An ingeniously constructed record of sunspot activity shows that the current episode is the most intense for several thousand years. But that does not let us off the anthropogenic hook of global warming.
Dark spots or ‘blemishes’ on the face of the Sun (Fig. 1) were recognized from the early seventeenth century, and have since been identified as places where strong magnetic fields emerge from the Sun's surface. Data on sunspot numbers provide the longest observational record of solar activity. But that record is too short to document changes in activity occurring on timescales longer than the recognized cycles of 11- and 88-year periods, or to support claims for a connection between solar activity and Earth's climate on centennial to millennial timescales.
On page 1084 of this issue, however, Solanki et al.1 describe how they have produced a reconstruction of sunspot number for the past 11,000 years. They have done so by connecting a series of models based on well-established physics, taking as their data the concentration of the carbon isotope 14C found in tree rings, which provides windows on atmospheric and solar trends at known points in time.
The reconstruction shows that the current episode of high sunspot number, which has lasted for the past 70 years, has been the most intense and has had the longest duration of any in the past 8,000 years. Based on the length of previous episodes of high activity, the probability that the current event will continue until the end of the twenty-first century is quite low (1%).
Each model used in the reconstruction makes a step in connecting the tree-ring 14C record2 to sunspot number using parameters that were fixed by independent measurements (direct or indirect). Carbon-14, and some other isotopes such as the beryllium isotope 10Be, are formed from the bombardment of the atmosphere by cosmic-ray particles. The 14C in the atmosphere is converted to 14CO2 and incorporated into the tree rings as they form; the year of growth can be precisely determined from dendrochronology. Production of cosmogenic isotopes is high during periods of low solar magnetic activity. But during the Sun's active phase (with high sunspot number), the more intense solar wind — the ions streaming out from the Sun — deflects charged particles so that fewer of them enter Earth's atmosphere.
Solanki and colleagues' first step was to determine the rate of 14C production using the tree-ring record of atmospheric 14C concentration after removing the long-term trend in Earth's magnetic field, which modulates the cosmic-ray flux. The concentrations of 14C in the atmosphere may also be affected by variations in ocean circulation, because carbon is partitioned between the atmosphere, the ocean and the biosphere. But there is no evidence of major oceanic variability over the past 11,000 years, and carbon fluxes in the biosphere are not sufficient to cause large changes in atmospheric 14C.
The second step was to calculate the cosmic-ray flux from the data for 14C production, by ‘inverting’ a model of the transport and modulation of galactic cosmic rays within the envelope of the solar wind; model inversion means working backwards from the answer to find the necessary input to produce that answer. Solanki et al. then reconstructed the Sun's open magnetic flux — the magnetic field that extends into the interplanetary medium — from a model of the effect of the open magnetic flux on the transport of galactic cosmic rays. Finally, a model describing the evolution of the open magnetic flux for a given sunspot number was inverted to produce estimates of sunspot number. Within well-defined limits of uncertainty, the series of models reproduce the observed record of sunspots extremely well, from almost no sunspots during the seventeenth century to the current high levels.
Climate variability on centennial to millennial timescales is documented in many palaeoclimate records going back at least as far as the end of the last glaciation, some 12,000 years ago. Whether solar activity is a dominant influence in these changes is a subject of intense debate3,4,5,6. The exact relationship of solar irradiance to sunspot number is still uncertain7,8, but the reconstructed sunspot number will nonetheless provide a much-needed record of solar activity. This can then be compared with palaeoclimate data sets to test theories of possible solar–climate connections, as well as enabling physicists to model long-term solar variability. A better understanding of the mechanisms responsible for past climate variability will also help those using global circulation models to predict future climate change.
So does the current episode of high sunspot number imply that the Sun has had a significant role in the global warming of the late twentieth century? The answer is no. Although climate models differ in their estimation of the Sun's contribution to recent warming, even those that include spectrally varying changes in solar irradiance conclude that anthropogenic causes are the prime factor9,10,11,12. The high probability that this episode will end soon is not likely to cut us much slack in controlling global warming unless we reduce greenhouse-gas emissions. But because the solar influence may be more regionally variable than the effects of greenhouse gases11, model-based predictions of regional climate change may be improved by this study. It is at the regional level that climate change will have the greatest impact on society.
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