A
grand solar minimum would barely make a dent in human-caused global
warming
Research
has shown that a grand solar minimum would offset no more than 0.3°C
of global warming
14
August, 2013
Recent
articles in the Danish newspaper Jyllands-Posten (translation
available here)
and in the Irish
Times
both ran headlines claiming that another grand solar minimum could
potentially trigger an "ice age" or "mini ice age"
this century.
These articles actually refer to the Little
Ice Age (LIA)
– a period about 500 to 150 years ago when global surface
temperatures were approximately
1°C colder than they are today.
This is quite different from an ice age, which are more like 5°C
colder than today. The LIA was not actually very cold on a global
scale.
So,
in order to trigger another LIA, a new grand solar minimum would have
to cause about 1°C cooling, plus it would have to offset the
continued human-caused
global warming of 1 to 5°C by 2100,
depending on how our greenhouse gas emissions change over the next
century.
In
the Jyllands-Posten article, Henrik Svensmark (the main scientist
behind the hypothesis that the sun has a significant indirect impact
on global climate via
galactic cosmic rays)
was a bit more measured, suggesting,
"I
can imagine that it will become 0.2°C colder. I would be surprised
if it became 1–2°C"
So
these two articles are suggesting that a grand solar minimum could
have a net cooling effect in the ballpark of 1 to 6°C, depending on
how human greenhouse gas emissions change over the next century. Is
it plausible that a grand solar minimum could make that happen?
The
short answer is, 'No.'
Fortunately,
Solar Output is Stable
We're
fortunate that the amount of solar radiation reaching the Earth's
surface is very stable. Climate contrarians will often ask if we'd
prefer if the planet were warming or cooling, suggesting that global
warming is a good thing because at least the planet isn't getting
colder. This is a false dichotomy - an ideal climate is a stable one.
The
relatively stable climate over the past 10,000 years has allowed
establishment of human civilization, by making it possible to create
large stationary agricultural farms because we could rely on stable
weather patterns. During that time, net
global surface temperatures changes haven't exceeded 1°C
from the coldest to the hottest climates, though we're now
approaching that degree of change, with 1°C warming since the LIA,
0.8°C of that over the past century, with much more to come.
What
difference would a grand solar minimum make in the amount of solar
energy reaching Earth? Two examples are the Maunder
Minimum,
a period of very low solar activity between 1645 and 1715, and the
Dalton
Minimum,
a period of low (but not as low as the Maunder Minimum) solar
activity between 1790 and 1830.
400
years of sunspot observations data. Created by Robert Rohde, via
Wikipedia.
Relative
to current levels, the Dalton Minimum represents a 0.08% decrease in
the amount of solar radiation reaching the Earth's surface, and the
Maunder Minimum represents a 0.25% decline. That's how stable solar
activity is. That's also why we're playing with fire by increasing
the greenhouse effect so much and so quickly. We're threatening the
stability of the climate that has been so favorable to our
development.
Peer-Reviewed
Research Says Global Warming will Continue
There
have been several studies in recent years investigating what impact
another grand solar minimum would have on global surface
temperatures, since solar research suggests it's possible we could be
due for another extended solar minimum. Generally these studies will
run climate model simulations under a given greenhouse gas emissions
scenario with stable solar activity, then run the same scenario with
the sun going into a grand minimum, and look at the difference in
resulting global surface temperature changes.
Using
this approach, Feulner
& Rahmstorf (2010)
(PDF
available here)
estimated that another solar minimum equivalent to the Dalton and
Maunder minima would cause 0.09°C
and 0.26°C cooling,
respectively.
The
global mean temperature difference is shown for the time period 1900
to 2100 for the IPCC A2 emissions scenario. The red line shows
predicted temperature change for the current level of solar activity,
the blue line shows predicted temperature change for solar activity
at the much lower level of the Maunder Minimum, and the black line
shows observed temperatures through 2010. Adapted from Feulner &
Rahmstorf (2010) by SkepticalScience.com
Jones
et al. (2012)
(PDF
available here)
arrived at a nearly identical result, with cooling from another
Dalton and Maunder minimum at 0.09°C and 0.26°C, respectively.
Similarly, a new paper by Anet
et al. (2013)
found that a grand solar minimum will cause no more than 0.3°C
cooling over the 21st century.
Consistent
with these previous studies, Meehl
et al. (2013)
(PDF
available here)
estimate a Maunder Minimum would cause about 0.26°C cooling, but as
soon as solar activity began to rise again, that cooling would be
offset by solar warming. This is a key point, because a grand solar
minimum would not be a permanent change. These solar minima last for
a few decades, but eventually solar activity rises once again. Thus
any cooling caused by a solar minimum would only be temporary.
The
cooling effect of a grand solar minimum can also be estimated very
easily without the aid of climate models, because the change in the
amount of solar radiation reaching the Earth's surface is directly
proportional to the temperature change it causes. Performing this
calculation yields the same result as the model-based research:
approximately 0.3°C cooling from another Maunder-type grand solar
minimum. Click
here to see the details behind the calculation.
The
Heating of the Deep Oceans
In
the Jyllands-Posten article, Svensmark also disputes the data showing
the accelerated accumulation of heat in the deep oceans.
"How
can the ocean below 700 meters be heated up, without the upper ocean
warming up accordingly?"
This
is an increasingly common argument made by climate contrarians, and a
bit of a strange one. The data are what they are - we've
measured the deep ocean warming,
including with reliable instruments on Argo
buoys
for close to a decade now. Even if we couldn't explain how the heat
got there, it's there.
5-year
averages of ocean heat content 0-700 meters (red) and 0-2000 meters
(black), from the National Oceanographic Data Center
But
let's address the question anyway - do we expect to have seen some
obvious indication of heat being transferred from the shallow to deep
ocean layers?
It's
certainly not clear that we should. Consider the analogy of a
bathtub. Water from the faucet represents heat entering the shallow
ocean layer. Water exiting the drain represents heat leaving the
shallow oceans and entering the deep oceans. The water level in the
bathtub represents the heat in the shallow ocean layer (which is what
we measure).
If
the amount of water entering the tub from the faucet is the same as
the amount of water draining out of the tub, the water level in the
tub won't change. Yet the water still flows down the drain. Climate
scientist Gavin Schmidt has discussed this point, summarized
here.
In
short, we wouldn't necessarily see the heat being transferred through
the shallow to the deep oceans. However, there has been plenty of
warming of the shallow oceans that could have been transferred to the
deeper oceans. In our case, the water is flowing into the tub faster
than it's draining out - the shallow oceans are warming fast, as the
figure above illustrates.
Svensmark
Gets Ocean Warming Wrong
Unfortunately
Svensmark appears to be unfamiliar with this ocean heating data,
saying,
"The
thousands of buoys that we have deployed after 2003 to measure the
ocean temperature, have not registered any temperature rise."
This
is just totally wrong, even if we ignore the rapid warming of the
deep oceans (as is clear from a simple examination of the figure
above). The ocean
heat content data can be downloaded from the National Oceanographic
Data Center here.
The heating trend since 2003 in the upper 700 meters of oceans is
equivalent to nearly 1 Hiroshima atomic bomb detonation per second
(plus another 3 per second in the deep oceans). Both the shallow and
deep oceans are accumulating a whole lot of heat, with no signs of
slowing whatsoever. If
anything, the heating of the oceans and the planet as a whole is
accelerating.
Human
Influence on Climate
Change
is Bigger than the Sun's
The
bottom line is that the sun and the amount of solar radiation
reaching Earth are very stable. Even during the Maunder and Dalton
grand solar minima, global cooling was relatively small - smaller
than the amount of global warming caused by human greenhouse gas
emissions over the past century.
A
new grand solar minimum would not trigger another LIA; in fact, the
maximum 0.3°C cooling would barely make a dent in the human-caused
global warming over the next century. While it would be enough to
offset to about a decade's worth of human-caused warming, it's also
important to bear in mind that any solar cooling would only be
temporary, until the end of the solar minimum.
The
science is quite clear that the human influence on climate change has
become bigger than the sun's. At this point, speculation about
another mini ice age is pure fantasy
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