You serious? I really feel like I shouldn't even have to explain this.
http://www.nucleardarkness.org/nucle...uclearweapons/
Here's just a climate effect and yes it can have a global impact. Yes those are based on a nuclear war scenario but even one weapon going off can be still have global consequences.
What are the effects on climate?
Massive absorption of warming sunlight by a global smoke layer would cause Ice Age temperatures on Earth. NASA computer models predict 40% of the smoke would stay in the stratosphere for 10 years. There the smoke would also destroy much of the protective ozone layer and allow dangerous amounts of UV light to reach the Earth's surface.
Half of 1% of the explosive power of the deployed nuclear arsenal can create nuclear darkness. 100 Hiroshima-size weapons exploded in the large cities of India and Pakistan would put 5 million tons of smoke in the stratosphere and drop average global temperatures to Little Ice Age levels. Shortened growing seasons could cause up to 1 billion people to starve to death.
A large nuclear war could put 150 million tons of smoke in the stratosphere and make global temperatures colder than they were 18,000 years ago during the coldest part of the last Ice Age. Killing frosts would occur every day for 1-3 years in the large agricultural regions of the Northern Hemisphere. Average global precipitation would be reduced by 45%. Earth’s ozone layer would be decimated. Growing seasons would be eliminated.
A large nuclear war would utterly devastate the environment and cause most people to starve to death. Already stressed ecosystems would collapse. Deadly climate change, radioactive fallout and toxic pollution would cause a mass extinction event, eliminating humans and most complex forms of life on Earth.