Deep-Time and Abrupt Climate Events

The extreme warm and cold states of the geological past and the sudden transitions, recorded in ice and sediment, that show how rapidly climate can change.

Definition

Deep-time climate refers to the major warm and cold states of Earth's distant geological past, and abrupt climate events are large reorganizations of the climate system that occur over years to centuries, far faster than orbital pacing.

Scope

This topic covers climate over the deep geological past and the abrupt events punctuating it. It treats the alternation between warm hothouse and cold icehouse states across millions of years, hyperthermal events such as the Paleocene-Eocene Thermal Maximum, and the abrupt millennial-scale transitions of the last glacial period, including Dansgaard-Oeschger events and the Younger Dryas, along with the ocean-circulation and feedback mechanisms thought to drive such rapid change.

Core questions

• What caused Earth to alternate between hothouse and icehouse states?
• How fast and how large were past hyperthermal events such as the PETM?
• What mechanisms produced abrupt millennial-scale transitions?
• What do these events imply about thresholds and tipping points?

Key theories

Carbon-cycle control of deep-time climate

Over millions of years, tectonics, weathering, and volcanism regulate atmospheric carbon dioxide, driving the slow transitions between warm hothouse and cold icehouse states.

Ocean-circulation thresholds and abrupt change

Rapid reorganizations of the Atlantic overturning circulation, triggered by freshwater input, can flip regional climate between states within decades, explaining abrupt glacial-age events.

Mechanisms

On the longest timescales, the balance between volcanic carbon release and removal by silicate weathering sets atmospheric carbon dioxide and thus whether the planet is in a warm or glaciated state. Superimposed are abrupt events: large, fast injections of carbon produce hyperthermals, while pulses of freshwater into the North Atlantic can shut down or restart the overturning circulation, switching regional climate between modes within years to decades as recorded in ice and sediment.

Clinical relevance

Hyperthermal events serve as natural analogues for the consequences of rapid carbon release, and the record of abrupt transitions demonstrates that the climate system can cross thresholds and change far faster than the gradual orbital pacing.

History

Greenland ice cores in the 1980s and 1990s revealed that the last glacial period was punctuated by abrupt Dansgaard-Oeschger warmings, overturning the assumption that climate changes only gradually; deep-sea records meanwhile mapped the long hothouse-to-icehouse trend of the past sixty-five million years and events such as the Paleocene-Eocene Thermal Maximum.

Debates

Source and rate of carbon in the PETM

The origin, total amount, and release rate of the carbon that drove the Paleocene-Eocene Thermal Maximum are debated, with direct relevance to how it compares with present emissions.

Key figures

• Richard Alley
• Wallace Broecker
• James Zachos
• Thomas Stocker

Seminal works

• zachos2001
• alley2000

Frequently asked questions

How fast can climate change naturally?

Ice cores show that regional climate has shifted by several degrees within decades during past abrupt events, far faster than the slow pacing of the ice ages.

What was the Paleocene-Eocene Thermal Maximum?

It was a rapid global warming about 56 million years ago, driven by a large carbon release, often studied as a natural analogue for present-day emissions.