In a paper recently published in Nature, Bryden et al. (2005) provide evidence which suggests, in their words, that "the Atlantic meridional overturning circulation has slowed by about 30 percent between 1957 and 2004," which they additionally suggest could have "profound implications for climate change." Nowhere is this postulated phenomenon expected to have a greater climatic impact than throughout northwest Europe, where some climate models suggest that a complete shutdown of the Atlantic branch of the thermohaline circulation could drop temperatures by 4°C. Adding fuel to the firestorm of concern that was ignited in the popular press by these suggestions was Quadfasel (2005), who in discussing Bryden et al.'s work in a News & Views article in Nature wrote that "palaeoclimate records show that northern air temperatures can drop by up to 10°C within decades, and that these abrupt changes are intimately linked to switches in the ocean circulation [our italics]."
Once again, we find ourselves confronted with the age-old "chicken-and-egg" question that similarly pervades the discussion of CO2 and climate change (see our Editorial of 30 Nov 2005): which leads which? Do changes in the global ocean's thermohaline circulation lead and thereby cause "intimately linked" climate change? Or is it the other way around? Does climate change lead and thereby cause changes in the earth's vast watery conveyer-belt?
This question was actually broached a few months earlier in Science by Piotrowski et al. (2005), who studied not only palaeo-climate and palaeo-circulation changes in South Atlantic deep-sea sediment cores, but also changes in global and Southern Ocean carbon budgets.
Their work revealed that during both the initiation and termination of the last great ice age, "climate and ice volume changed first, followed by the global carbon budget, which was in turn followed by ocean circulation." With respect to the timing of these events, Piotrowski et al.'s graphical representation of the last ice age's initiation depicts strong cooling for about 5500 years before a major rapid shift in carbon budget takes place, after which there is an hiatus of approximately 2500 years before there is a rapid decline in ocean circulation. Hence, from the start of major cooling to the start of major thermohaline circulation reduction, there was a gap of essentially 8000 years. In addition, there were no outstanding changes in global climate subsequent to the rapid and large decline in strength experienced by the thermo-haline circulation; the trip from full interglacial to full glacial conditions was by that time already over 80% complete, and the final 15-20% of the journey was gradually completed over the next 8000 years, thereby making the thermohaline circulation pretty much a non-factor with respect to both the initiation and the ultimate establishment of the last great ice age. Clearly, the meridional overturning circulation was neither a primary forcing nor a significant amplifier of global climate change.
In the case of interstadial climate changes, however, Piotrowski et al. report that "no consistent lead-lag relationships are observed during abrupt millennial warming events during the last ice age, allowing for the possibility [our italics] that ocean circulation triggered some [our italics] millennial climate changes," which just as readily allows for the possibility that changes in ocean circulation did not trigger any of the millennial climate changes nor act to amplify them. Last of all, the four researchers provide no data for abrupt mille-nnial warming or cooling events during the Holocene, which is the timeframe of pertinence to Bryden et al.'s investigation, probably because there were no millennial-scale warming or cooling events of comparable magnitude to those of the prior glacial era during this latter interglacial period.
In light of the findings and non-findings of Piotrowski et al., one ought not be too quick to consign Europe to a rapid cool-down any time soon based on the observations of Bryden et al., especially when the latter researchers openly acknowledge that the magnitude of the circulation change they observed is "uncomfor-tably close" to the magnitude of the uncertainties associated with their observations. Much to be prefer-red is the position reported by Kerr (2005) to have been articulated by MIT's Carl Wunsch in response to being asked if the thermohaline circulation has slowed recently or will slow in the future. He simply replied "we don't know," which is the pure and simple truth.
Sherwood, Keith and Craig IdsoBryden, H.L., Longworth, H.R. and Cunningham, S.A. 2005. Slowing of the Atlantic meridional overturning circulation at 25°N. Nature 438: 655-657. Kerr, R.A. 2005. The Atlantic conveyor may have slowed, but don't panic yet. Science 310: 1403-1404. Piotrowski, A.M., Goldstein, S.L., Hemming, S.R. and Fairbanks, R.G. 2005. Temporal relationships of carbon cycling and ocean circulation at glacial boundaries. Science 307: 1933-1938. Quadfasel, D. 2005. The Atlantic heat conveyor slows. Nature 438: 565-566.
For nine areas of contrasting environment within the Pampas region of Argentina, which accounts for over 90% of the country's grain production, the authors evaluated changes in climate over the 20th century along with changes in the yields of the region's chief crops (soybean, wheat, maize and sunflower). Then, after determining upward low-frequency trends in yield due to technological improvements in crop genetics and management techniques plus the aerial fertilization effect of the historical increase in the air's CO2 concentration, these annual yield anomalies and concomitant climatic anomalies were used to develop relations describing the effects of precipitation, temperature and solar radiation on crop yields, so that the effects of long-term changes in these climatic parameters on Argentina agriculture could be determined.
What was learnedAlthough noting that "technological improvements account for most of the observed changes in crop yields during the second part of the 20th century, which totaled 110% for maize, 56% for wheat and 102% for sunflower, Magrin et al. report that due to changes in climate between the periods 1950-70 and 1970-99, yields increased by 38% in soybean, 18% in maize, 13% in wheat, and 12% in sunflower.
What it meansTwentieth-century climate change, which is claimed by climate alarmists to have been unprecedented over the past two millennia and is often described by them as one of the greatest threats ever to be faced by humanity, has definitely not been a problem for agriculture in Argentina. In fact, it has helped it.
Ichii, K., Hashimoto, H., Nemani, R. and White, M. 2005. Modeling the interannual variability and trends in gross and net primary productivity of tropical forests from 1982 to 1999. Global and Planetary Change 48: 274-286.
What was doneIn a study of tropical forests, the authors say they "simulated and analyzed 1982-1999 Amazonian, African, and Asian carbon fluxes using the Biome-BGC prognostic carbon cycle model driven by National Centers for Environmental Prediction reanalysis daily climate data," after which they "calculated trends in gross primary productivity (GPP) and net primary productivity (NPP)."
What was learnedSolar radiation variability was found to be the primary factor responsible for interannual variations in GPP, followed by temperature and precipitation variability, while in terms of GPP trends, Ichii et al. report that "recent changes in atmospheric CO2 and climate promoted terrestrial GPP increases with a significant linear trend in all three tropical regions." In the Amazonian region, the rate of GPP increase was 0.67 PgC year-1 per decade, while in Africa and Asia it was about 0.3 PgC year-1 per decade. Likewise, they report that "CO2 fertilization effects strongly increased recent NPP trends in regional totals."
What it meansIn response to the supposedly most dramatic global warming of the past two millennia, which is claimed to have been driven by the even more unprecedented concomitant increase in atmospheric CO2 concentra-tion, earth's tropical forests appear to have fared remarkably well, growing ever more robustly and removing ever more carbon from the atmosphere in some of the hottest places on the planet.
Is this the stuff of which catastrophes are made? Or is this the stuff that reveals such claims of catastrophe to be pure and utter nonsense?Almeida-Lenero, L., Hooghiemstra, H., Cleef, A.M. and Van Geel, B. 2005. Holocene climatic and environ-mental change from pollen records of Lakes Zempoala and Quila, central Mexican highlands. Review of Palaeobotany and Palynology 136: 63-92.
What was doneThe authors analyzed pollen profiles derived from sediment cores retrieved from Lake Zempoala (19°03'N, 99°18'W) and nearby Lake Quila (19°04'N, 99°19'W) in the central Mexican highlands about 65 km south-west of Mexico City.
What was learnedAlmeida-Lenero et al. determined that it was generally more humid than at present in the central Mexican highlands during the mid-Holocene. Thereafter, however, there was a gradual drying of the climate; and their data from Lake Zempoala indicate that "the interval from 1300 to 1100 cal yr BP was driest and represents an extreme since the mid-Holocene," noting further that this interval of 200 years "coincides with the collapse of the Maya civilization."
Likewise, they report that their data from Lake Quila are also "indicative of the most arid period reported during the middle to late Holocene from c. 1300 to 1100 cal yr BP." In addition, they note that "climatic aridity during this time was also noted by Metcalfe et al. (1991) for the Lerma Basin [central Mexico]," that "dry climatic conditions were also reported from Lake Patzcuaro, central Mexico by Watts and Bradbury (1982)," and that "dry conditions were also reported for [Mexico's] Zacapu Basin (Metcalfe, 1995) and for [Mexico's] Yucatan Peninsula (Curtis et al., 1996, 1998; Hodell et al., 1995, 2001)."
What it meansBased on proxy temperature data from North America, it would appear that some of the driest conditions of the Late Holocene throughout much of Mexico may have occurred during the climatic transition between the Dark Ages Cold Period and the Medieval Warm Period.
ReferencesCurtis, J., Hodell, D. and Brenner, M. 1996. Climate variability on the Yucatan Peninsula (Mexico) during the past 3500 years, and implications for Maya cultural evolution. Quaternary Research 46: 37-47.
Curtis, J., Brenner, M., Hodell, D. Balser, R., Islebe, G.A. and Hooghiemstra, H. 1998. A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten Guatemala. Journal of Paleolimnology 19: 139-159.
Hodell, D., Curtis, and Brenner, M. 1995. Possible role of climate in the collapse of classic Maya civiliza-tion. Nature 375: 391-394.
Hodell, D., Brenner, M., Curtis, J. and Guilderson, T. 2001. Solar forcing of drought frequency in the Maya Lowlands. Science 292: 1367-1370.
Metcalfe, S.E. 1995. Holocene environmental change in the Zacapu Basin, Mexico: a diatom based record. The Holocene 5: 196-208.
Metcalfe, S.E., Street-Perrott, F.A., Perrott, R.A. and Harkness, D.D. 1991. Palaeolimnology of the Upper Lerma Basin, central Mexico: a record of climatic change and anthropogenic disturbance since 11,600 yr B.P. Journal of Paleolimnology 5: 197-218.
Watts, W.A. and Bradbury, J.P. 1982. Paleoecological studies at Lake Patzcuaro on the West Central Mexican plateau and at Chalco in the Basin of Mexico. Quaternary Research 17: 56-70.
