Learn about the sources of
human-emitted greenhouse gases:
Carbon dioxide (CO2 ) has both natural and human sources,
but CO2 levels are increasing primarily because of the combustion of fossil
fuels, cement production, deforestation (which reduces the CO2 taken up by
trees and increases the CO2 released by decomposition of the detritus), and
other land use changes. Increases in CO2 are the single largest contributor to
global warming.
Methane (CH4 ) has both human and natural sources,
and levels have risen significantly since pre-industrial times due to human
activities such as raising livestock, growing paddy rice, filling landfills,
and using natural gas (which is mostly CH4 , some of which may be released when
it is extracted, transported, and used).
Nitrous oxide (N2 O) concentrations
have risen primarily because of agricultural activities such as the use of
nitrogen-based fertilisers and land use changes.
Halocarbons, including chlorofluorocarbons
(CFCs), are chemicals used as refrigerants and fire retardants. In addition to
being potent greenhouse gases, CFCs also damage the ozone layer. The production
of most CFCs has now been banned, so their impact is starting to decline.
However, many CFC replacements are also potent greenhouse gases and their
concentrations and the concentrations of other halocarbons continue to
increase.
Learn about the ice ages:
Detailed
analyses of ocean sediments, ice cores, and other data show that for at least
the last 2.6 million years, Earth has gone through extended periods when
temperatures were much lower than today and thick blankets of ice covered large
areas of the Northern Hemisphere. These long cold spells, lasting in the most
recent cycles for around 100,000 years, were interrupted by shorter warm
‘interglacial’ periods, including the past 10,000 years.
Through a
combination of theory, observations, and modelling, scientists have deduced
that the ice ages* are triggered by recurring variations in Earth’s orbit that
primarily alter the regional and seasonal distribution of solar energy reaching
Earth. These relatively small changes in solar energy are reinforced over
thousands of years by gradual changes in Earth’s ice cover (the cryosphere),
especially over the Northern Hemisphere, and in atmospheric composition,
eventually leading to large changes in global temperature. The average global
temperature change during an ice-age cycle is estimated as 5 °C ± 1 °C (9 °F ±
2 °F).
*Note that
in geological terms Earth has been in an ice age ever since the Antarctic Ice
Sheet last formed about 36 million years ago. However, in this document we have
used the term in its more colloquial usage indicating the regular occurrence of
extensive ice sheets over North America and northern Eurasia.
Learn more about other human causes
of climate change:
In addition
to emitting greenhouse gases, human activities have also altered Earth’s energy
balance through, for example:
Changes in
land use. Changes in the way people use land — for example, for forests, farms,
or cities — can lead to both warming and cooling effects locally by changing
the reflectivity of Earth’s surfaces (affecting how much sunlight is sent back
into space) and by changing how wet a region is.
Emissions of pollutants (other than
greenhouse gases).
Some industrial and agricultural processes emit pollutants that produce
aerosols (small droplets or particles suspended in the atmosphere). Most
aerosols cool Earth by reflecting sunlight back to space. Some aerosols also
affect the formation of clouds, which can have a warming or cooling effect
depending on their type and location. Black carbon particles (or “soot”)
produced when fossil fuels or vegetation are burned generally have a warming
effect because they absorb incoming solar radiation.
Why are computer models used to study
climate change?
The future
evolution of Earth’s climate as it responds to the present rapid rate of
increasing atmospheric CO2 has no precise analogues in the past, nor can it be
properly understood through laboratory experiments. As we are also unable to
carry out deliberate controlled experiments on Earth itself, computer models
are among the most important tools used to study Earth’s climate system.
Climate
models are based on mathematical equations that represent the best
understanding of the basic laws of physics, chemistry, and biology that govern
the behaviour of the atmosphere, ocean, land surface, ice, and other parts of
the climate system, as well as the interactions among them. The most comprehensive
climate models, Earth-System Models, are designed to simulate Earth’s climate
system with as much detail as is permitted by our understanding and by
available supercomputers.
The
capability of climate models has improved steadily since the 1960s. Using physics-based
equations, the models can be tested and are successful in simulating a broad
range of weather and climate variations, for example from individual storms,
jet stream meanders, El NiƱo events, and the climate of the last century. Their
projections of the most prominent features of the long-term human-induced
climate change signal have remained robust, as generations of increasingly
complex models yield richer details of the change. They are also used to
perform experiments to isolate specific causes of climate change and to explore
the consequences of different scenarios of future greenhouse gas emissions and
other influences on climate.
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