"The climate is always changing. Climate is currently changing because of a combination of natural and human induced effects. The natural effects include variations of the sun, volcanic eruptions, and oscillations of the ocean. The human induced effects include the greenhouse gases such as carbon dioxide, pollution aerosols, and land use changes. The key scientific issue is determining how much of the climate change is associated with humans. This is not a simple thing to determine. The most recent IPCC assessment report states: “Most [50%] of the warming in the latter half of the 20th century is very likely [>90%] due to the observed increase in greenhouse gas concentrations.” There is certainly some contribution from the greenhouse gases, but whether it is currently a dominant factor or will be a dominant factor in the next century, is a topic under active debate, and I don’t think the high confidence level [>90%] is warranted given the uncertainties."
http://www.thegwpf.org/the-climate-record/5084-the-ipcc-may-have-outlived-its-usefulness-an-interview-with-judith-curry.html
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