How far do you agree with this statement that "Scientists believe global warming is caused only by human activity"?
Human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point. (National Oceanic and Atmospheric Administration)
The age of debate over whether global warming and the subsequent global climate change exist is past. The age of debate over whether it is caused by human agency is past. As the NOAA says, there "is no scientific debate" on these points any longer. While global warming has historically been caused by natural factors of chemical changes on Earth and changes in the Sun's solar activity, the present problem of global warming and the resultant global climate change is caused solely by human activity, whether you are discussing the effect of carbon dioxide, methane or chlorofluorocarbons, each of these is at problematic levels because of human agency and activity.
The questions that do remain are: What is the accurate model of causes and predictive developing results? Is the correlation between CO2 and global warming/ climate change the driving correlation? These questions reflect back on some of the old dissenting arguments to human activity caused global warming and climate change.
One dissenting argument was that there is and always has been a natural flow of climate changes that run throughout the history of the planet. Ocean floors reveal magnetic polar shifts that produced visible effects on topographic climate. Oxygen and carbon levels went through changes that produced climate change. Therefore climate change is a natural part of the global life-rhythm and lifespan.
Another dissenting argument was that variations in the Sun's 11-year cycle of solar activity (declining activity for 11 years until reaching the solar minimum, increasing activity for 11 years until reaching the solar maximum) has had a directly correlated relationship to changes in Earth's climate, for example, as during the time of the "Little Ice Age" spanning the 18th century when temperatures became significantly colder.
While these dissenting arguments posit events that are true, they do not correlate with nor explain the present warming and climate change situation. In regard to the questions that are being asked now about correct models and driving correlations, a new study has proposed that chlorofluorocarbons (CFCs) have driven global warming and climate change since the 1970s instead of carbon dioxide being the directly correlated driver.
Scientist Qing-Bin Lu of the University of Waterloo has developed a cosmic-ray-driven electron-reaction (CRE) theory that is supported by statistical analysis of observed climate data from the 19th, 20th and 21st centuries and by Lu's studies of Antarctic ozone depletion and surface temperatures.
Lu has found a nearly one-on-one correlation between CFC levels, ozone depletion and surface temperature in the Antarctic. Lu's CRE theory states that cosmic rays from space are interacting with CFCs, creating the driving factor of global warming and climate change. This replaces the model showing ultraviolet light from the Sun interacting with CO2 as the driving factor.