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Numerous ways to say "thousand years" in a scientific paper

by Martin Kölling, 2015. Version: 24.October 2020  found on:  https://www.sedgeochem.uni-bremen.de/kiloyears.html

If we want to express thousand meters, we call it "km" - a lowercase "k" for the multiplier 1000 and "m" as the generally accepted SI unit of length. If we want to express that two points are 1000 m apart, we would say one is 1 km from the other. If something is 25000 m away, we would say its 25 km away. And if a highway exit is at highway km 34 we would say its at highway km 34, no matter if it is only 1 km away. Very simple, very clear. If something is 20 km to the left, we might use -20 km and +20 km would be to the right e.g. like in a normal cartesian coordinate system. If a plane flies at 8840m, we could also say it flies 8.84 km high, and everybody would understand (ok pilots would call it 29000 ft, but this is a different story). If a borehole is 2000 m deep, you could call it 2 km deep. Nobody would dare to come up with a unit that is "kma" for km apart or "kmd" for km deep. And I have not seen people use "me", "mr" or "met" instead of "m" for meter. Its just "m". Clearly not a capital "M" which stands for Mega or the multiplier 106. The only complication is, that "m" also stands for the multiplier 10-3, but everybody seems to be perfectly fine with "mm" meaning millimeters or 0.001m - not m2.

Yet, when it comes to express thousand years scientifically, it gets complicated (maybe we scientists get complicated): It seems like a competition to find the one non-standard unit that nobody ever used before. We have actually found more than sixty (!) ways (and counting) of expressing thousand years (ago) in a scientific paper. And most of these different versions have been used in major peer reviewed journals.

There also seems to be some confusion about capitalizing units. Even when the title of axis should be capitalized as many journals request, this does not mean that units all of a sudden have to start upper case ! "K" stands for Kelvin, "k" stands for kilo.
So a simple correct age axis label is "Age [cal ka BP]"  - not " Cal Kyr B.P.".

Here is a list of what scientists write (or what editors force them to write) when they mean thousand years.
The rating is my personal opinion:

rating
unit name
unit for
meaning
example
 yes
age (ka)
age
age in kiloanni
common - NIST, ISO
1
no
age (ky)
age
age in kiloyears
2
ok
age (kyr)
age
age in kiloyears
3
no
cal ka
dated age
calibrated kiloanni before present
4
yes
cal ka BP
dated age
calibrated kiloanni before present
5
no
cal ka (BP)
dated age
calibrated kiloanni before present
6
no
cal ka B.P.
dated age
calibrated kiloanni before present
nn
7
no
cal. ka BP
dated age
calibrated kiloanni before present
8
no
cal. ka B.P.
dated age
calibrated kiloanni before present
9
NO
cal. BP
dated age
calibrated ???  before present
10
NO
cal kBP
dated age
calibrated kilo ???  before present
11
no
cal ky
dated age
calibrated kiloyears
12
no
cal ky B.P.
dated age
calibrated kiloyears before present
13
ok
cal kyr BP
dated age
calibrated kiloyears before present
14
no
Cal Kyr B.P.
dated age
calendar kiloyears before present
15
NO
Calendar Ka BP
dated age
calendar kiloanni before present
16
no
k.y.
age
calendar years
17
NO
k.y.
time
kiloyears
18
NO
k.y.a.
age
kiloyears ago
nn
19
NO
k.y. ago
age
kiloyears ago
20
NO
k.y. B.P.
age
calibrated kiloyears before present
21
NO
k.yr.
time
 kiloyears
22
yes
time
kiloanni
common - NIST, ISO
23
no
ka
age
kiloanni ago
24
NO
time
kiloanni
25
yes
ka ago
age
kiloanni ago
common
26
ok
ka before present
age
kiloanni before present
27
no
ka bp
age
kiloanni before present
28
no
ka b1950
age
kiloanni before 1950
29
yes
ka BP
age
kiloanni before present
common
30
no
ka, BP
age
kiloanni before present
31
no
ka B.P.
age
kiloanni before present
32
yes
ka cal BP
dated age
calibrated kiloanni before present
33
no
ky
time
kiloyears
34
NO
Ky
time
kiloyears
35
no
kya
age
kiloyears ago
36
 NO
Kya
age
thousand years ago
37
 NONO
KYA corrected !
age
kiloyears ago
38
 no
ky ago
age
kiloyears ago
39
 NO
KY ago
age
kiloyears ago
40
 NO
kybp
age
kiloyears before present
41
 NO
ky B.P.
age
kiloyears before the present
42
ok
age
kiloyears
43
ok
time
kiloyears
44
 No
time
kiloyears
wrong - (Kelvin years !!)
45
ok
kyr BP
age
kiloyears before present
46
no
kyr B.P.
age
kiloyears before present
47
no
kyr B.P.(where present is A.D. 1950)
age
kiloyears before present
48
 no
kyrs
time
kiloyears
49
ok
kyr ago
age
kiloyears ago
50
 no
kyr ago (ka)
age
kiloyears ago
51
ok
time
thousand year period
52
no
millenia
time
thousand year period
53
ok
time
thousand year period
54
ok
thousand years
time
thousand years
nn
55
ok
thousand years ago
age
thousand years ago
56
no
thousand years ago (ka)
age
thousand years ago or kiloanni
57
no
thousand years ago (ka)
time
thousand years ago or kiloanni
58
no
thousand years ago (kya)
age
thousand years ago
59
no
thousand years ago (kyr)
age
thousand years ago or kiloyears
60
no
thousands of years ago
age
thousand years ago
61
???
time b.p.
age
 ? time in ??? before present   ?
62
NoNo
tya
age
thousand years ago
63
NONo
Tya
age
thousand years ago
64
NONO
TYA
age
thousand years ago
65
no
Year (ka BP)
age
year ? in kiloanni before present
66
NONO
kaa
age
kiloanni ago
not used yet / name of a snake

NONO
cy
time
century=100 years or centiyears ?

no
cal. yr BP
time
calibrated years before present








Just when we thought, every single possible variety had been used, Science came up with a new spelling in 2015: "KYA" which most likely meant "kiloyears ago". (Strictly spoken, "KYA" means "Kelvin Yotta Ampere", with Yotta standing for the multiplier 1024). Nobody knows how this all uppercase spelling of a unit that is unusual already in its lowercase version (kya) could have been agreed on by dozens of co-authors and how it sneaked through a first class editing and reviewing process - but it did. 

Sorry Science. It seems like either this little rant, enlightenment from another side, or my hyping Science over Nature has led to correct this paper: In the online version, "ka" is consequently used now (although they eventually explain it with kiloyears ago). We are very happy for the correction and proud, if this website helped to stimulate it. Please send me an email if you cannot find the wrong usage of the units in the listed papers anymore, so I try to include the corrections.

Interestingly, that very same year the very same journal forced one of the major advocates for using "ka" and nothing else as a unit for 1000 years, Paul Renne, to use "ky" which is the favorite unit of the journal for geo-related papers. See here how to persuade authors of journal units.

If we could make a suggestion we recommend (like NIST and ISO) to use "a" for "annus / anni" which is Latin for "year / years". Normally events before today should extend to the left of a time axis and be negative time. Since this is impractical for most paleo-records, we suggest to consequently use "age [ka]" as the label of an age axis and use expressions such as "14.6 ka ago" in the text if you want to avoid negative time like "at -14.6 ka".

Here is a recent article on the proper use of time units in Pure and Applied Chemistry
The problem is much deeper in that the second is defined as a SI unit but the year is not, and for example the solar year changes in duration with time as this paper on time unit confusion in New Scientist nicely points out. See also the Wikipedia page on "year". ISO 80000-3 defines "a" as symbol for the time unit year, although interestingly it defines its length to be either 365 or 366 days (what kind of definition is that ??). NIST also recommends only "a" as a symbol for year.
Here is a link to an archived Time Unit Discussion page (2009) within the Geological Society of America (GSA) (- the original page has been lost in the recent redesign of GSA pages) a paper by Christie-Blick (2011) and a paper by Aubry et al. (2009) discussing this matter.

Most geoscientists like to have their scales get older from left to right, since they record their data in ages rather than in time.

All time units that include "ago" or "BP" or just the mysterious "a" in "kya" turn time units into negative time units, without making the numbers negative: So the scale gets older from left to right as the numbers count up.

Alternatively people label the scale "age [ka]" instead of "time [ka ago]" so its clear that increasing numbers mean you go back in time. This has the main reason that paleo-records are often from sediment cores, where depth corresponds to an age and usually age increases with depth.

In the paleo-community only modellers tend to do time scales, where ages are negative time and today is on the right hand end of the scale. For all other paleo-people it seems impossible to think of age as negative time as you would do easily with length. Nobody needs a separate unit for length, just because it extends to the left of the coordinate system.

And it seems impossible to use the correct Latin word "annus" for year, as it sounds too similar to "anus" which is used as  the medical expression for
the opening of the fecal canal in reference to the terminal cylindrical sphincter muscle (thanks, Paul to clarify on this !). Instead "annum" is used, which sounds very Latin but it actually stands for the duration "for one year" often used in "per annum" instead of "annual" or "per year".

So "a" might stand for the Latin "annus" for year or the plural "anni" for years.
"ka" stands for kiloanni which is thousand years. ok so far.. but then:

It is quite common that "ka" is used for "kiloyears ago", so the "a" somehow stands for both, "annus" and "ago" (??!!) and in contrast, "kyr" is used for a duration (which is always positive) or for a time in the future that would be positive anyway. Reimer et al., Radiocarbon (2009) used a unit, that elegantly solves the problem by simply not using any symbol for year: "cal kBP"  which stands for "calibrated kilosomething before AD 1950".

In the end, it is mostly accepted (although not precise and not backed by standards) to use "kyr" for time periods or durations and for time in the future and "ka" for age (or negative time) thus a period from 10.7 ka to 9.7 ka would be assumed to be in the past and have a duration of 1 kyr. Yet, most people use "kyr" and "ka" almost as synonyms and often two or three different ways of writing thousand years are used in the very same paper. And it seems to be perfectly normal to use different units in the text and in the graphs. Most journals have their favorite way of expressing age and time, but as you can see from the list above, this is not consistent and it changes with the field you are publishing in.

And: It is quite common to almost the rule to mix "kyr" for thousand years and "Ma" for million years, since "My" or even worse "my" looks like the possessive pronoun "my", it might be mixed up with the greek letter "µ", and "m" actually stands for "milli" according to SI conventions for multipliers-  Million or Mega have to be a capital "M". Thus "my" would be milliyears (or 8.766 hours). But then "Myr ago" is quite common. Talking about Gigayears, "Ga" might also look like the chemical element symbol for Gallium, while "Gy" is taken for Gray, a unit for the absorbed dose of ionizing radiation. And to complete confusion possibilities, "GY" is eventually used for galactic year, which is actually equivalent to 250 Ma and is based on the time that the sun takes to revolve once around the center of the Milky Way galaxy (which took more like 225 Ma for the last round).

The second very confusing way to use "a" in time/age  units is to explicitly abbreviate "ago" which turns time into age :
"kya" or "k.y.a." or  "KYA"  stands for "kiloyears ago". Its also not rare to ignore SI prefixes and use "tya" for "thousand years ago". There is also an all uppercase version "TYA" which would be "Tera Yotta Amperes" according to SI.
Luckily, as far as we know, nobody has used "kaa" for "kiloanni ago" - but we are sure this will happen soon.
In a 2014 nature communications paper, Grant et al. used "cy" not for centiyears (= 3.65 days) which would be weird but at least the prefix would be correct but for "century" - "cy" why not - who cares about conventions... so - logically,  millennium should be abbreviated "mm" or "ma" for millennia?? here we are... ?? where are we ??

Confused ? feel free to mail us more recent or new examples of additional ways to express thousand years in a yet more confusing way...

Even more confusing: Strictly spoken, "ka" for kiloyears is in conflict with SI regulations, since SI still officially allows to use "a" for "are" which is an old area unit and stands for Deka meters squared (1 a=1 dam2=100 m2). To my personal understanding both the (only) two letter prefix "da" for "deka" or 102 and the old unit "are" are quite confusing. I think we can live with m2 and maybe without "deka" if we have to abbreviate it "da". And the chance that somebody wants to use "ka" for kilo ares which would then be 105 m2 is rather low. But then nobody would use "Kelvin Yotta Ampere" in an archeological paper (see above)...

Conclusion: We still recommend "ka" but whatever you use, the copy editors will teach you, which unit to use in the journal and in the field you are trying to publish in.

A totally different discussion is worth another page: what exactly is "ago" or before present? In the radiocarbon community before present is defined as before 1950 CE (which opens another discussion page on the use of AD, CE, BC BCE ... which literally quickly gets religious - see below).

Carolin et al. (2019) just introduced "ka b1950" which clarifies the reference point, yet it is neither NIST nor SI  compatible. BTW - the unit "b" is used in nuclear physics for "barn" which is 10?28 m2 although not being an SI unit. And as Andy pointed out, it starts to get common to refer to "years before 2000 CE" as "b2k". Thats short, it is quite obvious (in the community) what is meant - but it is sloppy: like in Reimer's "cal kBP" there is no symbol for year in there! Could be "below 2 km" as well.

The expression "ago" is usually less precise. While it is probably irrelevant, whether 500 ka ago refers to 1950 AD or today (what exactly is today ? 2019 CE ?, 1950 AD ??) as reference point, there is a conceptual problem, whether or not we can agree on a fixed point in time that we refer to (like most of us refer to 0°C or 0 K with temperatures) - whether it is pre-industrial, pre-nuclear-bomb or pre-a-currently-popular-religion-protagonist's-wrong-birthdate), since "ago" would otherwise only refer to the time a paper was written.


Watanabe et al. (2019) just published a paper in Geology where already in the abstract I found:
"kyr B.P. (where present is A.D. 1950)". This is weird in many ways:
a) why would you use the restricted number of words of an abstract to start explaining what present in B.P. means?
b) If you assume people know you are not talking about a well known oil company when you say "B.P.", you might assume that the same people know what BP actually refers to. If you don't, you should probably explain that the "P." in "B.P." stands for "present" in the first place.
c) So if you decide you want to do this, you could write: " ka BP (kiloyears before present where present refers to 1950 CE) ", correct but horribly cumbersome to be used in an abstract.
d) B.P. is wrong - its BP - you would not use k.m. instead of km, likewise it is AD not A.D.
e) "present is A.D.1950" is wrong because present is not AD 1950  (it was present in 1950 CE), but the radiocarbon dating community agrees to refer to 1950 CE as present.
f) explaining BP is unnecessary especially maybe in an abstract
g)  to be religiously neutral AD (="Anno Domini - in the year of the Lord")
is out and CE is in
h) Yet, CE is actually not really free of religious bias since the common era (CE) is defined to start some time around the birth of Jesus Christ which is believed to be at 4±2 BCE - but at least you do not refer to "Lord" or "Christ" in the unit name.
The real advantage is that using BCE makes the sentence "Jesus Christ was born 4±2 BCE" (= between the year 6 and 2 before the Common Era)" sound much less stupid than "Jesus Christ was born 4±2 BC" (= between the year 6 and 2 before Christ ??!!).

Getting carried away....
Interestingly the "Common Era" was originally called the "Vulgar Era" (annus aerae nostrae vulgaris) when they were hunting witches in Europe and had to fight the Little Ice Age. They had no clue then, how vulgar exactly politicians would get as witch-hunts would re-emerge when mankind was fighting global warming (or not really). So now that impeachment turned out to be reconfirmation, I would suggest to switch to BVE / VE instead: And we could start to refer to 2016 VE as  0 a BT. 1950 VE would then be 66 a BT. If you are not comfortable with referring to the election year of an infamous president, we could give it a more positive spin: We could use 2012 - the year the model S was introduced - as the beginning of the EE Electric car Era so 1950 CE would then be 62 a BT (before Tesla). This is my recommendation as Tesla is an established SI unit already ......... Just kidding...  If you are confident that fighting global warming will succeed, we could also choose 2018
as a reference, the year when a collapse of CO2 emissions began as "Greata" started spending fridays on streets. So 1950 VE would then translate to 68 a BG (before Greta). But then G is already used as the gravitational constant (6.674×10?11 m3?kg?1?s?2) and for the multiplier prefix giga or 109. Yet, I personally would prefer BT (before Tesla). If the global perturbations get as severe as it currently looks, it might also make sense to complete confusion and redefine BC in a virological rather than in a theological way and refer to the year 2020 CE or -70 a BP as 0 BC (Before Corona) so "present" in a radiocarbon sense would become 70 BC and Jesus Christ would have been born in 2024±2 BC. Btw: Has anybody thought about the fact that if we refer to 1950 CE as present, we logically should refer to 2020 CE as 70 a AP (70 years after present).  Stay healthy and make sure there is a future for most of us !!


SCIENSE cover Jan 1977
Getting completely carried away:

Because in 1977, John Cleese was writing his groundbreaking Sciense paper about theories for a new programming language (monty Python), Graham Chapman got into the competition and successfully submitted his own elaboration on 100 ka cycles being nonsense after he had seen those stamp sized frequency analysis plots in Hays, Imbrie & Shackleton (Science, 1976).
In early shots for 'Life of Brian', Graham Chapman wanted to incorporate this recent experience of submitting a geo-related manuscript. So John Cleese helped him to give a short demonstration of what happens, if you think its o.k. to select a NIST / SI compatible unit for kiloyears when you want your paper published in Science. We superimposed subtitles that represent the rarely heard dialogue from the lost original script to this scene which only in 1979 evolved into the famous Latin lesson scene.
How to not get your story on the cover on the Science Mag has been recently explained here (long version)
.


One more thing: This website has already been cited as a reference for time units. If you want to use this webpage as a reference please use 2015 as a publishing year because that is the year it first went online. As you can see below, the site is constantly updated, so it makes sense to include the latest time stamp. 

Kölling, Martin (2015): Numerous ways to say "thousand years" in a scientific paper.
Version 24. October 2020 - www.sedgeochem.uni-bremen.de/kiloyears.html


You might also want to refer to the more serious papers of  Christie-Blick (2011) and  Aubry et al. (2009).
I personally recommend to join the "Popular Front for annus as single time unit (PFastu)" and cite Holden et al.(2011).


If this page helped you, you can help us by looking at our paper in nature geoscience
It is on CO2 release from continental shelves and you can read it for free here

Even if you are not interested in the shelf life of pyrite, the paper has a wonderful graph
that shows the relation between CO2 and sea-level during the last 800 ka.
It shows impressively how crazy our current CO2 levels are.

Martin Kölling, MARUM

One last thing: If you think I am what even educated Dutch would very metaphorically call a "mierenneuker" you have not seen any piece of tidied up art by Ursus Wehrli.

original time stamp 23.July 2015
updated 29.Aug15, 30.Aug15, 24.Sep15, 1.Oct15,  2.Feb16, 14.Sep16, 15.Dec16, 30.Mar17, 13.Jun17, 25.Oct17, 12.Nov17, 4.May18, 7.June18, 6.July18, 7.Aug18, 17.Dec18, 9.Jan19, 16.Jan19, 20.Mar19, 27.Mar19, 27.Jun19, 6.Sep19, 16.Oct19, 20.Oct19, 23.Oct19, 7.Nov19, 12.Nov19, 1.Apr20, 2.Apr20, 5.Apr20, 17.Jul20, 10.Sep20, 24.Oct20

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page created by M.Kölling & T.Feseker