[R-sig-Geo] Isolating only land areas on a global map for computing averages

[r@i@1290 m@iii@g oii @im@com]

Hi there,
I am interested in calculating precipitation medians globally. However, I only want to isolate land areas to compute the median. I already first created a raster stack, called "RCP1pctCO2median", which contains the median values. There are 138 layers, with each layer representing one year.  This raster stack has the following attributes:
class       : RasterStack 
dimensions  : 64, 128, 8192, 138  (nrow, ncol, ncell, nlayers)
resolution  : 2.8125, 2.789327  (x, y)
extent      : -181.4062, 178.5938, -89.25846, 89.25846  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
names       :    layer.1,    layer.2,    layer.3,    layer.4,    layer.5,    layer.6,    layer.7,    layer.8,    layer.9,   layer.10,   layer.11,   layer.12,   layer.13,   layer.14,   layer.15, ... 
min values  : 0.42964514, 0.43375653, 0.51749371, 0.50838983, 0.45366730, 0.53099146, 0.49757186, 0.45697752, 0.41382199, 0.46082401, 0.45516687, 0.51857087, 0.41005131, 0.45956529, 0.47497867, ... 
max values  :   96.30350,  104.08584,   88.92751,   97.49373,   89.57201,   90.58570,   86.67651,   88.33519,   96.94720,  101.58247,   96.07792,   93.21948,   99.59785,   94.26218,   90.62138, ...  

Previously, I was isolating a specific region by specifying a range of longitudes and latitudes to obtain the medians for that region, like this:
expansion1<-expand.grid(103:120, 3:15)lonlataaa<-extract(RCP1pctCO2Median, expansion1)Columnaaa<-colMeans(lonlataaa)

However, with this approach, too much water can mix with land areas, and if I narrow the latitude/longitude range on land, I might miss too much land to compute the median meaningfully.
Therefore, with this data, would it be possible to use an IF/ELSE statement to tell R that if the "center point" of each grid cell happens to fall on land, then it would be considered as land (i.e. that would be TRUE - if not, then FALSE)? Even if a grid cell happens to have water mixed with land, but the center point of the grid is on land, that would be considered land. But can R even tell what is land or water in this case?
Thank you, and I would greatly appreciate any assistance!

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