51
that we are only seeing the effects of longer-lived processes such as changing the
mass and volume of water in the ocean and the vertical motion of the land. In effect
we have managed to install someone on the dock for thousands of years, but they
were only inclined to report their observations after first averaging them over
decades. At first this might seem disappointing news, the annual tide-gauge data for
example provides a rich and detailed history of coastal change, but it is actually a
good thing. By smoothing away the high frequency processes, we are left with the
relative sea-level changes caused by processes that occurred because of significant
changes in conditions in North Carolina and the wider North Atlantic Ocean that
will be key drivers of future change.
On first appraisal, the North Carolina reconstruction (Fig. 3.3e) seems quite
straight forward, relative sea level rose continuously and at a steady rate, with some
seemingly subtle changes imposed on top of this long-term trend. The long-term
linear trend occurred because of GIA as Earth continued to respond to the most
recent deglaciation of North America and therefore the majority of relative sea-level
change had nothing to do with climate in the direct sense of the relationship. Since
we are confident that we know the rate of GIA, it can be removed from the relative
sea-level curve to leave us only with the parts caused by climate-driven processes
acting on decadal to centennial timescales (Fig. 3.3f). This so-called “detrended”
sea-level record shows some interesting features. There is a period of stability followed by a hump and then a marked rise beginning in the late 1800s. Let us first
tackle the recent acceleration because it is the most obvious feature in North
Carolina and also shows up time and time again in similar records produced elsewhere. Put simply, this is the onset of modern sea-level rise caused initially by
warming of the oceans and more recently by melting of ice. It is this acceleration of
global sea-level rise that has scientists and others concerned for the future of our
coasts because the rate of rise is predicted to accelerate, resulting in more and more
relative sea-level rise unless carbon dioxide emissions are curtailed. The earlier
bumps and wiggles are intriguing features, they probably represent periods of time
where changes in ocean and atmospheric circulation that were sustained for several
hundred years pushed water onto the coast of North Carolina and then allowed it to
return to the open ocean.
What Do We Know?
Relative sea level is what a passive observer (sitting, for example, on a dock or a
similar perch; Fig. 3.12) would observe and is the net outcome of a rich tapestry of
physical processes that cause the ocean and/or land surface to move up or down
depending on the time and place being considered. On timescales of hours to a few
years, the land surface is effectively static and relative sea level changes in response
to predictable astronomical tides and the effects of winds and ocean currents. On
timescales of a few decades to millennia, there can be large relative sea level changes
caused by vertical land motion. In North Carolina the land surface is subsiding
3 Time and Tide Wait for No Man
that we are only seeing the effects of longer-lived processes such as changing the
mass and volume of water in the ocean and the vertical motion of the land. In effect
we have managed to install someone on the dock for thousands of years, but they
were only inclined to report their observations after first averaging them over
decades. At first this might seem disappointing news, the annual tide-gauge data for
example provides a rich and detailed history of coastal change, but it is actually a
good thing. By smoothing away the high frequency processes, we are left with the
relative sea-level changes caused by processes that occurred because of significant
changes in conditions in North Carolina and the wider North Atlantic Ocean that
will be key drivers of future change.
On first appraisal, the North Carolina reconstruction (Fig. 3.3e) seems quite
straight forward, relative sea level rose continuously and at a steady rate, with some
seemingly subtle changes imposed on top of this long-term trend. The long-term
linear trend occurred because of GIA as Earth continued to respond to the most
recent deglaciation of North America and therefore the majority of relative sea-level
change had nothing to do with climate in the direct sense of the relationship. Since
we are confident that we know the rate of GIA, it can be removed from the relative
sea-level curve to leave us only with the parts caused by climate-driven processes
acting on decadal to centennial timescales (Fig. 3.3f). This so-called “detrended”
sea-level record shows some interesting features. There is a period of stability followed by a hump and then a marked rise beginning in the late 1800s. Let us first
tackle the recent acceleration because it is the most obvious feature in North
Carolina and also shows up time and time again in similar records produced elsewhere. Put simply, this is the onset of modern sea-level rise caused initially by
warming of the oceans and more recently by melting of ice. It is this acceleration of
global sea-level rise that has scientists and others concerned for the future of our
coasts because the rate of rise is predicted to accelerate, resulting in more and more
relative sea-level rise unless carbon dioxide emissions are curtailed. The earlier
bumps and wiggles are intriguing features, they probably represent periods of time
where changes in ocean and atmospheric circulation that were sustained for several
hundred years pushed water onto the coast of North Carolina and then allowed it to
return to the open ocean.
What Do We Know?
Relative sea level is what a passive observer (sitting, for example, on a dock or a
similar perch; Fig. 3.12) would observe and is the net outcome of a rich tapestry of
physical processes that cause the ocean and/or land surface to move up or down
depending on the time and place being considered. On timescales of hours to a few
years, the land surface is effectively static and relative sea level changes in response
to predictable astronomical tides and the effects of winds and ocean currents. On
timescales of a few decades to millennia, there can be large relative sea level changes
caused by vertical land motion. In North Carolina the land surface is subsiding
3 Time and Tide Wait for No Man