Email, John W. Nielsen-Gammon, Texas state climatologist, Texas A&M University, May 7, 2015
"You don't have to be as smart as a fifth-grader to know what causes the climate is the Sun. The Sun. I have people tell me, 'carbon dioxide warms the Earth.' No, it doesn't. The Sun warms the Earth."
We need the longer quote to understand what Mr. Zimmerman means by 'what causes the climate'. With that understanding:
The statement "what causes the climate is the sun" is true. Except for negligible amounts of heat generated within the Earth, the Sun is the source of the energy that drives the climate system.
Most other things that affect climate, such as volcanic eruptions, clouds, carbon dioxide, dust, etc., affect Earth's climate because they affect how much the Sun warms the Earth. If carbon dioxide concentrations increase, for example, the Sun drives the Earth to a higher temperature because the solar energy that the Earth absorbs cannot escape to space as efficiently.
For most members of the public, there's no difference between "carbon dioxide warms the Earth" and "carbon dioxide causes the Earth to be warmer". Apparently the distinction, and correct wording, matters a lot to Mr. Zimmerman.