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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 (N₂O) 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 Niño 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.