1. Accurate data on weather patterns and climate change has only been collected for less than 200 years, while the Earth has "climate" since it has had an oxygen enriched atmosphere, somewhere between 2 and 3 billion years. For any "scientist" to assert that they fully understand mankinds impact on climate is ludicrous.
2. Archaeological and geological evidence suggests that ever since the Earth has had "climate" (atmosphere), it has been constantly changing, without the effects of mankind.
3. To think that man's impact on climate is greater than natural forces (the Sun, galactic cosmic ray flux, planetary orbital mechanics and naturally occurring terrestrial events) is only a testament to mankind's extreme vanity.