Many lines of evidence support the conclusion that most of the observed warming since the start of the 20th century, and especially over the last several decades, can be attributed to human activities, including the following:
1. Earth's surface temperature has clearly risen over the past 100 years, at the same time that human activities have resulted in sharp increases in CO2 and other GHGs.
2. Both the basic physics of the greenhouse effect and more detailed calcula tions dictate that increases in atmospheric GHGs should lead to warming of Earth's surface and lower atmosphere.
3. The vertical pattern of observed warming—with warming in the bottommost layer of the atmosphere and cooling immediately above—is consistent with warming caused by GHG increases and inconsistent with other possible causes (see below).
4. Detailed simulations with state-of-the-art computer-based models of the climate system are only able to reproduce the observed warming trend and patterns when human-induced GHG emissions are included.
In addition, other possible causes of the observed warming have been rigorously evaluated:
5. As described above, the climate system varies naturally on a wide range of time scales, but a rigorous statistical evaluation of observed climate trends, supported by analyses with climate models, indicates that the observed warming, especially the warming since the late 1970s, cannot be attributed to natural variations.
6. Satellite measurements conclusively show that solar output has not increased over the past 30 years, so an increase in energy from the Sun cannot be responsible for recent warming. There is evidence that some of the warming observed during the first few decades of the 20th century may have been caused by a slight uptick in solar output, although this conclusion is much less certain.
7. Direct measurements likewise show that the number of cosmic rays, which some scientists have posited might influence cloud formation and hence climate, have neither declined nor increased during the last 30 years. Moreover, a plausible mechanism by which cosmic rays might influence climate has not been demonstrated.
Based on these and other lines of evidence, the Panel on Advancing the Science of Climate Change—along with an overwhelming majority of climate scientists (Rosenberg et al., 2010)—conclude that much of the observed warming since the start of the 20th century, and most of the warming over the last several decades, can be attributed to human activities.
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