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Surface melting has been contributing to the surface lowering and loss of firn air content on Larsen C Ice Shelf since at least the mid‐1990s. Where the amount of melting and refreezing is significant, the firn can become impermeable and begin to support ponds of surface meltwater such as have been implicated in ice shelf collapse. Although meteorological station data indicated an increase in melt on the Antarctic Peninsula over the second half of the 20th century, the existing Ku‐band Quick Scatterometer (QuikSCAT) time series is too short (1999–2009) to detect any significant 21st century trends. Here we investigate a longer 21st century period by extending the time series to 2017 using the C‐band Advanced Scatterometer (ASCAT). We validate our recent observations with in situ weather station data and, using a firn percolation model, explore the sensitivity of scatterometry to water at varying depths in the firn. We find that active microwave C‐band (5.6‐cm wavelength) instruments can detect water at depths of up to 0.75 m below a frozen firn layer. Our longer scatterometry time series reveals that Larsen C Ice Shelf has experienced a decrease in melt season length of 1–2 days per year over the past 18 years consistent with decreasing summer air temperatures. Only in western inlets, where föhn winds drive melt, has the annual melt duration increased during this period. Plain Language Summary: Antarctic ice shelves form where ice flows from the land and goes afloat on the sea. In recent decades, ice shelves along the Antarctic Peninsula have been disintegrating. Loss of an ice shelf allows faster flow of the land‐based ice to the oceans and adds to sea‐level rise. One possible cause of ice shelf breakup is increased surface melting; it is therefore important to monitor melt and the best way to do this is from space. We can detect melt from space using microwaves which are scattered back to the spaceborne instrument from the surface. A wet snow surface produces much lower backscatter than a dry one. We investigate trends in the number of days per year when Larsen C Ice Shelf on the Antarctic Peninsula experiences surface melting, using data from a new microwave instrument. On most of the ice shelf, the number of melt days each year has fallen by 1 or 2 days per year since 1999, consistent with decreasing summer air temperatures in this region. However, close to the mountains where the ice shelf is formed the number of melt days is increasing. These locations are where mountain winds known as föhn produce localized increases in surface temperatures. Key Points: We present the first use of enhanced resolution Advanced Scatterometer (ASCAT) data to identify melt on an Antarctic ice shelfThe analysis extends the Quick Scatterometer (QuikSCAT) melt record on Larsen C Ice Shelf from 1999–2009 to 20171999 to 2017 open‐shelf melt decreased by 1 or 2 days/year2, while western inlets saw a similar rate of melt increase [ABSTRACT FROM AUTHOR]