I’m sorry, but if this “scientist” says that CO2 has no effect on climate, then he can’t be a “leading” scientist in the field.
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He doesn’t say no effect, just negligible effect.
When Carbon dioxide is only four one thousandths of one percent of the other chemicals that make up our air, the the effect is only negligible. .004%
The idea that Carbon Dioxide has any meaningful effect on our climate is erroneous. If Carbon Dioxide has any effect then it is because plants require less water when there is more Carbon Dioxide for them. Plants then have an effect on climate. Think what might happen if the Sahara Desert turns green for instance. While you may not have heard it reported the Sahara is in fact greening. While it is not green the last decade has seen remarkable greening in the Sahara.
Geologist have found riverbeds in the Sahara.