What keeps Earth warm?
I’m sure you’ve seen the iconic photo of the beautiful, blue and green Earth suspended in the black of space. But have you wondered, how is that possible? Earth’s average temperature is about 60 degrees Fahrenheit, but the average temperature of empty space is minus-461.47 Fahrenheit, 1.8F above absolute zero. At the edge of Earth’s atmosphere the temperature is a little warmer, an object there will be 248F in sunlight and minus-148F in Earth’s shadow.
The “Snowball Earth” theory suggests 650 million years ago Earth’s surface, including the ocean, was almost entirely frozen. Heat from Earth’s formation, internal friction, and nuclear decay provides sufficient heat to keep Earth’s interior molten, but insufficient to keep its surface thawed.
Fortunately Earth’s gravity is able to hold an atmosphere, but it’s not that simple. The troposphere, the bottom layer of the atmosphere, which holds 80 percent of its mass, is only four miles thick at the poles and 12 miles thick at the equator, equivalent to a coat of shellac on a basketball.
But a thin atmosphere isn’t the only problem. Ninety-nine percent of Earth’s atmosphere is made up of two gases, nitrogen 78 percent and oxygen 21 percent, that are transparent to heat radiation, meaning they can’t trap heat.
Earth would remain frozen but for trace amounts of atmospheric greenhouse gases (GHGs). The primary naturally occurring GHG is water vapor (H2O), but its atmospheric concentration is very short lived and totally dependent on atmospheric temperature, when Earth cools H2O precipitates out of the atmosphere and when Earth warms H2O evaporates into the atmosphere.
Water vapor thus amplifies warming and cooling initiated by other forces. Before humans, these other forces were earth’s orbital cycles, solar variation, and naturally occurring GHGs — water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) which comprised only 300th of 1 percent of Earth’s pre-industrial atmosphere. Now the primary warming and climate forcing is us.
It’s the molecular structure of GHG molecules that allow them to trap heat. They always contain three or more atoms, which make them able to vibrate, absorbing longwave heat radiation from Earth’s surface and releasing it back to Earth’s surface and atmosphere, warming Earth. The abundant nitrogen and oxygen molecules have only two tightly bound atoms that cannot vibrate, thus they cannot absorb surface heat radiation to keep Earth warm.
Since such a small atmospheric GHG concentration warms Earth, it follows that small increases will make Earth warmer, and we have increased GHGs by 40 percent. Just as H2O will amplify warming, warming also releases CO2 and CH4 from natural sinks (ocean, forests, soil), which causes more warming, which releases more CO2 and CH4. These are “positive” feedbacks because they amplify warming in the climate system. Another feedback is the decrease in Earth’s “albedo,” or reflectivity, from the melting of ice and snow. There are many amplifying feedbacks.
Scientists know this because it’s basic physics and because Earth keeps records. Air bubbles trapped in ice provide records of atmospheric composition and temperature, among other things, over hundreds of thousands of years. Other “proxy measures” such as ocean and lake sediment contain records over hundreds of millions of years.
From ice records scientists know over the last 800,000 years Earth’s climate has swung from glacial to interglacial and back eight times. Interglacial warming is initiated approximately every 100,000 years by changes in Earth’s orbital cycles, and amplified by positive feedbacks. During the warm interglacial periods CO2 rose to approximately 300 ppm while temperature rose to approximately 5.4F above the Holocene average (the last 10,000 years).
Over the Holocene Earth’s temperature, climate, and ocean levels stabilized, allowing human civilization to flourish. During this period Earth’s temperature didn’t vary more than 1.8F from the Little Ice Age to the Medieval Warm Period.
The scientific understanding of the relationship between atmospheric CO2 and Earth’s temperature reaches back nearly two centuries. In science this theory is settled: if atmospheric CO2 goes up Earth’s temperature will follow. Currently, from our fossil-fuel burning, atmospheric CO2 is over 400 ppm, the highest it’s been in millions of years, so we can expect, at equilibrium, temperature to reach its highest in millions of years too. Natural variability continues, but human forcing dominates.
Venus once, much like Earth, had oceans before irreversible, runaway GHG-warming changed it forever. Now with its thick CO2 atmosphere and sulfuric acid clouds Venus is hot enough (860F) to melt lead and 100,000 times dryer than Earth.
Baba Dioum, a Senegalese forestry engineer, is often quoted: “In the end we will conserve only what we love, we will love only what we understand, and we will understand only what we are taught.”
Perhaps, if we are taught and understand how fragile and at risk Earth is, we will come to love Earth sufficiently to conserve for all future generations.
William Gran, now retired, was an adjunct instructor at Greenfield Community College on global warming and climate change. He can be reached at: whgran@gmail.com