How does late 20th-century warming compare with climate changes over the previous two millennia?

Details

This review examines evidence for climate change over the past several millennia using instrumental records, high-resolution climate proxy data, and climate modeling studies. Its focus is the past 1 to 2 millennia.

The authors assess reconstructions and modeling studies covering several climate fields, including atmospheric circulation diagnostics, precipitation, and drought. They give particular attention to proxy-based reconstructions of temperature patterns in past centuries to place recent large-scale warming in a longer-term context.

The assessment affirms that late 20th-century warmth is unprecedented at hemispheric and, likely, global scales. More tentative evidence suggests that modes of climate variability such as the El Niño/Southern Oscillation and the North Atlantic Oscillation may also have shown late 20th-century behavior that was anomalous in a long-term context.

Regional conclusions are more cautious, particularly for the Southern Hemisphere and parts of the tropics, where high-resolution proxy data are sparse. Past regional trends differ dramatically from hemispheric and global trends, and changes in surface temperature are distinct from changes in precipitation and drought. These differences limit the usefulness of labels such as the “Little Ice Age” and “Medieval Warm Period” for describing climate epochs during the last millennium.

A comparison of empirical evidence with proxy-based reconstructions indicates that natural factors appear to explain relatively well the major surface temperature changes of the past millennium through the 19th century, including hemispheric means and some spatial patterns. However, the review concludes that only anthropogenic climate forcing can explain the anomalous warming of the late 20th century.