A COMPARISON OF THE BARING HEAD AND MAUNA LOA … · Since then the overall rates have started to...
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A.J. Gomez, G.W. Brailsford, K. Riedel, and D.C. Lowe National Institute of Water & Atmospheric Research, Wellington, New Zealand; [email protected] Introduction The New Zealand atmospheric CO 2 record is the longest continuous measurement series in the Southern Hemisphere with its inception first at Makara (1970) followed by Baring Head (1972). Baring Head (41.4°S, 174.9°E), situated on a southern coastal cliff 85 m above sea-level, is a remote site located 12 km southeast of Wellington, New Zealand. Air masses arriving there from the South (onshore) are representative of a large part of the South West Pacific whereas northerlies are influenced mainly by local terrestrial biotic processes. The Southern Hemisphere, due to its large extent of ocean, plays an important role in processes relating to global climate and climate change. By the early 1960s the Mauna Loa record clearly demonstrated both seasonal cycles and a secular trend in atmospheric CO 2 in the Northern Hemisphere. In 1969 Dave Keeling sent a team of 3 staff from Scripps Institution of Oceanography (SIO) to set up the first continuous site for atmospheric CO 2 in the Southern Hemisphere located at Makara, New Zealand (Lowe, 1974). The team was led by Arnold Bainbridge and joined by Dave Lowe, a New Zealander, in 1970. This site was moved to Baring Head towards the end of 1971 when it became clear that the record was influenced by the local vegetation.. Analysis method The Baring Head and Mauna Loa CO 2 data are analysed using the STL procedure (Cleveland et al., 1990), a filtering procedure for decomposing seasonal time-series into trend, seasonal and remainder component series based on the loess (locally weighted regression) smoothers. The main features of STL are that it is robust to outliers in the data and it allows the seasonal component to evolve slowly over time in shape, amplitude and phase. Baring Head CO 2 measurements in southerly air, which have not deviated by more than 0.2 ppm for 6 hours, are averaged within each calendar month to form a baseline monthly time-series representative of the oceanic air to the south of New Zealand (Lowe et al., 1979). The Mauna Loa CO 2 monthly series used in this analysis was acquired from the SIO CO 2 website. The same STL smoothing parameters were applied to both series to derive the trend and seasonal components used in this comparison. A COMPARISON OF THE BARING HEAD AND MAUNA LOA ATMOSPHERIC CO 2 RECORDS 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year 310 320 330 340 350 360 370 380 390 CO 2 (ppm) Mauna Loa Baring Head 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year 1.0 1.5 2.0 2.5 3.0 3.5 CO 2 Trend difference (ppm) Smoothed Differences Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -2 -1 0 1 2 CO 2 Seasonality (ppm) Mauna Loa Baring Head 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 CO 2 Growth (ppm/year) Mauna Loa Baring Head Smoothed Smoothed 1980 1982 1984 1986 1988 1990 1992 1994 1996 Year -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 CO 2 Growth (ppm/year) Mauna Loa Baring Head 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year -1.0 -0.5 0.0 0.5 1.0 CO 2 Interannual variability (ppm) Mauna Loa Baring Head 1980 1982 1984 1986 1988 1990 1992 1994 1996 Year -1.0 -0.5 0.0 0.5 1.0 CO 2 Interannual variability (ppm) Mauna Loa Baring Head Fig 1. STL decomposition of the Baring Head (left) and Mauna Loa CO 2 (right) monthly time-series into trend, seasonal and remainder components using a 25-month smoother for the trend. A 25-month smoother allows structures larger in period than the annual cycle to be included in the trend component. The bars on the right of each plot represent scales of equal length. Note that the time domain and the measurement scales are different on these two plots.) The STL plots show changes in amplitude of the seasonal cycle with time which is more pronounced at Baring Head when compared to the larger seasonal cycle of the Mauna Loa monthly time-series. For both sites the remainder series shows some structure in periods less than the annual cycle with larger variations occurring in the Mauna Loa series. Trend and Seasonality at Mauna Loa and Baring Head The Mauna Loa record exhibits a slightly higher trend than its southern counterpart due to the greater source contribution from fossil fuel emissions in the northern hemisphere. The larger seasonality in the north is a result of changes in photosynthesis and respiration of the vegetation of the greater land mass whereas seasonality in the southern hemisphere is primarily oceanic. Difference in the trends As both trends increase over time due to mainly fossil fuel emissions, the smoothed difference shows a steadily increasing gap between the trends from 1.4 ppm in the early 1970s to around 2.9 ppm in 2000 where it remains constant. This could be an indication of a change in the sink processes such as a saturation of CO 2 absorption in the Southern Ocean (Le Quéré et al., 2007). Seasonal cycles This plot shows the larger peak to peak amplitude of the seasonal cycle in the northern hemisphere (~6 ppm) driven primarily by the seasonal changes in vegetation of the greater land mass. The larger variation and the trends in CO 2 measurements in the monthly sub-series at Mauna Loa reflect the larger variation in changes of vegetation from one year to the next and possible trends in these changes. The seasonality at Baring Head (~1.2 ppm) is not exactly 6 months out of phase with the northern hemisphere due to an influence of the northern hemisphere seasonality through the inter-hemispheric boundary. Fig 5. Growth rates in ppm/year at Mauna Loa and Baring Head. Interannual variability The interannual variability was determined by looking at deviations from the overall mean trend line which was derived using the STL procedure with a 121 month smoothing parameter and subtracted from original monthly series. Once this smooth mean trend has been subtracted the STL procedure is reapplied and the resulting trend forms the monthly interannual variability series. Note that the interannual variability is of the same magnitude as the Baring Head seasonal cycle and significantly less than the Mauna Loa seasonal cycle. As with the growth rates the magnitude and timing of the interannual variability for both sites is remarkably similar. An expanded view for the years 1980 to 1996 shows how similar the two series are. Given the different levels of CO 2 in the atmosphere and the different source and sink regimes in each hemisphere, may be related to a climate phenomenon affecting both hemispheres such as El Nino/La Nina events. Summary The STL procedure is a useful tool in extracting information from a regular time-series. In analysing the Baring Head and Mauna Loa CO 2 measurement records we find the seasonal cycles do vary in amplitude with time. The steadily increasing gap between the trends changed since 2000 and has remained constant. Despite the different levels of CO 2 in the atmosphere in the two hemispheres and the larger source of emissions in the north, the growth rates and interannual variability are remarkably similar both in magnitude and in timing. This leads to further insights into the roles of the large scale processes of the CO 2 sources and sinks. Reference Cleveland, R.B.; Cleveland, W.S.; McRae, J.E.; Terpenning, I. (1990). STL: A Seasonal-Trend Decomposition Procedure Based on Loess. Journal of Official Statistics Vol.6 No 1, 3-73. Lowe, D.C. (1974). Atmospheric carbon dioxide in the Southern Hemisphere. J. Clean Air Soc. Aust. and N.Z. 8, 12-15. Lowe, D.C.; Guenther, P.R.; Keeling, C.D. (1979). The concentration of atmospheric carbon dioxide at Baring Head, New Zealand. Tellus, 31, 58-67. Le Quere, C.; Rodenbeck, C.; Buitenhuis, E.T.; Conway, T.J.; Langenfelds, R.; Gomez, A.; Labuschagne, C.; Ramonet, M.; Nakazawa, T.; Metzl, N.; Gillett, N.; Heimann, M. (2007). Saturation of the Southern Ocean CO2 sink due to recent climate change. Science 316(5832): 1735–1738. Acknowledgements Scripps Institution of Oceanography for the use of the Mauna Loa CO 2 record. Martin Manning and Peter Pohl for their long involvement in the CO 2 measurements at Baring Head and to all those who have assisted during the past 38 years. Dave Keeling for his insight in beginning measurements of atmospheric CO 2 and his early support of the Baring Head CO 2 measurement programme. Foundation for Science, Research and Technology (Contract C01X0703). Fig 7. Interannual variability in the Mauna Loa and Baring Head CO 2 monthly series. Fig 8. Interannual variabilityfor 1980 to 1996. Baring Head Mauna Loa Growth rates The growth rates (Fig 5) are determined by a first-order calculation of the gradient at each monthly value in the trends. Despite the different levels of CO 2 in the atmosphere in the different hemispheres and the larger source of emissions in the north, the growth rates are remarkably similar both in magnitude and in timing. The expanded plot (Fig 6) shows the growth rates between 1985 and 1996 and it is not obvious that the northern hemisphere leads the south as would be expected. When it does, it leads by a few months despite the inter-hemispheric atmospheric transport exchange time for CO 2 known to be around 18 months. During this period the smoothed plots show the growth rates levelling off at about 1.4 ppm/year. Since then the overall rates have started to increase to 2.0 ppm/year near the end of 2007. Fig 4. De-trended Mauna Loa and Baring Head CO 2 monthly series plotted by each month. Fig 6. Growth rates in ppm/year for 1980 to 1996. Fig 2. Increasing trends and seasonality of both the Baring Head and Mauna Loa records. Fig 3. Difference in the trend at Mauna Loa to Baring Head where records match in time. Note that this does not take into account the altitude effect at Mauna Loa which would slightly under-estimate the difference. 330 350 370 data 330 350 370 fc.25 -0.6 -0.2 0.2 0.6 seasonal -0.3 -0.1 0.1 remainder 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year 320 340 360 380 data 320 340 360 380 fc.25 -2 -1 0 1 2 seasonal -0.5 0.0 0.5 remainder 1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year Baring Head Mauna Loa
Transcript of A COMPARISON OF THE BARING HEAD AND MAUNA LOA … · Since then the overall rates have started to...
A.J. Gomez, G.W. Brailsford, K. Riedel, and D.C. LoweNational Institute of Water & Atmospheric Research, Wellington, New Zealand; [email protected]
IntroductionThe New Zealand atmospheric CO2 record is the longest continuous measurement series in the Southern Hemisphere with its inception �rst at Makara (1970) followed by Baring Head (1972). Baring Head (41.4°S, 174.9°E), situated on a southern coastal cli� 85 m above sea-level, is a remote site located 12 km southeast of Wellington, New Zealand. Air masses arriving there from the South (onshore) are representative of a large part of the South West Paci�c whereas northerlies are in�uenced mainly by local terrestrial biotic processes. The Southern Hemisphere, due to its large extent of ocean, plays an important role in processes relating to global climate and climate change.By the early 1960s the Mauna Loa record clearly demonstrated both seasonal cycles and a secular trend in atmospheric CO2 in the Northern Hemisphere. In 1969 Dave Keeling sent a team of 3 sta� from Scripps Institution of Oceanography (SIO) to set up the �rst continuous site for atmospheric CO2 in the Southern Hemisphere located at Makara, New Zealand (Lowe, 1974). The team was led by Arnold Bainbridge and joined by Dave Lowe, a New Zealander, in 1970. This site was moved to Baring Head towards the end of 1971 when it became clear that the record was in�uenced by the local vegetation..
Analysis methodThe Baring Head and Mauna Loa CO2 data are analysed using the STL procedure (Cleveland et al., 1990), a �ltering procedure for decomposing seasonal time-series into trend, seasonal and remainder component series based on the loess (locally weighted regression) smoothers. The main features of STL are that it is robust to outliers in the data and it allows the seasonal component to evolve slowly over time in shape, amplitude and phase. Baring Head CO2 measurements in southerly air, which have not deviated by more than 0.2 ppm for 6 hours, are averaged within each calendar month to form a baseline monthly time-series representative of the oceanic air to the south of New Zealand (Lowe et al., 1979). The Mauna Loa CO2 monthly series used in this analysis was acquired from the SIO CO2 website. The same STL smoothing parameters were applied to both series to derive the trend and seasonal components used in this comparison.
A COMPARISON OF THE BARING HEAD AND MAUNA LOA ATMOSPHERIC CO2 RECORDS
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Fig 1. STL decomposition of the Baring Head (left) and Mauna Loa CO2 (right) monthly time-series into trend, seasonal and remainder components using a 25-month smoother for the trend. A 25-month smoother allows structures larger in period than the annual cycle to be included in the trend component. The bars on the right of each plot represent scales of equal length. Note that the time domain and the measurement scales are di�erent on these two plots.)
The STL plots show changes in amplitude of the seasonal cycle with time which is more pronounced at Baring Head when compared to the larger seasonal cycle of the Mauna Loa monthly time-series. For both sites the remainder series shows some structure in periods less than the annual cycle with larger variations occurring in the Mauna Loa series.
Trend and Seasonality at Mauna Loa and Baring HeadThe Mauna Loa record exhibits a slightly higher trend than its southern counterpart due to the greater source contribution from fossil fuel emissions in the northern hemisphere. The larger seasonality in the north is a result of changes in photosynthesis and respiration of the vegetation of the greater land mass whereas seasonality in the southern hemisphere is primarily oceanic.
Di�erence in the trendsAs both trends increase over time due to mainly fossil fuel emissions, the smoothed di�erence shows a steadily increasing gap between the trends from 1.4 ppm in the early 1970s to around 2.9 ppm in 2000 where it remains constant. This could be an indication of a change in the sink processes such as a saturation of CO2 absorption in the Southern Ocean (Le Quéré et al., 2007).
Seasonal cyclesThis plot shows the larger peak to peak amplitude of the seasonal cycle in the northern hemisphere (~6 ppm) driven primarily by the seasonal changes in vegetation of the greater land mass. The larger variation and the trends in CO2 measurements in the monthly sub-series at Mauna Loa re�ect the larger variation in changes of vegetation from one year to the next and possible trends in these changes. The seasonality at Baring Head (~1.2 ppm) is not exactly 6 months out of phase with the northern hemisphere due to an in�uence of the northern hemisphere seasonality through the inter-hemispheric boundary.
Fig 5. Growth rates in ppm/year at Mauna Loa and Baring Head.
Interannual variability
The interannual variability was determined by looking at deviations from the overall mean trend line which was derived using the STL procedure with a 121 month smoothing parameter and subtracted from original monthly series. Once this smooth mean trend has been subtracted the STL procedure is reapplied and the resulting trend forms the monthly interannual variability series. Note that the interannual variability is of the same magnitude as the Baring Head seasonal cycle and signi�cantly less than the Mauna Loa seasonal cycle.
As with the growth rates the magnitude and timing of the interannual variability for both sites is remarkably similar. An expanded view for the years 1980 to 1996 shows how similar the two series are. Given the di�erent levels of CO2 in the atmosphere and the di�erent source and sink regimes in each hemisphere, may be related to a climate phenomenon a�ecting both hemispheres such as El Nino/La Nina events.
SummaryThe STL procedure is a useful tool in extracting information from a regular time-series. In analysing the Baring Head and Mauna Loa CO2 measurement records we �nd the seasonal cycles do vary in amplitude with time. The steadily increasing gap between the trends changed since 2000 and has remained constant. Despite the di�erent levels of CO2 in the atmosphere in the two hemispheres and the larger source of emissions in the north, the growth rates and interannual variability are remarkably similar both in magnitude and in timing. This leads to further insights into the roles of the large scale processes of the CO2 sources and sinks.
ReferenceCleveland, R.B.; Cleveland, W.S.; McRae, J.E.; Terpenning, I. (1990). STL: A Seasonal-Trend Decomposition Procedure Based on Loess. Journal of O�cial Statistics Vol.6 No 1, 3-73.
Lowe, D.C. (1974). Atmospheric carbon dioxide in the Southern Hemisphere. J. Clean Air Soc. Aust. and N.Z. 8, 12-15.
Lowe, D.C.; Guenther, P.R.; Keeling, C.D. (1979). The concentration of atmospheric carbon dioxide at Baring Head, New Zealand. Tellus, 31, 58-67.
Le Quere, C.; Rodenbeck, C.; Buitenhuis, E.T.; Conway, T.J.; Langenfelds, R.; Gomez, A.; Labuschagne, C.; Ramonet, M.; Nakazawa, T.; Metzl, N.; Gillett, N.; Heimann, M. (2007). Saturation of the Southern Ocean CO2 sink due to recent climate change. Science 316(5832): 1735–1738.
AcknowledgementsScripps Institution of Oceanography for the use of the Mauna Loa CO2 record.
Martin Manning and Peter Pohl for their long involvement in the CO2 measurements at Baring Head and to all those who have assisted during the past 38 years.
Dave Keeling for his insight in beginning measurements of atmospheric CO2 and his early support of the Baring Head CO2 measurement programme.
Foundation for Science, Research and Technology (Contract C01X0703).
Fig 7. Interannual variability in the Mauna Loa and Baring Head CO2 monthly series.
Fig 8. Interannual variabilityfor 1980 to 1996.
Baring Head Mauna Loa
Growth ratesThe growth rates (Fig 5) are determined by a �rst-order calculation of the gradient at each monthly value in the trends. Despite the di�erent levels of CO2 in the atmosphere in the di�erent hemispheres and the larger source of emissions in the north, the growth rates are remarkably similar both in magnitude and in timing. The expanded plot (Fig 6) shows the growth rates between 1985 and 1996 and it is not obvious that the northern hemisphere leads the south as would be expected. When it does, it leads by a few months despite the inter-hemispheric atmospheric transport exchange time for CO2 known to be around 18 months. During this period the smoothed plots show the growth rates levelling o� at about 1.4 ppm/year. Since then the overall rates have started to increase to 2.0 ppm/year near the end of 2007.
Fig 4. De-trended Mauna Loa and Baring Head CO2 monthly series plotted by each month.
Fig 6. Growth rates in ppm/year for 1980 to 1996.
Fig 2. Increasing trends and seasonality of both the Baring Head and Mauna Loa records. Fig 3. Di�erence in the trend at Mauna Loa to Baring Head where records match in time. Note that this does not take into account the altitude e�ect at Mauna Loa which would slightly under-estimate the di�erence.
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