Option 1: Increased frequency of hurricanes is a likely effect of global warming. Warmer temperatures in the oceans provide more energy for hurricanes to form and intensify.

Option 2: Loss of fertile delta region for agriculture is less likely to be an effect of global warming. Global warming may impact agricultural productivity, but the direct loss of fertile delta regions is more related to factors like erosion, damming of rivers, and land-use changes.

Option 3: Decreased rate of photosynthesis in vegetation is a likely effect of global warming. Higher temperatures can lead to decreased photosynthesis and plant growth.

Option 4: Shrinking of the polar ice regions is a likely effect of global warming. Rising temperatures cause polar ice to melt, leading to the loss of ice sheets, glaciers, and sea ice.

In summary, option 2 is least likely to be an effect of global warming.