In the previous blog on this topic, I reached several conclusions:
(1)
the evidence that the planet is in a warming trend is overwhelming—at least up
until the last 10 years or so;
(2)
warming trends are historically correlated with increasing levels of CO2
(with the emphasis on “correlated”, meaning that the warming is not necessarily
caused by increasing CO2);
(3)
mathematical models of future global temperatures are now predicting much less
warming than previously thought;
(4)
historically, climate change has been cyclic, with small cycles within larger
cycles;
(5)
how the various natural “forcings” of climate change (such as the Milankovitch
cycle) will interact with increased CO2 and other greenhouse gases
is, of course, unknown.
I think it is reasonable to talk further about some of the
natural “forcing” mechanisms, as these can either potentially mitigate or
exacerbate whatever effects Homo sapiens
is contributing to the climate.
By natural forcings, I’m referring to things going on in the
earth and the universe that can impact our climate. One of these is the Milankovitch cycle that I mentioned
previously—the tilt, spin, and orbit of the earth around the sun. Another is sunspot activity, which is
cyclical too. Other forcings
include volcanic activity, oceanic currents, and cloud cover. And then, finally, there is the role of
greenhouse gases and how they interact with all of these other factors.
First sunspots.
Although everyone has heard of them, I don’t suppose you’ve ever seen
one, since without proper protection, you’d burn out your retinas. But with appropriate filters and
projectors, they can be seen. (As
kids we used to look at the sun through photographic negatives, but I’m sure
that’s not a good idea, so don’t try this at home!) How sunspots were first observed I don’t know, but they were
discovered in 364 BC by a Chinese astronomer and by 28 BC, the Chinese were
making regular observations.
Today we know that sunspots come and go in 11-year cycles
and that they are correlated with solar flares and the amount of radiation
coming from the sun. The radiation
we are talking about is not just what we can see (sunshine), but also heat
(infrared) as well as ultraviolet and cosmic rays (high-energy particles). The amount of solar radiation hitting
the earth’s upper atmosphere, as measured by satellites, varies from one year
to the next over the course of each 11-year sunspot cycle (more sunspots =
greater total radiation), but the difference between the minimum and maximum
amounts is only about 0.1%. That
doesn’t sound like much does it?
Especially for something that has the ability to make changes in
something as big as the earth’s climate.
By 1801, people were already trying to prove that sunspots
have something to do with the weather.
And, indeed, current data indicates that sunspot numbers have been
correlated with cooling and warming periods for at least a thousand years.
The following is a graph of solar activity measured by the
amount of 14C
produced by cosmic rays, which has been shown to be correlated with sunspot
activity. The peaks and valleys
roughly correlate with heating and cooling periods on earth. For example, the high levels of 14C after 1000 AD correlate with the
Medieval Warm Period, and the lower levels of 14C up to about 1900 correspond to the Little
Ice Age.
Presumably the sunspot cycle mentioned above has existed for
thousands of years, but the foregoing 14C graph does not reflect an 11-year pattern. However, you can easily see the 11-year
cycle in the next graph, which plots the last 400 years of actual sunspot
counts. And from the black line (which
represents an average), it is clear that the 14C data correlates rather
well with sunspot activity.
The basic conclusion from these two graphs is that the
current high level of sunspot activity has not been seen for over 1000 years.
And what about volcanic activity? This is relevant because volcanoes emit CO2, and
as anyone living today knows, CO2 is a greenhouse gas that traps
heat in the atmosphere. Volcanoes
also release sulfur dioxide, which gets converted in the
upper atmosphere into sulfuric acid.
Sulfuric acid in turn interacts with other particles in the atmosphere
to form “sulfate aerosols.” These aerosols
(gases) can reflect sunlight back into space, resulting in a cooling
effect. (Actually, there have been
some ambitious proposals to release sulfur into the stratosphere to cool down
the warming planet. This is on the
order of fertilizing the ocean with iron to stimulate plankton growth, which
would in turn absorb CO2, and thus, theoretically, causing cooling
by reducing greenhouse gases. This geoengineering, as it is called, seems like
a risky idea, don’t you think? The
law of unintended consequences and all that.)
Of these two results of volcanic activity—CO2 and
sulfur dioxide—the latter has a greater effect on climate. CO2 from volcanoes is
actually tiny in comparison to the CO2 released by humans; estimates
are that volcanoes account for only about 0.06% of what humans produce (0.2
gigatons vs. 35 gigatons). And
although volcanoes have certainly affected our climate, their effects are only
transitory: about one to three
years. Mount Pinatubo’s eruption
in 1991 lowered global temperatures, mainly through sulfur dioxide, about 0.7
degrees Fahrenheit for three years.
So it would seem volcanoes are not a bad actor in the long term. Of much larger importance are oceanic
currents.
Now the topic of oceanic currents is worthy of a blog all to
itself, but I can just cut to the chase and say that the effect of oceanic
currents on global climate is HUGE.
Have you heard of El Niño?
La Niña? Both are
caused by ocean currents.
And as far as I am concerned, a discussion of ocean currents
just has to include Benjamin Franklin.
That’s because Ben worked out the extent of what we now know as the Gulf
Stream—in fact, he gave it its name.
He did all this by consulting sea captains as well as taking direct
measurements of seawater temperature in the course of his various
peregrinations back and forth across the Atlantic. In fact, he supplied the British government with a map of
the Gulf Stream to assist in accelerating shipping across the ocean. Unfortunately, it took Great Britain
many years to adopt Franklin’s proposal and they didn’t do so until long after
the U.S. and France, but ultimately their cross-Atlantic transit times were
shortened by two weeks. (An early
example of “not invented here,” I suspect.)
The Gulf Stream is part of a larger, global system of
currents known as the “thermohaline circulation” or “THC”. When broken down into its Latin or
Greek roots, this scary name really means temperature (“thermo”) plus salt
(“haline”). So, as the name
implies, it is a circulating current driven by temperature and salt
concentrations. Simple as that….I
wish!
Critical to an understanding of the forces driving this
gigantic serpentine current that runs through the Atlantic, Indian, Pacific and
Antarctic oceans is the fact that both cold water and salty water sink, while
warm water and less-salty water rise. It’s all about water density: when coupled with the influence of
wind, differences in density (determined by the precise mixture of temperature
and salt at a given location) cause water to move. Well, that’s pretty simple, don’t you agree?
Because this gargantuan current is actually connected at
both ends (!), there really isn’t a “start” or “stop.” So let’s begin with the Gulf Stream,
which is a current of warm water that runs through the Gulf of Mexico, around
Florida, and up into the North Atlantic off Greenland. Let’s pause there, because at this
juncture, COLD FRESH WATER from melting sea ice flows into the North
Atlantic. (Although sea ice is
frozen salt water, the melt from sea ice is fresh water because all the salt is
left behind when the melting occurs.)
Cold, dry winds lower the temperature of the melt water and cause
evaporation, thus increasing the density of the water. This causes the formerly warm Gulf
Stream water to SINK, and the now warmer air blows east and moderates the
temperature of Europe.
So this 6,000-mile stretch of the Gulf Stream illustrates
how ocean currents can influence the earth’s climate. Interestingly, it is believed that the melting of North
American glaciers after the last ice age produced such huge volumes of water
that it caused a very cold period in Europe 12,800-11,500 years ago (known as
the Younger Dryas)—there was just so much cold water it overwhelmed the warm
Gulf Stream and the air moving east was colder.
The story continues: the now SINKING water of the Gulf
Stream falls to the bottom of the ocean in the North Atlantic and flows south
to Antarctica. Antarctica, the
earth’s freezer, produces cold water that also sinks. So we have “deep water formation” at both the northern and
southern ends of the planet, which causes the current to split in two. One branch of the current flows east,
resulting in an upwelling of cold water into the warm water of the Indian
Ocean. (The transit time for the flow
of water from the North Atlantic to the Indian Ocean is around 1,600
YEARS.) The other branch of the
“split” current flows west as a
deep, cold, salty current to the central Pacific Ocean, where solar warming and
wind cause ANOTHER upwelling, returning a warm current back across the top of
Australia, around the southern tip of Africa, across the Atlantic, and into the
Gulf of Mexico—thus completing the loop.
Whew!
OK, hold these thoughts regarding the THC. In 1994 it was discovered that over the
course of about 70 years, the surface temperature of the water in the Atlantic
Ocean varies by about 1 degree Fahrenheit; this variation is now known as the
Atlantic Multidecadal Oscillation (AMO).
After scientists remove the part of the temperature variation that is
attributed to the effect of greenhouse gases, what is left is an alternating
warming/cooling cycle believed to be caused by periodic natural processes. Here is a graph of the AMO.
You can see that we are in the warming part of the cycle,
with perhaps another 30 years to go.
When the AMO is in its warm phase, droughts in the U.S.
Midwest and Southwest are more frequent (think Dust Bowl of the 1930’s). The opposite occurs in Florida and the
Pacific Northwest—that is, more rain.
A warm AMO also strengthens
summer rainfall over India and may be correlated with increased numbers of
hurricanes out of the Atlantic.
Remember modern-day glacial melting that we talked about in
the last blog? Researchers have correlated glacial growth and retreat with
variations in the AMO, and in their view, present day glacial retreat is due
significantly to decreased precipitation in Europe.
OK, now back to the THC. It is believed that the THC is the driver of the AMO, or at
least one of them. And
remember that this huge current undergoes an upwelling in the Pacific? It is almost certainly coupled to El Niño/El
Niña
events, which bring another whole cascade of global weather events. There is also a connection between the
THC and greenhouse gases. That’s
right—it turns out that upwelling water brings lots of CO2 to
surface waters.
I think a pause right here is in order. Can you imagine how difficult it is to
model global climate change on just those parameters mentioned so far in the
last two blogs? Sunspot
activity. The Milankovitch Cycle.
Volcanic activity. The thermohaline circulation and other ocean
currents. Melting ice sheets and
the release of fresh water into the ocean. The complexity is staggering.
Mathematical modeling is a beautiful activity though. It attempts to quantify all parameters
that may impact the system you are interested in and predict an outcome. It is required in any field of science
that involves predicting what will happen when more than one parameter is altered. Consequently modeling has been used for
climate projections for years.
That’s how a weather forecaster makes predictions as to what the weather
will be 10 days from now—within a certain statistical probability.
Suppose that you want to know whether we are going to enter
an ice age based on the Milankovitch Cycle, even though we are currently in a
warming period due to sunspot activity and the Atlantic Multidecadal
Oscillation AND we have fresh water entering the oceans. How could you possibly integrate all
these factors without modeling them?
And what makes it even more challenging is that all of these parameters
have to be quantified—that is, equations and values have to be determined in
order to make a calculation.
As a result, climate change modeling is based on the work of thousands
of scientists and the model is constantly changing as it is updated to include
the latest findings. In my
opinion, modeling represents the very best of our scientific efforts—taking all
that we know and making projections.
Even so, modelers don’t get any respect. People assume that the projections made
by a mathematical represent “truth,” and they are irate when some of those
projections inevitably turn out to be incorrect. Well, folks, the “truth” is always going to be changing
based on new information. The beauty of modeling is that it lays out what we
know and what we don’t know. (But
even if we examine the results of the modeling closely, we won’t know what
assumptions have been made to “fill in the gaps” in order to make the model
work.)
The task is huge, and requires gigantic computing
power. Wouldn’t you love to
sit down with one of those computers and fiddle here, increase that, decrease
this, and listen to the click and clatter of the computer as it works to spit
out your answer?! (Well, computers
USED to click and clatter.)
And that brings us to greenhouse gases. They are the final batch of
“parameters” I’m going to address with respect to climate change. There are three main greenhouse gases
that I’d like to focus on: water
vapor, methane, and CO2.
They are called greenhouse gases because they trap heat, like a
greenhouse. In fact CO2
is used in commercial greenhouses to increase plant growth—not to increase the
temperature, but rather to promote photosynthesis.
This is a good place to stop, and we will continue with
greenhouse gases in the next blog.
Useful References: