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Here doesn't come the Sun

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Blaming the Sun for all the woes related to global warming (see letters) looks less tenable an argument following results reported today at the BA Festival of Science. Ted Nield reports.



Geoscientist Online 10 September 2008



There have been several ideas as to what has been driving climate change over the past 10,000 years, according to the lead researcher and radiocarbon dating expert Prof. Chris Turney (School of Archaeology, Geography and Earth Resources, University of Exeter). Speaking at a Society-sponsored session on Isotopes – silent witnesses to Earth History today, Turney said that he and his international co-researchers have concluded that solar activity is not the primary force.

Prof. Turney said the interest in what drives climate change has intensified in recent years with the identification of climate cycles, of which the Medieval Warm Period and Little Ice Age are the most recent examples.

A popular view pinpoints changes in the Sun's output for driving these changes, he said. Several records have been produced from North Atlantic Ocean sediment cores that show how climate cycles apparently correlate to the Sun's past activity. To get a handle on the long-term behaviour of the Sun, scientists have looked at how it controls changes in the radioactivity of the air. By measuring changes in the radioactive version of carbon preserved in tree rings, a precise year-by-year record of the Sun's activity can be made.
BA Turney said to test this idea he and his co-researchers at Queens University Belfast in the UK looked at the 7468 year-long Irish bog oak record. The trees have been preserved growing on the bog surfaces in the past when the surface was dry enough to colonise.

"By matching the distinctive tree ring patterns, an absolute, year-by-year record of the number of trees growing on the bogs can be made. Amazingly, this measure of tree population mirrors the climate cycles over the past 10,000 years. Basically, when the Atlantic waters get cooler, Ireland gets wetter. So when the North Atlantic sneezes, Ireland gets a cold" Prof. Turney said.

Irish trees have also been used to develop the global record of past changes in atmospheric radiocarbon content so the research team had no uncertainties in correlating changes in the climate cycles to changes in the Sun's output.

However, the team found no simple relationship between the Sun's activity and climate change in the North Atlantic region. "If it is the Sun, it's a lot more complicated than many people have claimed - and this is of enormous importance to the global warming debate” Turney told the BA. "Many who don't believe humans are causing what is happening in today's climate blame the Sun but it's not as simple as that.”
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The science bit – concentrate!


The current warm period (an interglacial), known as the Holocene, spans the last 11,600 years and was until recently thought to be a period of exceptional climatic stability. Although it was first suggested back in 1973 that the Holocene was not as climatically stable as implied by land and marine records it was not until the 1990s that this view was seriously challenged. Changes in the concentration of mineral grains through Holocene North Atlantic sediments have been suggested to represent pervasive 900 and 500 year cycles (loosely bundled together as a quasi-periodic '1500 year' cycle), and appear to record the southward advection of cold, icebearing waters from the Labrador and Nordic Seas.

These quasi-cycles have been interpreted as representing a climate pacemaker, independent of glacial-interglacial cycles and anthropogenic forcing, that appear to be driven by relatively small changes in solar activity, the latter measured by cosmogenic isotopes recorded either in the annually-resolved tree rings (14C) or the Greenland ice core (10Be). Possible explanations to account for the relatively large response to small changes in solar activity include amplification by other factors (such as stratospheric ozone concentration), though recently some workers have tended to favour cycles different to those implied for solar activity.

Several attempts have been made to test the hypothesis of the solar forcing of Holocene climate change, yet virtually all of these studies have used radiocarbon to derive a chronology for correlating palaeoclimatic datasets to proxies of past solar activity. A major uncertainty has been the extent to which high-precision correlation can be effected between palaeoclimatic and solar datasets due to inherent uncertainties in derived calendar ages as a result of calibrating radiocarbon ages. Ideally, both dataset types should be derived from the same sequence, since this keeps potential stratigraphic uncertainties to a minimum while leaving only time-lags inherent to the climate system. The number of such suitable sequences is extremely limited.

Ireland's maritime climate is dominated by prevailing westerly airflow and is effectively free of any influence of major ice masses and continents, making it extremely sensitive to past fluctuations in the strength of thermohaline circulation and to past freshwater pulses into the North Atlantic. We have developed Irish tree-ring chronologies using bog-grown oaks (Quercus spp.) and pines (Pinus sylvestris L.), the former contributing to the internationally-accepted radiocarbon calibration curve. The marginal environments of these species growing on bogs coupled with the effectively random nature of the sampling (in both space and time) allows us to robustly test whether tree populations in Ireland changed in response to varying hydrological conditions associated with North Atlantic millennial-scale cycles, while the 14C dataset can be used to derive a measure of past solar activity.

BA A continuous bog oak chronology of more than 750 trees exists back to 7468 years ago, obtained from numerous bog and lake sites across Northern Ireland. The synchronisation of sequences was achieved by visual and statistical correlation of tree ring-width patterns over a period of 30 years. The method is extremely robust and error-free, and as a result has been used to provide precisely-dated bi-decadal wood samples for the international radiocarbon calibration curve. Included in the oak chronology are trees collected from lake margin locations below present day water levels. In addition, there are Scots pine chronologies from bogs and lake-side sites, dated by tree rings.

“The oak dataset commences 7468 years ago and records a cyclic pattern throughout its length that we interpret to represent changes in moisture delivery to Ireland” said Turney. “Under 'dry' conditions, water table levels decrease, allowing the surface to be colonised by tree populations. The peaks in bog oak populations are complemented by the presence of stands of tree ring-dated oak identified below current lake levels in Northern Ireland, providing unambiguous evidence for drier conditions. The bog oaks and pines used to construct the above chronologies were by their nature dependent on post-glacial climate amelioration and the development of marginal vegetation and were hence not prevalent in the early Holocene landscape.”

“Earlier cycles in the Holocene are represented by extensive finds of bog pine and also a radiocarbon dated cohort of willow trees (Salix spp) below the current lake level in Lough Neagh, consistent with peaks in tree populations during dry phases. We undertook mean age analyses to identify whether the cycles had an effect on the population dynamics of the bog oaks and find that troughs in tree populations coincide in the first instance with peaks in the mean age of the populations, indicating recruitment failure. Saplings could not establish under pervasive 'wet' conditions, as a result skewing the mean age to older members of the population, which subsequently died, resulting in a significant drop in the mean age.

“Following climatic amelioration (interpreted here to be 'drying'), the trees regenerate, returning to intermediate values. Peaks in Irish bog and lake-edge tree populations are centred on 800 yrs ago (equivalent in time to the Medieval Warm Period), 1700 yrs ago (equivalent in time to the Roman Optimum), 2650, 3200, 4100, 5000, 5600, 6200, 7300, 8000 and 9000 years ago indicating that moisture delivery and/or water tables were significantly below present levels at these times. Thus, we can recognise that oak and pine populations obtained from bog and lake-edge contexts indicate alternating periods of sustained dry and wet conditions in Ireland.

“Recently, it has been argued that some Holocene cycles are a result of changes in solar activity. As a measure of the latter, we looked at the decadal production rate of 14C, a cosmogenic radionuclide, which is related to solar activity. Reduced solar activity results in higher production rates. As a result of the above, high precision comparisons are possible as the absolutely-dated Irish oak chronologies reported here underpin the radiocarbon calibration curve for this time period, providing a unique opportunity to precisely test the hypothesis of solar forcing.

“To test the above observation, we spectrally analysed 14C production over the same interval as that represented by the Irish oaks, and found different spectral peaks. We conclude that North Atlantic climate change at the millennial to centennial scale is not driven by a linear response to changes in solar activity. If solar activity is the primary forcing for Holocene climate change, it enters the system elsewhere and is propagated internally to the North Atlantic via one or more non-linear mechanisms. Alternative potential mechanisms must be investigated more fully before we can fully explain Holocene climate change and the potential influence of these cycles on future climate change.

“The key thing now is to develop records in other parts of the world to try and work out what happened when over the past 10,000 years. This will need detailed records from around the globe, peering into archives preserved on land, the ice and in the oceans. The problem the scientific community now faces is that many of the most climatically-sensitive regions of the world are threatened by climate change, endangering the very records that can give us this long-term perspective" Prof. Turney said.

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