Energy released directly from human activity, such as the heat released from decomposing landfills, can and does cause local warming. Global warming, however, is not a result of direct energy transfer from human activity; we do release a lot of energy, true, but the planet is a huge, open system, and we just don't have enough impact to do that. Your kitchen appliances might heat your home a few degrees, but you'll stop feeling them the instant you step outside.
To heat the whole atmosphere, it takes a really huge transfer of energy taking place over the entire planet or at least a really big part of the planet. There's only three really huge energy transfers that might do it.
First, the transfer of energy from the sun to the earth. We can see a change in this after major volcanic events; the dust reduces the energy earth receives for a year or so, and, indeed, global atmospheric temperatures take a temporary cooling dip. But other than volcanoes, solar input hasn't changed much in the last century, so it doesn't explain why global warming is happening.
Second, the transfer of heat to and from the ocean during the southern oscillation. This cycles back and forth, and balances out in periods longer than a few years, so while it might explain why 2012 is hotter than 2011, it doesn't explain why long-term global warming is happening.
Third, the transfer of energy from the earth into cold, dark space. This transfer is slowed down by greenhouse gases, and greenhouse gas concentrations have been steadily rising every year for decades. This is the change that most likely explains why global warming is happening.
So, knowing now that it's about greenhouse gases and not direct energy transfer, we're prepared to compare nuclear to the other options. Nuclear power does involve some indirect greenhouse gas emissions, construction, mining, transportation, etc., but they are relatively small. One estimate puts nuclear power's total greenhouse gas emissions, including indirect emissions, at 66 grams of carbon dioxide equivalent per kilowatt hour, or 66 gCO₂e/kWh. This is far below coal power's total emissions (960 gCO₂e/kWh), but higher than solar (32 gCO₂e/kWh) and wind (10 gCO₂e/kWh). Unlike solar and wind, however, nuclear can provide a consistent level of power without a form of battery storage, so makes a relatively easy build-one-plant tear-down-the-other replacement for coal.