A new study affirms coral reefs grow when seas are warm and rising. Growth “shuts off” during colder, falling sea level periods. Consequently, the Great Barrier Reef has experienced growth in the last 150 years – especially in recent decades.
During the Last Glacial Maximum (~30,000 to 15,000 years ago), when Great Barrier Reef (GBR) sea surface temperatures (SSTs) were sometimes 3-5°C colder and sea levels plummeted to 120 m lower than today, reefs experienced at least 5 “near death events” (Yokoyama et al., 2019).
Image Source: Yokoyama et al., 2019
As SSTs warmed and sea levels rose during the post-glacial period, GBR growth was “vigorous” (Humblet et al., 2019). The health of coral communities is significantly determined by sea level changes, with growth associated with rising sea levels and growth shut downs occurring as sea levels fall.
Image Source: Humblet et al., 2019
Coral reefs have thrived (“switch-on” phases) during warm periods with rising sea levels such as the Holocene Thermal Maximum, the Medieval Warm Period, and Current Warm Period (Yan et al., 2019). Growth dramatically ceases (“switch-off” phases) during cold periods like the Dark Ages Cold Period and Little Ice Age.
Image Source: Yan et al., 2019
A new GBR study (Leonard et al., 2020) suggests this region enjoyed “~1-2 °C warmer than present” temperatures and 0.7 to 1.0 m higher sea levels at multiple intervals during the last 7,000 years.
Coral growth occurs during warmer, rising sea level periods. Periods when coral growth “turned-off” are “associated with a rapid relative sea level fall” and colder SSTs (i.e., the Little Ice Age). Consequently, the GBR has experienced growth in the last 150 years – especially since the 1970s.
“It is notable that reef accretion at Middle Island appears to have increased in the past ~150 years, especially since the late 1970’s to early 1980’s, coincident with the onset of increasing SSTs on the GBR.”