A simple forcing-feedback model of the effect of CO2 on temperatures implies that the natural logarithm of the concentration of CO2 in the atmosphere,
, and satellite-measured lower tropospheric temperatures,
, should have the same order of integration. Multiple tests provide strong evidence that
has a unit root (it is integrated of order 1). However, the hypothesis that
has a unit root is strongly rejected both for the globe as a whole and for a set of non-overlapping geographic subsets. A possible explanation is that CO2 accumulation is associated with some other trending process that moderates its effects on atmospheric temperatures in such a way as to prevent
from also having a unit root. The analysis also reveals interactions between temperature anomalies in the non-overlapping geographic subsets that further limit the range of models consistent with the evidence.
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