to be a single-season crossing for another 38 years, when Sergeant Henry Larsen of the Royal Canadian Mounted Police managed it in a schooner.

So yes, while the Northwest Passage was successfully navigated before 2007, the current state of the sea ice means that the picture is now quite different. Part of the reason for that is down to the age of much of the Arctic sea ice today. Sea ice that has yet to experience a summer melting season is known as first-year ice. It's relatively thin, fragile and more vulnerable to melting compared to the ice that has withstood one or more melting-seasons, known as multiyear ice. Multiyear ice can even give a good ice-breaker a run for its money. But now there's a lot less of it.

During many recent summers the Northwest Passage has become open: freight ships and even cruise liners have steamed through. That doesn't mean it's risk-free of course - there are still icebergs to watch out for. Nevertheless, it's getting to the point where there are various concerns being voiced about the number of ships passing through the area, on both ecological and political grounds. For the Northwest Passage, global warming really is a mixed blessing.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Arctic sea ice has aptly been termed a "canary in the global warming coal mine". In other words, it is regarded as a sensitive indicator of manmade climate change. Because of that importance, climate science deniers will often attempt to fob off the decline of Arctic sea ice as a natural phenomenon.

Satellite measurements of Arctic sea ice extent (fig.1) reveal a rapid decline over the past 45 years, particularly at the end of each year's annual melt season. The downward trend and the increasing difference between seasons are both in keeping with predictions of the effects of global warming.

Monthly sea ice extent anomalies.

Fig. 1: Monthly sea ice extent anomalies (solid lines) and linear trend lines (dashed lines) for March (black) and September (red) from 1979 to 2021. The anomalies are relative to the 1981–2010 average for each month. Image: American Meteorological Society State of the Climate 2021 (index here).

As the Arctic warms, the volume of ice in the region gradually declines. In particular, the thicker multiyear ice that used to be present in abundance has declined significantly (fig 2). Instead, today's ice-pack is dominated by first-year ice that is far more prone to seasonal melt. Combined with natural factors such as storms, this vulnerability can sometimes produce huge melt years such as 2012's record-buster.

 Age of the ice pack.

Fig. 2: The age of the ice pack—which has been observed since the 1980s is used as a proxy for ice thickness. In mid-March 1985 (left), the winter maximum ice pack was dominated by ice at least 4 years old (white). In 2021 (right), only a small strip of very old ice remained tucked up against the islands of the Canadian Arctic. More than half of the winter ice pack was less than a year old (dark blue). NOAA image, based on data from the National Snow and Ice Data Center. Original can be found here.

So, how can one expect climate science deniers to claim all of this melting is simply natural? A popular misinformation-technique is to state that we are just seeing the effects of multidecadal natural cycles causing deep swings in Arctic sea ice coverage and volume. Direct observational support for such 'cycles' is of course impossible. Convenient, eh? Since 1979, we have relied upon direct observations of sea ice extent with contemporary instruments aboard satellites. That's not a long time in the grand scheme of things. What about before then?

In fact, thanks to ocean sediment cores and other physical clues left behind by past climate regimes, we do have pretty good insight into past Arctic sea ice extent. Combining various sources of information about past climate behaviour, we can figure out why changes in ice coverage have occurred in past times.

Natural climate variations have indeed caused significant changes in past Arctic ice extent. Of course they have. But it's important to remember that such changes in no way constitute airtight arguments against modern ice-loss being contributed to by anthropogenic global warming. Events of the pre-industrial past obviously had other causes. Today's losses of sea ice can in contrast be partly attributed to our disruption of the Carbon Cycle through fossil fuel-burning. Indeed, comparisons between past and present Arctic climate reveal different reasons for past and modern sea ice changes (Overpeck et al. 1997). Meanwhile, analysis of several hundred markers of past Arctic sea ice extent tells us that recent losses appear to have no parallel in records going back many thousands of years (Polyak et al. 2010).

Evidence from the past two hundred years shows how natural and anthropogenic influences on Arctic sea ice can be distinguished. Data demonstrate that the Arctic underwent an unusually cool period in the early 19th century. That was followed by recovery to milder conditions extending all the way into the 20th century. After the middle of that century, we see the accelerated warming of recent decades. This pattern is in good agreement with other observations of global warming such as increasing air and ocean temperatures.

It is worth saying again: both observations and model simulations indicate that the nature of the Arctic warming in the last two decades is distinct from the early twentieth-century mild period. Recent, additional reductions in Arctic sea ice are mostly caused by a new, anthropogenic mechanism (Johannessen et al. 2004) and are thus unique in Earth's history.

Last updated on 25 February 2024 by John Mason. 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 viewing

The following animation shows how the oldest thickest sea ice has been progressively flushed from the Arctic Ocean over the last two decades. The colours are an indication of the age of the sea ice. Lighter colours are older sea ice - white is 10 years old.


1  2  3  4  Next

Comments 1 to 25 out of 79:

  1. So the sudden drop in sea ice in 2007 was due to weather conditions, not climate change ( excepting weather is the end product of the climate process). How many times do we need to remind ourselves "one swallow does not a summer make"? Sea ice is a part of the negative feedback system that keeps climate (reasonably) stable. Melting requires heat (334J/gm if I remember rightly)and this mostly affects local sea temperature. It also keeps plankton et al very happy and bloomimg nicely, (they like it cool)which is good because they lock up a bit more CO2 and the food chain speeds up.
  2. John Well written. I like this one much better than the original.
  3. This whole piece is somewhat humorous. To even use arctic ice as evidence of a global warming trend as caused by CO2 over merely 3 decades of evidence is utterly ridiculous. Anthropogenic GW advocates will cite that the Northwest Passage has opened the for the first time since records began in 1978. Since records began. Sorry, but if a norweigan sailor by the name of Roald Amundsen could navigate the passage in 1906, then you're going to have to accumulate another century of evidence of so-called Anthropogenic Global Warming before the case is made. Remember people, satellite data is only available after satellites were invented...
  4. What jecht8 either doesn't know or acknowledge is that it took Amundsen 3 years to make the trip because only bits opened up at a time. Also, he took an extremely shallow water route sticking close to the mainland shore. When we speak of the NW passage opening now, we're talking about being able to go pretty much full speed, just steering around a few bergs - and it's the more northern, deep water route.
  5. Two recent Arctic Ice updates: Beginning in early January 2009, sensor drift caused an underestimation of ice that grew until the error was finally caught in the mid-February. Internet visitors who look to the NSIDC for data sent emails to the center and, it became clear that there was a significant problem—sea-ice-covered regions were showing up as open ocean. (See NSIDC) MORE . . . (May 4, 2009) Ice in the Arctic is often twice as thick as expected, report surprised scientists who returned last week from a major scientific expedition. The scientists - a 20-member contingent from Canada, the U.S., Germany, and Italy - spent one month exploring the North Pole as well as never-before measured regions of the Arctic. Among their findings: Rather than finding newly formed ice to be two metres thick, "we measured ice thickness up to four metres," stated a spokesperson for the Alfred Wegener Institute for Polar and Marine Research of the Helmholtz Association, Germany's largest scientific organization. More info to come from Polar 5.
  6. The Arctic seems to be warming up. Reports from fishermen, seal hunters and explorers … all point to a radical change in climate conditions, and hitherto unheard-of high temperatures in that part of the earth's surface. … Ice conditions were exceptional. In fact so little ice has never before been noted. The expedition all but established a record, sailing as far north as 81 degrees 29 minutes in ice-free water. … Many old landmarks are so changed as to be unrecognizable. Where formerly great masses of ice have been were found, there are now often moraines... At many points where glaciers formerly extended far into the sea, they have entirely disappeared. - - - The above alarming excerpts were taken from an October report to the US Weather Bureau. October 1922, that is, not 2009. So it has happened before, and will happen again. We should not think that everything is so special for our time: the contents of this site strike me as very centered around here and now. It is 10 years this, 30 years that, highest since record began in 1978, and so on. That is a very short time perspective. Somehow the lack of perspective in the climate discussions remind me of the 2000+ year old quote attributed to Socrates that most people would place in our time frame: "The children now love luxury; they have bad manners, contempt for authority; they show disrespect for elders and love chatter in place of exercise. Children are now tyrants, not the servants of their households. They no longer rise when elders enter the room. They contradict their parents, chatter before company, gobble up dainties at the table, cross their legs, and tyrannize their teachers." It is presumptuous to think that mankind rules the earth's climate. There are other, more powerful, forces in play. But we think we can raise or lower the average temperature of the earth at will! 0.33 degrees up, or why not 4 degrees. Or raise the oceans, 38 centimeters up, or why not 6 meters, or whatever ! Invent a figure, and people will bow to you in awe. Ridiculous. Anything could happen, and probably will, but we are not in control.
  7. Argus, yes, you can claim whatever you wish untill you look at the data. Which tell a different story. By the way, in the '20s the arctic was already warming.
  8. Well Riccardo, who was warming it so much then, when human CO2 emissions were only a fraction (less than 1/10) of what it is now? The cows and their methane outlets? What explanation do you have in your 'data'?
  9. Argus, it should come as no surprise that climate has changed before. Indeed, one has to look at how climate works, i.e. that there are several possible forcings other than CO2. In particular, in the first half of last century there has been a reduced volcanic activity (grey line) concomitant to an increase of total solar irradiance (top panel). The result is an increase in temperature till about 1950, overall and in the Arctic as well.
  10. Thank you very much for acknowledging that there are other forcings than CO2, and that climate changes similar to what we are experiencing now, have occurred before. It seems to me that 99% of what is presented here is focussed on CO2 only, as if there were no other explanation to anything at all happening on this planet. Also, thanks for the links! I am continually reading up on more facts presentations and connected debates within this great site (and some others). I am slowly learning, and by now I know a lot more than the average person in the street, but I also recognize how little I know compared to those who have studied this field seriously for years.
  11. Argus, I'm glad you now realize that we all think that there's not just CO2. It's an important point to make clear as did our host writing a post on it. It is only by looking at all the important factors that scientists can be so confident on the causes of recent and past climate variations. Please keep reading and asking, it won't take that much time :)
  12. Argus, a good overview is cce's The Global Warming Debate. It will give you a good base from which you can more efficiently and effectively pursue particular topics here.
  13. Is the graph from Stroeve 2007? I find it quite hard to read a graph that I don't know the source from.
  14. The Prince of Cherries is at it again. the ice itself is about to set a record high for the date in the DMI database (emphasis added). BTW, that database includes the years 2005-2010. We are about to set a record high for a specific date in a statistically insignificant 6 year period. Huzzah! With just 3 more years, we draw a different conclusion: Note that the annual rebound of new ice is always steeper than the melt. And yes, even with globally increasing temperatures, there will still be winter in the Arctic.
  15. Heh. "Prince of Cherries". Meanwhile the ice volume remained far below previous record lows through the end of September. Ice volume is now only about 20% of what it was in 1979 while extent is about 60%. However, the two factors ultimately ARE linked... if volume hits 0% extent perforce will as well. Unless the volume trend suddenly levels off for some reason Goddard only has a few more years (at best) of being able to play games with extent data.
  16. Response to NQuest from the ice age thread> You're missing the point. None of those examples suggest that the changes will be monotonic year-after-year. When discussing global warming we are referring inherently to long term trends. Given the degree of annual variation, you need about 15 years to establish statistical significance. A 2-3 year trend means little to nothing with regard to the long term trends being discussed. This is a common misunderstanding among many skeptics and this same discussion has played out many many times on this site. It is in no way new or "sudden". The natural variation referred to is of the inter-annual short-term variety. If you take a look at the graph provided in this post, you can see clearly that there has been a clear downward trend extending from about 1970. You may also notice that there are many 2-3 year "recoveries" amidst this trend. It is because of this variability in the signal that you must look at 10+ years of data to make any claims about whether the ice is melting or growing over the long term.
  17. e (16) - Could you please explain to me why is it that the glacialist, when discussing the expansion of the glacier made the statement, "We're not sure why this happens".
  18. @NQoA: because they're not certain about the particular characteristics of that glacier that make it resist the global trend towards glacier retreat. Again, no one said all glaciers would recede at the same time, or at the same rate. Given the number of glaciers on the world, some are bound to react differently. The fact remains, however, that an overwhelming majority of glaciers are retreating.
  19. NQuest @17: Perhaps without realizing that the discussion was to be moved to this thread, muoncounter provided you an explanation on the 'ice age' thread. Please note that he cites directly from the link you provided. The article's own tone seems to be in opposition to the use to which you wish to put it, a behaviour which seems lamentably common among contrarians who visit this site. The article suggests that this glacier is certainly anomalous, but since the rest of the world's ice (as documented in this very post or handily summarized with this search of SkS) continues to decline, I hardly see how it can present a major challenge to the science supporting AGW. Certainly I would conjecture that one factor in the Perito Monero glacier's stability would be an increase in precipitation (specifically, snowfall at the glacier's source), which follows from an increase in atmospheric water vapour, which follows from (wait for it...) warming temperatures. Sooner or later, though, if temperatures continue to rise, Perito Monero will follow its fellow glaciers into decline. At any rate, it seems to me that bringing up Argentine glaciers is a complete non sequitur - perhaps even a red herring - when it comes to discussions specifically focused on Arctic sea ice decline.
  20. @Argus: no one said CO2 was the only forcing. To claim otherwise would be a type of strawman fallacy. The point you're apparently missing is that, no, the climate change we are currently experiencing is not like what has happened before. We have a pretty good idea of why climate changed in the past, and none of the various circumstances that provoked past change is at play today What *is* different, of course, is that this time we're pouring gigatons of CO2 in the atmosphere, and that CO2 is causing temperatures to rise. Just to make things clear, though: are you in fact disagree with NQoA? Because the latter seems to think there is no warming, while you claim the warming is natural. Aren't going to argue with NQoA as well? After all, he's also disagreeing with you. I'd love to see some "skeptics" break the unspoken rule once in a while, but I don't think this is going to happen here...
  21. Re: NQuestofApollo (17) Perhaps if you had read Dr. Rivera's extended comments in the longer version of the article here:
    "One hypothesis for the 3-mile-wide (5 kilometer-wide) Perito Moreno's advance is the glacier's apparent insensitivity to changes in what glaciologists call the equilibrium line on glaciers, Rivera said. Roughly equivalent to the snow line, the equilibrium line is the elevation above which the glacier is growing, due to snow accumulation, and below which the glacier is melting. When this line moves higher up a hill or a mountain due to rising temperatures, for example, more of the glacier is situated in the melting zone, and the glacier retreats. But because Argentina's Perito Moreno glacier is so steep in the area where the equilibrium line falls, climate shifts don't impact the line's movement much, at least as it relates to the height of the mountain, Rivera noted. As a result, the amount of of ice lost or gained is minimal. It could also be that Perito Moreno simply hasn't got all that much to lose. The lake where Perito Moreno ends—Lago Argentino—is shallower than the bodies of water at the ends of most glaciers. Most glaciers calve, or release ice, in deep water, but not Perito Moreno, where the calving rates are higher than on other Patagonian glaciers. That means less of the glacier is in the melting zone below the equilibrium line. As heavy snowfall above the equilibrium line pushes the glacier downhill, the glacier breaks up when it hits the lake, Rivera explained. Such impacts kept the glacier from growing longer when the climate was cooler, and thus more likely to expand, he said. If Perito Moreno had extended into a deep lake area, it would have become a longer glacier, and Earth's recent warming trend would be causing the glacier to melt and its ice to retreat more easily, Richard Alley, a glaciologist at Pennsylvania State University in University Park, said in an email. "Instead, we have a shorter glacier, with less [of a] zone where the warming can cause melting, but a large high-elevation [snow and ice] accumulation zone," Alley added."
    Forming an opinion based on an incomplete news article on one glacier that happens to be advancing at a time when glaciers worldwide are in retreat is cherry-picking. No one said glacial retreat would be linear and uniform. Perito Moreno, for the reasons surmised, is one of the exceptions to the overall trend. Noise in the data. For more on glacial changes, go to Mauri Pelto's blog. The Yooper
  22. Natural cycle? Really? Not this year: Observations from the ground in the Eastern Arctic, ... and views taken by satellites at 500 kilometres above the earth’s surface showed ArcticNet participants that ice formation in 2010 is abnormally slow. ... “We have dramatic changes taking place,” with the Arctic becoming a place of rain instead of snow ...
  23. muoncounter, thanks for the AO link, but I already look at that almost every day since it affects my own weather (an aside: AO predictions this year have been less accurate than usual). That site has the long term trends here and we're still in strongly negative territory. As I said on the other thread, AGW is responsible for ice loss. AO is also a factor and negative AO should bring a recovery in ice. Another factor is last year's El Nino and a decline in ice. This year should see a continued recovery due to negative AO if that theory holds (paper linked on other thread). That still leaves the question on the other thread of the effects of AGW on AO according to models.
  24. #23: "we're still in strongly negative territory" What you're looking at is the three month running mean, which is strongly negative, but that's a hindcast. The daily record and the forward looks are here: -- replaces the auto-updating graph. Sure looks like it bottomed in mid December.
  25. Muoncounter, you are looking at short term fluctuations. Last year AO also looked like it bottomed in December but then hit an all-time low (since 1954) in February.

1  2  3  4  Next

Post a Comment

Political, off-topic or ad hominem comments will be deleted. Comments Policy...

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

Link to this page

The Consensus Project Website


(free to republish)

© Copyright 2024 John Cook
Home | Translations | About Us | Privacy | Contact Us