Unit 12: Earth's Changing Climate // Section 3: Climate Change: What the Past Tells Us
Throughout much of its 4.5 billion year history, Earth's climate has alternated between periods of warmth and relative cold, each lasting for tens to hundreds of millions of years. During the warmest periods, the polar regions of the world were completely free of ice. Earth also has experienced repeated ice ages—periods lasting for millions of years, during which ice sheets advanced and retreated many times over portions of the globe. During the most extreme cold phases, snow and ice covered the entire globe (for more details, see Unit 1, "Many Planets, One Earth").
From the perspective of geological time our planet is currently passing through a relatively cold phase in its history and has been cooling for the past 35 million years, a trend that is only one of many swings between hot and cold states over the last 500 million years. During cold phases glaciers and snow cover have covered much of the mid-latitudes; in warm phases, forests extended all the way to the poles (Fig. 5).
Figure 5. Ice sheet advance during the most recent ice age
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Source: Courtesy National Oceanic and Atmospheric Administration Paleoclimatology Program.
Scientists have analyzed paleoclimate records from many regions of the world to document Earth's climate history. Important sources of information about past climate shifts include:
- Mineral deposits in deep sea beds. Over time, dissolved shells of microscopic marine organisms create layers of chalk and limestone on sea beds. Analyzing the ratio of oxygen-18 (a rare isotope) to oxygen-16 (the common form) indicates whether the shells were formed during glacial periods, when more of the light isotope evaporated and rained down, or during warm periods.
- Pollen grains trapped in terrestrial soils. Scientists use radiocarbon dating to determine what types of plants lived in the sampled region at the time each layer was formed. Changes in vegetation reflect surface temperature changes.
- Chemical variations in coral reefs. Coral reefs grow very slowly over hundreds or thousands of years. Analyzing their chemical composition and determining the time at which variations in corals' makeup occurred allows scientists to create records of past ocean temperatures and climate cycles.
- Core samples from polar ice fields and high-altitude glaciers. The layers created in ice cores by individual years of snowfall, which alternate with dry-season deposits of pollen and dust, provide physical timelines of glacial cycles. Air bubbles in the ice can be analyzed to measure atmospheric CO2 levels at the time the ice was laid down.
Understanding the geological past is key to today's climate change research for several reasons. First, as the next sections will show, Earth's climate history illustrates how changing GHG levels and temperatures in the past shaped climate systems and affected conditions for life. Second, researchers use past records to tune climate models and see whether they are accurately estimating dynamics like temperature increase and climate feedbacks. The more closely a model can replicate past climate conditions, the more accurate its future predictions are likely to be.