What was the weather like in New York last week? You can look it up on weather.com. What was it like on March 7, 1953, in the pre-internet era? You might be able to find it at the library, in the New York Times edition of that day. In fact, with weather data, you can go back pretty far. People began recording daily temperatures in the 1850s.
Why does this matter? Data about weather conditions taken over a period of time determines the climate of a region. Recorded data helps us understand how global temperatures changed in our recent past. It tells us more about changes in biodiversity and geological conditions over this period. Similarly, having climate data from ancient times can help scientists understand what the environmental conditions were when mere molecules became life. But without any records, how can we estimate climate conditions from millions of years ago?
One way scientists try to answer this question is by using proxies. A proxy is any indirect indicator of prehistoric conditions. Proxies in climate science are usually a preserved object that is characteristic of the time period being studied. In this case, scientists are detectives, piecing together evidence to describe something not directly witnessed. The simplest example is the way we can tell the age of a tree by counting the number of rings in its trunk. More complex techniques involve using the properties of radioisotopes, like carbon dating and other dating methods. The dating method chosen depends on the time period being studied.
Today we enjoy an atmosphere that is dense enough to let us breathe, but not so dense that our bones shatter from the pressure. However, there is no reason to believe that the atmospheric pressure was the same now as it was 3.5 Billion years ago. To determine what these ancient environmental conditions were, scientists use gas bubbles trapped in volcanic rocks from that era as proxies.
The physics behind this can actually be observed by pouring a carbonated drink. The fizz in the drink is dissolved carbon dioxide stored under high pressure. The pop and hiss we get after opening the bottle is the sound of this gas escaping. The pressure drops when the container is opened, and the carbon dioxide gas molecules start gathering together to form bubbles — a process known as nucleation. These bubbles start to rise towards the top of the glass since they are far less dense than the beverage.
As the bubbles rise, they get bigger. A bubble that strikes the surface is larger than what it was at the bottom where its nucleation occurred. This happens for two reasons. First, the pressure is lower at the top of a glass, so gas in those bubbles face less opposition in their effort to expand. Secondly, the bubbles also get bigger because they collect more carbon dioxide as they rise.
A snapshot of a carbonated beverage shows larger bubbles at the top of the glass and smaller ones close to the bottom. The size of the bubbles at the top relative to the ones at the bottom tell us a lot about the fluid and the gas it contains. Using the drink example, beverages with more fizz will show a greater difference in bubble size than those with less. External conditions also determine how the bubble sizes change, such as the outside air pressure. On Earth, carbonated beverages all behave the same way because the air pressure is pretty much the same everywhere. However, if we conduct them on Mars, where the atmosphere is thinner and exerts a lower pressure, we would get different results. The bubbles would show a greater size difference from top to bottom, because they have a lower resistance against their quest to expand as they rise.
So what does this have to do with the climate of early Earth? We have no reason to believe that the atmospheric pressure has been the same on Earth for all of its lifetime. Nature has been performing such bubble-rising experiments on Earth for eons. Gas bubbles formed and rose to the surface of lava which flowed freely on the surface of a young Earth. As the lava gradually solidified to form rock, these bubbles were trapped inside, forming spaces inside the rock called vesicles. Over time, these vesicles were filled in by rain and minerals like quartz and chlorite to form what geologists refer to as ‘amygdales’. Amygdales can be studied using X-rays to detect where the spaces are and paint a picture of what the atmospheric pressure of early Earth might have been.
Using a technique called X-ray tomography, a team of geologists led by Dr. Sanjoy Som at the Blue Marble Space Institute of Science scanned through a set of amygdales unearthed from the Pilbara Craton in Western Australia. Independent techniques had already determined the age of these rocks to be around 2.7 billion years old. X-ray tomography accurately reveals the interior structures of a rock, including the sizes and locations of gas vesicles. Based on these sizes, they estimated that Earth’s atmospheric pressure was around one-third of what it is today.
What are the implications of such a finding? Every chemist knows that temperature and pressure are vital in determining the outcome of their chemical reactions. Chemistry labs have equipment specifically designed to control the external conditions of an experiment. Think of Earth as one giant laboratory where chemical reactions that functioned best in those conditions ultimately formed the building blocks of life about 3.5 billion years ago. Knowing the average temperature and pressure of ancient Earth lets scientists infer its climate. The more we learn about the ancient climate, the closer we come to understanding how life formed.