While the extent of sea ice covering the arctic varies every year depending on the weather, a general downward trend becomes visible when examining data on the extent of arctic sea ice in March since 1979. This is an effect of global climate change, which sees average and mean temperatures rise across the Earth, leading to decreasing extents and thickness of the ice covers around the poles.
March is the month where arctic sea is typically reaches its maximum extent, which is why it is referenced frequently in scientific studies and record-keeping, like the data from the National Oceanic and Atmospheric Administration used in the graphic.
Just like the arctic sea is, ice cover in the Antarctic is also decreasing on average. The same is true for snow cover on the poles, which has been decreasing on average since 1967, a trend that has been speeding up recently.