This shows that with upslope winds, the data is influenced by local CO2
depleted air. These data are rightfully discarded from the
daily/monthly/yearly averages, as they don't reflect the background CO2
levels, which we are interested in.
Does discarding of "contaminated" data influence the trend over a year
or several years? I have asked that question to Pieter Tans,
responsible for dataprocessing of the Mauna Loa data. His answer:
data selection method has been described in Thoning et al., J. Geophys.
Research, (1989) vol. 94, 8549-8565. Different data selection
methods are compared in that paper, including no selection. The methods give annual means differing by
a few tenths of 1 ppm. I assume that you have read the
The good, the bad and the ugly
] when downloading the data. The
hourly means are NOT pre-processed, but they are flagged when the
st.dev. of the minute averages is large.
Several stations are deemed "good", as these have minimal influence
from local vegetation and/or human emissions (traffic, heating). These
are stations in the middle of the oceans, sometimes at coastal points
(as long as the wind is not blowing from land side) and/or above the
inversion layer. These stations, after discarding outliers, differ from
each other within 5 ppmv for yearly averages, of which most is from the
delay between the NH and the SH, see next item. 10 of them, spanning
globe from near the North Pole (Alert, Canada) to the South Pole, are
used as reference for daily, monthly and yearly averages and yearly
trends. The graphs and the data can be found at [5
Some inland stations, like Schauinsland only give reliable "background"
levels, when the station (at 1200 m altitude) is above the inversion
layer and with enough wind speed. This happens only about 10% of the
And last, but not least, many inland stations are practically
unsuitable for background CO2 measurement, because of incomplete mixing
with the higher air layers, partly due to too many local sources/sinks
like vegetation and/or human use of fossil fuels, partly due to a
shielded location. This is the case for e.g. Diekirch (Luxemburg) [6
where the station is in a valley with forests,
urbanisation and traffic in the main upwind direction:
Diekirch (Luxemburg) CO2 measurements compared to wind speed
Graph from [6
As can be seen, even at inconvenient places with lots of local
sources/sinks, there is an inverse correlation between CO2 levels and
wind speed. With higher wind speeds, CO2 levels are better mixed with
higher air layers which have "background" CO2 content. This reduces the
CO2 content at ground level. The assymptote of CO2 levels at high speed
winds (as was seen during storm Franz, 11 January 2007) is about 385
ppmv, very close to the 382 ppmv level found at Mauna Loa in the same
period. The same is true for diurnal variance: at daytime and with high
enough wind speed (> 1 m/s), CO2 levels are lower and near
background, while at night under the inversion layer, CO2 levels are up
to 100 ppmv higher.
CO2 due to the seasons:
There are two main natural influences on the CO2 levels of the
atmosphere: the temperature of the ocean's surface waters and the
uptake of CO2 by plants in spring/summer and the release of CO2 by the
decay of dead plant material in fall/winter. This is most clear in the
NH (Northern Hemisphere), where most of vegetation on land is situated.
CO2 is continuously emitted by deep sea upwelling, especially in the
tropics, where temperatures are high and the partial pressure of CO2
(pCO2) in the upper oceans is higher than in the atmosphere above it.
CO2 is continuously absorbed in the upper ocean layers at higher
latitudes, where the colder temperatures reduce the pCO2 of the oceans,
lower than the pCO2 of the atmosphere. This is especially the case at
the sink places of the THC (thermohaline circulation) in the Nordic
Atlantic ocean. Colder water can retain more CO2 than warmer water, but
in the case of CO2 there are also a lot of chemical and biological
reactions which influence the solubility of CO2 and hence pCO2 at the
surface of the oceans. For more details on this, Wiki
quite good explanation.
The CO2 flow between the tropics and the colder places in the oceans is
relatively constant (more about that later), and doesn't influence the
seasonal variation that much. More variation is in the temperature (and
thus pCO2) of the mid-latitudes, where there is absorption of CO2 in
winter and release of CO2 in summer. The CO2 flow of vegetation
(including algues in the upper oceans) is in opposite direction: more
release in winter and more uptake in summer. The net effect in the NH
variation of +/-4 ppmv in Mauna Loa (mid Pacific Ocean, middle
troposphere) between summer and winter, up to +/-20 ppmv for Barrow
(Alaska, USA, sea level, near tundra) or even 35 ppmv at Schauinsland
(Germany, 1200 m high). The data of
Schauinsland are heavily contaminated by the nearby fully inhabited and
industrialised Rhine valley. And influenced by vegetation, in this case
the Black Forest of SW Germany. Only at night, when separated from the
valleys by an inversion layer, and with sufficient wind speed, the CO2
levels are better mixed with the rest of the troposphere and retained.
This is the case for only 10% of the data.
Data series from the SH (Southern Hemisphere) show much less seasonal
variation, because of the much smaller area of land/vegetation. The
smallest influence of the seasons is found at the South Pole.
Here follows some comparison of the Mauna Loa (selected) monthly
averages with these of other stations:
Monthly trends 1997-1999 of 2 NH stations (Barrow and Mauna Loa) and 2
SH stations (Samoa and South Pole)
As can be noticed, the variation at Mauna Loa is smaller than at Barrow
and the SH stations have a much smaller seasonal influence than the NH
stations. Also, although the trend of the SH stations is near the same,
there is some lag between the NH and SH stations. This is the first
indication that the source of the increase is situated in the NH, as
the ITCZ (intertropical convergence zone) forms a barrier for the
exchange of CO2 (and aerosols) between the NH and the SZ. This is even
more clear in the longer term yearly trends:
Trends in yearly averages of CO2 levels at different stations.
The trend of the SH stations has a growing delay of 6-12 months behind
the NH trend. But all yearly average data of the "best" stations (and
the average of least contaminated data from less suitable stations like
Schauinsland) are within 5 ppmv for similar growth.
4. Where to measure? The concept of
"background" CO2 levels.
The concept was launched by C.D. Keeling in the mid fifties, when he
made several series of measurements in the USA. He found widely varying
CO2 levels, sometimes in samples taken as short as 15 minutes from each
other. He also noticed that values in widely different places, far away
from each other, but taken in the afternoon, were much lower and much
closer resembling each other. He thought that this was because in the
afternoon, there was more turbulence and the production of CO2 by
decaying vegetation and/or emissions was more readily mixed with the
overlying air. Fortunately, from the first series on [2
he also measured 13C/12C ratios of the same samples, which did prove
that the diurnal variation was from vegetation decay at night, while
during the day photosynthesis at one side and turbulence at the other
side increased the 13C/12C ratio back to maximum values.
Keeling's first series of samples, taken at Big Sur State Park, showing
the diurnal CO2 and d13C cycle was published in [7
original data (of other series too) can be found in [8
Diurnal variation in the concentration and carbon
isotopic ratio of atmospheric
in a coastal redwood forest of California, 18-19 May
1955, Big Sur St. Pk.
(Keeling, 1958. Reproduced by permission of Pergamon Press.)
Several others measured CO2 levels/d13C ratios of the their own samples
too. This happened at several places in Germany (Heidelberg,
Schauinsland, Nord Rhine Westphalia). This confirmed that local
production was the origin of the high CO2 levels. The smallest CO2/d13C
variations were found in mountain ranges, deserts and near the oceans.
The largest in forests, urban neighbourhoods and non-urban, but heavely
industrialised neighbourhoods. When the reciproke of CO2 levels were
plotted against d13C ratios, this showed a clear relationship between
the two. Again from [7
Figure 3.5 Relation between carbon isotope ratio and concentration of
different air types from measurements summarized in Table 3.4
(Keeling, 1958, 1961: full squares; Esser, 1975: open circles; Freyer
and Wiesberg, 1975,
Freyer, 1978c: open squares). All 13
C measurements have not been
O contamination (Craig and Keeling, 1963), which is at
the most in the area of + 0.6‰
search for background places.
Keeling then sought for places on earth not (or not much) influenced by
local production/uptake, thus far from forests, agriculture and/or
urbanisation. He had the opportunity to launch two continuous
measurements: at Mauna Loa and at the South Pole. Later, other basic
stations were added, all together 10 from near the North Pole (Alert,
NWT, Canada) to the South Pole, most of them working continuous, some
working with regular flask sampling.
We are interested in CO2 levels in a certain year all over the globe
and the trends of the CO2 levels over the years. So, here we are at the
definition of the "background" level:
Yearly average data taken from places minimal influenced by vegetation
and human sources are deemed "background".
For convenience, the yearly average data from Mauna Loa are used as
reference. One could use any base station as reference or the average
of the stations, but as all base stations (and a lot of other stations,
even Schauinsland) are within 5 ppmv of Mauna Loa, with near identical
trends, and that station has the longest CO2 record, Mauna Loa is the
As the oceans represent about 70% of the earth's surface, and all
oceanic stations show near the same yearly averages and trends, already
70% of the atmosphere shows background behaviour (within 5 ppmv). This
can be extended to near the total earth for the part above the
Measurements above the inversion layer.
Above land, diurnal variations are only seen up to 150 m (according to [7
Seasonal changes reduce with altitude. This is based on years of
flights (1963-1979) in Scandinavia [7
] and between
Scandinavia and California [9
]. Further confirmed by
old and modern [10
] flights in the USA and Australia
(Tasmania). In the SH, the seasonal variation is much smaller and there
is a high-altitude to lower altitude gradient, where the high altitude
is 1 ppmv richer in CO2 than the lower altitude. This may be caused by
the supply of extra CO2 from the NH via the southern branch of the
Hadley cell to the upper troposphere in the SH.
], based on [9
Amplitude and phase shift of seasonal variations in
at different altitudes, calculated from direct observations by harmonic
(Bolin and Bischof, 1970. Reproduced by permission of the Swedish
Modern flight measurements in Colorado, CO2 levels below the inversion
in forested valleys and above the inversion layer at different
As one can see, again the values above the inversion layer are near
straight and agree within a few ppmv with the Mauna Loa data. Below the
inversion layer, the morning values are 15-35 ppmv higher, in the
afternoon, these may sink to background again.
If we take the 1000 m as the average upper level for the influence of
local disturbances, that represents about 10% of the atmospheric mass.
Thus the "background" level can be found at 70% of the earth's air mass
(oceans) + 90% of the remaining land surface (27%). That is in 97% of
the global air mass. Only 3% of the global air mass contains not-well
mixed amounts of CO2, which is only over land.
Background CO2 levels can be found
over all oceans and over land at 1000 m and higher altitudes
high mountain ranges, this may be higher).
5. Evidence of human
influence on the increase of CO2 in the atmosphere.
The amount of CO2 emitted by humans nowadays is about 7 GtC/yr (CO2
counted as carbon). The increase in the atmosphere is about 4 GtC/yr.
That implies that there is little to no increase in the atmosphere due
to other causes, or the amount in the atmosphere in the case of a
natural unbalance should be higher than the emissions, not lower. To
show this for the past near 50 years [11
The graph shows the increase in CO2 from emissions, land use and others
vs. the increase found
in the atmosphere, land and oceans, expressed in GtC/yr
The graph is based on calculations of emissions, sampled from national
inventories of fuel use and land use change. In the best case, these
are accurate, in the worst case, the emissions are underestimated (as
probably is the case for China). Inventories of the atmosphere are
based on very accurate measurements of CO2 at Mauna Loa. The difference
between CO2 emissions (expressed in gigaton carbon per year - GtC/yr)
and carbon increase in the atmosphere is what the oceans and/or
vegetation absorb each year. The partitioning between land and ocean is
certainly not known at the accuracy as shown, but not of interest here,
as in every year the sum of
land+oceans is more sink than source.
But to be honest, here is a better indication of the partitioning
between land and oceans by Battle ea. [12
], based on
changes of d13C and oxygen content in the atmosphere over the last
decade of the previous century and earlier estimates:
As the first graph shows, in any year of the past 40+ years, the
are larger than the increase in the atmosphere. That means that the
total mass balance of all natural variables (temperature, ocean pH,
vegetation) which influence CO2 levels, is always towards more sink
than source over any year:
Csources + Cemissions = Csinks + dCair
where Cemissions = 2.5-6.5 GtC/yr
and dCair = 1-6 GtC/yr
and Csinks = Csources + 3 GtC +/- 2.5 GtC
The natural seasonal exchange between vegetation and oceans at one side
and the atmosphere at the other side is estimated at about 150 GtC/yr.
But that is not of interest for what the change is over a year, as most
of the natural releases are absorbed within the same year. The
difference after a year is not more than +/- 2.5 GtC, mainly caused by
temperature changes (El Niño, Pinatubo eruption). Thus the
natural variations over a year are smaller than the emissions. No
matter how high the natural seasonal turnover might be, in all
years over the previous near 50 years, the natural CO2 sinks were
larger than the
natural CO2 sources... Thus it is impossible that natural sources were
responsible for (a substantial part of) the increase of CO2 in the past
50 years. The increase of CO2 in the atmosphere in the past near 50
years is about 70% of the increase since the start of the industrial
revolution. This is based on accurate measurements at Mauna Loa and a
lot of other places. The amount of CO2 in the atmosphere in
pre-industrial times is based on ice cores, which of course are less
certain and more smoothed, but there are other proxies with a better
resolution in time, which point to lower CO2 levels prior to the
This proves beyond doubt that human
emissions are the main cause of the increase of CO2
, at least
over the past near 50 years. But there is even more proof of that...
 Carbon Dioxide Concentrations at Mauna Loa
Observatory, Hawaii, 1958-1999, CDIAC NDP-001: http://ceos.neonet.nl/metadata/dif/CDIAC_NDP1.xml
 Rewards and penalties of monitoring the earth,
Charles D. Keeling, Ann. Rev. Energy. Envir. 1998.23.25-82: http://scrippsco2.ucsd.edu/publications/keeling_autobiography.pdf
Fascinating autobiographic story from C.D.Keeling about the history of
CO2 measurements and the struggle against the administrations to get
and continue funding for continuous measurements.
Of special interest:
- First measurements on 5 l flasks were done with enhanced barometric
equipment, with an accuracy of better than 0.1 ppmv.
- The same barometric equipment was used to test calibration gases and
NIR equipment. A change in calibration gases (air/CO2 vs. N2/CO2)
caused a jump in response of the NIR equipment. All previous collected
data were corrected for this change.