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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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Why a few degrees of global warmings matter

What the science says...

A few degrees of global warming has a huge impact on ice sheets, sea levels and other aspects of climate.

Climate Myth...

It's only a few degrees

"There might be some adverse outcomes from that eight tenths of a degree of temperature rise threatening my Grandchildren in 2050, but neither I nor anyone else knows what those outcomes might be. We’ll assuredly get an extra flood over here, and one less flood over there, it’s very likely to be drier somewhere and wetter somewhere else, in other words, the climate will do what climate has done since forever — change." (Willis Eschenbach)

There are 3 problems with even small sounding global warming. Firstly, 2 °C is a very optimistic assessment: if the skeptical Dr Roy Spencer is correct here then we’re on course to get more like 3.5 °C. If most climate science is correct then we’ll get 6 °C by doubling CO2 twice.

Secondly, if we cause a ~2 °C warming, some scientists think feedbacks such as melting permafrost releasing more greenhouse gases might kick in. Ice and sediment cores suggest we haven’t been this warm in at least 600,000 years so we’re not sure – but this could trigger a lot more warming.

Finally, 6 °C, the actual “best estimate” for eventual global warming from current CO2 trends still sounds small. But heating isn’t distributed evenly: as we came out of the last ice age, the temperature in northern countries rose by more than at the equator. When you average over the entire world it turns out to have only been about 6 °C global warming: for people living in Northern Europe and Canada it’s the difference between walking around in a t-shirt and a mile of ice over your head.

The graph below is the temperature calculated over the past 400,000 years in Antarctica from the Vostok ice core. The tiny peaks are a bit like today and the tiny troughs would force hundreds of millions from their homes. A few degrees of warming might sound small, but it can mean a lot and this is why scientists look at what the impacts of warming will be, rather than just saying “it doesn’t look like much so it can’t matter”.

Vostok temperature reconstruction

Basic rebuttal written by MarkR

Update August 2015:

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


Last updated on 4 August 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 9:

  1. If you take a tray of ice cubes and let them warm until they are partially melted, then they will be at a balance point where only a small change in temperature will make them either all melt or all freeze. The Earth is also partly ice and partly water, at a similar balance point.
  2. One cause of temperature increased is CO2 and these two are directly proportion to each other, which mean it will double each other. While temperature is increasing just 6 degrees it double CO2 amout. In my opinion I think this is and global problem that everyone have to be aware because just a few degrees can cause a huge problem, trigger dangerous and damage climate change. It can change our environment, including our world.  

  3. I am a teacher and many students of mine say that because it is a few degrees it does not matter. The change between summer and winter is much bigger.

    I get that a few degrees make a huge difference. I don't fully understand why a few degrees matter so much. Could somebody help me with this, so I can better explain it to my students?

    Thanks a lot.

  4. Jasper, NASA took a look at that.  Part of what they found is that the crops we grow to support our civilization are evolved for a narrow range of temperatures and are currently near their thermal limit.

    NASA also more recently took a deeper-dive into that subject, with the interactive results present in 2 parts:

    Part 1

    Part 2

  5. Jasper @3, I always like to point out the difference between our climate today and the climate during the height of glacial periods in past "ice ages"...that difference is only 4-5°C. The cartoonist XKCD calls this 1 "Ice Age Unit". He has another cartoon showing Earth's history from now back to this last glacial maximum.

    A few degrees may not be a big deal weather-wise, but for climate it is a very big deal.

  6. Jasper @3 , another take :-

    In a heavily populated world, the food supply is vulnerable.   The staple food crops are rice, wheat, maize.   Plant breeding and GMO can produce some benefit in "toughening" these species against higher temperatures ~ but it is uncertain how far this can be improved [maize yield is especially damaged by prolonged hot spells].

    Even a small rise in average temperature produces a disproportionate increase in hot spells (frequency, peak, and duration).   Add to that, the increased regional occurrence of droughts and/or floods affecting crops.

    And infrastructure and poor quality soil problems will make it difficult to simply move croplands to newly-warm territory in Siberia and nothern Canada.

    Seafood is also a vulnerability, from the [CO2 caused] rising acidity in oceans affecting the shells of planktonic creatures (which provide a large part of the "base of the pyramid" supporting our fish stocks).

    Then there is the political problem of "climate refugees".   Already we have pre-existing tensions / resentments / disruptions just from the 26 million international political/economic refugees (and also from the much larger number of "internally-displaced" refugees.)

    The most recent and accurate satellite estimate of coastal elevations does indicate that a 1 meter average rise in seal level would displace approx 200 million people.  With the storm surges and salination of low-lying farmlands, the result is that many of those people will become "climate refugees" well before the average rise of 1 meter is attained.  (That may be 100 - 150 years away ~ or perhaps distinctly earlier.)

    The other big increase in climate refugees would come from the near-equatorial regions of the world, as higher/longer hot spells make "outdoor work" more difficult for part of the year.   (Would you call them climate refugees or economic refugees?)

    Many nations already have more than enough internal social problems arising from us/them types of racial or religious or cultural differences.  The addition of large numbers of "foreign" climate refugees would multiply that social disruption !

    Add to all this, some further uncertainties about climate effects on "non-staple" foodcrops & insect predation . . . and in total we have a great deal of danger facing us from even 1 more degree of warming.   (And the global warming already occurred has locked-in more sea evel rise even if temperature rise halts immediately.)

  7. Jasper @3, Yet another take.

    You write "I get that a few degrees make a huge difference. I don't fully understand why a few degrees matter so much." Although a huge difference is suggestive that it does matter, I read your meaning that you are after an authoritiative take on the effects of "a few degrees" and something with a bit of meat on it.

    Warming the globe by "a few degrees" will make a big difference to the climate system which has been previously reasonably fixed for millenia and so will bring unprecedented change for human civilisation. But providing an authoritative account of what that change will amount to isn't so easy.

    Will those "few degrees" be enough to stop the AMOC and plunge Europe into a mini-ice age, enough to broaden the Hadley Cells and turn the central US lands and the Mediterranean lands into deserts, to green the Sahara and turn the Amazon into a treeless savannah? The answers are not straightforward. There is no long list if definitive outcomes set out in the headlines of the IPCC AR5 Synthesis Report. The word "risk" features too often when IPCC describes such outcomes.


    But there are a couple of definitive temperature-related outcomes from AGW.

    One is that Greenland will melt out somewhere between +1ºC and +2ºC threatening serious sea level rise. (The IPCC AR5 puts the upper bound at +4°C which is rather a fudge. Antarctica's ice caps are similarly a threat but how quickly they will react to global temperature rise is not well enough understood to be so predictable.) Another is the habitability of the tropics for humanity and perhaps a third is ocean acidification which would be unprecedented in tens of million of years.

    If Greenland were to melt down (a process that once started will not stop as the top of the Greenland ice sheet today sits happily frozen high up in the cold upper atmosphere), the oceans would rise by over seven metres. This compares with the last six thousand years (which spans the period of human civilisation) when changes in sea level could be measures in centimetres. A seven metre rise would be a big problem as so much of our populations today live close to sea coasts. (About a third of humanity inhabit land less than 100 metres above sea level while the loss of both Greenland and Antarctica would raise sea levens 75 metres.) The melt-down of Greenland would take a few centuries to make its mark but the process certainly becomes unstoppable if global warming remains two degrees centigrade above pre-industrial.

    The "few degrees" global temperature rise that accompanied the warming from the last glacial maximum 20,000 years ago and the dramatic impact on climate has been mentioned up-thread. The change in climate resulting from another similar-sized rise in global temperature would be just as dramatic for humanity. If global temperatures rose by six degrees celsius above pre-industrial, it could perhaps be described as a "Steam Age" as the increase in wet bulb temperatures would make the tropics a death trap for humans outside air conditioning. And such a six-degree temperature increase by 2100 is within the projection of the Business-As-Usual scenario of the IPCC.

    The ocean acidification would rival that of the PETM 55 million years ago but would happen in decades rather than tens-of-millenia.

    There is a big pile of reason not to let AGW run beyond +1.5°C. The implications for humanity and for much of the biosphere will be catastrophic if we let AGW run. It's a bit like jumping off a cliff. Predicting the height it would require for the fall to split your skull open is not straightforward but that is no reason to consider jumping. Besides, when you fall it's the intracranial hypertension that usually kills.

  8. Thank you for your help. English is not my native tongue so please forgive me if I make any language mistakes.

    I am a chemistry teacher in the Netherlands and I teach 14 till 18 year old students. On a regular basis I encounter young sceptics that are difficult to win over with the normal arguments because they live in a social environment where everybody thinks climate change is not a problem. You can think of them as the Dutch republicans (to put it in an American perspective).

    A few months ago a student said to me that an increase in 3, or 5 degrees C will not mean much because in winter in can be below 0 C and in summer it can be 40 C. It was a new argument for me. I thought about the average temperatures in the ice age, but that is for a 14 year old too abstract.

    I teach in an agricultural area and the arguments about droughts (The Netherlands was famous for its rains, but recent summers meant long hot dry spells, which means something to these farmer kids...) and the fact that because of temperature change crops might not be able to grow anymore or (more importantly) will give a lower yield might also mean something to them.

    Hopefully with this I will be able to start some students of mine on the track of re-evaluating their believes in climate change.

  9. Jasper,

    I am surprised that students in the Netherlands are not concerned about sea level rise.  In the last ice age the temperatures were about 5C less than the current temeperatures.  The sea level was 125 meters lower than it is today!  From "only" 5C temeprature change!!!  The current IPCC estimate for 2100 is a little over 1 meter sea level rise.  Dutch engineers think they can manage 1 meter sea level rise.  The high estimates of sea level rise by 2100 are well over 2 meters.  That would overwhelm the Dutch dykes!  What do farm boys think of that?

    Have the students check the estimates for sea level rise from all 5 IPCC reports.  Every report the sea level estimate is increased!  Currently the temperature response of the great ice sheets on Greenland and the Antarctic is not well understood.  Some sea level experts think that 6 meters of rise by 2100 is possible if the ice sheets respond rapidly.  (To my students I emphasize the high end of the IPCC.  The IPCC estimates are low compared to other scientific estimates).  If you post again I can look for other estimates for you.

    I had my students (the same age as yours) write me a report on the Arctic sea ice minimum in September.  The NSIDC web site is easy to read (unfortunately in English).  The summary of the yearly low in sea ice will be posted about October 7.  The retreat of sea ice does not raise sea level.  On the other hand, it is right next to Greenland which does raise sea level.  My students were invariably shocked at the rate of sea ice loss.

    I had my students write another report on global temperature using the NOAA web site.  This is also easily read in English.  The summary of yearly temperature will be posted about January 15 2021 for the year 2020.  There is probably a web site in Dutch (any readers know of a Dutch site?) but I like the NOAA web site a lot.

    Post again if we can help you.

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