Geography 140
Introduction to Physical Geography
Lecture: Climate Change
I. Much in the news of late has been discussion of climate change and the
possible rôle of humans in altering climate.
A. As we saw in the lecture about the chemical composition of the earth's
atmosphere, several trace gasses are "greenhouse gasses," that is,
they absorb long-wave radiation very well and delay its exit from the
earth system. I'll sketch a few highlights here, but you can review
the earlier lecture for more details.
1. Carbon dioxide (CO2) is the best-known and the most
abundant of these.
a. It is increasing in abundance quite significantly as a byproduct
of combustion (even clean-burning fuels release carbon dioxide
and water, another greenhouse gas).
b. Human combustive activities are the major source of the increase
in carbon dioxide levels, producing a doubling in concentration
since the advent of the Industrial Revolution.
c. This may be one reason for the ~0.5° C increase in global
average temperatures over the last century, and climate
modelling suggests that global average temperatures could rise
another 3° C over the course of the next century, if nothing
is done to stop build-up of carbon dioxide.
2. Methane (CH4)is another, even more powerful greenhouse
gas, but at least it is drastically less abundant in the earth's
atmosphere (though it, too, is increasing as a result of human
activities).
3. Nitrous oxide (N2) is even more powerful in its
greenhouse activity than carbon dioxide or methane and is also
increasing due to human activity, but it is very rare.
4. CFCs may be the most potent of all greenhouse gasses (in addition
to their notorious rôle in ozone depletion), accounting for an
estimated 5 percent of the overall greenhouse effect, and are also
increasing very rapidly.
B. So, these trace gasses known to have greenhouse properties are
increasing quite rapidly, and we have seen an overall global warming
of about 0.5° C in the last century. The question, then, is
whether the increase of known greenhouse gasses is responsible for
this global warming and to which extent.
1. If human activity is responsible for a signficant proportion of
global warming, it becomes imperative to reduce our output of
greenhouse gasses.
2. If we are not responsible for the lion's share of the warming and
that warming is being driven by secular climatic change, we may
still need to reduce our output of the greenhouse gasses in order
to counter, rather than assist, the natural increase in
temperatures.
3. This is not a simple question of just shutting off our production
of the culprit gasses, unfortunately: Reducing combustion will
almost certainly reduce economic growth, and, the way the global
economy is structured now (profit comes from growth), this
necessarily translates into slower economic development, a reduced
ability to feed a growing population, and condemning the
overwhelming majority of the world's population to wrenching
poverty and social turmoil. In short, this is an epochal decision
and one that, unfortunately, must be effectively acted on in the
very short-term future. If we make the wrong decision, drastic
environmental changes may make our planet a miserable and very,
very different place for our descendants -- or drastic economic
change may make their life chances pretty grim and strife-ridden.
II. To evaluate the possible magnitude of human impacts on the global
atmosphere, it helps to have a very long perspective. In this section of
the lecture, I'll summarize what is known about secular (centuries long)
and millenial climate change.
A. The planet's atmosphere and climate is not as stable as we would think
from the narrow perspective of our all-too-short human lifespans.
Over the last century or two, we have carefully recorded temperatures
and precipitation receipts for more and more places on Earth. We have
dutifully calculated averages and standard deviations and ranges for
each of these. In fact, that's what climate means: averaged
temperatures and precipitation amounts and their standard deviations
and ranges.
B. What if our sample of years is not representative of what we're
measuring? That is, what if our climate is not reflected in the
numbers we have but, instead, exists in some sort of dynamic
equilibrium, changing directionally over time? What if, instead, our
climate is in a metastable equilibrium, alternating between two more-
or-less stable equilibria with sharp transitions between them?
C. Let's have a look at the longue durée, then, at a quick
history of glaciation on this planet over the last billion years or
so. I'll use a lot of terms describing various geological time
frames, and it sometimes helps to visualize them as a whole. You can
see the geologic time scale by clicking here.
1. During the Archaean eon, 3.8 to 2.5 billion years ago, the planet
was warmer than it is now, which is weird, given that the young sun
was, like most young stars, some 20-30 percent less bright than it
became in maturity, so it was cooler.
a. This paradox may reflect more carbon dioxide in the early
earth's atmosphere before oxygen had been built up by plant
photosynthesis.
b. Another speculation is that the earth's axis was much more
extremely tilted back then (about 70° from the vertical of
the plane of ecliptic), due to the collision with some large
planetoid that broke off the material that would later collect
to form the moon. A more extreme tilt, coupled with a more
oceanic planet (the landmasses hadn't accumulated to their
present dimensions early during the plate tectonic process),
meant that the high specific heat of water on an extremely
tilted planet could have kept overall temperatures warmer than
now. Most of the landmasses of the day were concentrated over
the South Pole, which may have been the counterweight that
eventually brought the axial tilt to a less extreme angle.
2. The worst glaciations ever to hit this planet took place in the
late Proterozoic, some 950-600 million years ago. At least three
glaciations were so severe that glaciers formed even on the equator
of the day! I've heard this called "Snowball Earth," and it would
have been a crisis for the early lifeforms that had evolved by
then, a real bottleneck for life. Apparently, it was resolved by
volcanic activity, which built up enough carbon dioxide to warm the
earth enough to melt the ice (volcanoes also emit sulphur dioxide
and dust, which can over the short term, produce climatic cooling,
so this is a very complicated factor). The process of deglaciation
would have been chaotic for life, too, taking only a few hundred
years from Snowball Earth to greenhouse conditions. The resulting
genetic instability among surviving organisms may have been what
resulted in the amazing proliferation and diversification of the
first complex multi-cellular life in the years of recovery after
the Proterozoic glaciations: the Cambrian explosion of some 575 to
525 million years ago.
3. Another major ice age hit in the Carboniferous and Permian periods,
some 350 to 250 million years ago. What's interesting about this
one is that in the immediately preceding Devonian Period (410-360
million years ago), complex plants first began to invade the land
surfaces of the earth. The reason this is relevant is that plant
photosynthesis consumes carbon dioxide, and there is evidence that,
in fact, carbon dioxide did diminish in the earth's atmosphere,
causing a lowering of the earth's thermostat. Plant life then was
largely confined to a swampy tropical rainforest, where dead plant
organic matter did not decay very efficiently (which prevented the
release of carbon dioxide fixed in photosynthesis) and built up the
huge coal and other fossil fuels for which the Carboniferous is
famous (and which we're releasing rather suddenly in modern
combustion).
4. The Pleistocene glacial epoch is the most recent one, starting
about 2 million years ago and "ending" about 14,000 years ago.
a. The Pleistocene has seen at least 20 advances of the ice sheets,
which are called glacials, and as many retreats, which are
called interglacials.
b. The term, "ice age," can refer more generally to sustained time
periods that saw glacials and interglacials (such as the
Pleistocene or Proterozoic) or more specifically to glacial
advances (such as the one that peaked around 18,000 or 20,000
years ago).
c. We like to call the last, oh, 10,000 or 15,000 years the
"Recent" or the "Holocene," to differentiate it from the bad old
times of the Pleistocene, but, in reality, the Holocene is
nothing more than an ordinary interglacial within the
Pleistocene system. In fact, in some interglacials over the
last couple million years, things got much warmer, so much so
that ALL the polar ice disappeared. So, ours isn't even a full-
blown interglacial in a manner of speaking!
d. The Holocene interglacial has not been marked by a smooth, even
transition of gradually warming temperatures, either: There
have been a number of minor ice advances over the course of the
last ten or fifteen thousand years. I'll expand on these below.
5. Ice advances within the Holocene:
![[ animation of ice sheet extent in North America over
the last 18,000 years, Illinois Museum ]](http://www.museum.state.il.us/exhibits/ice_ages/images/loopglac.gif)
a. The Younger Dryas was a period of cooling and ice advance that
took place pretty early in the Holocene, about 12,000 to 10,500
years ago. This may have been the result of fresh water trapped
behind receding glaciers being released in sudden floods, which
would have changed the salinity of the oceans and thereby messed
up the thermohaline circulation, which moves heat along salt
gradients in the deep ocean. Warming resumed about 10,500 bp
(before the present) and reached its greatest levels around
7,000 to 5,000 bp, with global temperatures some 1-2° C
above modern levels. This very warm timeframe is called the
"Climatic Optimum."
b. A cooling and glacial advance took place around 5,000 to 4,000
bp, followed by warming from 4,000 to 3,500 years ago.
c. Colder temperatures are again seen from 3,500 to 2,750 years ago
(and this one was cold enough to cause sea levels to drop some 2
to 3 meters below modern levels). Things again warmed up (but
not to the levels of the Climatic Optimum) from around 2,750 to
2,150 years ago.
d. At the height of the Roman Empire, from 2,150 to 1,100 years
ago, cooling again resumed. At its worst, the Nile River in
Africa actually saw ice form on its surface! The Black Sea also
saw surface ice sheets form! The environmental stresses and
traumas may have set off the great migrations of peoples that
eventually ruined the Roman Empire and triggered the Dark Ages.
Things warmed up substantially from about 1,100 to 800 years
ago. In fact, this warming episode is sometimes called the
"Little Climatic Optimum."
e. A period of cooler and much more extreme weather hit from 800
years ago to about 600 years ago. In our own neck of the woods,
a tremendous drought struck about 700 years ago, which went on
for decades. It was so severe that Owens Lake completely dried
up and vegetation grew on its lakebed (the stumps of which are
still found at the bottom of the lake!). This period of cool
and fluctuating weather deepened into an ice advance, about 600
years ago, called the "Little Ice Age." Temperatures were about
1° C cooler than they are today. Another extreme, decades-
long drought hit California about 400 years ago. Famine from
crop failure was experienced in Iceland from 1753-1759, which
killed one quarter of its population! The Little Ice Age went
on until 1850, which makes it relevant to the question of global
warming today, since the global climate has been rebounding from
the Little Ice Age ever since about 1850.
6. Major plot complication: Rebound from the Little Ice Age.
a. Temperatures have been warming ever since about 1850, as you can
see in this graph of average temperatures since 1880 (the red
lines smooth out some of the spiky fluctuations from year to
year by representing 5 year averages):
![[ global temperature trends since 1880, NASA Goddard
SFC ]](http://web.archive.org/web/20020724221828/http://geog.ouc.bc.ca/physgeog/contents/images/1999_global_temp1.gif)
b. So, the warming of global temperatures coïncides with the
dramatic increase in carbon dioxide being released by human use
of fossil fuels during the Industrial Revolution, as shown in
these data plotted from carbon dioxide concentrations trapped in
bubbles in glacier core samples and from actual carbon dioxide
monitors in Hawai'i:
![[ carbon dioxide concentrations since 1744, several
sources, cited in M.J. Pidwirny, Okanagan University College ]](http://web.archive.org/web/20021021172029/http://geog.ouc.bc.ca/physgeog/contents/images/co2graph.gif)
c. Is it possible that global warming is nothing more than a
rebound from the Little Ice Age and that there is nothing humans
can do about it, so why try?
d. Alternatively, is it possible that human combustion activities
in fact caused the Little Ice Age to end?
e. Is it possible that the Little Ice Age was ending on its own,
but that human combustion activities artificially steepened the
rise of temperatures and that we are in danger of a runaway
greenhouse effect?
f. The majority of the scientific community now holds that carbon
dioxide, methane, nitrous oxide, and CFC emissions are very
significantly related to temperature increases over the last
century and a half, especially carbon dioxide. Furthermore, it
is held that reducing carbon dioxide levels dramatically now and
over the next several decades is critical to the stabilization
of global temperatures or at least a reduction in the rate of
warming. This consensus in the scientific community has arisen
over the course of the last twenty years that I've been teaching
this class (it used to be a pretty evenly divided toss-up
between global warming due to carbon dioxide and global cooling
due to particulates, another side-effect of a lot of human
activities). Attention in the sciences has begun to concentrate
on predicting the effects of global warming.
III. Possible ramifications of global warming include:
A. Intensification of some hurricanes. Hurricanes are powered by the
amount of water evaporating from the warm tropical waters over which
they form and travel. By increasing air temperature, you get more
evaporation which, when released during adiabatic expansion and
cooling, results in the release of latent heat, which accelerates the
original uplift. The jury is still out on whether there will be more
hurricanes; it's just that we may see more super hurricanes now and
again.
B. Some tornadoes, too, could become more violent. They depend on the
contrast between warm, humid air and denser, colder air. Global
warming may make the warm air warmer and more humid.
C. Some models predict more extreme weather, including more extreme
temperatures (including colder weather in certain places) and moisture
conditions. This could result both in greater flooding and greater
drought! The concentration of heating increases also is expected to
vary latitudinally, with more warming taking place in colder regions
than in hot regions.
D. Warmer conditions and increased flooding may be perfect conditons for
the migration of tropical diseases into the mid-latitudes, such as
mosquito-borne dengue fever, malaria, encephalitis, and yellow fever.
E. Extinctions, as species are pushed beyond their ranges of tolerance
and blocked in their poleward migration by human fragmentation of
potential migration corridors and habitats.
F. Sea levels will creep up, which will cause coastal flooding and the
loss of low-lying farmland that supports an awful lot of people (e.g.,
much of Bangladesh) and some urban real-estate, too (e.g., Venice,
Italy, and New York and Long Beach).
G. In California, there is much concern that global warming will increase
both flood hazard and drought hazard! Warmer temperatures will reduce
the Sierra snowpack, on which much of the state depends for water.
This is the major "reservoir" for the state. By diminishing the
snowpack, we can expect water shortages during our long summer
droughts. At the same time, the formation of the snowpack keeps
liquid water from pouring down the mountains in the winter storms. If
the precipitation does not fall as snow in the Sierra, there will be
increased runoff during the winter, raising the specter of winter
flooding.
H. On the positive side, there is a chance that there will be an
accelerated rate of photosynthesis by crop plants, resulting in more
productivity. This may slightly offset some of the carbon dioxide
releases, too.
I. Optimistically, global warming may also raise the rate at which ocean
water dissolves carbon dioxide and causes it to precipitate to the
ocean floors as carbonate rock beds.
IV. Other than pesky humans and their pyromaniac ways, there are other causes
for climate change over the long haul. Terra just isn't as firma as we
once thought!
A. One factor is variation in the tilt of the earth's axis. It's now
about 23½° from the perpendicular of the plane
of ecliptic. It varies, however, from about 22¼° to about 24½° in a
roughly 40,600 year cycle. Exaggerated
tilt really affects the higher latitudes where glaciation
concentrates, as summer radiation receipt is increased with the more
direct sun angle and with the increase in the area experiencing
midnight sun. This discourages glacier formation. Diminished tilt
means summers are cooler at higher latitudes (which keeps snow
accumulation from melting) and winters are warmer (which means more
snow, due to the increased moisture-holding capacity of warmer air),
which creates optimum conditions for glacier accumulation.
B. Another factor is the precession in the equinoces. The axis, as you
may remember from lecture 3, wobbles. It
causes the North Pole to point to different stars over the course of a
roughly 22,000 year cycle (actually a major cycle of 23,000 years and
a minor cycle of 19,000 years). More importantly, it causes the
direct ray of the sun to cross the equator at progressively earlier
and earlier dates (about 1° every 72 years). The reason this is
important is that this changes the relative timing of the equinoces
and solstices with respect to aphelion and perihelion
(which are discussed in lecture 2.
1. Perihelion now occurs around the 3rd of January, close to the
December solstice.
2. Aphelion now occurs around the 4th of July, close to the June
solstice.
3. So, perihelion kind of takes the edge off winter cooling in the
Northern Hemisphere, where most of the land of our planet is now
found ... and the high latitude land prone to glaciation.
4. If the equinoces precede to such an extent that perihelion takes
place in the Northern Hemisphere summer, it would slightly
exaggerate the heating of the hemisphere having most of the land
and slightly exaggerate the chilling of that hemisphere during the
winter.
C. On top of all that, the earth's orbit changes in eccentricity over
time (how elliptical or circular it is). It varies from a low of 1
percent to a high of 6 percent over the course of a roughly 100,000
year cycle. If the earth's orbit is more elongated, that means
there's a bigger difference in radiation between aphelion and
perihelion. This difference gets pretty dramatic if perihelion occurs
during the Northern Hemisphere summer.
D. So, there are all these cycles that affect the amount of solar
radiation incident on the hemisphere with the most land to support
glaciation: a ~22,000 year cycle (precession of the equinoces), a
~41,000 year cycle (change in axial tilt), and a ~100,000 year cycle
(eccentricity of the earth's orbital shape).
E. Volcanic eruptions are another plot complication. Often, after a
major explosive eruption, temperatures worldwide will drop noticeably,
and this effect can persist for as long as three years after the
eruption. An upsurge in volcanic activity, then, might possibly
trigger ice advances.
1. For a long time, it was thought that this was because of the
tremendous amount of dust lobbed into the atmosphere by a major
eruption. The idea was that this dust would shade the surface of
the earth and thereby allow temperatures to drop. This has proven
not quite the total picture, however, as it became obvious that
dust actually settles out over about a six month period, not enough
to account for the duration of the temperature drops.
2. It now appears that a major mechanism for this volcanic cooling is
the ejections of massive amounts of a gas called sulphur dioxide
(SO2). This is a significant component of volcanic
ejecta and a really violent eruption can shoot this stuff all the
way up into the stratosphere. Once in the stratosphere, this gas
can stay up there for years, and it tends to reflect a lot of
visible light back into space. This, then, reduces the amount of
radiation transmitted to the earth's surface, which hinders
absorption and reradiation and warming of the air.
F. Then, it turns out that the solar "constant" isn't so constant
(remember in lecture 16, I mentioned that the
solar constant was 1354.21 J/m2/s, if you calculate it from
the large round numbers used for the earth's mean distance from the
sun, perihelion, and aphelion, which is what was used to talk about
how sun angle affects radiation receipt. Directly measured, it's more
like 1,366 J/m2/s. The sun's output varies more than we
thought (so much for the solar constant being, well, "constant"). For
example, measurements of the sun's output in the early 1980s showed a
drop of 0.1 percent in just an 18 month period. If this were
sustained so that a drop could average 1 percent over the course of a
century, global temperatures could drop 0.5-1.0° C!
1. One cause of this variation in the sun's output could be the
sunspot cycle. Sunspots are huge magnetic storms reaching the
surface of the sun, and they show up as darker, cooler spots on the
surface of the sun (which are quite visible to careful amateur
astronomers). As sunspot numbers and intensity increase, the sun's
surface can cool as much as 6° C. This cooling, however, is
more than compensated by brighter, hotter spots nearby
(faculæ), with the result that the magnetic anomalies
are creating so much turmoil on the surface of the sun that the
solar energy leaving the sun increases!
2. Sunspots increase in number in consistent cycles that have been
recorded for centuries, with maxima every 11, 90, and 180 years.
G. As if all this weren't bad enough, glaciation can be a runaway process
once it gets going by whatever trigger mechanism sets it off, as
unusual snow accumulations increase the Bond albedo of the earth's
surface, which means less atmospheric heating, which means the process
just keeps on going, er, "snowballing" (sorry!).
H. So, at the present, we're at a conjunction of low orbital
eccentricity, perihelion in the Northern Hemisphere winter, and
roughly average axial tilt, which makes glaciation less likely over
the immediate future. We're also at a conjunction of warming from the
Little Ice Age and the increase in combustion of fossil fuels
associated with industrial activities and modern transportation. So,
global warming seems the more troublesome issue facing humanity over
the next several decades.
V. So, how the heck can we reconstruct past climates, anyhow, when no-one
was dutifully recording daily weather data way back when?
A. For the last century or two, there are records of temperature and
precipitation for certain places on Earth, usually well populated and
economically developed.
B. There are also all sorts of archival records, such as lab notebooks
from astronomers recording sunspot cycles, for instance, and ships'
logs noting weather conditions at particular times and places
(depending on how well they could pinpoint longitude ...), diaries,
old news accounts of floods and blizzards and other extreme events.
The problem is these sources are uneven and very subjective. They are
amenable to people trained in the archival methods of historical
geography, a branch of human geography, which is part of the social
science side of geography. So, if you're interested in climate change
but you don't want to pick up the math, physics, and chemistry
background to work in meteorology (study of weather) and climatology
(study of climate), you could make a valuable contribution through
archival work with old records like these.
C. Proxy data can also tell us a lot about past climates. These are
physical data that correlate with past temperature and precipitation
regimes and allow us to infer what those regimes were like. Here's a
very brief sampler:
1. People can remove deep ice cores from Greenland and Antarctica and
analyze the gas trapped in small bubbles that formed as the ice was
accumulating. This can yield carbon dioxide concentrations, for
example, and the balance among various isotopes of hydrogen and
oxygen, which correlate with global temperatures.
2. Fossils, including microscopically tiny ones, can be examined to
tell us about the kinds of organisms in a given area at a given
time, which often reflects the climate there. For example, you can
look at fossilized pollen to tell if the area was covered by forest
vegetation or scrubland or grassland by identifying the species
that produced the pollen. The vegetation can tell you a lot about
prevailing climates.
3. Living trees, dead wood, and fossilized trees can be subjected to
tree-ring analysis to reconstruct dry and wet periods in the past.
I had the pleasure of supervising a master's thesis at Chico State
(by the late Robert Erving), which reconstructed the climatic past
of the northern Sierra Nevada using dendrochronology (tree ring
dating).
4. Geomorphology and geology can tell you a lot about past climates
because certain kinds of landforms, such as striations in bedrock
and jumbled deposits of rock debris (moraines) are created by
glaciers, while now anchored sand dunes might originally have been
produced and been active during a drier climate.
5. Inorganic sediments can also yield climate information. Depending
on the local environment, sandstones might indicate a run of stormy
and flood-prone years, which have the force to move the relatively
larger sand grains and deposit them in lakes and seashores. Shales
or mudstones might indicate quieter conditions where water flow
could only just move really fine materials like clay and silt.
Well, that's enough for climate change. Make sure you understand the
connection between carbon dioxide and global temperatures and why scientists
are so concerned about humans' releasing a significant part of 65 million
years accumulation of fossil fuels (the Carboniferous) in a geological instant
(the last couple hundred years). Also, be aware that this is not a simple
relationship of more carbon dioxide equals warmer temperatures: Long term
climatic cycles intersect with carbon dioxide increases in as-yet unspecified
ways. Know what the Little Ice Age was and when it took place. Know the
timing and possible causes of the two really antique ice ages (the Proterozoic
and the Carboniferous/Permian). Be able to identify some of the factors that
might have triggered ice advances over the last couple million years or so.
Know some of the possible consequences of global warming, no matter what its
source. Also, have a general idea of some of the sources of data by which
past climates can be reconstructed.
Document and © maintained by Dr.
Rodrigue
First placed on web: 03/18/01
Last revised: 10/25/07