Meltwater Pulse 1A (MWP1a) is a term used by scientists, such as geologists, paleoclimatologists, and oceanographers, to describe a time period during the Quaternary era when sea levels rose rapidly after the last ice age. This event occurred between 14,700 and 13,500 years ago. During this time, global sea levels increased by 16 to 25 meters (52 to 82 feet) over about 400 to 500 years. This means the average rate of sea level rise was about 40 to 60 millimeters (1.6 to 2.4 inches) per year. In the Caribbean Sea, this event is also called Catastrophic Rise Event 1 (CRE1).

The sea level rise during MWP1a was the fastest recorded rise after the last ice age. MWP1a is the most widely accepted and least debated of the named post-glacial meltwater pulses. Other named meltwater pulses include Meltwater Pulse 1A0 (Meltwater Pulse 19ka), Meltwater Pulse 1B, Meltwater Pulse 1C, Meltwater Pulse 1D, and Meltwater Pulse 2. These events are called meltwater pulses because they are believed to have been caused by the sudden release of large amounts of meltwater into the oceans from the collapse of continental ice sheets.

Sea level and timing

MWP1a happened during a time of rising sea levels and quick climate changes at the end of the Last Glacial Period, called Termination I. Scientists have determined that this event occurred between 13,500 and 14,700 years ago, with its strongest point around 13,800 years ago. The beginning of this meltwater event matches or follows closely the sudden start of the Bølling–Allerød (B-A) interstadial and warming recorded in the NorthGRIP ice core in Greenland around 14,600 years ago. During MWP1a, sea level is estimated to have risen at a rate of 40 to 60 millimeters (0.13 to 0.20 feet) each year. This rate of sea level rise was much faster than today’s rate, which is estimated to be about 2 to 3 millimeters (0.0066 to 0.0098 feet) each year.