40
another scientist’s data) and look at year-on-year trends there is a clear and systematic relative sea-level rise at an average rate of ~4.4 mm/yr. (1.43 ft. in a century).
There are, broadly speaking, three processes that explain this long-term trend: (1)
expansion of water already in the ocean as it warms up; (2) additional water being
transferred to the ocean as ice on land melts; and (3) downward vertical motion of
the land (subsidence). The density of seawater is controlled mainly by temperature
with a smaller influence from salinity. Since the oceans absorb heat from the atmosphere, when the atmosphere becomes warmer so will the oceans, which causes
relative sea-level rise because the ocean water expands. Indeed, this thermal expansion of existing ocean water has been the main cause of global sea-level rise for the
75–100 years since the start of the Industrial Revolution in the mid-nineteenth
century.
A mass of water can only be in one place at a time. The hydrological cycle moves
water from one reservoir (in a figurative, not literal sense of the word) to another.
For example, energy from the Sun arriving on Earth at the ocean surface (as most of
it does on our Blue Planet), is put to work evaporating water from the ocean (a reservoir) into the atmosphere (another reservoir). At some point down the line, this
water condenses and falls back to the surface as precipitation. Over the course of a
year and averaged out over the globe (since precipitation often falls somewhere
other than where the water was originally evaporated from), the volume of water in
the ocean is about constant. However, if conditions permit, there can be a sustained
and pronounced imbalance in the hydrological cycle. If the precipitation falls as
snow that doesn’t melt in the summer, then over time ice accumulates into a glacier
or ice sheet and the amount of water in the ocean decreases resulting in a sea-level
fall. For context, enough water was removed from the global ocean in this fashion
between approximately 125,000 and 25,000 years ago that global sea level fell by
around 130 m (425 ft). In North America, the Laurentide Ice Sheet covered much of
Canada and contained up to 2.65 × 10
7
km of ice, that was up to 2 miles (3.2 km)
thick in some places. This disequilibrium also runs in the opposite direction, when
the meltwater from shrinking ice masses on land runs off the landscape and into the
ocean, which raises relative sea level. This process happens faster than the ice
growth, such that the 130 m of water stored on land as ice returned to the ocean in
about 20,000 years. In recent years our glaciers and ice sheets have begun to melt at
an accelerating rate and are once again delivering water to the oceans. Between
1992 and 2017, an estimated three trillion tons of ice melted from Antarctica, which
is equivalent to almost 1 cm of sea-level rise around the world. Changing the volume and mass of the ocean causes the sea-surface to rise even if the land beneath the
dock is stable and our observer on the dock experiences relative sea-level rise on
timescales of a few decades or longer.
Once we begin to think on century and millennial timescales, our assumptions of
a stable land surface rapidly become untenable; the surface on which our dock is
built is in motion and able to move up and down. In the case of North Carolina specifically and the U.S. Atlantic coast more widely, a key process driving this relative
sea-level change is called glacio-isostatic adjustment (GIA). Over the past few
thousand years GIA drove subsidence and therefore relative sea-level rise at the
A. C. Kemp and B. P. Horton
another scientist’s data) and look at year-on-year trends there is a clear and systematic relative sea-level rise at an average rate of ~4.4 mm/yr. (1.43 ft. in a century).
There are, broadly speaking, three processes that explain this long-term trend: (1)
expansion of water already in the ocean as it warms up; (2) additional water being
transferred to the ocean as ice on land melts; and (3) downward vertical motion of
the land (subsidence). The density of seawater is controlled mainly by temperature
with a smaller influence from salinity. Since the oceans absorb heat from the atmosphere, when the atmosphere becomes warmer so will the oceans, which causes
relative sea-level rise because the ocean water expands. Indeed, this thermal expansion of existing ocean water has been the main cause of global sea-level rise for the
75–100 years since the start of the Industrial Revolution in the mid-nineteenth
century.
A mass of water can only be in one place at a time. The hydrological cycle moves
water from one reservoir (in a figurative, not literal sense of the word) to another.
For example, energy from the Sun arriving on Earth at the ocean surface (as most of
it does on our Blue Planet), is put to work evaporating water from the ocean (a reservoir) into the atmosphere (another reservoir). At some point down the line, this
water condenses and falls back to the surface as precipitation. Over the course of a
year and averaged out over the globe (since precipitation often falls somewhere
other than where the water was originally evaporated from), the volume of water in
the ocean is about constant. However, if conditions permit, there can be a sustained
and pronounced imbalance in the hydrological cycle. If the precipitation falls as
snow that doesn’t melt in the summer, then over time ice accumulates into a glacier
or ice sheet and the amount of water in the ocean decreases resulting in a sea-level
fall. For context, enough water was removed from the global ocean in this fashion
between approximately 125,000 and 25,000 years ago that global sea level fell by
around 130 m (425 ft). In North America, the Laurentide Ice Sheet covered much of
Canada and contained up to 2.65 × 10
7
km of ice, that was up to 2 miles (3.2 km)
thick in some places. This disequilibrium also runs in the opposite direction, when
the meltwater from shrinking ice masses on land runs off the landscape and into the
ocean, which raises relative sea level. This process happens faster than the ice
growth, such that the 130 m of water stored on land as ice returned to the ocean in
about 20,000 years. In recent years our glaciers and ice sheets have begun to melt at
an accelerating rate and are once again delivering water to the oceans. Between
1992 and 2017, an estimated three trillion tons of ice melted from Antarctica, which
is equivalent to almost 1 cm of sea-level rise around the world. Changing the volume and mass of the ocean causes the sea-surface to rise even if the land beneath the
dock is stable and our observer on the dock experiences relative sea-level rise on
timescales of a few decades or longer.
Once we begin to think on century and millennial timescales, our assumptions of
a stable land surface rapidly become untenable; the surface on which our dock is
built is in motion and able to move up and down. In the case of North Carolina specifically and the U.S. Atlantic coast more widely, a key process driving this relative
sea-level change is called glacio-isostatic adjustment (GIA). Over the past few
thousand years GIA drove subsidence and therefore relative sea-level rise at the
A. C. Kemp and B. P. Horton