Saturday, December 17, 2016

Global Warming - The View from Deep Time

Geologists routinely talk about geologic epochs millions of years in the past like everyone else talks about last week's sports scores.

The Earth has been around for 4.55 billion years. If all of Earth's history were compressed into a 24-hour clock, all of recorded history would take place in the last tenth of a second.

In a nutshell, this is the concept of deep time.

The geologic record tells us that the climate has been radically different from what we consider normal today. Based on fossils found in Alaska, if you lived in Anchorage, Alaska just 3 million years ago, you would have been walking under palm trees and dodging alligators.

But since then the climate has undergone a lot of instability, shifting quickly between brutal cold and climate similar to, or somewhat warmer than today's climate in the space of just a few thousand years, a long time relative to a human life, but a few tenths of a second on the 24-hour geologic clock.

We see evidence of these shifts from a variety of sources including deep sea cores, layers extracted from stalagmites in limestone caverns, cross sections through corals and ice cores drilled through the Greenland and Antarctic ice caps.

These data, gleaned from many different sources, tell the same story. Not only that climate has shifted, but the reason why.

The first part of the reason is how currents flow in the ocean. Three million years ago, the connection between the Pacific and Atlantic oceans was blocked by geologic uplift and volcanic activity that created Panama in Central America.

This change in ocean circulation amplified the second part: the effects of small changes in the amount of sunlight hitting the earth due to variations in Earth's orbit around the sun, as well as the Earth's tilt, relative to the sun.

In case you didn't know, Earth wobbles a bit while it spins, and its orbit is not a perfect circle (it's an ellipse), so sometimes the northern hemisphere is closer to the sun in summer and sometimes in winter.

When everything lines up, the Earth plunges into one of several ice ages which have periodically buried the northern hemisphere under miles of ice.

The very last piece of this puzzle is carbon dioxide (C02).

The amount of CO2 in the atmosphere is very small, about 400 parts per million (ppm) or 0.04% of all gases in the air, as of today. Incidentally, the last time CO2 was at 400 ppm was 3 million years ago . . . when you could find alligators in Alaska.

So why is CO2 so important? CO2 absorbs infrared (heat) radiation. The sun bombards the earth with a lot of light, which is absorbed by the earth's surface and re-radiated as heat. CO2 acts like a filter, trapping some of that heat. A French scientist by the name of Joseph Fourier discovered this property of CO2 in the 1820s. Scientists also figured out that a little CO2 goes a very long way.

If we had no CO2 in the atmosphere, Earth would be mostly covered in ice. At least twice in Earth's history, it DID practically freeze solid in a phenomenon called "Snowball Earth."

The freeze was most likely due to the first photosynthesizing bacteria consuming most of the CO2, plunging the Earth into a deep freeze that was eventually replenished by long-term volcanic activity.

The last "snowball" occurred about 700 million years ago.

You may or may not know that the colder water gets, the more gas it can absorb. As the Earth started to cool during one of those times when it was getting less sunshine, the oceans absorbed more CO2, which in turn made the air grow colder, then made the oceans grow colder, then the oceans could absorb more CO2 and so on, until at the depths of an ice age, there was only about 180 ppm of CO2 in the atmosphere.

This process is called a feedback loop. In this case, a negative feedback loop.

During the start of an ice age, declining CO2 did not trigger cooling, but the decline accelerated the cooling and vice versa. Once more sunlight started to hit the Earth during periodic shifts in Earth's orbit and rotation, CO2 outgassing from the oceans accelerated the warming - a positive feedback loop. The CO2 feedback loop is one among many in nature.

At the depths of the last ice age, about 25,000 years ago, CO2 concentration was at about 180 ppm. As the ice age ended, it took about 15,000 years to get to 270 ppm, the concentration at the beginning of the industrial revolution in 1800.

Since 1800, the concentration of CO2 has gone from 270 to 400 ppm, an increase of over 40 percent, in just two centuries, and a good chunk of that has occurred in just the last 50 years.

Scientists have been able to make good estimates on the concentration of CO2 in the atmosphere based on deep sea sediment and ice cores. In the last 65 million years, there is nothing close to this rate of change in the geologic record.


CO2 has increased so quickly that the rest of the Earth's systems have not yet caught up, which is why alligators have not yet taken up residence in Anchorage.

The view from deep time shows us that CO2 plays a very big role in our climate, either by causing change (Snowball Earth) or accelerating change (our current ice age cycle).

And that is why every major scientific organization on the planet is very, very, concerned about what is happening right now.

Originally published in the Westborough News 08/15/2015.

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