why the human contribution to greenhouse gases in our atmosphere is so small that it does not matter.
Most critics who are also climate scientists don't disagree with an anthropogenic factor in principle, but say that weak experimental results leave us currently unable to say what specific climate outcomes will or can be the result of specific human actions, to the point that there is not enough evidence that curbing absolute CO2 emissions alone would mitigate any observed trends (whether or not these trends should be interpreted as "catastrophic"). Another example:

A hypothesis to the effect that humans cause some warming, or
even that most current global warming is very likely to be anthropogenic, is not—and does
not necessarily imply—a hypothesis to the effect that current warming, if continued over
some unspecified period, might prove sufficiently damaging to justify any climate policy to
address climate change, still less any public support for it.