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Climate FAQ 10.1 | Climate changes
The causes of observed long-term climate changes (on time scales of over a decade) are estimated by determining whether the expected “fingerprints” of various causes of climate change are present in historical data sets. These fingerprints are derived from computer model simulations of the various climate change patterns caused by individual climate forcing. On time scales spanning several decades, these drives include processes such as greenhouse gas increases or changes in the brightness of the sun. By comparing the simulated fingerprint patterns with observed climate changes, we can determine whether the observed changes are best explained by these fingerprint patterns or by natural variability that occurs without any forcing.
The fingerprint of man-made greenhouse gas increases is clearly recognizable in the pattern of observed climate change in the 20th century. The observed change cannot otherwise be explained by the fingerprints of natural forcing or natural variability simulated in climate models. Assignment studies therefore support the conclusion that "it is extremely likely that human activity caused more than half of the observed increase in global mean surface temperature from 1951 to 2010".
The earth's climate is constantly changing, and it can happen for many reasons. To determine the main causes of observed changes, we must first determine whether an observed climate change is different from other variations that occur without any forcing. Climate variability without a driving factor - called internal variability - is the result of processes within the climate system. Large-scale oceanic variability, such as B. Fluctuations caused by El Niño-Southern Oscillation (ENSO) in the Pacific, represents the dominant source of internal climate variability on time scales from decades to centuries.
Climate change can also result from natural forcings outside the climate system such as volcanic eruptions or changes in the brightness of the sun. Drives like these are responsible for the major climate changes that are clearly demonstrated in geological data. Man-made drives include greenhouse gas emissions or the pollution of the atmosphere with particles. Any of these forcing, natural or human, could affect both internal variability and cause a change in the average climate. Assignment studies attempt to determine the causes of a proven change in the observed climate. We know that the mean global temperature has increased over the past century. Therefore, if the observed change was triggered by a drive, the main drive must cause heating and not cooling.
Formal mapping studies on climate change are carried out through controlled experiments with climate models. The responses simulated by the model to certain climate forcing systems are often referred to as the fingerprints of these forcing systems. A climate model must reliably simulate the fingerprint patterns associated with individual drives and the patterns of natural internal variability without a drive in order to obtain a meaningful assessment of the climate change allocation. No model can perfectly depict all the characteristics of the climate, but many detailed studies show that simulations with the current models are actually sufficiently reliable to carry out allocation studies.
FAQ 10.1, Figure 1 shows part of a fingerprint assessment of the global temperature change at the earth's surface during the late 20th century. The change observed in the second half of the 20th century, represented by the black time series in the left-hand panels, is greater than would have been expected from internal variability alone. Simulations based only on natural forcing (yellow and blue lines in the upper left panel) do not reproduce global warming on the earth's surface at the end of the 20th century and generate a spatial change pattern (top right) that is completely different from the observed change pattern (Middle right). Simulations, which include both natural and man-made forcing, provide a much better representation of the temporal course (bottom left) and the spatial distribution pattern (bottom right) of the observed changes in the earth's surface temperature.
Both panels on the left show that computer models reproduce the naturally-induced cooling on the earth's surface observed for a year or two after major volcanic eruptions, such as those that occurred in 1982 and 1991. Simulations with natural propulsion capture the short-term temperature changes after eruptions, but only the simulations that take into account natural and man-made propulsion depict the longer-lasting warming trend.
A more complete mapping study would, in addition to the results shown in FAQ 10.1, Figure 1, regarding the earth's surface temperatures, also consider the temperature above the earth's surface and possibly other climate variables. The fingerprint patterns associated with individual drives become easier to differentiate when more variables are taken into account in the assessment.
Overall, FAQ 10.1, Figure 1, shows that the pattern of the observed temperature change differs greatly from the pattern of the reaction to exclusively natural drives. The simulated response to all forcing, including man-made, gives a good match with the observed changes in the earth's surface. We cannot properly simulate observed recent climate change without taking into account the response to man-made forcing, including greenhouse gases, stratospheric ozone, and aerosols. Natural causes of change are still at work in the climate system, but recent temperature trends are largely due to man-made forcing.
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