Global Warming and Sea Level Change


THERE ARE AREAS OF THE WORLD
THAT ARE VERY FLAT, WHERE A CHANGE IN SEA LEVEL OF 50
CENTIMETERS WOULD MAKE A BIG DIFFERENCE. AREAS WITH STEEP COASTS WHERE
YOU WOULDN’T BE SO CONCERNED. BUT IF YOU OWNED A BEACH LOFT
WITH A SLOPE OF ONE IN 1000, THEN FOR EVERY FOOT SEA LEVEL
GOES UP, YOU LOSE 1,000 FEET OF HORIZONTALLY. YOU WOULDN’T LIKE IT. AND SO THERE ARE CERTAINLY
EFFECTS OF THIS KIND. I HAVE BEEN WORKING
ON THAT PROBLEM. I’M MORE THAN INTERESTED IN IT
IN THE LAST YEAR, AND I’D LIKE TO SAY FEW WORDS ABOUT IT. SEA LEVEL RESPONDS TO GLOBAL
WARMING IN TWO DISTINCTLY DIFFERENT WAYS. ONE IS IF THE WATER GETS WARMER,
THERE’S A THERMAL EXPANSION AND THE
SEA LEVEL RISES. AND THAT, BY THE WAY, MAY BE
ANOTHER GOOD REASON WHY YOU WANT TO KNOW WHETHER THE OCEAN
IS WARMING THAT I FORGOT TO MENTION WHEN YOU ASKED THE
QUESTION PREVIOUSLY. THE SECOND IS THAT, AS THE
ATMOSPHERE GETS WARMER, GLACIERS AND ICE CAPS MELT. THE TWO ARE CONTRIBUTORS
TO SEA LEVEL. AN INTERESTING QUESTION IS TO
WHAT EXTENT IS ONE OR THE OTHER IMPORTANT? IT WAS THOUGHT UNTIL VERY, VERY
RECENTLY THAT THERMAL EXPANSION WAS THE ESSENTIAL
PROBLEM, UNTIL, FOR THE FIRST TIME, THERE WERE ENOUGH
MEASUREMENTS MADE THAT WE COULD ACTUALLY KNOW HOW MUCH
THE OCEAN HAS WARMED. AND THAT, FOR THE 20TH CENTURY,
OR LET’S SAY FOR THE LAST 50 YEARS, GAVE ONLY THREE
CENTIMETERS, ONLY A LITTLE MORE THAN AN INCH. THE GENERAL IDEA OF WHAT SEA
LEVEL IS DOING IS TWO MILLIMETERS PER YEAR, OR 20
CENTIMETERS, TWO-THIRDS OF A FOOT PER CENTURY. THE FACT THAT THE WARMING HAD
ONLY CONTRIBUTED THREE CENTIMETERS, A SMALL AMOUNT,
CAME AS A DISTINCT SURPRISE. THE OTHER PART, THEN, COULD THE
REMAINING 17 CENTIMETERS OF SEA LEVEL RISE BE
DUE TO MELTING? IT TURNS OUT TO BE VERY, VERY
DIFFICULT TO ACCEPT. THE ANTARCTIC, IN FACT, APPEARS
TO BE INCREASING ITS TOTAL SNOW MASS, MUCH TO
PEOPLE’S AMAZEMENT. AND THAT APPEARS TO HAPPEN
BECAUSE GLOBAL WARMING, IF YOU WISH, PUTS MORE MOISTURE IN THE
AIR AND MORE SNOWFALL, AND THERE IS A FEEDBACK THAT
IS UNEXPECTED. GREENLAND SEEMS TO BE
MELTING SOMEWHAT. EUROPEAN GLACIERS AND AMERICAN
GLACIERS HAVE CERTAINLY LOST A LOT OF WATER, BUT NOT ENOUGH
TO ACCOUNT FOR WHAT HAS BEEN OBSERVED. AND I CLAIM THAT, AT THIS
TIME, THE SEA LEVEL PROBLEM IS AN ENIGMA. WE DON’T UNDERSTAND IT. THERE’S A BIG DIFFERENCE BETWEEN
WHAT WE THINK HAS HAPPENED AND WHAT WE THINK
WE CAN EXPLAIN. I DO HAVE TO ADD, ESPECIALLY FOR
THE BENEFIT OF STUDENTS, THAT MELTING OR FLOATING ICE
DOES NOT CHANGE SEA LEVEL. THERE IS SOMETHING CALLED
ARCHIMEDES’ PRINCIPLE. IF YOU PUT AN ICE CUBE FLOATING
IN YOUR GLASS, AND LET IT MELT, THE WATER LEVEL
IN THE GLASS WILL NOT RISE, BECAUSE THE ICE, BEING LIGHTER,
DISPLACES JUST THE SAME AMOUNT OF WATER THAT IT
WILL OCCUPY AFTER IT’S MELTED. AND SO THE REDUCTION OF FLOATING
ICE IN THE ARCTIC, WHICH HAS CERTAINLY TAKEN PLACE
AND WHICH WE MAY WANT TO TALK ABOUT– IT’S
VERY DRAMATIC– THAT AMOUNT OF MELTING
DOES NOT CONTRIBUTE TO SEA LEVEL RISE. THE THING THAT DOES CONTRIBUTE
IS MELTING OF LAND-BASED ICE MASSES.

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