Climate change is a major issue facing our planet today, but it is not the first time Earth has undergone dramatic shifts in climate. Studying how and why Earth’s climate has oscillated in the past is an active area of research.
Climate refers to average weather conditions over long periods of time. Climate forcing describes processes that alter the balance of Earth’s energy budget. Earth’s energy budget and climate are determined by incoming radiation from the sun, the planet’s reflectivity or “albedo”, and the atmospheric greenhouse effect which traps heat. On very long time scales radiation reaching Earth from the sun has changed and may have altered Earth’s climate. The faint young sun conundrum is a puzzle to scientists, referring to the fact that early Earth was very warm but less radiation was reaching Earth from the Sun than today.
Earth underwent a shift in climate from a hothouse state to an icehouse state during the Cenozoic Era beginning 66 million years ago. During that time solar irradiance did not change, so the two most likely factors responsible for this climatic shift are the greenhouse effect and changes in Earth’s albedo. Unfortunately, there is no record of Earth’s long-term albedo, but through chemical proxies scientists can estimate how much CO2, a major greenhouse gas, was present in the atmosphere millions of years ago. During hothouse conditions, CO2 concentrations were over 1000 ppm, and decreased to 180 – 280 ppm during the Pleistocene Icehouse Earth, which began 2.6 million years ago. Today, atmospheric CO2 concentrations are around 400 ppm. Scientists study sources and sinks of CO2 in Earth’s atmosphere to understand how environmental processes alter atmospheric CO2 concentrations, such as volcanic degassing, respiration of organic material, and chemical weathering.
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