Both natural and human-induced influences have changed twentieth-century climate, but their relative roles and regional impacts are still under debate.
For example, most model-based studies point to increasing human-generated greenhouse gas and aerosol concentrations as the dominant cause of global surface warming after 1967, while some empirical analyses suggest that solar variability accounts for as much as 69 percent of warming seen in the past 100 years and 25-35 percent of recent warming.
To help resolve this, Lean and Rind analyze the best available estimates of both natural and human-induced climate influences and compare them with observed surface temperatures across the globe from 1889 to 2006. They find that solar forcing contributed negligible long-term warming in the past 25 years and 10 percent of the warming in the past 100 years.
Additionally, in contrast with recent model results by the Intergovernmental Panel on Climate Change, which estimates that anthropogenic warming has minimum values in the tropics and increases steadily from 30 degrees N to 70 degrees N, the authors find that the zonal surface temperature changes from the historical surface temperature record are more pronounced between 45 degrees S and 50 degrees N.-American Geophysical Union
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