Greenhouse gases affect Earth’s energy
balance and climate.
The Sun
serves as the primary energy source for Earth’s climate. Some of the incoming
sunlight is reflected directly back into space, especially by bright surfaces
such as ice and clouds, and the rest is absorbed by the surface and the
atmosphere. Much of this absorbed solar energy is re-emitted as heat (longwave
or infrared radiation). The atmosphere in turn absorbs and re-radiates heat,
some of which escapes to space. Any disturbance to this balance of incoming and
outgoing energy will affect the climate. For example, small changes in the
output of energy from the Sun will affect this balance directly.
If all heat
energy emitted from the surface passed through the atmosphere directly into
space, Earth’s average surface temperature would be tens of degrees colder than
today. Greenhouse gases in the atmosphere, including water vapour, carbon
dioxide, methane, and nitrous oxide, act to make the surface much warmer than
this because they absorb and emit heat energy in all directions (including
downwards), keeping Earth’s surface and lower atmosphere warm. Without this
greenhouse effect, life as we know it could not have evolved on our planet.
Adding more greenhouse gases to the atmosphere makes it even more effective at
preventing heat from escaping into space. When the energy leaving is less than
the energy entering, Earth warms until a new balance is established.
Greenhouse
gases emitted by human activities alter Earth’s energy balance and thus its
climate. Humans also affect climate by changing the nature of the land surfaces
(for example by clearing forests for farming) and through the emission of
pollutants that affect the amount and type of particles in the atmosphere.
Scientists
have determined that, when all human and natural factors are considered,
Earth’s climate balance has been altered towards warming, with the biggest
contributor being increases in CO2.
Human activities have added
greenhouse gases to the atmosphere.
The
atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have
increased significantly since the Industrial Revolution began. In the case of
carbon dioxide, the average concentration measured at the Mauna Loa Observatory
in Hawaii has risen from 316 parts per million (ppm)1 in 1959 (the first full year
of data available) to more than 411 ppm in 2019 [Figure B2]. The same rates of
increase have since been recorded at numerous other stations worldwide. Since
preindustrial times, the atmospheric concentration of CO2 has increased by over
40%, methane has increased by more than 150%, and nitrous oxide has increased
by roughly 20%. More than half of the increase in CO2 has occurred since 1970.
Increases in all three gases contribute to warming of Earth, with the increase
in CO2 playing the largest role. See page B3 to learn about the sources of
human emitted greenhouse gases.
Scientists
have examined greenhouse gases in the context of the past. Analysis of air
trapped inside ice that has been accumulating over time in Antarctica shows
that the CO2 concentration began to increase significantly in the 19th century,
after staying in the range of 260 to 280 ppm for the previous 10,000 years. Ice
core records extending back 800,000 years show that during that time, CO2
concentrations remained within the range of 170 to 300 ppm throughout many “ice
age” cycles to learn about the ice ages—and no concentration above 300 ppm is
seen in ice core records until the past 200 years.
Measurements
of the forms (isotopes) of carbon in the modern atmosphere show a clear
fingerprint of the addition of “old” carbon (depleted in natural radioactive
14C) coming from the combustion of fossil fuels (as opposed to “newer” carbon
coming from living systems). In addition, it is known that human activities
(excluding land use changes) currently emit an estimated 10 billion tonnes of
carbon each year, mostly by burning fossil fuels, which is more than enough to
explain the observed increase in concentration. These and other lines of
evidence point conclusively to the fact that the elevated CO2 concentration in
our atmosphere is the result of human activities.
Climate records show a warming trend.
Estimating
global average surface air temperature increase requires careful analysis of
millions of measurements from around the world, including from land stations,
ships, and satellites. Despite the many complications of synthesising such
data, multiple independent teams have concluded separately and unanimously that
global average surface air temperature has risen by about 1 °C (1.8 °F) since
1900 [Figure B4]. Although the record shows several pauses and accelerations in
the increasing trend, each of the last four decades has been warmer than any
other decade in the instrumental record since 1850. Going further back in time
before accurate thermometers were widely available, temperatures can be
reconstructed using climate-sensitive indicators “proxies” in materials such as
tree rings, ice cores, and marine sediments. Comparisons of the thermometer
record with these proxy measurements suggest that the time since the early
1980s has been the warmest 40-year period in at least eight centuries, and that
global temperature is rising towards peak temperatures last seen 5,000 to
10,000 years ago in the warmest part of our current interglacial period.
Many other
impacts associated with the warming trend have become evident in recent years.
Arctic summer sea ice cover has shrunk dramatically. The heat content of the
ocean has increased. Global average sea level has risen by approximately 16 cm
(6 inches) since 1901, due both to the expansion of warmer ocean water and to
the addition of melt waters from glaciers and ice sheets on land. Warming and
precipitation changes are altering the geographical ranges of many plant and
animal species and the timing of their life cycles. In addition to the effects
on climate, some of the excess CO2 in the atmosphere is being taken up by the
ocean, changing its chemical composition (causing ocean acidification).
Many complex processes shape our
climate.
Based just on
the physics of the amount of energy that CO2 absorbs and emits, a doubling of
atmospheric CO2 concentration from pre-industrial levels (up to about 560 ppm)
would by itself cause a global average temperature increase of about 1 °C (1.8
°F). In the overall climate system, however, things are more complex; warming
leads to further effects (feedbacks) that either amplify or diminish the
initial warming.
The most
important feedbacks involve various forms of water. A warmer atmosphere
generally contains more water vapour. Water vapour is a potent greenhouse gas,
thus causing more warming; its short lifetime in the atmosphere keeps its
increase largely in step with warming. Thus, water vapour is treated as an
amplifier, and not a driver, of climate change. Higher temperatures in the
polar regions melt sea ice and reduce seasonal snow cover, exposing a darker
ocean and land surface that can absorb more heat, causing further warming.
Another important but uncertain feedback concerns changes in clouds. Warming and
increases in water vapour together may cause cloud cover to increase or
decrease which can either amplify or dampen temperature change depending on the
changes in the horizontal extent, altitude, and properties of clouds. The
latest assessment of the science indicates that the overall net global effect
of cloud changes is likely to be to amplify warming.
The ocean
moderates climate change. The ocean is a huge heat reservoir, but it is
difficult to heat its full depth because warm water tends to stay near the
surface. The rate at which heat is transferred to the deep ocean is therefore
slow; it varies from year to year and from decade to decade, and it helps to
determine the pace of warming at the surface. Observations of the sub-surface
ocean are limited prior to about 1970, but since then, warming of the upper 700
m (2,300 feet) is readily apparent, and deeper warming is also clearly observed
since about 1990.
Surface
temperatures and rainfall in most regions vary greatly from the global average
because of geographical location, in particular latitude and continental
position. Both the average values of temperature, rainfall, and their extremes
(which generally have the largest impacts on natural systems and human
infrastructure), are also strongly affected by local patterns of winds.
Estimating
the effects of feedback processes, the pace of the warming, and regional
climate change requires the use of mathematical models of the atmosphere,
ocean, land, and ice (the cryosphere) built upon established laws of physics
and the latest understanding of the physical, chemical and biological processes
affecting climate, and run on powerful computers. Models vary in their
projections of how much additional warming to expect (depending on the type of
model and on assumptions used in simulating certain climate processes,
particularly cloud formation and ocean mixing), but all such models agree that
the overall net effect of feedbacks is to amplify warming.
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