CLIMATE CHANGE - UNIVERSITY OF EXETER - Week 2

This is the reflection for the second week (January 20-26) since I am a bit behind (ha!).  The second weeks lesson was really fascinating because it was covering primarily paleo-climate information. Prior to week 2 readings I had only the haziest notion of paleo-climate, so it was really informative. 

I understand that of necessity the paleo-climate models they provided us were simplified (nothing about ocean currents for example in discussing mechanisms of climate), but despite being simplified they were quite powerful in providing explanations for the major long term changes in earth's climate. 

First important discovery: our sun, Sol, puts forth substantially more energy today than it did 4.4 Billion years ago when the solar system and Earth was formed. Now before someone goes "ah ha! that's what's driving global warming today" let's quickly put that to rest. We are talking a slow, gradual warming for the entire 4.4 Billion year period - which covers both much warmer and much colder geological periods, and has little to do with the dramatic increase in temperatures observed in just the past 150 years since the start of the industrial age. 

So 4.2 Billion years ago (once the surface of the planet settled down enough to result in rock evidence we can still find today), the sun was substantially weaker, and earth received substantially less solar radiation. The estimate is 20 to 30 percent less energy output 4.2 Billion years ago (https://www.sciencenews.org/article/faint-young-sun).  This creates a bit of a puzzle: if we were to suddenly lose 20 to 30 percent of the sun's energy today, our world would plunge into a bitter freeze. The earth's surface would be so cold that there would be no liquid water left. But geologists and paleo-biologists can demonstrate to us in the form of fossils, and evidence of rain and water weathering that there was abundant liquid water on the surface of the earth 4.2 Billion years ago. So how come the earth was warm enough for liquid oceans, rain, streams and lakes even though there was less radiation from the sun. The most accepted hypothesis is that the earth's atmosphere was much denser and had much higher levels of carbon dioxide, methane, and combinations of nitrogen and oxygen that facilitated heat absorption. One source of these greenhouse gases would have been volcanoes. This is still an area of research and discussion to find the precise mechanisms for the warming.  

A new puzzle arises when one asks the question, well if the earth were as least as warm as it is today, with less sunlight and more greenhouse gases, why didn't the earth get consistently hotter over time?  The key to that is in chemical and geological processes that remove carbon dioxide from the air and lock it up in rocks in the earth's surface.  Particles of carbon dioxide are absorbed by water vapor and create rain, a lightly acidic rain (carbonic acid rain) that over time weathers (chemically wears down) the rocks of the earth's surface. Rain run-off in streams and rivers carries that rock and carbon bearing water into the oceans where the carbon and other minerals from the rock are used by tiny living creatures in the oceans to build their bodies and their shells. When they die, their remains filter to the bottom of the ocean and provide the sediments that become sedimentary rocks.  Constant weathering and the growth of living species removes carbon from the atmosphere over the billions of years that the sun grew brighter. 

About 2.2 Billion years ago a new puzzle emerges. For the first 2 billion years the earth's climate experienced swings from warmer to cooler, with the shrinking and growing of the earth's icy poles bearing witness to those swings. But at about 2.2 Billion years ago the geological evidence strongly supports the idea that the whole surface of the earth froze over, creating what is called "snowball earth". There are a variety of things that might have caused a cooling cycle, and physicists have determined that if as much as two-thirds of the earth's surface became covered with ice and snow, then the dramatically increased albedo (reflection back into space of sunlight) would reach a tipping point and there would be nothing to stop the earth from freezing entirely, which is apparently exactly what happened. The true puzzle becomes, how did the earth emerge from that frozen state to return to millions of years of much warmer climates? The key seems to be in the continuation of volcanic action throwing more greenhouse gasses into the atmosphere, but with all surface water frozen there was no rain to wash the carbon dioxide out of the air, create carbonic acid rain, and engage in rock weathering that would sequester the carbon in the earth's surface.  So the concentrations of greenhouse gases increased, and increased and increased until the air was warm enough to begin melting the ice ball. 

To me the most significant aspect of all this information is that the basic mechanism of climate - sunlight and greenhouse gases were the same 4.2 billion years ago as they are today, the only difference is that today we industrial humans are inputting significant additional carbon, methane, etc. into the atmosphere with our economic activity. We are taking carbon that was sequestered in the earth's crust for billions of years, hauling it out, burning it and returning it to the atmosphere. So that in 2013 the Mauna Loa monitoring station measured 400 ppm of atmospheric carbon dioxide, a level not seen on earth for the past 2 million years.