“We’re heading into a new [little] ice age!”. This meme is a favourite of the denialosphere, I suppose because it is considered by them to be the ultimate counter to global warming. An inactive sun is fingered as the potential culprit in this alternative-universe prognostication hypothesis. But just how likely is such solar-driven cooling? What if the sun really did shut off its 11-year sunspot cycle for some reason, and move into a new extended (multi-decadal) period of low activity like was observed during the Maunder Minimum – would this be sufficient to offset the warming induced by an increased build-up of long-lived greenhouse gases from recent human industrial output and land use change?
The basic answer (“no, an inactive sun will not cause an ice age“) is actually remarkably easy to demonstrate. Jim Hansen did this recently in his occasional blog. This ‘trip report’ (printable PDF) covers a wide range of topics – why coal is the climate lynchpin, what industrial nations are (not) doing, what palaeoclimate tells us about climate sensitivity, and the prospects for fourth-generation nuclear power – and is worth reading for all of these gems. But given the prevalance with which the ice age meme appears in non-greenhouse theorist Op-Eds these days, I’ll reproduce his section on solar forcing here in full:
Temperature and Solar Data (extract from Hansen 2008: Trip Report, p11-14)
Figure 4 updates global and low latitude temperature at seasonal resolution. Red rectangles, blue semi-circles and green triangles at the bottom of the plot show the timing of El Ninos, La Ninas and large volcanic eruptions. Oscillation from El Ninos to La Ninas is the main cause of the big fluctuations of low latitude temperature. These fluctuations are also apparent, albeit muted, in the global mean temperature change.
The most recent few seasons (Figure 4) have been cool relative to the previous five years, on average ~0.25°C cooler. If one takes the recent peak (early 2007) and recent low point (early 2008), the change is about -0.5°C. This drop is the source of recent contrarian assertions that all global warming of the past century has been lost and the world is now headed into an ice age. Figure 4 reveals that it is silly to use a peak and valley as an indication of the trend. Peak to valley drops and rises of 0.3-0.5°C in seasonal mean temperature anomalies are common (Figure 4), usually associated with ENSO (El Nino Southern Oscillation) fluctuations.
The recent La Nina was strong, but tropical temperatures in mid-2008 have returned nearly to ENSO neutral conditions and global temperature is heading back to the high level of the past few years. The low temperatures in the first half of 2008 lead us to estimate that the mean 2008 global temperature will be perhaps in the range about 10th to 15th warmest year in our record.
A majority of the critical e-mails asserted emphatically that global temperature change is due mainly to solar changes, not human-made effects. They also state or imply that, because of ongoing solar changes, the Earth is entering a long-term cooling period (following the warming of the past 30 years, which they presume to be due to increases of solar energy). One e-mail virtually shouted: “THE SUN IS GOING OUT!”
Images from SOHO (Figure 5) might be the basis for that conclusion. The sun is inactive at the present, at a minimum of the normal ~11 year solar cycle. The solar cycle has a measureable effect on the amount of solar energy received by Earth (Figure 6). The amplitude of solar cycle variations is about 1 W/m2 at the Earth’s distance from the sun, a bit less than 0.1% of the ~1365 W/m2 of energy passing through an area oriented perpendicular to the Earth-sun direction.
The Earth absorbs ~235 W/m2, of solar energy, averaged over the Earth’s surface. So climate forcing due to change from solar minimum to solar maximum is about ¼ W/m2. If equilibrium climate sensitivity is 3°C for doubled CO2 (¾°C per W/m2), the expected equilibrium response to this solar forcing is ~0.2°C. However, because of the ocean’s thermal inertia less than half of the equilibrium response would be expected for a cyclic forcing with ~11 year period. Thus the expected global-mean transient response to the solar cycle is less than or approximately 0.1°C.
Is there some way that the small variations of energy coming from the sun could be amplified, so that the ‘solar exponents’ are actually correct and the sun is driving our climate changes? There are indirect effects of solar variability, e.g., solar radiation varies most at ultraviolet wavelengths that affect ozone. Indeed, empirical data on ozone change with the solar cycle and climate model studies indicate that induced ozone changes amplify the direct solar forcing (J. Geophys. Res. 102, 6831, 1997; ibid 106, 77193, 2001), but the amplification is by a factor of one-third or less.
Other mechanisms to amplify the solar forcing have been hypothesized, such as induced changes of atmospheric condensation nuclei and thus changes of cloud cover. However, if such mechanisms were effective, then an 11-year signal should appear in temperature observations (Figure 4). In fact a very weak solar signal in global temperature has been found by many investigators, but only of the magnitude (~0.1°C or less) expected due to the direct solar forcing. So the sun is only a minor contributor to the temperature fluctuations in Figure 4.
The possibility remains that the sun could be an important cause of climate change on longer time scales. (The source of nuclear energy at the sun’s core is essentially continuous, in fact increasing at a rate of about 1% in 100 million years, which is a negligible rate of change for our purposes. But the photosphere, the upper layers of the sun, can slightly impede or speed the emission of energy as the strength of magnetic fields fluctuates.) Perhaps the normal solar cycle evidenced in Figure 6 is about to be interrupted. Sunspots seemed to nearly disappear for a long period in the 17th century, which may have contributed (along with volcanic eruptions) to the “little ice age”. And the current solar minimum is already longer than the previous two (Figure 6). Perhaps the e-mailer who shouted “THE SUN IS GOING OUT!” is correct!
Fortunately, we can compare quantitatively the climate forcing due to the sun (if its irradiance does not recover from its present minimum) and the forcing due to human-made greenhouse gases. Solar irradiance seems to be slightly less at its current minimum than in earlier minima (Figure 6), but, at most, the decrease from the mean irradiance of recent decades is ~0.1% yielding a climate forcing of about -0.2 W/m2. The current rate of atmospheric CO2 increase is ~2 ppm/year, yielding an annual increase of climate forcing of about +0.03 W/m2 per year.
Thus if the sun remains “out”, i.e., stuck for a long period in the current solar minimum, it can offset only about 7 years of CO2 increase. The human-made greenhouse gas climate forcing is now relentlessly, monotonically, increasing at a rate that overwhelms variability of natural climate forcings. Unforced variability of global temperature is great, as shown in Figure 4, but the global temperature trend on decadal and longer time scales is now determined by the larger human-made climate forcing. Speculation that we may have entered a solar-driven long-term cooling trend must be dismissed as a pipe-dream.
Another good read which explains the solar cycle is this news feature from NASA, which shows that there is nothing particularly remarkable about the current solar cycle, and so there is no reasonable expectation that we are heading into a new Maunder Minimum anyway.