The magnitude of temperature change in response to increased forcing (heat absorption in the atmosphere) must be consistent with climate changes in the past. In this module, students will learn about past climates, how we know what we know about them, and how that knowledge is useful for understanding current climate change. In particular, we will use past changes to estimate how sensitive the Earth’s climate is to radiative forcing — including all the real-world feedbacks.
Over billions of years, Earth’s climate co-evolved with the planet itself and with the chemistry of life. Early Earth was sterile and chemically reduced. Photosynthesis, burial of organic debris, and the deposition of limestone produced free oxygen in the oceans, which eventually oxidized the oceans and then the air. The Great Oxidation Event severely degraded the greenhouse effect and led to several catastrophic “snowball Earth” episodes. Life and climate eventually stabilized around 0.6 billion years ago.
Over hundreds of millions of years, climate is stabilized by slow but strong negative feedback involving sources and sinks of atmospheric CO2 associated with plate tectonics. Volcanic outgassing increases CO2 and temperature. Chemical weathering of fresh rock exposed in new mountain ranges consumes CO2 and lowers temperature. Sometimes volcanism exceeds weathering and Earth enters a “hothouse” climate for 100 million years or more. Conversely, an excess of mountains and weathering plunge Earth into an “icehouse” climate.
On timescales of hundreds of thousands of years, Earth’s climate has recently experienced about 20 huge oscillations between glacial (ice age) and interglacial (warm) climates. The stage was set for this by steady cooling that followed the collision of India and Asia 50 million years ago. The timing of ice ages is controlled by tiny variations in Earth’s orbit around the Sun, with strong positive feedbacks amplifying subtle seasonal changes into worldwide swings from ice to heat.