Throughout its long history, Earth has warmed and cooled
time and again. Climate has changed when the planet received more or less
sunlight due to subtle shifts in its orbit, as the atmosphere or surface
changed, or when the Sun’s energy varied. But in the past century, another
force has started to influence Earth’s climate: humanity.
How does this warming compare to previous changes in
Earth’s climate? How can we be certain that human-released greenhouse gases are
causing the warming? How much more will the Earth warm? How will Earth respond?
Answering these questions is perhaps the most significant scientific challenge
of our time.
Global warming is the unusually rapid increase in Earth’s
average surface temperature over the past century primarily due to the
greenhouse gases released as people burn fossil fuels. The global average
surface temperature rose 0.6 to 0.9 degrees Celsius (1.1 to 1.6° F) between
1906 and 2005, and the rate of temperature increase has nearly doubled in the
last 50 years. Temperatures are certain to go up further. Earth’s temperature
begins with the Sun. Roughly 30 percent of incoming sunlight is reflected back
into space by bright surfaces like clouds and ice. Of the remaining 70 percent,
most is absorbed by the land and ocean, and the rest is absorbed by the
atmosphere. The absorbed solar energy heats our planet.
As the rocks, the air, and the seas warm, they radiate
“heat” energy (thermal infrared radiation). From the surface, this energy
travels into the atmosphere where much of it is absorbed by water vapor and
long-lived greenhouse gases such as carbon dioxide and methane. When they
absorb the energy radiating from Earth’s surface, microscopic water or
greenhouse gas molecules turn into tiny heaters— like the bricks in a
fireplace, they radiate heat even after the fire goes out. They radiate in all
directions. The energy that radiates back toward Earth heats both the lower
atmosphere and the surface, enhancing the heating they get from direct
sunlight.
This absorption and radiation of heat by the
atmosphere—the natural greenhouse effect—is beneficial for life on Earth. If
there were no greenhouse effect, the Earth’s average surface temperature would
be a very chilly -18°C (0°F) instead of the comfortable 15°C (59°F) that it is
today. What has scientists concerned now is that over the past 250 years,
humans have been artificially raising the concentration of greenhouse gases in
the atmosphere at an ever-increasing rate, mostly by burning fossil fuels, but
also from cutting down carbon-absorbing forests. Since the Industrial
Revolution began in about 1750, carbon dioxide levels have increased nearly 38
percent as of 2009 and methane levels have increased 148 percent.
The atmosphere today contains more greenhouse gas
molecules, so more of the infrared energy emitted by the surface ends up being
absorbed by the atmosphere. Since some of the extra energy from a warmer
atmosphere radiates back down to the surface, Earth’s surface temperature
rises. By increasing the concentration of greenhouse gases, we are making
Earth’s atmosphere a more efficient greenhouse.
The average surface temperature of Earth is maintained by
a balance of various forms of solar and terrestrial radiation. Solar radiation
is often called “shortwave” radiation because the frequencies of the radiation
are relatively high and the wavelengths relatively short—close to the visible
portion of the electromagnetic spectrum. Terrestrial radiation, on the other
hand, is often called “longwave” radiation because the frequencies are
relatively low and the wavelengths relatively long—somewhere in the infrared
part of the spectrum. Downward-moving solar energy is typically measured in
watts per square metre. The energy of the total incoming solar radiation at the
top of Earth’s atmosphere (the so-called “solar constant”) amounts roughly to
1,366 watts per square metre annually. Adjusting for the fact that only
one-half of the planet’s surface receives solar radiation at any given time,
the average surface insolation is 342 watts per square metre annually.
The amount of solar radiation absorbed by Earth’s surface
is only a small fraction of the total solar radiation entering the atmosphere.
For every 100 units of incoming solar radiation, roughly 30 units are reflected
back to space by either clouds, the atmosphere, or reflective regions of
Earth’s surface. This reflective capacity is referred to as Earth’s planetary
albedo, and it need not remain fixed over time, since the spatial extent and
distribution of reflective formations, such as clouds and ice cover, can
change. The 70 units of solar radiation that are not reflected may be absorbed
by the atmosphere, clouds, or the surface. In the absence of further
complications, in order to maintain thermodynamic equilibrium, Earth’s surface
and atmosphere must radiate these same 70 units back to space. Earth’s surface
temperature (and that of the lower layer of the atmosphere essentially in
contact with the surface) is tied to the magnitude of this emission of outgoing
radiation according to the Stefan-Boltzmann law.
Earth’s energy budget is further complicated by the
greenhouse effect. Trace gases with certain chemical properties—the so-called
greenhouse gases, mainly carbon dioxide (CO2), methane (CH4), and nitrous oxide
(N2O)—absorb some of the infrared radiation produced by Earth’s surface. Because
of this absorption, some fraction of the original 70 units does not directly
escape to space. Because greenhouse gases emit the same amount of radiation
they absorb and because this radiation is emitted equally in all directions
(that is, as much downward as upward), the net effect of absorption by
greenhouse gases is to increase the total amount of radiation emitted downward
toward Earth’s surface and lower atmosphere. To maintain equilibrium, Earth’s
surface and lower atmosphere must emit more radiation than the original 70
units. Consequently, the surface temperature must be higher. This process is
not quite the same as that which governs a true greenhouse, but the end effect
is similar. The presence of greenhouse gases in the atmosphere leads to a
warming of the surface and lower part of the atmosphere (and a cooling higher
up in the atmosphere) relative to what would be expected in the absence of
greenhouse gases.
It is essential to distinguish the “natural,” or
background, greenhouse effect from the “enhanced” greenhouse effect associated
with human activity. The natural greenhouse effect is associated with surface
warming properties of natural constituents of Earth’s atmosphere, especially
water vapour, carbon dioxide, and methane. The existence of this effect is
accepted by all scientists. Indeed, in its absence, Earth’s average temperature
would be approximately 33 °C (59 °F) colder than today, and Earth would be a
frozen and likely uninhabitable planet. What has been subject to controversy is
the so-called enhanced greenhouse effect, which is associated with increased
concentrations of greenhouse gases caused by human activity. In particular, the
burning of fossil fuels raises the concentrations of the major greenhouse gases
in the atmosphere, and these higher concentrations have the potential to warm
the atmosphere by several degrees.
Global warming is related to the more general phenomenon
of climate change, which refers to changes in the totality of attributes that
define climate. In addition to changes in air temperature, climate change
involves changes to precipitation patterns, winds, ocean currents, and other
measures of Earth’s climate. Normally, climate change can be viewed as the
combination of various natural forces occurring over diverse timescales. Since
the advent of human civilization, climate change has involved an
“anthropogenic,” or exclusively human-caused, element, and this anthropogenic
element has become more important in the industrial period of the past two centuries.
The term global warming is used specifically to refer to any warming of
near-surface air during the past two centuries that can be traced to
anthropogenic causes.
To define the concepts of global warming and climate
change properly, it is first necessary to recognize that the climate of Earth
has varied across many timescales, ranging from an individual human life span
to billions of years. This variable climate history is typically classified in
terms of “regimes” or “epochs.” For instance, the Pleistocene glacial epoch
(about 2,600,000 to 11,700 years ago) was marked by substantial variations in
the global extent of glaciers and ice sheets. These variations took place on
timescales of tens to hundreds of millennia and were driven by changes in the distribution
of solar radiation across Earth’s surface. The distribution of solar radiation
is known as the insolation pattern, and it is strongly affected by the geometry
of Earth’s orbit around the Sun and by the orientation, or tilt, of Earth’s
axis relative to the direct rays of the Sun.
Worldwide, the most recent glacial period, or ice age,
culminated about 21,000 years ago in what is often called the Last Glacial
Maximum. During this time, continental ice sheets extended well into the middle
latitude regions of Europe and North America, reaching as far south as
present-day London and New York City. Global annual mean temperature appears to
have been about 4–5 °C (7–9 °F) colder than in the mid-20th century. It is
important to remember that these figures are a global average. In fact, during
the height of this last ice age, Earth’s climate was characterized by greater
cooling at higher latitudes (that is, toward the poles) and relatively little
cooling over large parts of the tropical oceans (near the Equator). This
glacial interval terminated abruptly about 11,700 years ago and was followed by
the subsequent relatively ice-free period known as the Holocene Epoch. The
modern period of Earth’s history is conventionally defined as residing within
the Holocene. However, some scientists have argued that the Holocene Epoch
terminated in the relatively recent past and that Earth currently resides in a
climatic interval that could justly be called the Anthropocene Epoch—that is, a
period during which humans have exerted a dominant influence over climate.
Though less dramatic than the climate changes that
occurred during the Pleistocene Epoch, significant variations in global climate
have nonetheless taken place over the course of the Holocene. During the early
Holocene, roughly 9,000 years ago, atmospheric circulation and precipitation
patterns appear to have been substantially different from those of today. For
example, there is evidence for relatively wet conditions in what is now the
Sahara Desert. The change from one climatic regime to another was caused by
only modest changes in the pattern of insolation within the Holocene interval
as well as the interaction of these patterns with large-scale climate phenomena
such as monsoons and El Niño/Southern Oscillation (ENSO).
During the middle Holocene, some 5,000–7,000 years ago,
conditions appear to have been relatively warm—indeed, perhaps warmer than
today in some parts of the world and during certain seasons. For this reason,
this interval is sometimes referred to as the Mid-Holocene Climatic Optimum.
The relative warmth of average near-surface air temperatures at this time,
however, is somewhat unclear. Changes in the pattern of insolation favoured
warmer summers at higher latitudes in the Northern Hemisphere, but these changes
also produced cooler winters in the Northern Hemisphere and relatively cool
conditions year-round in the tropics. Any overall hemispheric or global mean
temperature changes thus reflected a balance between competing seasonal and
regional changes. In fact, recent theoretical climate model studies suggest
that global mean temperatures during the middle Holocene were probably 0.2–0.3
°C (0.4–0.5 °F) colder than average late 20th-century conditions.
Over subsequent millennia, conditions appear to have cooled
relative to middle Holocene levels. This period has sometimes been referred to
as the “Neoglacial.” In the middle latitudes this cooling trend was associated
with intermittent periods of advancing and retreating mountain glaciers
reminiscent of (though far more modest than) the more substantial advance and
retreat of the major continental ice sheets of the Pleistocene climate epoch.
In my opinion The World is either warming up or not. If
it is warming then the cause is natural or due at least in part to human
activity i.e. air pollution. If the World is warming and the warming is due to
natural causes then we will just have to adapt. If it is due to human activity
then we can do something about it by eliminating the air pollution that we
cause. Either way we will have clean air which is a good idea in itself. If our
air pollution can warm the Earth then we may have a mechanism that could
prevent another Ice Age. Another Ice Age could kill most of us. Ordinary people
should not get into pointless “yes it is” or “no it isn’t” arguments.
Scientists and history will decide the issue eventually. We don’t have to stop
burning fossil fuels. Burning coal and oil does not pollute the atmosphere. The
smoke does that. Eliminate the smoke and other exhaust gases and air pollution
will cease.
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