Final answer:
The Great Global Warming Swindle points towards natural factors such as volcanic activity and solar variations as responsible for the decrease in global temperatures during the 1940s. Volcanic eruptions eject aerosols that can reduce temperatures, as observed with the Pinatubo eruption. However, the consensus is that the recent global warming trend is predominantly due to human activities like fossil fuel combustion.
Step-by-step explanation:
The documentary The Great Global Warming Swindle suggests that natural factors, such as changes in solar activity and volcanic eruptions, contributed to decreases in global temperatures.
In particular, the decrease in global temperatures during the 1940s may be attributed to factors other than human activities. Volcanic activity, for instance, can lead to a significant amount of aerosols being ejected into the atmosphere, which efficiently reflect sunlight and can cool the Earth. Examples of this include the Tambora eruption in 1815 and the Mount Pinatubo eruption in 1991, both of which caused a temporary reduction in global temperatures due to the reflection of sunlight from sulfur dioxide (SO₂) aerosols.
While volcanic activity has historically had a cooling effect on the Earth's climate, most observed warming since the mid-20th century is extremely likely caused by human activities, particularly the combustion of fossil fuels. This scientific consensus is based on evidence showing a steady increase in greenhouse gas levels due to human actions, which in turn enhance the greenhouse effect and lead to global warming.