Is there any chance that the Earth might be warming due to slight variations
In the trajectory of the Earth's orbit around the Sun? Maybe the Earth is
slightly closer? What about the chance that the Sun might be brighter than it has been?
Is Venice sinking or the sea rising?
The first question: variations in Earth's orbital shape around the sun (eccentricity), changes in the Earth's tilt, and precession of the equinoxes all are considered as contributors to long-term climate change on the Earth (called Milankovitch Cycles).
A good review of the Milankovitch Cycles is available at: http://en.wikipedia.org/wiki/Milankovitch_cycle.
These "cycles" are generally agreed to serve as key drivers in the advance and retreat of the ice sheets (Ice Ages) and the primary climate drivers of the past half-million years or more. But the general consensus is that these cycles run on too long a time-scale (10,000 to 120,000 years) to account for the recent 100-200 years of "significant" warming -- the warming has been too large (even at just 1.0-1.5F) to be a function of these cycles.
Solar irradiance (and related features like "brightness", "flares", "sunspots") appear to be elevated compared to past centuries (there is a sunspot record that extends back at least 400-500 years), but we must acknowledge that the data prior to about the 20th Century (and probably all data prior to the mid-20th Century) are of somewhat questionable quality.
Recent work by a number of research groups suggests that the energy variations of these solar events alone (likely less than 1-2%) would not account for all of the "observed" warming. However, there is some new work suggesting that there may be some significant "positive" feedback processes related to incoming solar energy.
For example, some fairly recent (and highly controversial) work suggests that elevated incoming sunshine may reduce, rather than increase, cloud cover. Less clouds would mean even MORE incoming solar energy ... a "positive" feedback that would produce MORE warming than just the 1-2% bump alone.
Also, several reports including one by NASA shows that the aerosol content (dust particles, water droplets, etc.) of the atmosphere has been falling since the early 1990s (after the explosion of volcano Mt. Pinatubo) ... less aerosols means more sunshine reaching the surface. Some scientists are suggesting that this is a large contributor to the fact that something like 11 of the last 12 years rank among the very warmest on record! More work needs to be done here ...
Finally, the Venice question: my understanding is that Venice is not too unlike south Louisiana. Both are sinking while the oceans are indeed rising. That results in a higher "apparent sea-level rise" than the actual "absolute sea-level rise."
"Apparent" sea rise indicates the compounded effect of sinking land in a place where waters are indeed rising VERY slowly.
My understanding is that south Louisiana, for example, has one of the highest, if not the highest, rates of "apparent sea-level rise" in North America. Sea-level rise is a function of two processes: (1) more water in the ocean as the ice caps and glaciers of the world melt, and (2) thermal expansion of water -- the same amount of water takes up more space because like most objects, water expands when heated.