What's the evidence humans are causing climate change?

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Human activities – mainly fossil fuel burning and deforestation – emit greenhouse gases such as carbon dioxide into the atmosphere. Scientists know from well-understood physical principles that an increase in the atmospheric concentration of these gases leads to an increase in heat near the surface of the planet. The recent observed temperature trends are broadly consistent with what scientists expect from global warming caused by increased greenhouse gas concentrations superimposed on the natural variability of the climate system.