Trump decided to withdraw from the Paris Agreement because he understood its implications - a racheting commitment that will undermine American self-determination and prosperity. He was advised that even though America is the one country that can probably meet its commitments (ironically because of its move to fracked natural gas instead of coal to generate power), the legal and constitutional risks of staying in the agreement far outweighed the diplomatic benefits of staying in it. If you want to understand more about why this is the case, listen to James Delingpole's interview of constitutional lawyer, Chris Horner, here).
It is interesting that just as the climate change lobby works itself into a lather about Trump's announcement, new scientific studies have appeared that cast more doubts on the basic AGW theory. The first of these studies (see published paper here) found that almost all of the warming in recent years in datasets published by leading climate research organisations such as NOAA and the UK MetOffice is due to adjustments to the raw temperature readings and that these almost exclusively made recent temperatures warmer. One would expect the adjustments, if statistically sound, to produce as many decreases as increases and, in fact, there is a strong argument for net reductions in recent readings to account for heat island effects from modern infrastructure.
The second study (see published paper here) is more of a bombshell, claiming that the entire physics of global-warming theory – the assumption that greenhouse gases warm the atmosphere by trapping outgoing heat – is wrong, and that the Earth's greenhouse effect is actually due to the intensity of the Sun's radiation combined with surface atmospheric pressure. The scientists and the publication seem credible, and even if you rightly take a sceptical viewpoint (as I always do on AGW matters irrespective of whether it is towards claims that support or contradict my existing views), it is further evidence that there is not an overwhelming scientific consensus on AGW theory.
I believe that mankind has an impact on the climate. I believe that carbon emissions from fossil fuels contribute to this impact. However, even without the new studies, there is strong evidence that the AGW effect is small and that the effects of increases in atmospheric carbon dioxide and modest global warming are likely to be beneficial overall to both humans and other living things. I also believe that mankind's best defence against any adverse climatic events is prosperity - and the best way of ensuring prosperity is free enterprise in a deregulated economy, which is the exact opposite of what those promoting solutions for AGW (such as the Paris Agreement) want to achieve.