Sea levels change over different spatial and temporal scales. The images produced by altimetry illustrate well the spatial variations, and also provide important data on relatively recent temporal changes. We can examine sea level changes over short term and longer term. Examination of tide gauge data gives us a detailed look at sea level change over a short period of history. These are valuable, but do not show us the whole picture.
If we want to look back at the planet’s ocean levels before people began making measurements, we must use proxy, or indirect measurement. This is the basis of the science of paleoclimatology. Before looking at more information on paleoclimate, we need to understand how these data are obtained.
How do we know what the climate was like 500 million years ago? To reconstruct and understand the fluctuations in climate that have taken place on Earth, scientists use proxy, or indirect data, including data obtained in ice cores, coral, tree rings, and ocean and lake sediment cores.
Paleoclimatologists use various forms of environmental evidence to understand the Earth’s past climate. Earth’s past climate conditions are preserved in tree rings, skeletons of tropical coral reefs, sediment layers in lakes and the ocean, and in the ice of glaciers and ice caps. Using these records, paleoclimatologists can reconstruct climate conditions going back hundreds of millions of years to create graphs such as the one in Figure 4.4 on the previous page.
It was the examination and analysis of ice cores and their trapped molecular contents that revealed the connection between Earth’s atmospheric CO2 and temperature. In order to unlock the information contained in the ice, scientists collect cores and analyze them in slices representing small increments of time, using very precise methods. This way patterns that identify changes in the atmosphere's composition and temperature can be revealed.
For example, the ratio of oxygen isotopes present in the cores ("light" oxygen-16 to "heavy" oxygen-18) can tell the story of global temperatures when the ice formed. Colder temperatures are needed to produce precipitation when water vapor in the atmosphere contains higher levels of oxygen 16.
The paleorecord shows that the Earth’s climate is always changing and that in the distant past (such as the Cretaceous – think end of the dinosaurs’ reign - from 145.5 to 65.5 million years ago), the climate on Earth was much warmer than today and sea levels would have been significantly higher. See Figure 4.4 on the previous page.
The paleoclimate record also shows that in relatively recent geologic time (within the last 2 million years), the Earth underwent a series of glacial periods, which locked much of the Earth’s water in ice which covered the northern hemisphere landmasses. This caused the sea level to drop much lower than today (more than 400 ft. below current levels). We are currently in an “interglacial” period during which the Earth has warmed and the sea level has risen.
Paleoclimate records can also help to shed light on the more recent changes and provide evidence for the anthropomorphic effects on climate and sea level, correlating an unprecedented rapid rise in sea level with increased carbon dioxide in the atmosphere. More on that later.
Please read the article on the how scientists use ice cores to reconstruct past climates, "Climate at the core: how scientists study ice cores to reveal Earth’s climate history," that is linked from the Module 4 Overview Roadmap.