Expert answer:Global Nature of Climate Change and The Greenhouse

  

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1.CLIMATE CHANGE
Catherine Gautier
The global environmental crisis of climate change has resulted from a process
called the greenhouse effect. This is a disturbance of the energy balance of the
Earth that results in a rise in surface temperature when there is an atmospheric
increase in the concentration of greenhouse gases such as carbon dioxide (CO2).
As humans burn fossil fuels and deforest the land, CO2 is released, altering for a
long time the chemistry of the atmosphere. This change in atmospheric
composition has led to a discernible human influence on our planet’s climate, to
what is referred to as global climate change. It represents changes over time in
the averages and variability of surface temperature, precipitation, and wind, as
well as associated changes in the Earth’s atmosphere, oceans and natural water
supplies, snow and ice, land surface, ecosystems, and living organisms. The
climate is a complex system evolving under natural and anthropogenic forces; its
change produces a variety of impacts felt all over the world. This [essay]
discusses the changes and impacts, as well as other concerns, associated with
climate change on the Earth.
GLOBAL NATURE OF CLIMATE CHANGE AND THE GREENHOUSE EFFECT
The presence of greenhouse gases (particularly water vapor but also carbon
dioxide, ozone, and methane) in the atmosphere leads to an Earth temperature
significantly warmer than it would otherwise have been without their greenhouse
effect. As human activities add more or new (chlorofluorocarbons for instance)
greenhouse gases, the Earth’s average temperature increases.
The Earth is naturally warmed by solar (shortwave, SW) radiation absorption and
cooled by emission of infrared (longwave, LW) radiation. It is the balance
between the warming from SW absorption and the cooling from LW emission that
governs the Earth’s temperature. When this balance is altered, the Earth’s
temperature adjusts to the new equilibrium. Such perturbation of the Earth’s
energy balance is called a radiative forcing, and the greenhouse effect is the
main perturbation whereby the presence of greenhouse gases in the atmosphere
blocks the emission of LW radiation to space, therefore reducing the cooling this
escaping radiation normally causes. This process leads to a warming of the
Earth.
Of the CO2 emitted by human activities, nearly half is quickly taken up by land
and ocean, and the remainder is transported by atmospheric motion around the
globe. This CO2 remains in the atmosphere for hundreds of years before being
taken up by chemical weathering on land and sediments deposition in the ocean
over geological time scales. CO2 can therefore be considered as having a long
lifetime, and the location where it is added to the atmosphere has essentially no
influence over where the greenhouse effect will occur. The greenhouse effect
occurs globally.
REGIONALIZATION OF THE WARMING
Surface observations averaged over the entire globe show a general temperature
increase (∼0.8°C) during the last century as is expected from the greenhouse
effect. A map of surface temperature changes during the last several decades,
however, displays some spatial variations in the warming. . . . In particular, the
high latitudes of the Northern Hemisphere display a warming two to three times
larger than the globally averaged one. These spatial variations do not result from
local greenhouse or other radiative forcing but are the consequence of a regional
feedback (a self-reinforcing reaction to a forcing that either enhances or reduces
the original forcing) process that tends to regionalize the warming. The so-called
snow/ice albedo feedback effect is responsible for the large temperature increase
in Northern Hemisphere high latitudes. In regions covered by snow or ice, a large
part of the radiation from the sun is reflected (the surface is said to have a high
albedo) and little radiation penetrates into the surface. As the surface
temperature increases from the greenhouse effect, some snow or ice melts,
exposing the surface (whether land or ocean) and allowing solar radiation to
penetrate the upper layer where the radiation can be absorbed and warm it. This
induces an additional melting of the adjacent snow/ice covered areas, giving rise
to more solar radiation absorption. This self-enhancing process is called a
positive feedback: Under an initial fluctuation (the warming from increased CO2 in
this case), a domino effect gets established that expands the original melting.
The result is a larger temperature increase in these regions than in surrounding
regions not covered by snow or ice.
Another source of regionalizing of the temperature change originates from other
radiative forcings. One major example is the addition of aerosols (particles
suspended in the air) resulting from industrial activities and the burning of wood,
or from natural processes such as volcanic eruptions or sand storms. Their
impact on climate differs from that of greenhouse gases in two major ways. First,
the aerosol lifetime is short (of the order of weeks to months, except for volcanic
aerosols that can last a few years in the stratosphere); thus, they do not have
time to extend globally, and therefore, their impact remains more regional. A
consequence of this short lifetime is that their impact can decrease or even
disappear as soon as the source of emission is reduced or eliminated. This is not
the case for CO2 that, once in the atmosphere, can remain there long after the
source of emission has disappeared. Second, most aerosols have a cooling
effect on the surface. This is particularly true with the prevailing aerosols,
including the sulfate aerosols (SO4) produced through the burning of sulfurcontaining coal. The cooling effect of aerosol can in part compensate for the
greenhouse warming and therefore mask its real strength. So, as we “clean” our
atmosphere from aerosols, for health or other reasons, we might experience a
larger warming from the greenhouse effect.
GLOBAL CLIMATE CHANGES
Sea Ice Melting, Glaciers Retreating, and Sea Level IncreaseA major aspect of
climate change is the melting of sea ice and the retreat of glaciers as a result of
the enhanced warming in high latitudes resulting from the snow/ice albedo
feedback. These effects have already been observed in most parts of the globe,
with varying intensity and timing. Maybe the most compelling example is the rapid
melting of Arctic ice. The extent of Arctic ice, at its minimum in September, has
generally been decreasing, one year after the next with however some recovery
over short periods of time (e.g., in 2008). Ice, however, has been thinning
everywhere. The consequences are multiple on humans and natural systems. The
animals accustomed to living on sea ice must adjust to smaller ice areas in the
summer. For instance, polar bears have to modify their habits as the large ice
platforms from which they hunt are reduced to almost nothing and they now have
to swim over longer distances to reach their prey. Fishing conditions and catches
are also modified, and people have to adapt to those, while they can enjoy longer
crop-growing seasons.
Melting glaciers is different in its nature, and it can add large amounts of fresh
water to the ocean, increasing the sea level, which is not the case for sea ice. Only
a negligible sea level increase occurs when ice already in the water melts. Glacier
melt and its associated retreat occurs in both the Arctic and Antarctic. Shorter
glaciers flow faster, which results in glaciers breaking up and large icebergs
detaching from the ice sheet. Other processes such as melt water percolation
through pores and fractures can accelerate the glacier flow through lubrication of
its base. Because the Antarctic is a continent covered with a thick layer of ice (in
fact the thickest . . . in the world), only glacier retreat occurs. Already, nearly 90%
of glaciers have retreated. But the retreat of these glaciers cannot be entirely
attributed to global warming, and the slowing down of the retreat in the late 1980s
and early 1990s is still unexplained.
Ocean Heat Accumulation, Expansion, and Sea Level ChangeThe ocean plays a
major role in climate, storing heat in the tropical regions, redistributing it toward the
poles and exchanging it with the atmosphere. It also plays a role in the carbon
cycle as it absorbs a significant part of the CO2 emitted by human activities, as
mentioned. As the greenhouse effect adds heat to the Earth system it is in the
ocean that the heat is accumulated and the ocean warms. This has two major
effects. First, a warmer ocean evaporates more, therefore putting additional water
vapor in the atmosphere. Because water vapor is a greenhouse effect, this leads
to an additional warming and therefore this is a positive feedback, possibly the
most important one. The second effect of the ocean warming is ocean expansion.
This expansion is the second component that induces sea level rise: Ice melting
and ocean expansion have contributions nearly similar to this rise.
Temperature Extremes and Extreme EventsAtmospheric temperatures are
expected to change in most places in the world, with extremes (both low and high)
becoming more common and periods of heat waves occurring more frequently and
over longer periods during the summer in many locations. The Northern
Hemisphere is more likely to experience these heat waves and, because of the
higher population density, their impacts will be more severe. Warmer ocean
surface temperatures are expected to lead to increased hurricane intensity and
duration, possibly leading to enhanced devastation, particularly in highly populated
and more developed coastal regions.
Hydrological CycleDespite these major changes in temperature, it is probably the
changes in the hydrological cycle that will be affecting human and natural systems
the most. Precipitation distribution and intensity have already been modified, and
the prediction is for direr changes with, in particular, reduced precipitation in
already rain-deficient regions such as the sub-Saharan regions, the Mediterranean
surroundings, and the southwestern United States, associated with enhanced
likelihood of wild fires. Higher precipitation in northern midlatitudes and in the
tropics associated with flooding and landslides are also forecasted. In general,
when precipitation falls, it will be more abundant and separated by longer periods
of reduced or no rain.
Another major aspect of the hydrological cycle change relates to the nature of
precipitation. In a warmer atmosphere, precipitation will fall more often as rain
instead of snow. This will lead to a reduction of the snow pack and its ability to
store water over long period of times before it is released when it is the most
needed in spring and summer. When combined with glaciers melting discussed
previously, regions that rely on alpine water sources, such as the countries at the
foot of Himalayas, could experience floods followed by water shortages.
Other climatological changes related to shorter scale variability of the atmosphere
are expected to occur but are more difficult to predict because of the chaotic nature
of atmospheric dynamics. Among them, intense mesoscale wind events are
expected to occur more frequently but have limited predictability.
HUMAN-INDUCED CHANGES?
One question often asked is as follows: How do we know that climate change
has an anthropogenic origin because climate has always changed? The first
thing to note is that the observed temperature change is much faster than any
ever observed in the past. Second, the observed increase in carbon dioxide
(CO2) concentration from 280 to nearly 385 parts per million (ppm) has been
clearly documented since the beginning of the 20th century as intense industrial
activities emitters of CO2 was ramping up. Moreover, an analysis of carbon
isotopes in CO2 allows us to verify its origin as that produced from burning fossil
fuels or burning forests. But because correlation is not causation, the third
element of this argumentation is that a theory—the greenhouse effect—links CO2
(and greenhouse gases) increase with temperature increase. Finally, it has been
suggested that the sun could be responsible for a large part of the observed
warming. This can be assessed by jointly examining the temperature changes at
the surface and in the stratosphere. In the case of a greenhouse-induced
change, the stratospheric temperature decreases with increasing CO2, while for a
sun-induced effect, it increases, like at the surface. Observations can therefore
help us differentiate an observed warming as a result of the greenhouse effect
from one originating mostly from solar variability; the temperature change
observed during the last few decades in the stratosphere clearly indicates a
decrease.
SOME SIGNIFICANT IMPACTS RESULTING FROM GLOBAL CHANGES
Ocean AcidificationApproximately one third of the CO2 emitted by human activity
has already been taken up by the ocean and thus moderated the atmospheric CO2
concentration increase and consequently global warming. In addition, as CO2
dissolves in sea water, carbonic acid is formed. This has the effect of acidifying, or
lowering, the pH of the ocean. Although not directly caused by warming,
acidification is related to it because, like the greenhouse effect, it is the result of
the increase of CO2 in the atmosphere. Ocean acidification has many impacts on
marine ecosystems, most of them highly detrimental to a substantial number of
species ranging from corals to lobsters and from sea urchins to mollusks and will
eventually affect the entire marine food chain.
Ecosystem Modification and Adaptation and Biodiversity ReductionThe list of
impacts of climate change on ecosystems, plants, and animals is long. Combined
changes in temperature and precipitation, as well as changes in extremes, can
have an impact on the survival of species and lead to a rapid reduction of
biodiversity. Ecological niches can disappear or the survival of some species can
be at risk because of the appearance of new pests that can survive milder winter,
for instance. But not all changes [are bad]: A lengthening growing season in higher
latitude, as discussed before, could be beneficial for some crops; increased CO2
concentration can, up to a certain point, have a fertilizing action. Overall, however,
the modification of ecosystems will affect the services they provide such as the
provision of food, clean water production, waste decomposition, regulation of
diseases, nutrient cycles, and crop pollination support, or even spiritual and
recreational benefits. Those services rely on complex interactions among many
species, each species having its unique function and DNA that has evolved to help
it respond to natural challenges. Once a species goes extinct, however, it does not
come back. And some losses of biodiversity then become irreversible.
Ecosystems can adjust over time to their climatic environment. Individual species
making an ecosystem can adapt by moving away from detrimental conditions. To
avoid increasing temperatures, they can grow further north in the Northern
Hemisphere and/or at higher altitude. Species that cannot adjust fast enough or
have nowhere to go (e.g., alpine ecosystems) will become stressed and at high
risk of extinction. Such range shift has happened in the past but during long periods
of time. The growing rate of both CO2 concentration and temperature is now
occurring at a faster pace than ever before, and this might be too fast for some
species; entire ecosystems might disappear as a result.
CLIMATE CHANGE PREDICTIONS
One question of significance for society is: How much CO2 in the atmosphere is
too much considering that CO2 concentration is already well outside the envelope
of the concentration that has occurred during the past 1 million years? An
associated question is: how large a temperature increase is too large? Although
no definitive answer can be offered to those questions because of the complexity
of the climate system, they can be addressed from a probabilistic perspective
using data from paleo-climatological records for guidance based on what has
happened in the past and climate models for predictions.
Climate models suggest that if we continue to use fossil fuel energy to fuel our
economic growth at the current rate, the global temperature change by the end of
the century would be between 2°C and 6°C with a most likely value around 4°C.
Most scientists agree that if the temperature increases beyond 2°C, the climate
might change in dramatic yet unknown ways. This is why this 2°C figure has
been selected as part of recent international discussions. CO2 emissions that
would correspond to a 2°C increase would correspond to releasing approximately
twice as much CO2 as we have done thus far, a far cry from the five times that
are predicted if we continue on our current energy usage trajectory. Almost all
computations converge to a needed cut of 80% to 85% in CO2 emissions in
highly emitting countries and 50% globally if we want to avoid dangerous climate
change.
ADDITIONAL CONCERNS: KNOWN UNKNOWNS
Several unknowns in the climate system can potentially trigger rapid and possibly
irreversible global changes. One important one is the possibility of methane
release as a result of permafrost—the soil that remains frozen year round in high
latitudes—melting. The top layer melts in the summer. This melting has been
observed to be expanding with time from rapidly increasing surface temperature.
Large amounts of methane are stored under the permafrost, and as it melts, this
very powerful greenhouse gas escapes into the atmosphere, significantly adding
to the greenhouse effect because it is approximately 25 times more potent than
CO2.
The exact amount of permafrost hydrate methane is not known so this process is
among the known unknowns, about which our knowledge might improve over
time. However, this is a powerful feedback process that cannot be stopped once
it is initiated by a general warming such as that produced by increasing CO2
concentration. And, naturally, there are unknown unknowns as well, about which
not much can be said except that we know they will develop at some point.
CONCLUSION
Global climate change connects people across space by its global nature and
time from the long CO2 lifetime. If unabated, it will be felt by future generations
well over the next century. Although climate change is one of the foremost issues
of our time that needs immediate attention, it is also the manifestation of another
significant concern, that of the overuse of the Earth’s resources and their nearterm exhaustion that may usher us into unknown territories.
2.TURNING THE TIDE
Ron Fujita
The ocean is alive. The waves change shape constantly, mesmerizing us with
their infinite variations on a theme. The sound of the surf is soothing background
music for contemplation. The vast panorama and the roar of breaking waves
inspire awe and expansive thoughts.
Changes in perspective offer glimpses of the ocean’s nature. At night, the waves
sometimes glow with the light of tiny organisms excited by the surf. I leave
bioluminescent swirls behind as I swim through warm water in the darkness.
Bouncing along in a boat, I am moved by the sight of a pod of dolphins, or of
young humpback whales leaping out of the water. But the ocean hides most of its
treasures below its mirrored surface.
Putting on a mask and snorkel can induce a startling revelation. Life is
everywhere. Clouds of small silvery fishes part as I approach. Tiny damselfish
fiercely chase me away from their carefully tended gardens. Elegant sea fans
wave in the surge. With the aid of an air tank, I can take the time to return the
inquisitive stares of cuttlefish and swim with graceful eagle rays. I meditate on
the sound of my breath and the bubbles I leave, and become aware of a school
of big …
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