The natural range of sea level for the past 3 million years is from a high of 35 meters above present sea level, to a low of 120 meters below present sea level:
The +35 m value is from the warm period that occurred during the mid-Pliocene, before the Earth plunged into the Quaternary cycle of alternating ice ages and interglacial periods (we live in an interglacial). In the mid-Pliocene, lowlands such as much of Florida and the lower Mississippi River valley were under water.
The -120 m value is from peak-glacial times, when large quantities of water were stored on the continents in continental ice sheets which were over 1000 meters thick covering millions of square kilometers. In this time, large areas of continental shelf were exposed, such as a wide area west of the current Florida coast, and the Bering Land Bridge that linked Alaska and Siberia.
The slight changes in sea level that have been observed during the past century (2-3 mm per year) are largely due to the expansion of sea water as global temperatures have increased. The wide range of sea level over the past three million years is mostly due to changes in the amount of water stored on the continents as ice. Because of the high density of human settlement and development along coastlines, even modest changes in sea level, on the order of 0.5 to 1.0 meters, could cause havoc in low-lying areas.
The geological perspective is: change is the norm in the Quaternary.
Grace and peace
The US Geological Survey has a news release regarding climate during the mid-Pliocene Epoch, between 3.0 and 3.3 million years ago: Getting Warmer? Prehistoric Climate Can Help Forecast Future Changes. Scientists used paleontological data (i.e. fossils) to reconstruct surface water and deep-ocean temperatures, as well as ocean circulation patterns. Here are some of the findings:
- Global average temperatures in the mid-Pliocene were 2.5°C (4.5°F) greater than today. This is in the range of temperatures predicted by the Intergovernmental Panel on Climate Change (IPCC) for later in the 21st century.
- CO2 levels were only slightly higher than what is found today. This implies that the atmosphere currently has enough CO2 to cause a couple of degrees of warming. It could be the other way around: the higher CO2 levels could have been caused by the higher temperatures; either way, there is a correlation between high CO2 levels and higher temperatures.
- Warming was much greater in the North Atlantic and Arctic than in other oceanic areas. While worldwide temperatures in the Pliocene were on the order of 2.5°C warmer than today, temperatures in the North Atlantic were up to 18°C warmer, bringing the average annual temperature in some areas from -2°C to 16°C, which is temperate rather than polar. This is radical–and in this case, natural–climate change. It is also consistent with computer models that predict greater warming in polar regions than in the rest of the world during the 21st century.
- One of the conclusions was that “the likely cause of mid-Pliocene warmth was a combination of several factors, including increased heat transport from equatorial regions to the poles and increased greenhouse gases.”
Here’s a map showing the sea surface temperature (SST) anomaly for August, which is a comparison of the Pliocene SST with today’s SST. The dark blotch over the North Atlantic is the area that experienced the most extreme warming in the Pliocene compared to today. The yellow areas were about 2°C warmer than today. Note also the warmer area off the Pacific coast of South America. This indicates an el Niño-like warming of the east Pacific surface waters in the Pliocene.
This study shows again the importance of a geological perspective when talking about climate change:
- Not only is the present a key to the past, the past is a key to the present. By better understanding climate change in the Pliocene, we can get a better idea of the effects of warming in the 21st century. Being that the geometry of ocean basins has not changed appreciably since the Pliocene, the temperature and circulation patterns present in the Pliocene could be a good model for changes that could occur if global temperatures do increase by 2°C in the 21st century.
- Geology gives us a context for climate change in the present. We cannot hope to distinguish between natural climate fluctuations and human-caused climate change if we don’t have a good grasp of natural climate change that has occurred over the past few millions of years.
Grace and peace