The thinning of the arctic ice cap is taken as strong evidence for and need for concern about global warming. Although knowledgable people understand that the melting of the floating ice of the arctic will not affect the level of the ocean there are people who erroneously tie the possible melting of the arctic ice cap to the flooding of coastal areas around the world.

There are two measures of the extent of the polar ice caps and both have relevance. One is the area of the ice caps and the other is the volume. For concern about the the volume of fresh water being added to the world's oceans the volume of the ice caps is the pertinent measure. For concern about the amount of solar radiation reflected back into space the area of the ice caps is the relevant measure.

The trends in the area and volume of an ice cap may not be the same. For example, the edges of the Greenland ice cap may be melting yet the thickness of the ice cap in interior of the island may be increasing and thus possibly increasing the volume. The altitude of the ice cap in the interior of the island is about ten thousand feet. Thus global warming that increases the humidity of the atmosphere may lead to increased percipitation in a region well below freezing. Thus global warming could increase the thickness of the ice cap and its volume while at the same time melting the thinner edges.

But the uncertainties about what is happening to the polar ice caps is not just a matter of area changes versus volume changes. Below are shown the statistics for the areas of the arctic and the antarctic ice caps.

While there is a decline in the Arctic ice there is an increase in the Antarctic ice. The sum is shown below.

The National Oceanic and Atmosphere Administration (NOAA) also provides data from satellites on the sea ice. This time series is shown below:

Over the thirty year period there is slight downward trend for Arctic sea ice and a slight upward trend for Antarctic sea ice, for a very slight downward trend for the to total sea ice. For the more recent years the trend is downward for both regions but extrapolating from a five year period is of doubtful validity. The amount of sea ice is the cumulative sum of the annual changes. The statisitcs for such variables always appear to have trends even when there is no underlying trend. For such statistics it is more appropriate to look at the first differences, the annual changes. These statistics for the total sea ice are shown below.

In this display there is no indication that there was any significant shift in the structure of the variable in recent years. For more on the statistical problem see Statistics.

The preceding pertains only to the sea ice. There is a good chance that what is happening to the sea ice in each polar region is also occurring for the total ice in the regions. Most of the arctic ice is floating and its melting would not add to the volume of the oceans whereas most of the antarctic ice is resting on land its increase would deduct water from the oceans.