Climate change

September 25, 2008

What if the sun got stuck?

Filed under: Climate Change — Barry Brook @ 9:20 pm

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:


4. Seasonal-mean global and low-latitude surface temperature, based on
an update of the analysis of Hansen et al. (J. Geophys. Res. 106,
23947, 2001).

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

5. Comparison of the sun at solar minimum (right side, July 2008) and
at solar maximum (left, August 2002) as seen in extreme ultraviolet
light from SOHO (Solar Heliospheric Observatory). Active regions during
solar maximum are producing a number of solar storms. The sun in 2008
is quiet, with no active regions, part of the normal 11-year solar

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

6. Solar irradiance from composite of several satellite-measured time
series based on Frohlich & Lean (1998;

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.


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