Meltwater Pulse 1A (MWP1a) is a term used by scientists who study Earth's history, climate, and oceans to describe a time period during the last ice age when global sea levels rose rapidly between 14,700 and 13,500 years ago. During this time, sea levels increased by about 16 to 25 meters (about 52 to 82 feet) over 400 to 500 years, which averages to about 40 to 60 millimeters (about 1.6 to 2.4 inches) per year. This event is also called Catastrophic Rise Event 1 (CRE1) in the Caribbean Sea.

The sea level rise during MWP1a is the fastest known rise after the last ice age. Scientists widely agree that MWP1a is the most well-known and least debated of the named rapid sea level rise events from the post-glacial period. Other similar events are named 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 scientists believe they were caused by large amounts of water from melting ice sheets flowing into the oceans.

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 took place between 13,500 and 14,700 years ago, with its strongest point around 13,800 years ago. The start of this meltwater event matches or happens just after the sudden beginning of the Bølling–Allerød (B-A) warm period and rising temperatures recorded in the NorthGRIP ice core in Greenland around 14,600 years ago. During MWP1a, sea levels are believed to have risen at a rate of 40–60 mm (0.13–0.20 ft) per year. This rate of sea level rise was much faster than today’s rate, which is estimated to be about 2–3 mm (0.0066–0.0098 ft) per year.