  There is evidence that a natural control system influences global atmospheric
surface temperature (Leggett and Ball, 2020). The present paper sets up and
tests a hypothesis concerning the physical makeup of the sequential elements of
the control system and its outcomes. The final outcome the control system
influences is defined as global atmospheric surface temperature. The terms used
for the control system element types in the hypothesis are, in sequence,
leading element, controller and actuator. Actuators are hypothesised to affect
in turn the final outcome either directly or via penultimate outcomes. The
existence of the control system is evidenced by demonstration of statistically
significant one way Granger causality across each step of the hypothesised
control system sequence. Evidence is presented that the leading element of the
control system, represented by the Normalized Difference Vegetation Index, is
the global biosphere. The biosphere as a control system has previously been
referred to as Gaia (Lovelock and Margulis, 1974). A fourth (second derivative)
term is found to enhance the Proportional, Integral and Derivative process
terms of the control system shown in Leggett and Ball (2020). The main
actuators of the control system found are shown to be wind speed and cloud
cover. Cloud cover is shown to influence the final outcome, global surface
temperature, directly. It and wind speed also influence the penultimate
outcomes found, those of enhanced ocean heat uptake and enhanced outgoing
longwave radiation. These together lead to control system output to the final
outcome, global atmospheric temperature. Overall, evidence for the activity of
the control system is present across many major physical dimensions of the
global atmosphere.
