When will the next ice age happen? – Lorraine Lisiecki

Twenty thousand years ago, the Earth was a frigid landscape where
woolly mammoths roamed. Huge ice sheets,
several thousand meters thick, encased parts of North America,
Asia, and Europe. We commonly know it as the “Ice Age.” But geologists call it
the Last Glacial Maximum. That’s because it’s the most recent
time that ice reached such a huge extent, and “ice age” is an informal term
without a single agreed-upon definition. Over the last million years, there have actually been
about 10 different glacial maxima. Throughout Earth’s history,
climate has varied greatly. For hundreds of millions of years, the planet had no polar ice caps. Without this ice,
the sea level was 70 meters higher. At the other extreme,
about 700 million years ago, Earth became almost
entirely covered in ice during an event known
as “Snowball Earth.” So what causes these massive swings
in the planet’s climate? One of the main drivers
is atmospheric carbon dioxide, a greenhouse gas that traps heat. Natural processes, such as volcanism, chemical weathering of rocks, and the burial of organic matter, can cause huge changes in carbon dioxide
when they continue for millions of years. Over the past million years,
carbon dioxide has been relatively low, and repeated glacial maxima have been caused by cycles
in Earth’s movement around the sun. As Earth rotates, it wobbles on its axis
and its tilt changes, altering the amount of sunlight that
strikes different parts of its surface. These wobbles, combined
with the planet’s elliptical orbit, cause summer temperatures to vary depending on whether the summer solstice
happens when Earth is closer or farther from the sun. Approximately every 100,000 years, these factors align to create dramatically
colder conditions that last for millennia. Cool summers that aren’t warm enough
to melt the preceding winter’s snow allow ice to accumulate year after year. These ice sheets produce
additional cooling by reflecting more solar energy
back into space. Simultaneously, cooler conditions
transfer carbon dioxide from the atmosphere into the ocean, causing even more cooling
and glacier expansion. About 20,000 years ago, these trends reversed when changes
in Earth’s orbit increased summer sunshine over the giant ice sheets,
and they began to melt. The sea level rose 130 meters and carbon dioxide was released
from the ocean back into the atmosphere. By analyzing pollen and marine fossils, geologists can tell that temperatures
peaked about 6,000 years ago, before another shift in Earth’s orbit
caused renewed cooling. So what’s coming next? Based on the repeated natural cycle seen
in the climate record, we’d normally expect the Earth
to continue a trend of gradual cooling for the next few thousand years. However, this cooling abruptly
reversed about 150 years ago. Why? Carbon dioxide levels in the atmosphere
have been rising since the 19th century, when fossil fuel use increased. We know that from studying air bubbles
trapped in Antarctic ice. This surge in carbon dioxide
also coincides with a global temperature increase
of nearly one degree Celsius. Ice cores
and atmospheric monitoring stations show us that carbon dioxide levels
are rising faster, and to higher levels, than at any point
in the last 800,000 years. Computer models forecast another one to
four degrees Celsius of warming by 2100, depending on how much
additional fossil fuel we burn. What does that mean for the ice currently
on Greenland and Antarctica? Past climate changes suggest that even
a small warming shift can begin a process of ice melt
that continues for thousands of years. By the end of this century, ice melt is expected to raise
the sea level by 30 to 100 centimeters, enough to impact many coastal cities
and island nations. If a four-degree Celsius warming persisted
for several millennia, the sea level could rise
by as much as 10 meters. By studying past climates, scientists learn more about what drives
the shifts in ice that have shaped our planet
for millions of years. Research suggests that
by taking action now to reduce carbon dioxide
emissions quickly, we still have the opportunity to curb
ice loss and save our coastal communities.

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