"It is very unlikely that the 20th-century warming can be explained by natural causes. The late 20th century has been unusually warm. Palaeoclimatic reconstructions show that the second half of the 20th century was likely the warmest 50-year period in the Northern Hemisphere in the last 1300 years. This rapid warming is consistent with the scientific understanding of how the climate should respond to a rapid increase in greenhouse
gases like that which has occurred over the past century, and the warming is inconsistent with the scientific understanding
of how the climate should respond to natural external factors
such as variability in solar output and volcanic activity."
[...]
"The natural external factors that affect climate include volcanic
activity and variations in solar output. Explosive volcanic
eruptions occasionally eject large amounts of dust and sulphate aerosol high into the atmosphere, temporarily shielding
the Earth and reflecting sunlight back to space. Solar output has an 11-year cycle and may also have longer-term variations.
Human activities over the last 100 years, particularly the burning of fossil fuels, have caused a rapid increase in carbon dioxide and other greenhouse gases in the atmosphere. Before the industrial age, these gases had remained at near stable concentrations
for thousands of years. Human activities have also caused increased concentrations of fine reflective particles, or ‘aerosols’, in the atmosphere, particularly during the 1950s and 1960s.
Although natural internal climate processes, such as El Niño, can cause variations in global mean temperature for relatively short periods, analysis indicates that a large portion is due to external factors. Brief periods of global cooling have followed major volcanic eruptions, such as Mt. Pinatubo in 1991. In the early part of the 20th century, global average temperature rose, during which time greenhouse gas concentrations started to rise, solar output was probably increasing and there was little volcanic activity. During the 1950s and 1960s, average global temperatures levelled off, as increases in aerosols from fossil fuels and other sources cooled the planet. The eruption of Mt. Agung in 1963 also put large quantities of reflective dust into the upper atmosphere. The rapid warming observed since the 1970s has occurred in a period when the increase in greenhouse gases has dominated over all other factors."
Quelle: AR4, WG1, Chapter 9, Seite 702
Vincent Gray
Diese Aussage von Vincent Gray ist nachzulesen:
AR4, WG1, Chapter 1, Seite 116
"The same is true for the cold, dry upper atmosphere where a small increase in water vapour has a greater influence on the greenhouse effect than the same change in water vapour would have near the surface."
[...]
"In the industrial era, human activities have added greenhouse gases to the atmosphere, primarily through the burning of fossil fuels and clearing of forests.
Adding more of a greenhouse gas, such as CO2, to the atmosphere
intensifies the greenhouse effect, thus warming Earth’s climate. The amount of warming depends on various feedback mechanisms. For example, as the atmosphere warms due to rising
levels of greenhouse gases, its concentration of water vapour increases,
further intensifying the greenhouse effect. This in turn causes more warming, which causes an additional increase in water
vapour, in a self-reinforcing cycle. This water vapour feedback
may be strong enough to approximately double the increase in the greenhouse effect due to the added CO2 alone.
Additional important feedback mechanisms involve clouds. Clouds are effective at absorbing infrared radiation and therefore exert a large greenhouse effect, thus warming the Earth. Clouds are also effective at reflecting away incoming solar radiation, thus cooling the Earth. A change in almost any aspect of clouds, such as their type, location, water content, cloud altitude, particle size and shape, or lifetimes, affects the degree to which clouds warm or cool the Earth. Some changes amplify warming while others diminish it."
Soviel also zur Begründung des IPCC.
MfG
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