A: Climate changes observed over recent decades are inconsistent with trends caused by natural forces but are totally consistent with the increase in human-induced heat-trapping gases. In fact, without human influences, Earth’s climate actually would have cooled slightly over the past 50 years.
Natural forces cause Earth’s temperature to fluctuate on long timescales due to slow changes in the planet’s orbit and tilt. Such forces were responsible for the ice ages. Other natural forces sometimes cause temperatures to change on short timescales. For example, major volcanic eruptions can cause short-term cooling lasting two to three years. Changes in the Sun’s output over the past 30 years have followed the typical 11-year cycle, with no net increase, while temperatures were warming strongly.
Many independent lines of evidence (from basic physics to the patterns of temperature change through the layers of the atmosphere) have shown that the warming of the past 50 years is primarily due to the human-caused increase in heat-trapping gases.