In response to the letter of 18 March by Stephen Foster
, I would like to point out that while the sun is the main driver for Earth’s climate, variations in sunspot activity provide only a minor modification to Earth’s global temperature. The main changes in global temperature through geological time are driven by (a) variations in insolation forced by changes in the Earth’s orbit and the tilt of the Earth’s axis, and (b) variations in atmospheric CO2
As shown by Wanner et al. (2015), orbital forcing has declined significantly over the past 10,000 years, in parallel with global temperature, as deduced from a wide range of proxies. It can be considered the main driver for Holocene climate change, because CO2 changed very little over the same period. The changes in insolation due to solar activity are much smaller than those caused by orbital change, so are superimposed as wiggles on this downward cooling trend. Following a Holocene Climatic Optimum, about 4,000 years ago, this cooling led Earth into a neoglacial phase, characterized by glacial readvance, which culminated in the Little Ice Age (LIA) (1250-1850). The coldest phases of the LIA occurred during periods of prolonged sunspot minima (Wolf Minimum: 1280 to 1345 AD; Spörer Minimum: 1420 to 1540 AD; Maunder Minimum: 1645-1715 AD; Dalton Minimum: 1790-1820 AD; and Gleissberg Minimum: 1890-1910 AD). The orbital data suggest that this neoglacial phase should persist for the next 5000 years (Loutre and Berger, 2000). Indeed, recognizing the dominance of the orbital signal, we should still be in the LIA – albeit in one of its warm phases.
Foster is correct in drawing attention to the current decline in sunspot numbers, which may be leading towards a further grand sunspot minimum like those mentioned above. We can see the pattern of recent sunspot activity in Figure 1 (from Clette et al., 2014). Clette et al. (2014) recalibrated the original sunspot series to show that by the mid 18th century solar activity had returned to levels typical of recent solar cycles in the 20th century, but is now in decline (see Figure 1, which shows sunspot groups rather than actual sunspot numbers - for explanation see Clette et al., 2014). Aside from the 11-year sunspot cycle, the Figure shows some of the grand sunspot minima mentioned above, which are caused by the fluctuations of either the 208-year Suess (or de Vries) Cycle, or the 88-year Gleissberg Cycle, along with their ‘combination tones’ (where signals of different wavelengths interfere with one another to accentuate or diminish the underlying signal) at 152 and 62 years.
Foster assumes from the current decline in sunspot activity from its peak in 1990, that “this energy reduction has initiated an expected reversal from the past warm era to a new cold era”. Is he correct? In fact, global temperatures have been more or less stable for the past 14 years, with 2014 at least as warm if not warmer than preceding years. Argo float data show that the ocean continued to gain heat at a rate of 0.4-0.6W/m2 between 2006 and 2013 (Roemmich et al., 2015). And observations by satellite radar altimeter show that the loss of ice from ice shelves around Antarctica was minimal in 1994-2003 (25±64 km3/yr), but rapid in 2003-2012 (310±74 km3) (Paolo et al. 2015). Arctic sea ice also exhibited a decline through the 2000s. These are not signs that the world is cooling. It seems counterintuitive that Antarctic sea ice is expanding slightly, but that expansion may be connected to the melting of West Antarctic ice shelves, mentioned above, which freshens the surface waters around the continent.
So what is going on? I suggest that what we are experiencing is a balance between the cooling to be expected from a growing sunspot minimum (which may or may not turn out to be long), combined with the warming to be expected from growing emissions of greenhouse gases. If I am right, then Dr Foster’s calculation that temperatures will cool by “1.0 and 1.5 degrees C lower than the peak year of 1998” may turn out to be a poor guess.
Dr Foster seems not to be convinced (despite experimental evidence) that CO2 has any effect on atmospheric temperatures. I recommend that he examine carefully the record of climate change across the Palaeocene-Eocene boundary, 55 million years ago (Zachos et al., 2008). There we find ample evidence for a massive increase in the emission of carbon into the atmosphere, a significant rise in temperature, acidification of the ocean causing a rise in the carbonate compensation depth, and a significant rise in sea level. There is no extraneous cause for the rise in temperature, so it is highly unlikely that it drove the rise in CO2.
There is abundant evidence that there were times in Earth’s history when plate tectonic processes provided a CO2 source, through enhanced volcanism, and times when the absorption of CO2 from the atmosphere by chemical weathering in growing mountain chains formed a sink for CO2, as part of the slow carbon cycle described elegantly by the late Bob Berner (2004), and that these changes modified Earth’s temperature. Equally, there were times when changes in orbital insolation forced changes in Earth’s temperature, which drove changes in CO2 through the loss of CO2 to the air from warming water, and its re-solution by colder water, which accentuated orbital warming and cooling by positive feedback. Thus CO2 can both drive temperature change and be driven by it. This is all fundamental geochemistry.
To conclude, the graph above shows that the peak sunspot numbers of the late 20th century were about the same as those of the warm periods of the LIA (1780-90; 1840-70). Given that the late 20th Century was about 0.8°C warmer than the warm temperatures in the 1870s, the late 20th Century warmth must have had some other source than solar – most likely the rise in CO2 stemming from the industrial revolution and its aftermath. If not that, then what?
Colin Summerhayes Scott Polar Research Institute, Cambridge
- Wanner, H., Mercolli, L., Grosjean, M, and Ritz, S.P., (2011) Holocene climate variability and change – a data based review. J. Geol. Soc London 172, 254-26
- Loutre, M.F., and Berger, A., 2000 Loutre, M.-F., and Berger, A. (2000) Future climate changes: are we entering an exceptionally long interglacial? Clim. Change 46, 61-90.
- Clette, F., Svalgaard, L., Vaquero, J.M., and Cliver, E.W. (2014) Revisiting the sunspot number. Space Science Reviews 186 (1-4), 35-103
- Roemmich, D., Church, J., Gilson, J., Monselesan, D., Sutton, P., Wijffels, S., (2015) Unabated planetary warming and its ocean structure since 2006. Nature Climate Change 5 (3): 240-245.
- Paolo, F.S., Fricker, H.A., Padman, L., (2015) Volume Loss from Antarctic Ice Shelves is Accelerating. Science : DOI: 10.1126/science.aaa0940.
- Zachos, J.C., Fickens, G.R., and Zeebe, R E., (2008) An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature 451, 279-283.
- Berner, R.A., (2004) The Phanerozoic Carbon Cycle: CO2 and O2. Oxf. Univ. Press, 124pp.