Welcome back to ESA’s Earth from Space programme.
Today, all five of the Great Lakes are featured in this spectacular image
captured by the Copernicus Sentinel-3 mission.
The Great Lakes are a chain of deep, freshwater lakes in North America.
They are, from west to east,
Lake Superior,
Lake Michigan,
Lake Huron,
Lake Erie
and Lake Ontario.
With a combined area of around 244 000 sq km,
the lakes represent the largest surface of freshwater in the world
covering an area exceeding that of the United Kingdom.
Around 100 000 years ago,
a major ice sheet formed over most of Canada and part of the US.
As it formed,
giant glaciers flowed into the land
carving out valleys and levelling mountains.
As higher temperatures began to melt the ice sheet,
meltwater filled the holes left by the glaciers.
Many of these holes today still contain water
and formed the thousands of lakes across central USA and Canada.
The biggest remnants of this process are the Great Lakes.
Lake Superior is the largest and deepest of the Great Lakes.
It drains into Lake Huron via the St. Marys River
at an average rate of 2000 cubic metres per second.
Lake Michigan lies south of Lake Superior.
It connects with Lake Huron
through the six km-wide channel
of the Straits of Mackinac in the north.
Lake Huron is the second largest of the Great Lakes
and is bounded by Michigan to the north
and by Ontario to the east.
Lake Erie, visible in the bottom,
is the shallowest and southernmost of the Great Lakes.
Green algal blooms are very visible in the lake.
These toxic blooms have been a major problem in recent years.
This image of the algal blooms,
captured by the Copernicus Sentinel-2 mission on the same day,
allows us to have a closer look at the blooms.
Caused by heightened levels of phosphorus,
found in fertilisers and common household products
finding its way into the water,
these blooms have caused harm to the lake’s fish population.
Lake Ontario is the easternmost of the Great Lakes
and also the smallest in terms of surface area.
It is bounded on the north by Ontario
and on the south by New York,
whose water boundaries meet in the middle of the lake.
In this image, captured in March 2020,
a large quantity of ice and snow coverage is visible north of the lakes,
yet the amount of ice cover on the lakes is minimal
which is extremely unusual for the ice season.
The image here shows the same view of the Great Lakes in March 2019,
exactly one year prior,
where there is significantly more ice in comparison.
Ice cover on the Great Lakes can fluctuate dramatically from year to year,
depending on several patterns of climate variability.
Years with lower-than-normal ice cover
appear to have become more frequent during the past two decades.
The data suggests that 1979 was the year with the highest ice coverage,
while 2002 had the lowest ice coverage
with just 11.8%
which means that the Great Lakes were almost ice-free for the entire year.
As Earth’s climate changes,
warmer air and water temperatures have led to less ice cover on many lakes in North America
with the Great Lakes being just a few of them.
Sentinel-3 is a two-satellite mission
to supply the coverage and data delivery needed for Europe’s Copernicus programme.
Each satellite’s instrument package
includes an optical sensor
to monitor changes in the colour of Earth’s surfaces.
It can be used,
for example,
to monitor ocean biology
and water quality.