Introduction and Key Results
Because the instrumental record of climate change (i.e., measurements of temperature, precipitation, etc., from various weather stations and monitoring centers around the world) extends only to the 19th century at the earliest, our knowledge of climate of the geologic past largely depends on proxy records contained in climate archives. Although these indirect measurements of past climate come with significant uncertainty—for example, the temperature dependence of certain processes cannot be perfectly known—and there are not proxies for all climate parameters, paleoclimate records and models have given us a much longer perspective on natural climate variability throughout the Earth’s history. With this information, it is possible to evaluate how unusual anthropogenic climate change is and gain insight into how certain parts of climate system respond to forcings. The key findings of this chapter of the IPCC report are summarized below:
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Concentrations of the greenhouse gases CO2, CH4, and N2O are higher now than at anytime in the last 800,000 years, based on measurements of air bubbles trapped in Antarctic ice cores.
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The rate of increase of CO2, CH4, and N2O is unprecedented in the last 22,000 years and probably the last 800,000 years.
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Although orbital variations primarily drove the glacial-interglacial cycles of the last ~2.6 million years, CO2 played an important role in amplifying temperature changes to the degree observed.
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Estimates of climate sensitivity from the Last Glacial Maximum (LGM; 21,000–19,000 years ago) indicate that a doubling of CO2 results in a temperature change between 1 and 6ºC.
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During past periods of high atmospheric CO2, temperatures were commensurately higher. For example, the Mid-Pliocene (3.3 to 3.0 million years ago) had atmospheric CO2 concentrations that were between 350 and 450 ppm (2013: 396.5 ppm [co2now.org]) and temperatures were 1.9 to 3.6ºC higher than today. During the Early Eocene (52 to 48 million years ago), atmospheric CO2 was likely above 1000 ppm and temperatures were likely ~9-14ºC higher than today.
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Various paleoclimate reconstructions show a significant amplification of the temperature response to CO2 in polar regions (see polar amplification).
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The current rate of sea level rise is unprecedented in the last 2,000 years.
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During the Last Interglacial period (a relatively warm period 129,000 to 116,000 years ago), sea level was 5-10 m higher than today due mostly to melt of the Greenland and Antarctic Ice Sheets. Similarly, during the warm Pliocene (3.3 to 3.0 million years ago), sea level was up to 20 meters higher than today.
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Temperatures during the Last Interglacial were never more than 2ºC above pre-industrial values (values from ~1750, before human activities began influencing climate in a significant way).
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Modern warming has reversed a 5,000 year cooling trend that was a result of minor changes in the Earth’s orbit.
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The recent decrease of sea ice and elevated sea surface temperatures in the Arctic is unprecedented in the last 1,450 years at least.
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Glaciers are shrinking despite orbital parameters being favorable for glacier growth, and Northern Hemisphere glaciers are likely to shrink to a minimum extent similar to that 8,000-6,500 years ago within this century.
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The period 1983-2012 was likely the warmest 30-year period in the last 1,400 years.
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In the executive summary of Chapter 5, the authors state that “it is virtually certain that orbital forcing will be unable to trigger widespread glaciation during the next 1000 years.” If CO2 stays above 300 ppm, glaciation is unlikely for the next 50,000 years.
A researcher examines a Greenland ice core, one of the most valuable climate archives in the Northern Hemisphere. Photo from www.bbc.com.